Solvatochromic behavior of D-π-A bithiophene carbonitrile derivatives

Journal of Molecular Liquids 286 (2019) 110856

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Solvatochromic behavior of D-π-A bithiophene carbonitrile derivatives Asmaa M. Dappour a, Maha A. Taha a, Mohamed A. Ismail b, Ayman A. Abdel-Shafi a,⁎ a b

Department of Chemistry, Faculty of Science, Ain Shams University, 11566 Abbassia, Cairo, Egypt Department of Chemistry, Faculty of Science, Mansoura University, 35516 Mansoura, Egypt

a r t i c l e

i n f o

Article history: Received 21 February 2019 Received in revised form 10 April 2019 Accepted 24 April 2019 Available online 29 April 2019 Keywords: Photophysical properties Solvent effect Intramolecular charge transfer Bithiophene carbonitrile Kamlet-Taft Catalán Laurence

a b s t r a c t Photophysical properties of 5-(5-(4-methoxyphenyl)thiophen-2-yl)thiophene-2-carbonitrile (MTTC) and 5-(5(3,5-dimethoxyphenyl)thiophen-2-yl)thiophene-2-carbonitrile (DMTTC) were investigated in different solvents. The photophysical properties of MTTC and DMTTC were correlated with the solvent polarity parameters N viz., EN T and reaction field factor Δf. Dependence of the photophysical properties on Δf and ET was best studied when the solvents are classified to protic and aprotic solvents. Dipole moment changes, Δμ, were determined using Lippert–Mataga and Reichardt-Ravi equation in protic and aprotic solvents. Multiple linear regression analysis of solvent dependent photophysical parameters using Kamlet-Taft, Catalán and Laurence et al. have shown that specific hydrogen bonding interactions and non-specific dipolar interaction play important roles in determining the photophysical properties of MTTC and DMTTC. Kamlet-Taft and Catalán treatments have shown that the non-specific interactions have high impact on the emission energy, νf, and the non-radiative rate constants, knr, while the specific interactions dominate the absorption energy, νa, Stokes shift, Δν, and the radiative rate constants, kr. Laurence treatment shows that the specific interactions have the important contribution only on the absorption energy, νa, while the dispersion and induction interactions, DI, and ES parameter have high impact on the emission energy, νf, the Stokes shift, Δν, radiative, kr, and non-radiative rate constants, knr. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Conjugated donor-(π-spacer)-acceptor (D–π–A) compounds have attracted tremendous research interest [1–6]. Such push–pull systems usually show intramolecular charge-transfer (ICT) characteristics which is responsible for the polarization of these compounds and generation of a molecular dipole. The notable spectral and electrical properties of such systems make them promising materials in many applications such as fluorescent biochemical probes, photovoltaic cells and organic light-emitting diodes (OLEDs) [7–12]. Proper choice of a strong electron donating group, strong electron withdrawing acceptor and suitable π-spacer is expected to enhance the charge transfer properties of the D-π-A fluorophore. 2,2′Bithiophene and its derivatives were reported to be among the most suitable structures for modeling of push-pull fluorophores due to the long-chain conjugation with substituents in 5 and 5′-positions [13]. The electronic transition in the majority of thiophene-based systems are simple π-π* transitions resulting in the population of the excited singlet state [14]. In solution, they exhibit structureless absorption spectra [15–17], while the fluorescence spectra tend to exhibit structured bands which are typically attributed to the rigid planar quinoid-like structure for the excited singlet state, in comparison to the twisted aromatic structure of the ground state [14–25]. Usually, the π-conjugated systems are ⁎ Corresponding author. Tel.: +201097998330 E-mail address: aaashafi@sci.asu.edu.eg (A.A. Abdel-Shafi).

https://doi.org/10.1016/j.molliq.2019.04.133 0167-7322/© 2019 Elsevier B.V. All rights reserved.

