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Red-emitting NLOphoric carbazole-coumarin hybrids - Synthesis, photophysical properties and DFT studies
Dyes and Pigments 129 (2016) 174e185
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
Red-emitting NLOphoric carbazole-coumarin hybrids - Synthesis, photophysical properties and DFT studies Abhinav B. Tathe, Nagaiyan Sekar* Tinctorial Chemistry Group, Department of Dyestuff Technology, Institute of Chemical Technology, N. P. Marg, Matunga, Mumbai, MH 400019, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2016 Received in revised form 15 February 2016 Accepted 25 February 2016 Available online 2 March 2016
The carbazole core is known to be a very efficient donor and can assist the coumarin core in achieving the red emission by pushing electrons in the chromophoric system. 9-ethyl-9H-carbazole-3-carbaldehyde and 9-ethyl-9H-carbazole-3,6-dicarbaldehyde were reacted with 3-acetyl-4-hydroxy-coumarin and 7(N,N-diethylamino)-3-acetyl-4-hydroxy-coumarin to give the corresponding hybrids. The BF2-complexes of the 4-hydroxy-3-acetyl-coumarin derivatives were reacted to give the coumarin-carbazole hybrids. The molecules synthesized emit in the red region. Their photophysical properties were studied in various solvents of different polarity. The solvent polarity function plots were studied and show linearity. The introduction of two units of coumarin core on carbazole improves the molar extinction co-efficient of the dye. The BF2-complexation has enhanced the quantum efficiency and non-linear optical properties of the molecules. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Carbazole-coumarin DFT TD-DFT Red emitting dyes NLO properties
1. Introduction Extensively conjugated organic molecules are widely researched for use in organic electronics [1e3] in the light of the apparent disadvantages of the widely used inorganic materials like lithium niobate in optoelectronic devices [4e8]. Organic nonlinear optical (NLO) materials are used for NLO devices and applications like optical switches, modulators, devices for frequency mixing processes, optical sensors, integrated optical circuits for telecommunications, optical computing and optical poling [9,10]. NLOphoredoped polymers are being increasingly studied as electrooptic materials in electrooptic devices [11], frequency doubling in optical storage devices operating near 400 nm rather than 800 nm with increased storage densities [12], and photorefractive materials in holographic storage and intensity-dependent filters [13]. Strategic design of NLOphoric molecules is mostly guided by the experience, and such molecules are largely of pushepull type with donor and acceptor groups over a p-conjugated core [11e14]. Coumarin class of chromophores has been successfully exploited for research in the above areas [18]. Methacrylate polymers having coumarin substitution have been shown to have reasonable electrooptic coefficients [19,20]. Similar coumarin-based materials
* Corresponding author. E-mail addresses: [email protected], [email protected] (N. Sekar). http://dx.doi.org/10.1016/j.dyepig.2016.02.026 0143-7208/© 2016 Elsevier Ltd. All rights reserved.
have been shown to have frequency doubling properties. Yankelevich et a1 have demonstrated frequency doubling [21]. Photorefractive properties as well as excellent charge mobility have been demonstrated with coumarin-doped polysilanes [22]. The bulk nonlinear properties are crucially dependent on the molecular hyperpolarizability and dipole moment of the molecule. Several amino substituted coumarins with donor-acceptor framework have been characterized for their nonlinear optical properties using EFISH as well as semiempirical methods [23]. The NLO properties of pentaammineruthenium complexes of coumarins 510 and 523 have been shown to have a large hyperpolarizability value [24]. Use of fluoro-coumarin and substituted styrylcoumarins is examined with specific examples for NLO properties [25]. Several p-aryl/heteroaryl conjugated coumarin-thiazoles have also been studied for their nonlinear optical properties [26]. The dye coumarin 6 doped potassium hydrogen phthalate (KAP) crystals has been used as NLO material [27]. Coumarin derived chromophores in the donor acceptoredonor format have been shown to show fluorescence enhancement and large two-photon activity in presence of specific metal ions [28,29]. The DFT and TDDFT methods have been employed in computing the NLO properties of coumarin derivatives [30,31]. Coumarin based polymer films and copolymers with large nonlinear optical properties have been reported [32,33]. Carbazole containing donor-acceptor systems are also known to show molecular nonlinearities [34,35]. The manifestation of high NLO properties of donoreacceptor
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organic p-eelctron conjugated systems bears an intrinsic relation to the intra-molecular charge transfer excited states [15e20]. The simplified two-state model developed by Oudar and Chemla based on the equivalent field model for b (Equation (1)) demonstrates that not only the energy of this transition but also the corresponding oscillator strength f as well as the change in dipole moment m are the deciding factors as exemplified by Equation (1).
DEeg f Dmge bf 2 2 DEeg ð2ðEexc Þ2 ½ DEeg ðEexc Þ2
(1)
where DEeg is electronic transition energy, f is oscillator strength and Dm is change of dipole moment between ground and excited state, respectively. Eexc is the frequency of the laser used for excitation. The above equation can be reduced to Equation (2) under the static conditions,
f Dmge bf 3 DEeg
(2)
The above model proposes that a major portion of the second order response in p-organic chromophores could be originating from the ground state deformation of the conjugated p-electron distribution modulated by the appended substituents. The subtle perturbation of the p-electron cloud caused by a particular substituent is defined as a substituent mesomeric moment (mR). The relationship between the p-distortion and the b is given by,
bf
3gDmge a
(3)
where a is polarizability, g is second order hyperpolarizability and b is the static hyperpolarizability. The origin of the hyperpolarizability, b, in such systems is rationalised in this model by Equation (4),
mge Dmge bf 3 DEeg
(4)
where, mge and DEeg are the dipole matrix element and transition energy, respectively, between the ground state and the first strongly allowed charge-transfer excited state. This model has been the guiding one in the design of second-order NLO chromophores for quite some time and has been reviewed extensively. The maximum value of b can thus be realized by a decrease in DE or by an increasing the product of mge Dmge . Therefore, in any chromophoric system the red and near infrared absorption are expected to give enhanced NLO properties. Introduction of the red shift to coumarin molecules can be achieved by a few strategies. Those include placing electron donor at 7position [21], electron acceptors at 3 and 4-position [22], rigidizing the donor and acceptor groups and increasing the p-conjugation [23]. The typical donors used are dialkylamino, eOH group and acceptors are eCN, benzoxazole, bezothiazole, benzimidazole, eCOOEt etc [24,25]. In this paper we have demonstrated the synthesis, photophysical properties and DFT computations of some red emitting (~600 nm) coumarins where carbazole core was used as a donor and coumarin acts as an acceptor. Further the electron accepting capacity of coumarin core was enhanced with BF2-complexation (Fig. 1). 2. Materials and methods All the commercial reagents were procured from SD Fine Chemicals (Mumbai) and were used without further purification.
175
Fig. 1. BF2-complexation to enhance red shift
Laboratory reagent grade solvents were purchased from Rankem, Mumbai. The reactions were monitored by TLC using on 0.25 mm EMerck silica gel 60 F254 precoated plates, which were visualized with UV light (254 nm and 344 nm). Melting points were measured on standard melting point apparatus from Sunder Industrial products, Mumbai and are uncorrected. 1H NMR and 13CNMR spectra were recorded on Varian 500 MHz instrument using TMS as an internal standard. Mass spectra were recorded on FINNIGAN LCQ ADVANTAGE MAX instrument from Thermo Electron Corporation (USA). The absorption spectra of the compounds were recorded on a Perkin Elmer Lambda 25 UVeVisible spectrophotometer; emission spectra were recorded on Varian Inc. Cary Eclipse spectrofluorometer. The relative quantum yields were calculated using rhodamine 6G as standard. The ground state (S0) geometry of all the compounds was optimized in the gas phase using Density Functional Theory (DFT) [26]. The popular hybrid functional B3LYP was used, which combines Becke's three parameter exchange functional (B3) [27] with the nonlocal correlation functional by Lee, Yang, and Parr (LYP) [28]. All the atoms were treated with 6e31 G(d) basis set, which deems to be sufficient for the molecules involved. The validity of the structures as local minima on potential energy surface was verified with vibrational analysis and confirmed they are with no imaginary frequencies. TD-DFT (Time Dependent Density Functional Theory) with same hybrid functional and basis set was used to estimate vertical excitation and their oscillator strength. The lowest singlet excited state (S1) was relaxed using TD-DFT to get optimised geometry of the excited state. Emissions were computed by taking ground state vertical excitations of lowest singlet excited state (S1) geometry. All the computations in solvents were carried out using the Polarizable Continuum Model (PCM) [29]. Gaussian 09W program [30] was used for all the quantum chemical computations and results were visualized with GaussView 5.0 [31]. 3. Result and discussion The molecules 2a-2b were synthesized by refluxing compound 1a-1b in benzene with BF2eEt2O at 60 C. The compounds 1a and 1b as well as their BF2-complexes (2a-2b) were reacted with nethyl carbazole mono aldehyde (Fig. 2 and Fig. 3) and n-ethyl carbazole dialdehyde. The carbazole core acts as a donor in these molecules. The compounds obtained from 1b has an additional donor group i.e. 7-n,n-diethylamino on the coumarin core. The conversions were carried out at 60e80 C, which is relatively a moderate condition. The conversion takes place without the need of any base, which could be detrimental to the stability of eBF2complex [32]. There are two sets of molecules one with single donor (includes 4a, 5a, 7a, 8a) and another is with two donors (includes 4b, 5b, 7b, 8b) (see Fig. 4e6). 3.1. Synthesis The compounds 2a-2b were synthesized from the compounds 1a-1b by heating in benzene at 60 C [32]. Carbazole aldehydes 3 and 6 were synthesized by Vilsmeier Haack reaction on N-ethyl carbazole [33,34].
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Fig. 2. Synthesis of carbazole-mono coumarin hybrids and their BF2-complex.
3.2. General procedure for synthesis of compound 4a, 4b, 5a, 5b Compound 3 (0.5 mmol) and compounds 1a-1b, 2a-2b (0.5 mmol) were refluxed in chloroform for a period of 12e24 h. The reaction mixture was clear pale coloured initially, turns darker (orange to red) at the end. This mixture was then concentrated and residue obtained was suspended in 5% CHCl3 ePet Ether and stirred for 1 h. The suspension was filtered and dried to give product in good yields. 3.2.1. 3-(3-(9-Ethyl-9H-carbazol-3-yl)acryloyl)-4-hydroxy-2Hchromen-2-one (4a) Yield: 70% M.P.: 210 C 1 HNMR (CDCl3, 500 MHz): d 1.462 (t, 3H), 4.392 (q, 2H), 7.25e7.322 (m, 3H), 7.416e7.440 (m, 2H), 7.890 (d, J ¼ 6 Hz, 1H), 8.098 (d, J ¼ 2 Hz, 1H), 8.168 (d, J ¼ 6 Hz, 1H), 8.366 (d, J ¼ 15 Hz, 1H),
8.438 (s,1H), 8.490 (d, J ¼ 15 Hz, 1H). 13 CNMR (CDCl3, 125 MHz): d 13.8, 37.9, 53.4, 109.0, 116.9, 118.6, 120.1, 120.8, 123.4, 124.2, 125.8, 126.5, 127.5, 135.6, 140.5, 142.2, 149.8, 154.6, 160.5, 182.0, 191.5. HRMS: 410.1387 (M þ Hþ) (Calculated for: C26H20NO4: 410.1392). 3.2.2. 4-(2-(9-Ethyl-9H-carbazol-3-yl)vinyl)-2,2-difluoro-5-oxo2,5-dihydro- [1,3,2]dioxaborinino [5,4-c]chromen-3-ium-2-uide (4b) Yield: 70% M.P.: 235 C 1 HNMR (CDCl3, 500 MHz): d 1.239(t, 6H), 1.481(t, 3H), 3.456(q, 4H), 4.396(q, 2H), 6.471(d, J ¼ 7 Hz, 1H), 6.6925(dd, J ¼ 7 Hz & 2 Hz, 1H), 7.295(t, 1H), 7.420(d, J ¼ 8 Hz, 2H), 7.439(t, J ¼ 2 Hz, 1H), 7.885(d, J ¼ 8 Hz, 1H), 8.157(d, J ¼ 15 Hz, 1H). 13 CNMR (CDCl3, 125 MHz): d 12.3, 13.8, 37.8, 45.6, 97.5, 98.5,
Fig. 3. Synthesis of carbazole-dicoumarin hybrids and their BF2-complexes.