polarized by the donor (D) and acceptor (A) substituents, resulting in stronger intramolecular charge transfer and large dipole moment [26–32]. It has been demonstrated that the solute-solvent interactions have a significant influence on the UV/vis absorption spectra of D-π-A compounds [33–35], hence they are called the solvatochromic dyes. Among the large variety of donor-acceptor end capped oligothiophenes, 5dimethylamino-5′-nitro-2,2′-bithiophene (Me2N-TT-NO2) was found to show a pronounced positive solvatochromic shift of about 130 nm on going from n-hexane to formamide [33,34]. Thus, Me2N-TT-NO2 was found to be a suitable for use as a probe for the determination of solvent polarity [35]. Thiophene, oligothiophene, polythiophene and its substituted derivatives have been the subject of several photophysical studies concerning the influence of the micro-structural changes on their luminescent spectral properties [21,22,33–40]. Becker et al. [21,22] found that the spectral properties of unsubstituted oligothiophene depended on the number of thiophene rings that were linked together. Danko et al. have shown that simple mono-substitution of bithiophenes with proper substituent might dramatically change the photophysics of the bithiophene moiety [36]. Lukeš et al. have recently prepared series of mono-substituted 2,2′bithiophene and found that while the parent 2,2′-bithiophene exhibits weak fluorescence, mono-substituted 2,2-bithiophene in position 5 with substituents containing the carbonyl group exhibited intense fluorescence [37]. The pronounced solvatochromic properties of donor-acceptor bithiophenes were synthesized and studied by Effenberger et al. [33]

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especially aminonitro substituted bithiophenes that have reported to be suitable to investigate the solvent polarity by means of their absorption wavenumbers. Since dimethylaminonitro bithiophene [33,34] excellently correlates with the π* values defined by Kamlet and Taft, π* values are usable but not perfect as the solvatochromic parameter to express the nonspecific contribution of the solvent polarity. Different groups have also studied the solvatochromic behavior of dimethylaminonitro bithiophene in various solvents, by employing different theoretical methods [29,38,39]. Bolduc et al. [40] have studied the solvatochromic and electrochemical properties of push–pull 2-aminobithiophenes consisting of an aldehyde and nitro withdrawing groups. They have found that these derivatives were highly fluorescent, provided the nitro group was not located in the 4′-position. High fluorescence yields were observed regardless of solvent, except for alcohols, notably methanol and ethanol. They have also reported that internal conversion participates in the deactivation of the excited singlet state. Absorption and fluorescence emission energies were found to be solvatochromic with more pronounced solvent dependent shifts being observed with the emission energy. Fluorescence emission energy and Stokes shifts were linearly dependent on the ET(30) solvent parameter. Deviations from the linear trend of the Stokes shift with ET(30) were observed for ethanol and methanol as a result of intermolecular hydrogen abstraction from the solvent and by the excited nitro group [40]. Rettig et al. [41] studied ionic and nonionic donor–acceptor bichromophors linked by a single bond among which bithiophene derivatives with electron withdrawing cyano group and dimethyl amine as electron donating group. In the excited state, they deactivate through twisting towards a 90° energetic minimum with charge shift or twisted intramolecular charge transfer (TICT) properties. The fate of this twisted excited state, i.e., its radiative or nonradiative properties, is probably determined by the presence (or absence) of an energetically close-lying conical intersection. In this study, 2,2′-bithiophene is linked to cyano group as an electron acceptor group for its strong electron-withdrawing ability and either pmethoxyphenyl or m-dimethoxyphenyl groups as an electron donor to examine the effect of such structural changes on the extent of the intramolecular charge transfer (ICT) characteristics in both compounds. Therefore, photophysical properties of MTTC and DMTTC were studied in a variety of protic and aprotic solvents and correlated with a number of solvent parameters such as the solvent polarity parameters EN T and the reaction field factor Δf. Three linear solvation energy relationships developed by Kamlet-Taft [42], Catalán [43,44], and Laurence et al. [45] were also employed to assess the effect of specific and nonspecific interactions on the ICT characteristics in MTTC and DMTTC.

2. Experimental 5-(5-(4-Methoxyphenyl)thiophen-2-yl)thiophene-2-carbonitrile (MTTC) and 5-(5-(3,5-dimethoxyphenyl)thiophen-2-yl)thiophene-2carbonitrile (DMTTC) were available from a previous study [46] and recrystallized three times from ethanol. Organic solvents were of highest purity grade from Aldrich and were used as received. Deionized water with resistivity N10 MΩ/cm and pH of 6.8 was used. MTTC and DMTTC stock solutions (2 mM) were prepared by dissolving the appropriate amount of each in ethanol, and 0.1 ml of this solution was transferred into 10 ml volumetric flasks and diluted with different solvents. All experiments were performed at room temperature (25 °C). Absorption spectra were recorded on Shimadzu 1800-UV–visible spectrophotometer. Steady state fluorescence measurements were obtained using Thermo Scientific Lumina spectrofluorophotometer. Excited state lifetimes were measured with Easylife from OBB using 375 nm LED as an excitation light source. The fluorescence quantum yield (Φf) was estimated using the following relationship:
 −As n2 F f 1−10   f  Φs 
Φf ¼ Fs 1−10−A f n2s

ð1Þ

where the subscripts f and s refer to the sample and the standard, respectively. Φ is the fluorescence quantum yield, F is the integrated emission area across the band, A is the absorbance at the excitation wavelength and nf and ns are the indices of refraction of the solvent containing the sample and standard, respectively. The fluorescence quantum yields in different solvents were measured relative to diphenylanthracene in cyclohexane (Φs = 0.90) or quinine sulfate in 1 N H2SO4 (Φs = 0.55) [47]. Absorbance of the sample and reference were the same at the excitation wavelength which was taken as λmax abs of MTTC or DMTTC in each solvent. The fluorescence decay of MTTC and DMTTC were measured over the entire emission spectra in all solvents. The fluorescence decay was found to be fitted very well with mono-exponential function in all solvents except in water which was found to fit with bi-exponential function according to Eq. (2) with χ2 of about 1.0 ± 0.1 and Durbin Watson parameter N1.8 in all cases.   −t ; and: I ðt Þ ¼ a exp τ     −t −t I ðt Þ ¼ a1 exp þ a2 exp τ1 τ2

ð2Þ

where τ is the lifetime of the fluorescence decay. 3. Results and discussion

5-(5-(4-methoxyphenyl)thiophen-2-yl)thiophene-2-carbonitrile (MTTC).

5-(5-(3,5-dimethoxyphenyl)thiophen-2-yl)thiophene-2-carbonitrile (DMTTC).

Absorption and fluorescence emission spectra were recorded in a series of neat solvents. Fig. 1a and b show the electronic absorption spectra of MTTC and DMTTC in dilute solutions of different solvents. It has been found that the absorption spectra show a red shift of about 12 nm on changing from non-polar aprotic (chx) to high polar aprotic solvents (DMSO) for MTTC and DMTTC. In addition, the absorption spectra of MTTC in different solvents show a red shift of about 10 ± 2 nm relative to DMTTC. The wavelength of maximum absorption, λmax abs , in different solvents are summarized in Table 1. MTTC and DMTTC show structureless longwavelength absorption bands with their maxima in the range 369–381 and 360–372 nm in nonaqueous solvents, respectively, which are signed to π-π* transition to the first excited singlet state [29]. In water DMTTC has an absorption peaks at 338 nm and a shoulder at about 414 nm while MTTC has an absorption peaks at 343 nm and a shoulder

A.M. Dappour et al. / Journal of Molecular Liquids 286 (2019) 110856

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Fig. 2. Normalized fluorescence emission spectra of (A) MTTC and (B) DMTTC in different solvents collected with excitation wavelength, λex, equals to λmax abs .