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177
Fig. 4. Uvevisible spectrum for compounds 4a, 5a, 7a, 8a in various solvents.
108.9, 119.9, 120.8, 122.7, 126.3, 127.2, 140.5, 141.9, 147.7, 152.2, 157.1, 161.4, 169.6, 180.5, 191.0, 197.2. HRMS: 458.1504 (M þ Hþ) (Calculated for: C26H19BF2NO4: 458.1375).
3.2.3. 7-(Diethylamino)-3-(3-(9-ethyl-9H-carbazol-3-yl)acryloyl)4-hydroxy-2H-chromen-2-one (5a) Yield: 64% M.P.: 188 C 1 HNMR (CDCl3, 500 MHz): d 1.317 (t, 3H), 4.418(q, 2H), 5.624(d,
Fig. 5. Fluorescence emission spectrum for compound 4a, 5a, 7a, 8a in various solvents.
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Fig. 6. Uvevisible spectrum for compounds 4b, 5b, 7b, 8b in various solvents.
J ¼ 10.9 Hz, 1H), 6.922(d, J ¼ 10.9 Hz, 1H), 7.44e7.4937(m, 3H), 7.68(t, 1H), 8.199(d, J ¼ 7.75 Hz, 1H), 8.264(d, J ¼ 10 Hz, 1H), 8.595(d, J ¼ 10 Hz, 1H), 8.819(d, J ¼ 10 Hz, 1H). 13 CNMR (CDCl3, 125 MHz): d 13.1, 37.6, 54.2, 99.9, 108.5, 115.4, 119.5, 120.5, 122.3, 123.4, 125.1, 126.1, 135.4, 139.9, 154.1, 182.0, 190.7. HRMS: 481.2098 (M þ Hþ) (Calculated for C30H29N2O4: 481.2127).
3.2.4. 8-(Diethylamino)-4-(2-(9-ethyl-9H-carbazol-3-yl)vinyl)-2,2difluoro-5-oxo-2,5-dihydro- [1,3,2]dioxaborinino [5,4-c]chromen-3ium-2-uide (5b) Yield: 72% M.P.: 224 C 1 HNMR (CDCl3, 500 MHz): d 1.283 (t, 6H), 1.481 (t, 3H), 3.514(q, 4H), 4.407(q, 2H), 6.391(d, J ¼ 9.5 Hz, 1H), 6.641(d, J ¼ 9.5 Hz, 1H), 7.331e7.436 (m, 1H), 7.446 (t, J ¼ 4.5 Hz, 2H), 7.521 (t, J ¼ 4.5 Hz, 1H), 7.981(d, J ¼ 10 Hz, 1H), 8.168(d, J ¼ 10 Hz, 1H), 8.537e8.732(m, 3H). 13 CNMR (CDCl3, 125 MHz): d 12.5, 13.9, 22.1, 38.0, 45.4, 53.4, 109.3, 110.1, 116.0, 120.5, 121.1, 122.9, 123.9, 124.4, 125.9, 126.8, 128.9, 140.6, 143.0, 155.4, 158.5, 159.7, 173.4, 181.2. HRMS: 529.2106 (M þ Hþ) (Calculated for C30H28BF2N2O4 529.2110).
3.3. General procedure for synthesis of compound 7a, 7b, 8a, 8b Compound 6 (0.5 mmol) and compounds 1a-1b, 2a-2b (1.0 mmol) were refluxed in chloroform for a period of 12e24 h. The reaction mixture was clear pale coloured initially, turns darker (orange to red) at the end. This mixture was then concentrated and residue obtained was suspended in 5% CHCl3 ePet Ether and stirred for 1 h. The fine suspension was filtered and dried to give product.
3.3.1. 3,30 -((2,20 )-3,30 -(9-Ethyl-9H-carbazole-3,6-diyl)bis(acryloyl)) bis(4-hydroxy-2H-chromen-2-one) (7a) Yield: 80% M.P.: 248 C 1 HNMR (CDCl3, 500 MHz): d 1.361(t, 3H), 4.555(q, 2H), 7.820e7.855(m, 2H), 7.62(s, 2H), 8.029(d, J ¼ 8 Hz, 1H), 8.291(m, 2H), 8.749(s, 1H), 8.87(s,1H). 13 CNMR (CDCl3, 125 MHz): d 13.9, 22.3, 37.7, 44.7, 109.6, 110.0, 116.8, 122.9, 123.6, 124.3, 125.7, 129.3, 144.1, 191.5. HRMS: 624.1625 (M þ Hþ) (Calculated for C38H26NO8:624.1658). 3.3.2. 4,40 -((1,10 )-(9-Ethyl-9H-carbazole-3,6-diyl)bis(ethene-2,1diyl))bis(2,2-difluoro-5-oxo-2,5-dihydro- [1,3,2]dioxaborinino [5,4c]chromen-3-ium-2-uide) (7b) Yield: 80% M.P.: 272 C 1 HNMR (CDCl3, 500 MHz): d 1.273 (t, 12H), 1.486 (t, 6H), 3.476 (q, 8H), 4.416 (q, 4H), 6.391 (s, 2H), 6.645 (d, J ¼ 9 Hz, 2H), 7.436 (d, J ¼ 5 Hz, 2H), 7.446 (t, J ¼ 5 Hz, 2H), 7.970 (d, J ¼ 10 Hz, 2H), 8.101 (d, J ¼ 10 Hz, 2H), 8.639e8.688 (m, 2H), 8.761e8.791 (m, 2H). 13 CNMR (CDCl3, 125 MHz): d 12.7, 14.9, 25.5, 46.5, 95.1, 108.6, 111.1, 115.7, 120.7, 121.1, 123.9, 126.4, 129.6, 136.3, 137.4, 141.3, 143.1, 150.2, 156.6, 159.1, 183.5, 191.4. HRMS: 742.1469 (M þ Naþ) (Calculated for C38H23B2F4NNaO8:742.1444). 3.3.3. 3,30 -((2,20 )-3,30 -(9-Ethyl-9H-carbazole-3,6-diyl) bis(acryloyl))bis(7-(diethylamino)-4-hydroxy-2H-chromen-2-one) (8a) Yield: 64% M.P.: 214 C 1 HNMR (CDCl3, 500 MHz) d 1.535(t, 3H), 4.474(q, 2H), 6.35(d, J ¼ 12.4 Hz, 2H), 6.645(d, J ¼ 12.4 Hz, 2H), 7.450e7.501(m, 2H), 7.431(t, J ¼ 8 Hz, 1H), 7.55(d, J ¼ 10 Hz, 2H), 8.244 (m, 1H), 8.025(d,
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179
Table 1 Photophysical properties of compounds 4a, 5a, 7a and 8a. Solvent
Experimental
4a
5a
7a
8a
ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol
Computational
lem
(nm)
Е (L mol1 cm1)
414 451 448 348 346 447 411 408 448 529 536 523 539 540 524 535 545 527 424 455 442 366 367 438 392 393 439 515 524 507 563 560 508 505 507 510
26,000 36,000 33160 19,600 18,400 38,800 12 640 9200 34,400 36,000 42,000 42,800 18,800 23,200 35,600 21,200 16,000 44,000 42,400 70,800 70,800 72,000 48,000 48,000 27,600 31,600 66,800 12,400 13,600 15,200 2440 2280 14,400 6000 5600 17,200
labs
F
(nm)
Stokes shift (cm1)
578 554 534 e e 541 517 517 525 653 619 594 e e 610 e e 571 e 544 530 e e 542 e e 522 623 604 576 e e 588 e e 563
6854 4122 3595 e e 3887 4989 5167 3274 3590 2502 2285 e e 2691 e e 1462 e 3596 3757 e e 4381 e e 3622 3366 2528 2363 e e 2678 e e 1846
0.121 0.264 0.152 e e 0.176 <0.001 <0.001 0.099 <0.001 0.758 0.789 e e 0.048 e e 0.884 e 0.078 0.109 e e 0.027 e e 0.294 0.035 0.731 0.712 e e 0.426 e e 0.870
J ¼ 8 Hz, 1H), 8.630(d, J ¼ 10hz, 1H), 8.79(d, J ¼ 10 Hz, 1H). 13 CNMR (CDCl3, 125 MHz): d d 13.9, 30.1, 35.7, 38.5, 97.0, 109.8, 110.3, 116.4, 117.2, 125.2, 126.7, 130.1, 135.6, 136.1, 138.2, 157.3, 166.6, 191.4. HRMS: 766.3086 (M þ Hþ) (Calculated for: C46H44N3O8: 766.3128). 3.3.4. 4,40 -((1,10 )-(9-Ethyl-9H-carbazole-3,6-diyl)bis(ethene-2,1diyl))bis(8-(diethylamino)-2,2-difluoro-5-oxo-2,5-dihydro- [1,3,2] dioxaborinino [5,4-c]chromen-3-ium-2-uide) (8b) Yield: 62% M.P.: 238 C 1 HNMR (CDCl3, 500 MHz): d 1.263(t, 12H), 2.84(t, 3H), 3.51(q, 8H), 4.126(q, 2H), 4.450(q, 2H), 6.365(d, J ¼ 7 Hz, 2H), 6.661(d, J ¼ 7 Hz, 2H), 7.527(t, J ¼ 8 Hz, 2H), 7.930(d, J ¼ 9 Hz, 3H), 8.123(d, J ¼ 9 Hz, 1H), 8.579(s, 2H), 8.688(s, 1H). 13 CNMR (CDCl3, 125 MHz): d 12.5, 13.7, 16.4, 26.3, 45.6, 96.6, 110.5, 117.5, 129.4, 142.9, 147.7, 156.2, 158.0, 159.1, 191.5. HRMS: 862.3094 (M þ Hþ) (Calculated for C46H42B2F4N3O8: 862.3094). The compounds synthesized show a large red shift in absorption from their parent compounds. Visually the precursors are pale yellow to colourless in appearance and do not show intense colour in solution. The synthesized compounds show yellow to orange colour for compounds 4a, 7a, 4b and 7b. The BF2-complexed compounds (5a, 5b, 8a, 8b) are even more red shifted and were maroon to purple in colour.