Fig. 1. Absorption spectra of (A) MTTC and (B) DMTTC in different solvents.

at about 420 nm. In other words, MTTC shows bathochromic shift in the absorption maxima relative to DMTTC. Room temperature fluorescence spectra of MTTC and DMTTC in different solvents are shown in Fig. 2a and b and the corresponding band maxima are collected in Table 1 which are in the range 445–484 nm

and 437–452 nm, respectively, while in water both have a fluorescence emission maximum at about 529 nm. In addition to the bathochromic shift observed in the absorption spectra of DMTTC and MTTC, the fluorescence spectra show significant bathochromic shift in polar solvents

Table 1 Wavelength of maximum absorption, λabs, wavelength of maximum emission, λem, Stokes shift, Δν, fluorescence quantum yield, Φf, excited state lifetime, τ, radiative rate constant, kr, and non-radiative rate constant, knr, for MTTC and DMTTC in different solvents (Errors unless otherwise stated are ≤10%). Solvent

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Water a Methanol Ethanol 2-Propanol 1-Butanol 1,4-Dioxane Ethylacetate Acetonitrile DMF DMSO Chloroform Dichloromethane Toluene Cyclohexane

MTTC

DMTTC

λabs /nm

λem /nm

Δν /cm−1

Φf

τ/ns

kr /108 s−1

knr /109 s−1

λabs /nm

λem /nm

Δν /cm−1

Φf

τ/ns

kr /108 s−1

knr /109 s−1

340 374 375 376 377 373 372 373 379 381 376 377 376 369

529 476 474 471 471 455 461 476 479 484 465 468 450 445

9331.4 5729.6 5569.6 5364.3 5293.8 4831.6 5189.7 5801.2 5508.4 5585.6 5090.4 5157.7 4373.5 4628.4

0.03 0.24 0.23 0.23 0.23 0.09 0.11 0.25 0.38 0.49 0.17 0.22 0.08 0.07

0.91, 3.02 1.064 0.93 0.89 0.91 0.38 0.44 0.97 1.44 1.76 0.6 0.74 0.34 0.23

0.33 2.26 2.48 2.58 2.53 2.37 2.50 2.58 2.64 2.78 2.83 2.97 2.35 3.04

1.06 0.71 0.83 0.86 0.85 2.39 2.02 0.77 0.43 0.29 1.38 1.05 2.71 4.04

337 364 365 365 367 366 365 364 370 372 367 366 366 360

529 448 443 441 443 437 440 443 447 452 441 442 439 437

10,770.0 5151.1 4823.9 4721.5 4674.6 4439.1 4670.0 4899.2 4655.7 4757.8 4572.2 4698.0 4543.4 4894.5

0.02 0.10 0.10 0.10 0.10 0.09 0.09 0.11 0.12 0.16 0.10 0.09 0.09 0.09

1.02, 3.0 0.25 0.24 0.23 0.24 0.23 0.22 0.24 0.28 0.35 0.22 0.23 0.22 0.22

0.24 4.04 4.18 4.48 4.41 4.11 4.19 4.53 4.14 4.50 4.40 4.04 4.24 4.24

0.96 3.60 3.68 3.90 3.80 4.00 4.09 3.70 3.19 2.43 4.19 3.90 4.18 4.18

a Excited state decay lifetime is fit by biexponential function with amplitudes (0.86 and 0.14) and (0.87 and 0.13) for MTTC and DMTTC respectively. kr and knr were calculated using the lifetime with higher amplitude in both cases.

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and being higher for MTTC than for DMTTC. It can be seen from Fig. 2a and b that MTTC shows a bathochromic shift of about 30 nm relative to DMTTC in protic (except in water) and polar aprotic solvents. Fig. 1 and Table 1 show that the absorption peaks of both compounds are slightly shifted to longer wavelengths with increasing solvent polarity. On the other hand, the emission maxima (Fig. 2 and Table 1) show significant bathochromic shift on going from lower to higher polarity solvents due to emission from the intramolecular charge transfer state [48]. The slight bathochromic shift of the absorption spectra indicates a low polar character of the ground state, whereas the larger bathochromic of the emission spectra indicates that the dipole moment of the excited state is larger than that of the ground state for both compounds [48]. The absorption and fluorescence spectra of MTTC and DMTTC do not show a vibrational structure in all solvents (except in water, see Figs. 1 and 2). The absorption and emission spectra of MTTC and DMTTC show an approximate mirror symmetry, providing convincing evidence that there is only a single excited electronic state contributing to the absorption and fluorescence spectra. MTTC and DMTTC emit in polar protic and aprotic solvents only a single and broad long wavelength emission band which is generally attributed to the ICT state [48–61]. In addition, DMTTC show a Stokes shift of about 77 ± 6 nm in all solvents except in water, while MTTC's Stokes shift was found to be more sensitive to the type of the solvent and increases as the solvent polarity increases both in protic and aprotic solvents with the highest value observed in DMSO of about 103 nm. Evaluation of intramolecular charge transfer process strength can be obtained by analysis of the spectroscopic data as a function of solvent polarity parameters. Since the charge distributions in the ground state and the first excited singlet state are different (different dipole moments), the stabilization of both states involved in the electronic transitions is different in solvents of different polarity, hence, the spectral band position depends on solvent polarity characteristics. Therefore, the analysis of spectral characteristics allows us to evaluate the changes in the charge distributions and dipole moment change upon electronic excitation. In the present work, several solute-solvent treatments will be used. Effect of solvent on the energy difference between the ground and excited singlet states of a molecule are correlated to the refractive index (n) and dielectric constant (ε) of the solvent as previously described by Lippert–Mataga formalism (Eq. (3)) [62]. Δν ¼ ν a −ν f ¼

2Δμ 2 Δf þ constant hca3

ð3Þ

The dipole moment change, Δμ, can be obtained from the slope of the plot of Stokes shift (Δν) versus solvent polarity parameter (Δf) (Fig. 3). Fig. 3 shows that the dependence of Δν for both compounds versus Δf did not result in good linear fit but slightly improved when the solvents are classified into aprotic and protic solvents. The dipole moment difference between the excited state and ground state dipole moments (Δμ) can be calculated from the slope of Eq. (3) and assuming a value of 6 Å for the radius of the solvent cavity based on dimensionally similar compounds [64,65]. The dipole moment change, Δμ, was found to be 15.2 ± 5.0 D and 8.8 ± 4.0 D in protic and aprotic solvents for MTTC and 13.6 ± 5.0 D and 4.8 ± 3.0 D in protic and aprotic solvents for DMTTC. The higher dipole moment changes in the region of high polarity and protic solvents reflect the higher stability of the ICT state in these highly polar solvents. It has been shown that solvent polarity has significant influence on energetics and dynamics of intramolecular charge transfer by static effect (change in the potential energy surface on which the reaction occurs) and dynamic effect (dielectric friction caused by polar solute/ solvent interactions) [66–71]. Reichardt introduced an empirical solvent polarity parameter, EN T [72], using tetramethylsilane as the least polar solvent and water as the most polar solvent and classified the solvents to three groups: protic solvents with EN T values from 0.5 to 1.0, dipolar non-hydrogen donating N solvents with EN T values from 0.3 to 0.5 and aprotic solvents with ET values from 0.0 to 0.3 [73]. Dependence of different spectroscopic properties, namely, absorption energy, νa, emission energy, νf, Stokes shift, Δν, fluorescence quantum yield, Φf, radiative rate constant, kr, and non-radiative rate constant, knr, on the dimensionless solvent polarity scale, EN T , were much pronounced than on the solvent polarity function, Δf, when the solvents are divided to protic and aprotic solvents as seen from Figs. 4 and 5 and the following equations:   νa cm−1 ¼ 26; 094:6ð217:0Þ þ 842:7ð338:0ÞEN T; R2 ¼ 0:76; MTTC; protic solvents   νa cm−1 ¼ 27; 156:6ð70:3Þ−1977:5ð242:6ÞEN T; R2 ¼ 0:93; MTTC; aprotic solvents   νa cm−1 ¼ 26; 974:1ð250:0Þ þ 635:3ð524:5ÞEN T; R2 ¼ 0:41; DMTTC; protic solvents