labs
%D
(nm) 465 464 454 467 467 461 465 464 456 505 503 489 508 507 499 505 504 493 472 471 463 473 473 469 472 471 465 533 531 513 536 536 526 533 533 516
lem
%D
(nm) 12.3 2.9 1.3 34.2 35.0 3.1 13.1 13.7 1.8 4.5 6.2 6.5 5.8 6.1 4.8 5.6 7.5 6.5 11.3 3.5 4.8 29.2 28.9 7.1 20.4 19.8 5.9 3.5 1.3 1.2 4.8 4.3 3.5 5.5 5.1 1.2
485 485 483 488 488 482 485 484 484 563 570 514 565 574 575 564 562 542 493 493 485 495 495 490 493 493 487 551 548 530 553 553 543 551 551 533
16.1 12.5 9.6 e e 10.9 6.2 6.4 7.8 13.8 7.9 13.5 e e 5.7 e e 5.1 e 9.4 8.5 e e 9.6 e e 6.7 11.6 9.3 8.0 e e 7.7 e e 5.3
In solutions the absorption and emission wavelengths differs according to solvents used and the substituents present on the molecules. The compound 4a which is simplest of all, bearing a nethyl carbazole and 3-acetyl-4-hydroxy coumarin derivative shifts the absorption in visible region. The absorption wavelength ranges from 414 nm for acetonitrile (ACN) and 448 nm for toluene. The highly polar solvents such as DMF and DMSO shows a blue shifted absorption. In non-polar solvents such as toluene and dioxane the absorption wavelength is the most red shifted (448 nm). The molar absorptivitiesare higher in non-polar solvents, i.e. 38,880 L mol1 cm1 in ethyl acetate, 36,000 L mol1 cm1 in dichloromethane and 34,400 L mol1 cm1 for toluene. A similar trend is observed in emission properties, however a larger Stokes shift is observed in polar solvents such as acetonitrile (6854 cm1) and a lower stokes shifts in non-polar solvent like toluene (3274 cm1). The molecule was non-fluorescent in DMF and DMSO and shows low to moderate quantum yields in other solvents. The dichloromethane gives highest quantum yield at 0.264 (26.4%) (Table 1). Compound 5a which is BF2-complexed derivative of 4a, clearly shows a huge red shift in the absorption wavelength. This observation can be attributed to increase in electron pulling capacity of the eOeBF2eOe core. The red shift induced is between 75 and 194 nm in various solvents. The red shift observed in emission wavelength is not at the same level as absorption (46e75 nm), as a result the Stokes shift has decreased significantly. The reason of this difference in Stokes shift is due to decreased structural freedom as a
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Table 2 Photophysical properties of compounds 4b, 5b, 7b and 8b. Solvent
Experimental
4b
5b
7b
8b
ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol
Computational
(nm)
Е (L mol1 cm1)
444 451 448 456 462 447 444 439 448 529 536 524 541 541 524 540 545 527 418 448 438 364 364 436 389 389 437 514 519 507 581 582 505 495 495 510
58,800 58,800 68,000 22,000 64,000 65,600 48,000 35,200 54,400 36,400 40,000 42,800 18,400 23,200 35,600 2040 1960 44,000 42,800 72,000 72,000 50,000 72,000 50,400 28,000 32,320 68,000 12,400 14,000 15,200 2400 2480 14,400 6400 5720 17,320
labs
lem
F
labs (nm)
%D
lem (nm)
%D
(nm)
Stokes shift (cm1)
e 553 537 e e 527 571 570 517 618 592 574 626 638 582 612 614 566 552 536 532 e e 536 e e 520 614 590 570 e e 586 e e 550
e 4090 3699 e e 3396 5009 5235 2979 2722 1765 1662 2510 2810 1902 2179 2062 1307 5808 3665 4034 e e 4279 e e 3653 3169 2319 2180 e e 2737 e e 1426
e 0.663 0.556 e e 0.519 0.295 0.398 0.466 <0.001 0.108 0.427 <0.001 <0.001 0.104 <0.001 <0.001 0.815 0.002 0.234 0.111 e e 0.018 e e 0.232 <0.001 0.603 0.940 e e 0.272 e e 0.960
446 444 434 448 448 441 446 445 436 508 505 488 511 511 500 508 507 491 471 469 458 473 473 466 471 471 460 546 541 519 548 549 535 545 545 522
0.5 1.6 3.1 1.8 3.0 1.3 0.5 1.4 2.7 4.0 5.8 6.9 5.5 5.5 4.6 5.9 7.0 6.8 12.7 4.7 4.6 29.9 29.9 6.9 21.1 21.1 5.3 6.2 4.2 2.4 5.7 5.7 5.9 10.1 10.1 2.4
533 541 571 535 545 535 533 569 533 535 532 520 538 538 530 534 534 522 498 495 480 500 500 491 498 497 482 573 568 545 576 576 562 573 573 548
e
consequence of BF2-complexation. However the decrease in Stokes shift is accompanied by a many fold increase in quantum yield. At the excited state more the structural freedom, more is the probability of higher Stokes shift and at the same time more is the probability of non-radiative de-excitation. Compound 7a has two acceptor cores in the form of two 4hydroxy-3-acetyl-coumarin moieties. There is a smaller red shift of 10 nm and 5 nm observed in acetonitrile and dichloromethane respectively when compared to compound 4a. However this red shift is not observed in other solvents. The molar extinction coefficients are almost doubled from 4a. The quantum yields have come down to 0.027 to 0.294 across the solvents. The two acceptor groups have significantly increased the absorptivity of the carbazole-coumarin compounds. The effect of BF2-complexation on two acceptor system is similar to the one acceptor system (5a). The red shift in absorption and emission and increase in the quantum yield is also observed in compound 8b. The computational estimation of absorption (vertical excitation) also predicts a similar trend in various solvents. The deviations normally ranges from 1.2% to 6.5% in non-polar solvents. But on the other hand in polar solvents the deviations ranges from 4.8% to 35%. The red shift induced by BF2-complexation and introducing second acceptor core is well predicted by the theoretical method. The emission computations are fairly in agreement with the experimental emissions and deviations are given in Table 1, which ranges from 5.1% to 16.0%. Another set of compounds i.e 4b, 5b, 7b, and 8b has a distinction from last set in placement of the donor and acceptor groups. The
2.2 6.3 e e 1.5 6.7 0.2 3.1 13.4 10.1 9.4 14.1 15.7 8.9 12.7 13.0 7.8 9.8 7.6 9.8 e e 8.4 e e 7.3 6.7 3.7 4.4 e e 4.1 e e 0.4
typical donor is carbazole core and acceptor is 4-hydroxy 3-acetyl coumarin or its BF2-complex. The addition to this is a 7-n,ndiethylamino group which acts as a donor. The systems discussed here become D-A-D (4b and 5b) or D-A-D-A-D (7b and 8b) systems. Compound 4b is the most simple compound in this series which has only one additional substituent i.e. diethylamino group at 7position in the compound 4a. The red shift of around 30 nm in non-polar solvents is observed. Stokes shift is lowered in case of compound 4b; however there is a significant increase in the quantum yields (Table 2). Higher molar extinction co-efficients and quantum yields are two clear advantages the addition of diethylamino group provides. The compounds 7b and 8b has similar correlation as that of 7a and 8a. Computational absorption and emission are in good agreement with the observed ones. The deviation in absorption from the observed one remains the lowest at 0.5%e29%. A higher deviation is observed in case of the polar solvents. Deviation in emission values ranges from 0.4% to 15.7%. The absorption properties of the compounds can also be expressed in the parameters like FWHM (full width at half maxima), IAC (integrated absorption co-efficient), s (one photon absorption cross-section). FWHM is the measure of broadness of the absorption peak. In case of compound 4a the FWHM is lower for the polar solvents and opposite is the case for compound 5a. Similarly in compound 7a the absorption peak is narrower in polar solvents, whereas 8a has broader peak in the same solvents. The one photon cross section of the molecules also indicates that the compound 4a and 7a absorbs well in non-polar solvents. Compound 8b shows
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181
Fig. 7. Fluorescence emission spectrum for compound 4b, 5b, 7b, 8b in various solvents.
lowest one photon absorption cross section. Oscillator strengths in non-polar solvents are higher for all the molecules. The second set of molecules also show similar trend but the difference is marginal (Supporting Information) (see Fig. 7).
to relatively electron deficient coumarin core Fig. 8. In HOMOs the electron density is located on the N-ethyl carbazole core and shifts to the coumarin core in LUMO. In BF2complex the electron density is more concentrated on the BF2-core and hence proved to be more efficient electron acceptor.
3.4. FMO 3.5. Transition dipole moment The FMO diagram of the synthesized molecules support the assumption that the coumarin and coumarin-BF2-core act as acceptors. The carbazole core being a strong donor pushes electrons
The charge transfer character of the fluorophore can be understood from the oscillator strength (f) and transition dipole moment
Fig. 8. FMO diagram of compound 4a and 5a.
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Table 3 Transition dipole moments (meg) obtained from absorption properties. Solvent
4a
5a
7a
8a
4b
5b
7b
Table 7 Experimental and computed b xxx values for compounds 4a, 5a, 7a and 8a in various solvents.
8b
Solvent
Debye ACN DCM DIOX DMF DMSO EtOAc EtOH MeOH Tol
7.74 7.12 8.55 4.67 5.51 9.67 5.19 3.77 8.38
7.74 6.88 8.03 5.93 6.64 6.40 2.29 1.82 7.12
10.15 13.07 12.46 10.75 6.59 9.46 6.60 7.11 11.98
4.75 5.12 5.23 2.38 2.14 5.30 4.21 3.77 5.50
11.43 11.40 11.87 8.32 10.68 10.33 10.01 8.61 9.59
12.96 12.17 12.67 10.87 10.49 12.48 10.69 7.46 12.73
8.35 10.95 10.18 6.97 6.84 9.51 8.00 6.73 9.91
10.14 11.53 10.35 10.20 8.37 10.31 9.14 8.49 9.45
Method
4a
5a
7a
8a
4b
5b
7b
8b
Bilot-Kawski Bakhshiev Liptay
0.17 0.15 0.14
0.20 0.17 0.16
0.71 0.74 0.77
0.24 0.21 0.20
0.20 0.17 0.16
0.25 0.21 0.20
0.68 0.67 0.65
0.32 0.29 0.27
Table 5 Linear polarizability aCT calculated by solvatochromic method and computed aCT for compound 4a, 5a, 7a and 8a.
ACN DCM DIOX DMF DMSO EtOAc EtOH MeOH Tol
4a
5a
7a
8a
E axx
C axx
E axx
C axx
E axx
C axx
E axx
C axx
16.9 14.5 21.0 4.9 6.5 27.9 7.5 4.1 19.2
22.6 21.7 19.6 22.6 22.7 21.3 22.5 22.6 19.8
21.6 16.0 21.7 12.0 14.8 14.3 1.9 1.2 16.3
74.4 81.8 71.0 74.5 68.6 79.6 85.1 74.3 71.7
29.7 49.2 44.1 27.0 9.9 26.1 11.5 13.7 38.5
213.9 208.4 190.5 213.9 214.3 205.0 213.0 213.7 191.9
7.9 8.8 8.9 2.1 1.6 9.6 6.1 5.0 9.5
264.8 254.5 224.2 265.0 265.7 248.5 263.1 264.4 226.5
of the dyes (meg). The effective number of electrons transition from the ground to excited state is usually described by the oscillator strength, which provides the absorption area in the electronic spectrum. The oscillator strength (f) can be calculated using the following Equation (5) [35].
Z εðvÞdv
(5)
Where, ε is the molar absorptivity(L mol1 cm1). v represents the Table 6 Linear polarizability aCT calculated by solvatochromic method and computed aCT for compound 4b, 5b, 7b and 8b. Solvent
ACN DCM DIOX DMF DMSO EtOAc EtOH MeOH Tol
4b
5b
7b
8b
8a
E bxxx
C bxxx
E bxxx
C bxxx
E bxxx
C bxxx
4.9 4.5 6.5 1.2 1.5 8.5 2.1 1.2 6.0
2.8 2.6 1.8 2.8 2.8 2.4 2.8 2.8 1.9
11.6 9.1 12.0 6.6 8.4 7.6 1.1 0.7 8.8
52.8 19.4 11.4 52.9 41.6 17.5 22.6 52.6 11.9
24.3 43.2 37.3 18.4 6.9 22.3 8.7 10.0 33.1
0.4 0.3 0.1 0.4 0.4 0.2 0.4 0.4 0.1
4.4 5.1 5.0 1.3 1.0 5.4 3.4 2.8 5.3
0.1 1.1 0.5 0.1 0.1 1.0 0.1 0.1 0.5
numerical value of wavenumber (cm1). Transition dipole moments for absorption (meg) which is a measure of the probability of radiative transitions have been calculated for the dyes different solvent environments using the Equation (6) [36].
m2eg ¼
f 4:72 107 v
(6)
The values of transition dipole moment (meg) for compounds for each solvent is given in Table 3. The transition dipole moment (meg) values are solvent dependent. Compound 4a and 7a show higher transition dipole moments in non-polar solvents in general. When compounds with and without additional diethylamino group, the later (4b, 5b, 7b and 8b) has higher transition dipole moment in all the solvent as compared to former (4a, 5a, 7a and 8a). This indicates a higher probability of excitation.
The solvatochromic behaviour of the molecules 4a-4c and 5a-5c was studied with the help of Lippert [37], Weller [38] and Rettig's [38] plots. Lippert's plot is the plot of stokes shift in cm1 vs orientation polarizability (Equations given in supplementary information). T function (NorAlso the stokes shift was plotted against the EN malised DimrotheReichardt parameter) [39] of the respective solvents. The solvent polarity parameters plots are given in supporting information. The solvent polarity plots gives good regression coefficients (~0.8e0.9) and suggests a very good linearity. The non-polar
Table 8 Experimental and computed b xxx values for compounds 4b, 5b, 7b and 8b in various solvents. Solvent
E axx
C axx
E axx
C axx
E axx
C axx
E axx
C axx
39.5 37.1 40.6 20.2 32.6 31.8 29.9 22.5 25.2
62.0 60.4 55.4 62.0 62.1 59.5 61.7 61.9 55.8
60.5 50.3 54.2 40.9 36.8 54.5 41.5 20.9 52.2
196.2 187.7 163.2 196.3 196.9 183.0 194.9 195.9 165.1
19.9 33.9 29.2 11.3 10.5 26.3 16.8 12.1 26.2
284.3 276.1 251.5 284.4 285.0 271.3 283.0 284.0 253.4
36.0 43.7 35.0 38.7 25.2 35.8 27.9 24.6 27.8
358.1 343.1 301.0 358.4 359.5 335.1 355.8 357.6 304.1
E axx ¼ Experimental, C axx ¼ Computational. Values are in the order of 1030 esu
7a
C bxxx
3.6. Solvatochromism
E axx ¼ Experimental, C axx ¼ Computational. Values are in the order of 1030 esu
f ¼ 4:32 109
5a
E bxxx
E bxxx ¼ Experimental, C bxxx ¼ Computed. Values are in the order of 1030 esu.
Table 4 Dipole moment ratio me =mg calculated with different methods.
Solvent
ACN DCM DIOX DMF DMSO EtOAc EtOH MeOH Tol
4a
ACN DCM DIOX DMF DMSO EtOAc EtOH MeOH Tol
4b
5b
7b
8b
E bxxx
C bxxx
E bxxx
C bxxx
E bxxx
C bxxx
E bxxx
C bxxx
19.8 18.9 20.4 10.1 16.6 16.1 15.2 10.9 13.1
70.8 62.2 41.8 70.9 71.6 57.7 69.4 70.5 43.1
29.0 24.2 25.4 19.9 18.3 25.5 21.0 10.2 25.0
6.9 2.2 0.4 6.9 7.0 4.9 6.7 6.8 0.5
3.5 6.7 5.5 1.8 1.7 4.9 2.9 2.1 5.0
41.6 40.3 22.9 41.6 41.5 37.1 42.9 41.7 24.1
21.4 25.9 20.4 25.4 17.3 20.7 15.8 13.8 16.4
5.2 4.4 0.1 5.2 5.2 0.1 5.0 5.1 0.1
E bxxx ¼ Experimental, C bxxx ¼ Computed. Values are in the order of 1030 esu.
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Table 9 Total first order hyperpolarizability calculated with B3LYP/6-31G (d). Solvent
4a
5a
7a
8a
4b
5b
7b
8b
572.7 499.0 315.2 573.9 458.7 458.7 560.9 570.1 327.1 481.8 1309
359.3 323.3 226.0 359.8 362.4 303.0 353.6 358.0 232.7 319.8 869
576.0 503.9 325.9 577.1 582.5 464.6 564.4 573.5 337.5 500.6 1360
175.4 158.0 110.6 175.6 176.9 148.2 172.6 174.8 113.9 156.2 425
310.6 269.8 176.4 311.1 314.0 251.3 304.4 309.2 182.6 269.9 734
295.3 267.4 192.9 295.7 297.8 251.8 290.8 294.3 198.0 264.9 720
489.0 430.9 290.1 489.9 494.3 401.4 479.6 487.0 299.4 429.1 1166
bo (1030) ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol Average Ratio with Urea (Urea: Compound)
437.6 386.7 253.1 438.4 442.1 358.3 429.6 435.9 262.1 382.7 1040
Table 10 Solvatochromic descriptor < g > of third order hyperpolarizability of compounds. Solvent
4a
5a
7a
8a
4b
5b
7b
8b
ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol
1.4 1.1 2.3 0.1 0.2 4.2 0.2 0.1 1.9
2.5 1.3 2.5 0.7 1.1 1.0 0.1 0.1 1.3
2.9 10.5 8.0 2.3 0.1 2.1 0.1 0.3 5.7
0.2 0.3 0.3 0.1 0.1 0.3 0.1 0.1 0.3
7.8 7.0 8.4 1.9 5.4 5.0 4.3 2.3 3.0
23.1 16.0 18.3 10.5 8.4 18.5 10.8 2.6 17.0
2.0 6.4 4.6 0.6 0.5 3.7 1.3 0.7 3.7
7.3 11.1 6.7 9.4 3.6 7.1 4.0 3.1 4.1
All the molecules in this series give dipole moments ratio. me =mg less than unity. This suggests the excited state dipole moment is lower than the ground state dipole moment. 3.8. NLO properties Coumarin molecules can be good candidates for use as NLO (non-linear optical) materials [43e45]. The synthesized molecules show sensitivity towards the solvent polarity and the property is used to determine the NLO properties of the dyes. 3.9. Calculation of aCT from the solvatochromic data
solvents only interact with the solute by dipole interactions unlike the polar solvents which can interact through some of the specific interactions such as H-bonding. As the polarity of the solvent increases there is higher stokes shift is observed in all the molecules. The plots such as Weller and Rettig plot which consider only the emission wavelength also behave linearly. The trend here is in polar solvents emission is at longer wavelength. The polar solvents facilitates the higher degree of relaxation at excited state. 3.7. Dipole moment ratio All the molecules shows sensitivity to solvent polarity, not only the emission but Stokes shift is also sensitive to the solvent polarity. This behaviour of the molecules can be exploited to obtain the me =mg ratio. The following equations are used to obtain the me =mg ratio. The equations are introduced by Bilot-Kawski [40], Bakhshiev [41] and Liptay [42] for the estimation of ratio of excited state dipole moment and ground state dipole moment i.e. me =mg . The dipole moment ratios calculated with the equations are given in Table 4.
Table 11 Second order static hyperpolarizability obtained computationally using B3LYP/631G(d). Solvent
4a
5a
7a
8a
4b
5b
7b
8b
ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol
1610.8 1356.9 766.6 1614.8 1634.0 1222.1 1569.5 1601.7 802.7
2134.9 1763.7 946.2 2140.8 1558.9 1571.5 2073.8 2121.4 994.3
3445.0 3056.3 2033.7 3450.9 3479.5 2839.8 3383.4 3431.5 2102.8
5540.5 4706.8 2787.7 5553.7 5617.6 4266.4 5403.6 5509.7 2906.3
1990.4 1716.4 1053.8 1994.6 2015.2 1568.9 1946.1 1980.7 1095.9
3169.6 2623.2 1443.4 3178.4 3220.7 2346.4 3079.4 3149.8 1512.7
4908.2 4354.8 2956.9 4916.7 4957.9 4052.7 4819.2 4888.7 3049.4
9548.9 8121.2 4901.0 9571.2 9681.5 7392.9 9314.4 9497.7 5098.1
The linear polarizability aCT was evaluated experimentally for synthesized extended carbazole coumarin hybrids. These values are obtained by two-level model using UVevis absorption/emission spectroscopy. The solvatochromic method can also be utilized in the determination of dipole moment of the lowest lying charge transfer excited state [46]. The aCT are values calculated for carbazole-coumarin hybrids, which are compared with theoretically obtained aCT or axx (Table 5). 3.10. Calculation of bxxx from solvatochromic shifts The values for first hyperpolarizability obtained using the solvatochromic method is based on several assumptions and thus allow only approximate estimate of dominant tensor of total hyperpolarizability along the direction of charge transfer which is the major contributor to the total hyperpolarizability, though the values are approximate it has advantages over the other wellknown expensive methods to understand the trend (see Table 6). The calculated values of b xxx for compound 4a, 5a, 7a and 8a are given in Table 7. The bxxx calculated and experimental shows similar trends. The BF2-complexation has decreased the bxxx value as compared to the non-complexed compounds (4a and 7a). However the trends predicted across solvents only holds true for the BF2complexed molecules (5a and 8a). The possible reason to this may be the structural flexibility of the molecules, which with interaction with solvents gives different real world observation. In the case of second set of molecules where the additional donor is present the similar thing is not observed. The trends predicted by the computational method is contrary to the experimental observation. But the solvent effect on the bxxx is well predicted by the theoretical method despite some deviations Table 8. The value of b0 or total first order hyperpolarizability of the molecule gives an estimate of the NLO properties of the organic molecules. The computed values were compared with the values obtained for urea. The comparison of the values is given below in Table 9.
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The synthesized organic molecules show 425 to 1360 times greater hyperpolarizability than urea calculated at the same level. The addition of BF2-core in the molecule has increased the hyperpolarizability by 40e60%. The additional donor group in the form of diethylamino group on the coumarin core has lowered the total first order static hyperpolarizability. 3.11. Calculation of < g > from solvatochromic shifts The second order hyperpolarizability < g > SD (Equations are given in supporting information) at molecular level originating from the electronic polarization in the non-resonant region can be treated by a three-level model [19,47e50]. The values of “solvatochromic descriptor of second order hyperpolarizability” are calculated and are given in Table 10. The solvatochromic descriptor < g > of the second order hyperpolarizability of compounds show higher values in case of a two donor system found in compounds 4b, 5b, 7b and 8b. The values of the molecules with single donor remains 0.1 to 10.5. The electron flow enhancement in the chromophoric system seems to be enhancing the third order hyperpolarizability descriptor < g > . 3.12. Computational g value (second order static hyperpolarizability) The individual components of the second order static hyperpolarizability g were obtained computationally. The values are considered to be proportional to the solvatochromic descriptor < g > of the second order hyperpolarizability. The values obtained are given in Table 11. The values has a trend where higher polarity solvents show higher values of g, but the similar trend is not followed by the solvatochromically obtained values. 4. Conclusion In conclusion, we have successfully synthesized the carbazolecoumarin hybrids which emit in red region. Coumarin core is utilised as an efficient acceptor in these novel hybrids. The BF2complexation assists in pulling electrons from the carbazole core. The molecules synthesized have very good response to the solvent polarity. There is also improvement in quantum yields in BF2complexed compounds, over their parent uncomplexed compounds. In polar solvents molecules are either have very low fluorescence intensity or are non-fluorescent. The molecules were also evaluated for their NLO properties and found to be 425 to 1360 times greater in first order static hyperpolarizability than the urea. Thus these molecules can be considered as good candidates for non-linear optical materials (NLO). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2016.02.026. References [1] Hany R, Fan B, Araujo F, Castro D, Heier J, Kylberg W, et al. Strategies to improve cyanine dye multi layer organic solar cells. Prog Photovolta Res Appl 2011;19:851e7. http://dx.doi.org/10.1002/pip. [2] Cells E. Polymer light-emitting. Science 1995;269:1086e8. [3] Benmansour H, Castro FA, Nagel M, Heier J, Hany R, Nüesch F. Ionic space charge driven organic photovoltaic devices. Chim (Aarau) 2007;61:787e91. http://dx.doi.org/10.2533/chimia.2007.787. [4] Dalton LR, Benight SJ, Johnson LE, Knorr DB, Kosilkin I, Eichinger BE, et al. Systematic nanoengineering of soft matter organic electro-optic materials. Chem Mater 2011;23:430e45. http://dx.doi.org/10.1021/cm102166j.
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Red-emitting NLOphoric carbazole-coumarin hybrids - Synthesis, photophysical properties and DFT studies Abhinav B. Tathe, Nagaiyan Sekar* Tinctorial Chemistry Group, Department of Dyestuff Technology, Institute of Chemical Technology, N. P. Marg, Matunga, Mumbai, MH 400019, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2016 Received in revised form 15 February 2016 Accepted 25 February 2016 Available online 2 March 2016
The carbazole core is known to be a very efficient donor and can assist the coumarin core in achieving the red emission by pushing electrons in the chromophoric system. 9-ethyl-9H-carbazole-3-carbaldehyde and 9-ethyl-9H-carbazole-3,6-dicarbaldehyde were reacted with 3-acetyl-4-hydroxy-coumarin and 7(N,N-diethylamino)-3-acetyl-4-hydroxy-coumarin to give the corresponding hybrids. The BF2-complexes of the 4-hydroxy-3-acetyl-coumarin derivatives were reacted to give the coumarin-carbazole hybrids. The molecules synthesized emit in the red region. Their photophysical properties were studied in various solvents of different polarity. The solvent polarity function plots were studied and show linearity. The introduction of two units of coumarin core on carbazole improves the molar extinction co-efficient of the dye. The BF2-complexation has enhanced the quantum efficiency and non-linear optical properties of the molecules. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Carbazole-coumarin DFT TD-DFT Red emitting dyes NLO properties
1. Introduction Extensively conjugated organic molecules are widely researched for use in organic electronics [1e3] in the light of the apparent disadvantages of the widely used inorganic materials like lithium niobate in optoelectronic devices [4e8]. Organic nonlinear optical (NLO) materials are used for NLO devices and applications like optical switches, modulators, devices for frequency mixing processes, optical sensors, integrated optical circuits for telecommunications, optical computing and optical poling [9,10]. NLOphoredoped polymers are being increasingly studied as electrooptic materials in electrooptic devices [11], frequency doubling in optical storage devices operating near 400 nm rather than 800 nm with increased storage densities [12], and photorefractive materials in holographic storage and intensity-dependent filters [13]. Strategic design of NLOphoric molecules is mostly guided by the experience, and such molecules are largely of pushepull type with donor and acceptor groups over a p-conjugated core [11e14]. Coumarin class of chromophores has been successfully exploited for research in the above areas [18]. Methacrylate polymers having coumarin substitution have been shown to have reasonable electrooptic coefficients [19,20]. Similar coumarin-based materials
* Corresponding author. E-mail addresses: [email protected], [email protected] (N. Sekar). http://dx.doi.org/10.1016/j.dyepig.2016.02.026 0143-7208/© 2016 Elsevier Ltd. All rights reserved.
have been shown to have frequency doubling properties. Yankelevich et a1 have demonstrated frequency doubling [21]. Photorefractive properties as well as excellent charge mobility have been demonstrated with coumarin-doped polysilanes [22]. The bulk nonlinear properties are crucially dependent on the molecular hyperpolarizability and dipole moment of the molecule. Several amino substituted coumarins with donor-acceptor framework have been characterized for their nonlinear optical properties using EFISH as well as semiempirical methods [23]. The NLO properties of pentaammineruthenium complexes of coumarins 510 and 523 have been shown to have a large hyperpolarizability value [24]. Use of fluoro-coumarin and substituted styrylcoumarins is examined with specific examples for NLO properties [25]. Several p-aryl/heteroaryl conjugated coumarin-thiazoles have also been studied for their nonlinear optical properties [26]. The dye coumarin 6 doped potassium hydrogen phthalate (KAP) crystals has been used as NLO material [27]. Coumarin derived chromophores in the donor acceptoredonor format have been shown to show fluorescence enhancement and large two-photon activity in presence of specific metal ions [28,29]. The DFT and TDDFT methods have been employed in computing the NLO properties of coumarin derivatives [30,31]. Coumarin based polymer films and copolymers with large nonlinear optical properties have been reported [32,33]. Carbazole containing donor-acceptor systems are also known to show molecular nonlinearities [34,35]. The manifestation of high NLO properties of donoreacceptor
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organic p-eelctron conjugated systems bears an intrinsic relation to the intra-molecular charge transfer excited states [15e20]. The simplified two-state model developed by Oudar and Chemla based on the equivalent field model for b (Equation (1)) demonstrates that not only the energy of this transition but also the corresponding oscillator strength f as well as the change in dipole moment m are the deciding factors as exemplified by Equation (1).
DEeg f Dmge bf 2 2 DEeg ð2ðEexc Þ2 ½ DEeg ðEexc Þ2
(1)
where DEeg is electronic transition energy, f is oscillator strength and Dm is change of dipole moment between ground and excited state, respectively. Eexc is the frequency of the laser used for excitation. The above equation can be reduced to Equation (2) under the static conditions,
f Dmge bf 3 DEeg
(2)
The above model proposes that a major portion of the second order response in p-organic chromophores could be originating from the ground state deformation of the conjugated p-electron distribution modulated by the appended substituents. The subtle perturbation of the p-electron cloud caused by a particular substituent is defined as a substituent mesomeric moment (mR). The relationship between the p-distortion and the b is given by,
bf
3gDmge a
(3)
where a is polarizability, g is second order hyperpolarizability and b is the static hyperpolarizability. The origin of the hyperpolarizability, b, in such systems is rationalised in this model by Equation (4),
mge Dmge bf 3 DEeg
(4)
where, mge and DEeg are the dipole matrix element and transition energy, respectively, between the ground state and the first strongly allowed charge-transfer excited state. This model has been the guiding one in the design of second-order NLO chromophores for quite some time and has been reviewed extensively. The maximum value of b can thus be realized by a decrease in DE or by an increasing the product of mge Dmge . Therefore, in any chromophoric system the red and near infrared absorption are expected to give enhanced NLO properties. Introduction of the red shift to coumarin molecules can be achieved by a few strategies. Those include placing electron donor at 7position [21], electron acceptors at 3 and 4-position [22], rigidizing the donor and acceptor groups and increasing the p-conjugation [23]. The typical donors used are dialkylamino, eOH group and acceptors are eCN, benzoxazole, bezothiazole, benzimidazole, eCOOEt etc [24,25]. In this paper we have demonstrated the synthesis, photophysical properties and DFT computations of some red emitting (~600 nm) coumarins where carbazole core was used as a donor and coumarin acts as an acceptor. Further the electron accepting capacity of coumarin core was enhanced with BF2-complexation (Fig. 1). 2. Materials and methods All the commercial reagents were procured from SD Fine Chemicals (Mumbai) and were used without further purification.
175
Fig. 1. BF2-complexation to enhance red shift
Laboratory reagent grade solvents were purchased from Rankem, Mumbai. The reactions were monitored by TLC using on 0.25 mm EMerck silica gel 60 F254 precoated plates, which were visualized with UV light (254 nm and 344 nm). Melting points were measured on standard melting point apparatus from Sunder Industrial products, Mumbai and are uncorrected. 1H NMR and 13CNMR spectra were recorded on Varian 500 MHz instrument using TMS as an internal standard. Mass spectra were recorded on FINNIGAN LCQ ADVANTAGE MAX instrument from Thermo Electron Corporation (USA). The absorption spectra of the compounds were recorded on a Perkin Elmer Lambda 25 UVeVisible spectrophotometer; emission spectra were recorded on Varian Inc. Cary Eclipse spectrofluorometer. The relative quantum yields were calculated using rhodamine 6G as standard. The ground state (S0) geometry of all the compounds was optimized in the gas phase using Density Functional Theory (DFT) [26]. The popular hybrid functional B3LYP was used, which combines Becke's three parameter exchange functional (B3) [27] with the nonlocal correlation functional by Lee, Yang, and Parr (LYP) [28]. All the atoms were treated with 6e31 G(d) basis set, which deems to be sufficient for the molecules involved. The validity of the structures as local minima on potential energy surface was verified with vibrational analysis and confirmed they are with no imaginary frequencies. TD-DFT (Time Dependent Density Functional Theory) with same hybrid functional and basis set was used to estimate vertical excitation and their oscillator strength. The lowest singlet excited state (S1) was relaxed using TD-DFT to get optimised geometry of the excited state. Emissions were computed by taking ground state vertical excitations of lowest singlet excited state (S1) geometry. All the computations in solvents were carried out using the Polarizable Continuum Model (PCM) [29]. Gaussian 09W program [30] was used for all the quantum chemical computations and results were visualized with GaussView 5.0 [31]. 3. Result and discussion The molecules 2a-2b were synthesized by refluxing compound 1a-1b in benzene with BF2eEt2O at 60 C. The compounds 1a and 1b as well as their BF2-complexes (2a-2b) were reacted with nethyl carbazole mono aldehyde (Fig. 2 and Fig. 3) and n-ethyl carbazole dialdehyde. The carbazole core acts as a donor in these molecules. The compounds obtained from 1b has an additional donor group i.e. 7-n,n-diethylamino on the coumarin core. The conversions were carried out at 60e80 C, which is relatively a moderate condition. The conversion takes place without the need of any base, which could be detrimental to the stability of eBF2complex [32]. There are two sets of molecules one with single donor (includes 4a, 5a, 7a, 8a) and another is with two donors (includes 4b, 5b, 7b, 8b) (see Fig. 4e6). 3.1. Synthesis The compounds 2a-2b were synthesized from the compounds 1a-1b by heating in benzene at 60 C [32]. Carbazole aldehydes 3 and 6 were synthesized by Vilsmeier Haack reaction on N-ethyl carbazole [33,34].
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Fig. 2. Synthesis of carbazole-mono coumarin hybrids and their BF2-complex.
3.2. General procedure for synthesis of compound 4a, 4b, 5a, 5b Compound 3 (0.5 mmol) and compounds 1a-1b, 2a-2b (0.5 mmol) were refluxed in chloroform for a period of 12e24 h. The reaction mixture was clear pale coloured initially, turns darker (orange to red) at the end. This mixture was then concentrated and residue obtained was suspended in 5% CHCl3 ePet Ether and stirred for 1 h. The suspension was filtered and dried to give product in good yields. 3.2.1. 3-(3-(9-Ethyl-9H-carbazol-3-yl)acryloyl)-4-hydroxy-2Hchromen-2-one (4a) Yield: 70% M.P.: 210 C 1 HNMR (CDCl3, 500 MHz): d 1.462 (t, 3H), 4.392 (q, 2H), 7.25e7.322 (m, 3H), 7.416e7.440 (m, 2H), 7.890 (d, J ¼ 6 Hz, 1H), 8.098 (d, J ¼ 2 Hz, 1H), 8.168 (d, J ¼ 6 Hz, 1H), 8.366 (d, J ¼ 15 Hz, 1H),
8.438 (s,1H), 8.490 (d, J ¼ 15 Hz, 1H). 13 CNMR (CDCl3, 125 MHz): d 13.8, 37.9, 53.4, 109.0, 116.9, 118.6, 120.1, 120.8, 123.4, 124.2, 125.8, 126.5, 127.5, 135.6, 140.5, 142.2, 149.8, 154.6, 160.5, 182.0, 191.5. HRMS: 410.1387 (M þ Hþ) (Calculated for: C26H20NO4: 410.1392). 3.2.2. 4-(2-(9-Ethyl-9H-carbazol-3-yl)vinyl)-2,2-difluoro-5-oxo2,5-dihydro- [1,3,2]dioxaborinino [5,4-c]chromen-3-ium-2-uide (4b) Yield: 70% M.P.: 235 C 1 HNMR (CDCl3, 500 MHz): d 1.239(t, 6H), 1.481(t, 3H), 3.456(q, 4H), 4.396(q, 2H), 6.471(d, J ¼ 7 Hz, 1H), 6.6925(dd, J ¼ 7 Hz & 2 Hz, 1H), 7.295(t, 1H), 7.420(d, J ¼ 8 Hz, 2H), 7.439(t, J ¼ 2 Hz, 1H), 7.885(d, J ¼ 8 Hz, 1H), 8.157(d, J ¼ 15 Hz, 1H). 13 CNMR (CDCl3, 125 MHz): d 12.3, 13.8, 37.8, 45.6, 97.5, 98.5,
Fig. 3. Synthesis of carbazole-dicoumarin hybrids and their BF2-complexes.
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177
Fig. 4. Uvevisible spectrum for compounds 4a, 5a, 7a, 8a in various solvents.
108.9, 119.9, 120.8, 122.7, 126.3, 127.2, 140.5, 141.9, 147.7, 152.2, 157.1, 161.4, 169.6, 180.5, 191.0, 197.2. HRMS: 458.1504 (M þ Hþ) (Calculated for: C26H19BF2NO4: 458.1375).
3.2.3. 7-(Diethylamino)-3-(3-(9-ethyl-9H-carbazol-3-yl)acryloyl)4-hydroxy-2H-chromen-2-one (5a) Yield: 64% M.P.: 188 C 1 HNMR (CDCl3, 500 MHz): d 1.317 (t, 3H), 4.418(q, 2H), 5.624(d,
Fig. 5. Fluorescence emission spectrum for compound 4a, 5a, 7a, 8a in various solvents.
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Fig. 6. Uvevisible spectrum for compounds 4b, 5b, 7b, 8b in various solvents.
J ¼ 10.9 Hz, 1H), 6.922(d, J ¼ 10.9 Hz, 1H), 7.44e7.4937(m, 3H), 7.68(t, 1H), 8.199(d, J ¼ 7.75 Hz, 1H), 8.264(d, J ¼ 10 Hz, 1H), 8.595(d, J ¼ 10 Hz, 1H), 8.819(d, J ¼ 10 Hz, 1H). 13 CNMR (CDCl3, 125 MHz): d 13.1, 37.6, 54.2, 99.9, 108.5, 115.4, 119.5, 120.5, 122.3, 123.4, 125.1, 126.1, 135.4, 139.9, 154.1, 182.0, 190.7. HRMS: 481.2098 (M þ Hþ) (Calculated for C30H29N2O4: 481.2127).
3.2.4. 8-(Diethylamino)-4-(2-(9-ethyl-9H-carbazol-3-yl)vinyl)-2,2difluoro-5-oxo-2,5-dihydro- [1,3,2]dioxaborinino [5,4-c]chromen-3ium-2-uide (5b) Yield: 72% M.P.: 224 C 1 HNMR (CDCl3, 500 MHz): d 1.283 (t, 6H), 1.481 (t, 3H), 3.514(q, 4H), 4.407(q, 2H), 6.391(d, J ¼ 9.5 Hz, 1H), 6.641(d, J ¼ 9.5 Hz, 1H), 7.331e7.436 (m, 1H), 7.446 (t, J ¼ 4.5 Hz, 2H), 7.521 (t, J ¼ 4.5 Hz, 1H), 7.981(d, J ¼ 10 Hz, 1H), 8.168(d, J ¼ 10 Hz, 1H), 8.537e8.732(m, 3H). 13 CNMR (CDCl3, 125 MHz): d 12.5, 13.9, 22.1, 38.0, 45.4, 53.4, 109.3, 110.1, 116.0, 120.5, 121.1, 122.9, 123.9, 124.4, 125.9, 126.8, 128.9, 140.6, 143.0, 155.4, 158.5, 159.7, 173.4, 181.2. HRMS: 529.2106 (M þ Hþ) (Calculated for C30H28BF2N2O4 529.2110).
3.3. General procedure for synthesis of compound 7a, 7b, 8a, 8b Compound 6 (0.5 mmol) and compounds 1a-1b, 2a-2b (1.0 mmol) were refluxed in chloroform for a period of 12e24 h. The reaction mixture was clear pale coloured initially, turns darker (orange to red) at the end. This mixture was then concentrated and residue obtained was suspended in 5% CHCl3 ePet Ether and stirred for 1 h. The fine suspension was filtered and dried to give product.
3.3.1. 3,30 -((2,20 )-3,30 -(9-Ethyl-9H-carbazole-3,6-diyl)bis(acryloyl)) bis(4-hydroxy-2H-chromen-2-one) (7a) Yield: 80% M.P.: 248 C 1 HNMR (CDCl3, 500 MHz): d 1.361(t, 3H), 4.555(q, 2H), 7.820e7.855(m, 2H), 7.62(s, 2H), 8.029(d, J ¼ 8 Hz, 1H), 8.291(m, 2H), 8.749(s, 1H), 8.87(s,1H). 13 CNMR (CDCl3, 125 MHz): d 13.9, 22.3, 37.7, 44.7, 109.6, 110.0, 116.8, 122.9, 123.6, 124.3, 125.7, 129.3, 144.1, 191.5. HRMS: 624.1625 (M þ Hþ) (Calculated for C38H26NO8:624.1658). 3.3.2. 4,40 -((1,10 )-(9-Ethyl-9H-carbazole-3,6-diyl)bis(ethene-2,1diyl))bis(2,2-difluoro-5-oxo-2,5-dihydro- [1,3,2]dioxaborinino [5,4c]chromen-3-ium-2-uide) (7b) Yield: 80% M.P.: 272 C 1 HNMR (CDCl3, 500 MHz): d 1.273 (t, 12H), 1.486 (t, 6H), 3.476 (q, 8H), 4.416 (q, 4H), 6.391 (s, 2H), 6.645 (d, J ¼ 9 Hz, 2H), 7.436 (d, J ¼ 5 Hz, 2H), 7.446 (t, J ¼ 5 Hz, 2H), 7.970 (d, J ¼ 10 Hz, 2H), 8.101 (d, J ¼ 10 Hz, 2H), 8.639e8.688 (m, 2H), 8.761e8.791 (m, 2H). 13 CNMR (CDCl3, 125 MHz): d 12.7, 14.9, 25.5, 46.5, 95.1, 108.6, 111.1, 115.7, 120.7, 121.1, 123.9, 126.4, 129.6, 136.3, 137.4, 141.3, 143.1, 150.2, 156.6, 159.1, 183.5, 191.4. HRMS: 742.1469 (M þ Naþ) (Calculated for C38H23B2F4NNaO8:742.1444). 3.3.3. 3,30 -((2,20 )-3,30 -(9-Ethyl-9H-carbazole-3,6-diyl) bis(acryloyl))bis(7-(diethylamino)-4-hydroxy-2H-chromen-2-one) (8a) Yield: 64% M.P.: 214 C 1 HNMR (CDCl3, 500 MHz) d 1.535(t, 3H), 4.474(q, 2H), 6.35(d, J ¼ 12.4 Hz, 2H), 6.645(d, J ¼ 12.4 Hz, 2H), 7.450e7.501(m, 2H), 7.431(t, J ¼ 8 Hz, 1H), 7.55(d, J ¼ 10 Hz, 2H), 8.244 (m, 1H), 8.025(d,
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179
Table 1 Photophysical properties of compounds 4a, 5a, 7a and 8a. Solvent
Experimental
4a
5a
7a
8a
ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol
Computational
lem
(nm)
Е (L mol1 cm1)
414 451 448 348 346 447 411 408 448 529 536 523 539 540 524 535 545 527 424 455 442 366 367 438 392 393 439 515 524 507 563 560 508 505 507 510
26,000 36,000 33160 19,600 18,400 38,800 12 640 9200 34,400 36,000 42,000 42,800 18,800 23,200 35,600 21,200 16,000 44,000 42,400 70,800 70,800 72,000 48,000 48,000 27,600 31,600 66,800 12,400 13,600 15,200 2440 2280 14,400 6000 5600 17,200
labs
F
(nm)
Stokes shift (cm1)
578 554 534 e e 541 517 517 525 653 619 594 e e 610 e e 571 e 544 530 e e 542 e e 522 623 604 576 e e 588 e e 563
6854 4122 3595 e e 3887 4989 5167 3274 3590 2502 2285 e e 2691 e e 1462 e 3596 3757 e e 4381 e e 3622 3366 2528 2363 e e 2678 e e 1846
0.121 0.264 0.152 e e 0.176 <0.001 <0.001 0.099 <0.001 0.758 0.789 e e 0.048 e e 0.884 e 0.078 0.109 e e 0.027 e e 0.294 0.035 0.731 0.712 e e 0.426 e e 0.870
J ¼ 8 Hz, 1H), 8.630(d, J ¼ 10hz, 1H), 8.79(d, J ¼ 10 Hz, 1H). 13 CNMR (CDCl3, 125 MHz): d d 13.9, 30.1, 35.7, 38.5, 97.0, 109.8, 110.3, 116.4, 117.2, 125.2, 126.7, 130.1, 135.6, 136.1, 138.2, 157.3, 166.6, 191.4. HRMS: 766.3086 (M þ Hþ) (Calculated for: C46H44N3O8: 766.3128). 3.3.4. 4,40 -((1,10 )-(9-Ethyl-9H-carbazole-3,6-diyl)bis(ethene-2,1diyl))bis(8-(diethylamino)-2,2-difluoro-5-oxo-2,5-dihydro- [1,3,2] dioxaborinino [5,4-c]chromen-3-ium-2-uide) (8b) Yield: 62% M.P.: 238 C 1 HNMR (CDCl3, 500 MHz): d 1.263(t, 12H), 2.84(t, 3H), 3.51(q, 8H), 4.126(q, 2H), 4.450(q, 2H), 6.365(d, J ¼ 7 Hz, 2H), 6.661(d, J ¼ 7 Hz, 2H), 7.527(t, J ¼ 8 Hz, 2H), 7.930(d, J ¼ 9 Hz, 3H), 8.123(d, J ¼ 9 Hz, 1H), 8.579(s, 2H), 8.688(s, 1H). 13 CNMR (CDCl3, 125 MHz): d 12.5, 13.7, 16.4, 26.3, 45.6, 96.6, 110.5, 117.5, 129.4, 142.9, 147.7, 156.2, 158.0, 159.1, 191.5. HRMS: 862.3094 (M þ Hþ) (Calculated for C46H42B2F4N3O8: 862.3094). The compounds synthesized show a large red shift in absorption from their parent compounds. Visually the precursors are pale yellow to colourless in appearance and do not show intense colour in solution. The synthesized compounds show yellow to orange colour for compounds 4a, 7a, 4b and 7b. The BF2-complexed compounds (5a, 5b, 8a, 8b) are even more red shifted and were maroon to purple in colour.
labs
%D
(nm) 465 464 454 467 467 461 465 464 456 505 503 489 508 507 499 505 504 493 472 471 463 473 473 469 472 471 465 533 531 513 536 536 526 533 533 516
lem
%D
(nm) 12.3 2.9 1.3 34.2 35.0 3.1 13.1 13.7 1.8 4.5 6.2 6.5 5.8 6.1 4.8 5.6 7.5 6.5 11.3 3.5 4.8 29.2 28.9 7.1 20.4 19.8 5.9 3.5 1.3 1.2 4.8 4.3 3.5 5.5 5.1 1.2
485 485 483 488 488 482 485 484 484 563 570 514 565 574 575 564 562 542 493 493 485 495 495 490 493 493 487 551 548 530 553 553 543 551 551 533
16.1 12.5 9.6 e e 10.9 6.2 6.4 7.8 13.8 7.9 13.5 e e 5.7 e e 5.1 e 9.4 8.5 e e 9.6 e e 6.7 11.6 9.3 8.0 e e 7.7 e e 5.3
In solutions the absorption and emission wavelengths differs according to solvents used and the substituents present on the molecules. The compound 4a which is simplest of all, bearing a nethyl carbazole and 3-acetyl-4-hydroxy coumarin derivative shifts the absorption in visible region. The absorption wavelength ranges from 414 nm for acetonitrile (ACN) and 448 nm for toluene. The highly polar solvents such as DMF and DMSO shows a blue shifted absorption. In non-polar solvents such as toluene and dioxane the absorption wavelength is the most red shifted (448 nm). The molar absorptivitiesare higher in non-polar solvents, i.e. 38,880 L mol1 cm1 in ethyl acetate, 36,000 L mol1 cm1 in dichloromethane and 34,400 L mol1 cm1 for toluene. A similar trend is observed in emission properties, however a larger Stokes shift is observed in polar solvents such as acetonitrile (6854 cm1) and a lower stokes shifts in non-polar solvent like toluene (3274 cm1). The molecule was non-fluorescent in DMF and DMSO and shows low to moderate quantum yields in other solvents. The dichloromethane gives highest quantum yield at 0.264 (26.4%) (Table 1). Compound 5a which is BF2-complexed derivative of 4a, clearly shows a huge red shift in the absorption wavelength. This observation can be attributed to increase in electron pulling capacity of the eOeBF2eOe core. The red shift induced is between 75 and 194 nm in various solvents. The red shift observed in emission wavelength is not at the same level as absorption (46e75 nm), as a result the Stokes shift has decreased significantly. The reason of this difference in Stokes shift is due to decreased structural freedom as a
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Table 2 Photophysical properties of compounds 4b, 5b, 7b and 8b. Solvent
Experimental
4b
5b
7b
8b
ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol
Computational
(nm)
Е (L mol1 cm1)
444 451 448 456 462 447 444 439 448 529 536 524 541 541 524 540 545 527 418 448 438 364 364 436 389 389 437 514 519 507 581 582 505 495 495 510
58,800 58,800 68,000 22,000 64,000 65,600 48,000 35,200 54,400 36,400 40,000 42,800 18,400 23,200 35,600 2040 1960 44,000 42,800 72,000 72,000 50,000 72,000 50,400 28,000 32,320 68,000 12,400 14,000 15,200 2400 2480 14,400 6400 5720 17,320
labs
lem
F
labs (nm)
%D
lem (nm)
%D
(nm)
Stokes shift (cm1)
e 553 537 e e 527 571 570 517 618 592 574 626 638 582 612 614 566 552 536 532 e e 536 e e 520 614 590 570 e e 586 e e 550
e 4090 3699 e e 3396 5009 5235 2979 2722 1765 1662 2510 2810 1902 2179 2062 1307 5808 3665 4034 e e 4279 e e 3653 3169 2319 2180 e e 2737 e e 1426
e 0.663 0.556 e e 0.519 0.295 0.398 0.466 <0.001 0.108 0.427 <0.001 <0.001 0.104 <0.001 <0.001 0.815 0.002 0.234 0.111 e e 0.018 e e 0.232 <0.001 0.603 0.940 e e 0.272 e e 0.960
446 444 434 448 448 441 446 445 436 508 505 488 511 511 500 508 507 491 471 469 458 473 473 466 471 471 460 546 541 519 548 549 535 545 545 522
0.5 1.6 3.1 1.8 3.0 1.3 0.5 1.4 2.7 4.0 5.8 6.9 5.5 5.5 4.6 5.9 7.0 6.8 12.7 4.7 4.6 29.9 29.9 6.9 21.1 21.1 5.3 6.2 4.2 2.4 5.7 5.7 5.9 10.1 10.1 2.4
533 541 571 535 545 535 533 569 533 535 532 520 538 538 530 534 534 522 498 495 480 500 500 491 498 497 482 573 568 545 576 576 562 573 573 548
e
consequence of BF2-complexation. However the decrease in Stokes shift is accompanied by a many fold increase in quantum yield. At the excited state more the structural freedom, more is the probability of higher Stokes shift and at the same time more is the probability of non-radiative de-excitation. Compound 7a has two acceptor cores in the form of two 4hydroxy-3-acetyl-coumarin moieties. There is a smaller red shift of 10 nm and 5 nm observed in acetonitrile and dichloromethane respectively when compared to compound 4a. However this red shift is not observed in other solvents. The molar extinction coefficients are almost doubled from 4a. The quantum yields have come down to 0.027 to 0.294 across the solvents. The two acceptor groups have significantly increased the absorptivity of the carbazole-coumarin compounds. The effect of BF2-complexation on two acceptor system is similar to the one acceptor system (5a). The red shift in absorption and emission and increase in the quantum yield is also observed in compound 8b. The computational estimation of absorption (vertical excitation) also predicts a similar trend in various solvents. The deviations normally ranges from 1.2% to 6.5% in non-polar solvents. But on the other hand in polar solvents the deviations ranges from 4.8% to 35%. The red shift induced by BF2-complexation and introducing second acceptor core is well predicted by the theoretical method. The emission computations are fairly in agreement with the experimental emissions and deviations are given in Table 1, which ranges from 5.1% to 16.0%. Another set of compounds i.e 4b, 5b, 7b, and 8b has a distinction from last set in placement of the donor and acceptor groups. The
2.2 6.3 e e 1.5 6.7 0.2 3.1 13.4 10.1 9.4 14.1 15.7 8.9 12.7 13.0 7.8 9.8 7.6 9.8 e e 8.4 e e 7.3 6.7 3.7 4.4 e e 4.1 e e 0.4
typical donor is carbazole core and acceptor is 4-hydroxy 3-acetyl coumarin or its BF2-complex. The addition to this is a 7-n,ndiethylamino group which acts as a donor. The systems discussed here become D-A-D (4b and 5b) or D-A-D-A-D (7b and 8b) systems. Compound 4b is the most simple compound in this series which has only one additional substituent i.e. diethylamino group at 7position in the compound 4a. The red shift of around 30 nm in non-polar solvents is observed. Stokes shift is lowered in case of compound 4b; however there is a significant increase in the quantum yields (Table 2). Higher molar extinction co-efficients and quantum yields are two clear advantages the addition of diethylamino group provides. The compounds 7b and 8b has similar correlation as that of 7a and 8a. Computational absorption and emission are in good agreement with the observed ones. The deviation in absorption from the observed one remains the lowest at 0.5%e29%. A higher deviation is observed in case of the polar solvents. Deviation in emission values ranges from 0.4% to 15.7%. The absorption properties of the compounds can also be expressed in the parameters like FWHM (full width at half maxima), IAC (integrated absorption co-efficient), s (one photon absorption cross-section). FWHM is the measure of broadness of the absorption peak. In case of compound 4a the FWHM is lower for the polar solvents and opposite is the case for compound 5a. Similarly in compound 7a the absorption peak is narrower in polar solvents, whereas 8a has broader peak in the same solvents. The one photon cross section of the molecules also indicates that the compound 4a and 7a absorbs well in non-polar solvents. Compound 8b shows
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181
Fig. 7. Fluorescence emission spectrum for compound 4b, 5b, 7b, 8b in various solvents.
lowest one photon absorption cross section. Oscillator strengths in non-polar solvents are higher for all the molecules. The second set of molecules also show similar trend but the difference is marginal (Supporting Information) (see Fig. 7).
to relatively electron deficient coumarin core Fig. 8. In HOMOs the electron density is located on the N-ethyl carbazole core and shifts to the coumarin core in LUMO. In BF2complex the electron density is more concentrated on the BF2-core and hence proved to be more efficient electron acceptor.
3.4. FMO 3.5. Transition dipole moment The FMO diagram of the synthesized molecules support the assumption that the coumarin and coumarin-BF2-core act as acceptors. The carbazole core being a strong donor pushes electrons
The charge transfer character of the fluorophore can be understood from the oscillator strength (f) and transition dipole moment
Fig. 8. FMO diagram of compound 4a and 5a.
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Table 3 Transition dipole moments (meg) obtained from absorption properties. Solvent
4a
5a
7a
8a
4b
5b
7b
Table 7 Experimental and computed b xxx values for compounds 4a, 5a, 7a and 8a in various solvents.
8b
Solvent
Debye ACN DCM DIOX DMF DMSO EtOAc EtOH MeOH Tol
7.74 7.12 8.55 4.67 5.51 9.67 5.19 3.77 8.38
7.74 6.88 8.03 5.93 6.64 6.40 2.29 1.82 7.12
10.15 13.07 12.46 10.75 6.59 9.46 6.60 7.11 11.98
4.75 5.12 5.23 2.38 2.14 5.30 4.21 3.77 5.50
11.43 11.40 11.87 8.32 10.68 10.33 10.01 8.61 9.59
12.96 12.17 12.67 10.87 10.49 12.48 10.69 7.46 12.73
8.35 10.95 10.18 6.97 6.84 9.51 8.00 6.73 9.91
10.14 11.53 10.35 10.20 8.37 10.31 9.14 8.49 9.45
Method
4a
5a
7a
8a
4b
5b
7b
8b
Bilot-Kawski Bakhshiev Liptay
0.17 0.15 0.14
0.20 0.17 0.16
0.71 0.74 0.77
0.24 0.21 0.20
0.20 0.17 0.16
0.25 0.21 0.20
0.68 0.67 0.65
0.32 0.29 0.27
Table 5 Linear polarizability aCT calculated by solvatochromic method and computed aCT for compound 4a, 5a, 7a and 8a.
ACN DCM DIOX DMF DMSO EtOAc EtOH MeOH Tol
4a
5a
7a
8a
E axx
C axx
E axx
C axx
E axx
C axx
E axx
C axx
16.9 14.5 21.0 4.9 6.5 27.9 7.5 4.1 19.2
22.6 21.7 19.6 22.6 22.7 21.3 22.5 22.6 19.8
21.6 16.0 21.7 12.0 14.8 14.3 1.9 1.2 16.3
74.4 81.8 71.0 74.5 68.6 79.6 85.1 74.3 71.7
29.7 49.2 44.1 27.0 9.9 26.1 11.5 13.7 38.5
213.9 208.4 190.5 213.9 214.3 205.0 213.0 213.7 191.9
7.9 8.8 8.9 2.1 1.6 9.6 6.1 5.0 9.5
264.8 254.5 224.2 265.0 265.7 248.5 263.1 264.4 226.5
of the dyes (meg). The effective number of electrons transition from the ground to excited state is usually described by the oscillator strength, which provides the absorption area in the electronic spectrum. The oscillator strength (f) can be calculated using the following Equation (5) [35].
Z εðvÞdv
(5)
Where, ε is the molar absorptivity(L mol1 cm1). v represents the Table 6 Linear polarizability aCT calculated by solvatochromic method and computed aCT for compound 4b, 5b, 7b and 8b. Solvent
ACN DCM DIOX DMF DMSO EtOAc EtOH MeOH Tol
4b
5b
7b
8b
8a
E bxxx
C bxxx
E bxxx
C bxxx
E bxxx
C bxxx
4.9 4.5 6.5 1.2 1.5 8.5 2.1 1.2 6.0
2.8 2.6 1.8 2.8 2.8 2.4 2.8 2.8 1.9
11.6 9.1 12.0 6.6 8.4 7.6 1.1 0.7 8.8
52.8 19.4 11.4 52.9 41.6 17.5 22.6 52.6 11.9
24.3 43.2 37.3 18.4 6.9 22.3 8.7 10.0 33.1
0.4 0.3 0.1 0.4 0.4 0.2 0.4 0.4 0.1
4.4 5.1 5.0 1.3 1.0 5.4 3.4 2.8 5.3
0.1 1.1 0.5 0.1 0.1 1.0 0.1 0.1 0.5
numerical value of wavenumber (cm1). Transition dipole moments for absorption (meg) which is a measure of the probability of radiative transitions have been calculated for the dyes different solvent environments using the Equation (6) [36].
m2eg ¼
f 4:72 107 v
(6)
The values of transition dipole moment (meg) for compounds for each solvent is given in Table 3. The transition dipole moment (meg) values are solvent dependent. Compound 4a and 7a show higher transition dipole moments in non-polar solvents in general. When compounds with and without additional diethylamino group, the later (4b, 5b, 7b and 8b) has higher transition dipole moment in all the solvent as compared to former (4a, 5a, 7a and 8a). This indicates a higher probability of excitation.
The solvatochromic behaviour of the molecules 4a-4c and 5a-5c was studied with the help of Lippert [37], Weller [38] and Rettig's [38] plots. Lippert's plot is the plot of stokes shift in cm1 vs orientation polarizability (Equations given in supplementary information). T function (NorAlso the stokes shift was plotted against the EN malised DimrotheReichardt parameter) [39] of the respective solvents. The solvent polarity parameters plots are given in supporting information. The solvent polarity plots gives good regression coefficients (~0.8e0.9) and suggests a very good linearity. The non-polar
Table 8 Experimental and computed b xxx values for compounds 4b, 5b, 7b and 8b in various solvents. Solvent
E axx
C axx
E axx
C axx
E axx
C axx
E axx
C axx
39.5 37.1 40.6 20.2 32.6 31.8 29.9 22.5 25.2
62.0 60.4 55.4 62.0 62.1 59.5 61.7 61.9 55.8
60.5 50.3 54.2 40.9 36.8 54.5 41.5 20.9 52.2
196.2 187.7 163.2 196.3 196.9 183.0 194.9 195.9 165.1
19.9 33.9 29.2 11.3 10.5 26.3 16.8 12.1 26.2
284.3 276.1 251.5 284.4 285.0 271.3 283.0 284.0 253.4
36.0 43.7 35.0 38.7 25.2 35.8 27.9 24.6 27.8
358.1 343.1 301.0 358.4 359.5 335.1 355.8 357.6 304.1
E axx ¼ Experimental, C axx ¼ Computational. Values are in the order of 1030 esu
7a
C bxxx
3.6. Solvatochromism
E axx ¼ Experimental, C axx ¼ Computational. Values are in the order of 1030 esu
f ¼ 4:32 109
5a
E bxxx
E bxxx ¼ Experimental, C bxxx ¼ Computed. Values are in the order of 1030 esu.
Table 4 Dipole moment ratio me =mg calculated with different methods.
Solvent
ACN DCM DIOX DMF DMSO EtOAc EtOH MeOH Tol
4a
ACN DCM DIOX DMF DMSO EtOAc EtOH MeOH Tol
4b
5b
7b
8b
E bxxx
C bxxx
E bxxx
C bxxx
E bxxx
C bxxx
E bxxx
C bxxx
19.8 18.9 20.4 10.1 16.6 16.1 15.2 10.9 13.1
70.8 62.2 41.8 70.9 71.6 57.7 69.4 70.5 43.1
29.0 24.2 25.4 19.9 18.3 25.5 21.0 10.2 25.0
6.9 2.2 0.4 6.9 7.0 4.9 6.7 6.8 0.5
3.5 6.7 5.5 1.8 1.7 4.9 2.9 2.1 5.0
41.6 40.3 22.9 41.6 41.5 37.1 42.9 41.7 24.1
21.4 25.9 20.4 25.4 17.3 20.7 15.8 13.8 16.4
5.2 4.4 0.1 5.2 5.2 0.1 5.0 5.1 0.1
E bxxx ¼ Experimental, C bxxx ¼ Computed. Values are in the order of 1030 esu.
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Table 9 Total first order hyperpolarizability calculated with B3LYP/6-31G (d). Solvent
4a
5a
7a
8a
4b
5b
7b
8b
572.7 499.0 315.2 573.9 458.7 458.7 560.9 570.1 327.1 481.8 1309
359.3 323.3 226.0 359.8 362.4 303.0 353.6 358.0 232.7 319.8 869
576.0 503.9 325.9 577.1 582.5 464.6 564.4 573.5 337.5 500.6 1360
175.4 158.0 110.6 175.6 176.9 148.2 172.6 174.8 113.9 156.2 425
310.6 269.8 176.4 311.1 314.0 251.3 304.4 309.2 182.6 269.9 734
295.3 267.4 192.9 295.7 297.8 251.8 290.8 294.3 198.0 264.9 720
489.0 430.9 290.1 489.9 494.3 401.4 479.6 487.0 299.4 429.1 1166
bo (1030) ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol Average Ratio with Urea (Urea: Compound)
437.6 386.7 253.1 438.4 442.1 358.3 429.6 435.9 262.1 382.7 1040
Table 10 Solvatochromic descriptor < g > of third order hyperpolarizability of compounds. Solvent
4a
5a
7a
8a
4b
5b
7b
8b
ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol
1.4 1.1 2.3 0.1 0.2 4.2 0.2 0.1 1.9
2.5 1.3 2.5 0.7 1.1 1.0 0.1 0.1 1.3
2.9 10.5 8.0 2.3 0.1 2.1 0.1 0.3 5.7
0.2 0.3 0.3 0.1 0.1 0.3 0.1 0.1 0.3
7.8 7.0 8.4 1.9 5.4 5.0 4.3 2.3 3.0
23.1 16.0 18.3 10.5 8.4 18.5 10.8 2.6 17.0
2.0 6.4 4.6 0.6 0.5 3.7 1.3 0.7 3.7
7.3 11.1 6.7 9.4 3.6 7.1 4.0 3.1 4.1
All the molecules in this series give dipole moments ratio. me =mg less than unity. This suggests the excited state dipole moment is lower than the ground state dipole moment. 3.8. NLO properties Coumarin molecules can be good candidates for use as NLO (non-linear optical) materials [43e45]. The synthesized molecules show sensitivity towards the solvent polarity and the property is used to determine the NLO properties of the dyes. 3.9. Calculation of aCT from the solvatochromic data
solvents only interact with the solute by dipole interactions unlike the polar solvents which can interact through some of the specific interactions such as H-bonding. As the polarity of the solvent increases there is higher stokes shift is observed in all the molecules. The plots such as Weller and Rettig plot which consider only the emission wavelength also behave linearly. The trend here is in polar solvents emission is at longer wavelength. The polar solvents facilitates the higher degree of relaxation at excited state. 3.7. Dipole moment ratio All the molecules shows sensitivity to solvent polarity, not only the emission but Stokes shift is also sensitive to the solvent polarity. This behaviour of the molecules can be exploited to obtain the me =mg ratio. The following equations are used to obtain the me =mg ratio. The equations are introduced by Bilot-Kawski [40], Bakhshiev [41] and Liptay [42] for the estimation of ratio of excited state dipole moment and ground state dipole moment i.e. me =mg . The dipole moment ratios calculated with the equations are given in Table 4.
Table 11 Second order static hyperpolarizability obtained computationally using B3LYP/631G(d). Solvent
4a
5a
7a
8a
4b
5b
7b
8b
ACN DCM Diox DMF DMSO EtOAc EtOH MeOH Tol
1610.8 1356.9 766.6 1614.8 1634.0 1222.1 1569.5 1601.7 802.7
2134.9 1763.7 946.2 2140.8 1558.9 1571.5 2073.8 2121.4 994.3
3445.0 3056.3 2033.7 3450.9 3479.5 2839.8 3383.4 3431.5 2102.8
5540.5 4706.8 2787.7 5553.7 5617.6 4266.4 5403.6 5509.7 2906.3
1990.4 1716.4 1053.8 1994.6 2015.2 1568.9 1946.1 1980.7 1095.9
3169.6 2623.2 1443.4 3178.4 3220.7 2346.4 3079.4 3149.8 1512.7
4908.2 4354.8 2956.9 4916.7 4957.9 4052.7 4819.2 4888.7 3049.4
9548.9 8121.2 4901.0 9571.2 9681.5 7392.9 9314.4 9497.7 5098.1
The linear polarizability aCT was evaluated experimentally for synthesized extended carbazole coumarin hybrids. These values are obtained by two-level model using UVevis absorption/emission spectroscopy. The solvatochromic method can also be utilized in the determination of dipole moment of the lowest lying charge transfer excited state [46]. The aCT are values calculated for carbazole-coumarin hybrids, which are compared with theoretically obtained aCT or axx (Table 5). 3.10. Calculation of bxxx from solvatochromic shifts The values for first hyperpolarizability obtained using the solvatochromic method is based on several assumptions and thus allow only approximate estimate of dominant tensor of total hyperpolarizability along the direction of charge transfer which is the major contributor to the total hyperpolarizability, though the values are approximate it has advantages over the other wellknown expensive methods to understand the trend (see Table 6). The calculated values of b xxx for compound 4a, 5a, 7a and 8a are given in Table 7. The bxxx calculated and experimental shows similar trends. The BF2-complexation has decreased the bxxx value as compared to the non-complexed compounds (4a and 7a). However the trends predicted across solvents only holds true for the BF2complexed molecules (5a and 8a). The possible reason to this may be the structural flexibility of the molecules, which with interaction with solvents gives different real world observation. In the case of second set of molecules where the additional donor is present the similar thing is not observed. The trends predicted by the computational method is contrary to the experimental observation. But the solvent effect on the bxxx is well predicted by the theoretical method despite some deviations Table 8. The value of b0 or total first order hyperpolarizability of the molecule gives an estimate of the NLO properties of the organic molecules. The computed values were compared with the values obtained for urea. The comparison of the values is given below in Table 9.
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The synthesized organic molecules show 425 to 1360 times greater hyperpolarizability than urea calculated at the same level. The addition of BF2-core in the molecule has increased the hyperpolarizability by 40e60%. The additional donor group in the form of diethylamino group on the coumarin core has lowered the total first order static hyperpolarizability. 3.11. Calculation of < g > from solvatochromic shifts The second order hyperpolarizability < g > SD (Equations are given in supporting information) at molecular level originating from the electronic polarization in the non-resonant region can be treated by a three-level model [19,47e50]. The values of “solvatochromic descriptor of second order hyperpolarizability” are calculated and are given in Table 10. The solvatochromic descriptor < g > of the second order hyperpolarizability of compounds show higher values in case of a two donor system found in compounds 4b, 5b, 7b and 8b. The values of the molecules with single donor remains 0.1 to 10.5. The electron flow enhancement in the chromophoric system seems to be enhancing the third order hyperpolarizability descriptor < g > . 3.12. Computational g value (second order static hyperpolarizability) The individual components of the second order static hyperpolarizability g were obtained computationally. The values are considered to be proportional to the solvatochromic descriptor < g > of the second order hyperpolarizability. The values obtained are given in Table 11. The values has a trend where higher polarity solvents show higher values of g, but the similar trend is not followed by the solvatochromically obtained values. 4. Conclusion In conclusion, we have successfully synthesized the carbazolecoumarin hybrids which emit in red region. Coumarin core is utilised as an efficient acceptor in these novel hybrids. The BF2complexation assists in pulling electrons from the carbazole core. The molecules synthesized have very good response to the solvent polarity. There is also improvement in quantum yields in BF2complexed compounds, over their parent uncomplexed compounds. In polar solvents molecules are either have very low fluorescence intensity or are non-fluorescent. The molecules were also evaluated for their NLO properties and found to be 425 to 1360 times greater in first order static hyperpolarizability than the urea. Thus these molecules can be considered as good candidates for non-linear optical materials (NLO). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.dyepig.2016.02.026. References [1] Hany R, Fan B, Araujo F, Castro D, Heier J, Kylberg W, et al. Strategies to improve cyanine dye multi layer organic solar cells. Prog Photovolta Res Appl 2011;19:851e7. http://dx.doi.org/10.1002/pip. [2] Cells E. Polymer light-emitting. Science 1995;269:1086e8. [3] Benmansour H, Castro FA, Nagel M, Heier J, Hany R, Nüesch F. Ionic space charge driven organic photovoltaic devices. Chim (Aarau) 2007;61:787e91. http://dx.doi.org/10.2533/chimia.2007.787. [4] Dalton LR, Benight SJ, Johnson LE, Knorr DB, Kosilkin I, Eichinger BE, et al. Systematic nanoengineering of soft matter organic electro-optic materials. Chem Mater 2011;23:430e45. http://dx.doi.org/10.1021/cm102166j.
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