ð5Þ

ð6Þ

ð7Þ

In this equation, Δν is the Stokes shift; νa and νf are wavenumbers of the absorption and fluorescence peaks, respectively, c is the velocity of light, h is Planck's constant, a is Onsager cavity radius, and Δμ (Δμ = μe − μg) is the difference between the excited and ground state dipole moments. The wavenumbers of absorption and emission peaks were calculated from their corresponding absorption and fluorescence emission peak maxima, respectively. Angulo et al. [63] have shown the correct representation in the frequency or wavenumber scale of the electronic spectra for the extraction of the 0–0 transition energy especially in the case of broad spectra. Recalculation of directly obtained νa and νf based the correction scale given by Angulo et al. showed an error in νf of ≤2% and ≤1% in νa. Therefore, we used the directly obtained values of νa and νf from their spectral maxima in the following discussion for simplicity and comparison with other literature. Δf which is the solvent polarity parameter can be calculated using the dielectric constant, ε, and refractive indices, n, of the solvent using the following equation: Δf ¼

ε−1 n2 −1 − 2ε þ 1 2n2 þ 1

ð4Þ Fig. 3. Dependence of Δν on the solvent polarity parameter Δf for MTTC and DMTTC.

A.M. Dappour et al. / Journal of Molecular Liquids 286 (2019) 110856

5

that supports the substantial role of the hydrogen bonding interactions. This has been explained as the energy barrier for formation of the intramolecular charge transfer state is strongly governed by the hydrogen bonding interactions [54]. The linear dependence of the emission energy on the normalized Reichardt's polarity scale, EN T , shown in Fig. 4b and slopes of Eqs. (9)– (12) shows similar sensitivity in protic and aprotic solvents with slopes much higher than observed for the dependence of the absorption energy on EN T in case of DMTTC and much higher sensitivity in aprotic solvents than in protic solvents for MTTC.   ν f cm−1 ¼ 21; 859:1ð121:3Þ−1125:6ð188:9ÞEN T; R2 ¼ 0:95; MTTC; protic solvents   ν f cm−1 ¼ 22; 554:0ð93:1Þ−3907:6ð310:6ÞEN T; R2 ¼ 0:96; MTTC; aprotic solvents   ν f cm−1 ¼ 23; 506:1ð218:7Þ−1522:9ð340:5ÞEN T; R2 ¼ 0:91; DMTTC; protic solvents   ν f cm−1 ¼ 22; 970:3ð102:0Þ−1312:7ð340:1ÞEN T; R2 ¼ 0:68; DMTTC; aprotic solvents

ð9Þ

ð10Þ

ð11Þ

ð12Þ

Fig. 4c shows that the Stokes shift, Δν, increases markedly as the solvent polarity parameter, EN T , increases. Fig. 4c and Eqs. (13)–(16) also show similar dependence of the Stokes shift in protic solvents for both compounds while in aprotic solvents the slope is higher for MTTC than DMTTC.   Δν cm−1 ¼ 3939:3ð465:3Þ þ 2385:1ð693:2ÞEN T; R2 ¼ 0:92; MTTC; protic solvents   Δν cm−1 ¼ 4210:0ð120:6Þ þ 3345:3ð379:4ÞEN T; R2 ¼ 0:93; MTTC; aprotic solvents   Δν cm−1 ¼ 3468:0ð323:5Þ þ 2158:2ð503:7ÞEN T; R2 ¼ 0:99; DMTTC; protic solvents   Δν cm−1 ¼ 4387:5ð74:9Þ þ 909:1ð235:4ÞEN T; R2 ¼ 0:71; DMTTC; aprotic solvents Fig. 4. Plot of (a) the absorption energy, νa, (b) emission energy, νf, and (c) Stokes' shift, Δν, versus the solvent polarity parameter EN T.

  νa cm−1 ¼ 27; 762:4ð77:0Þ−1870:9ð266:0ÞEN T; R2 ¼ 0:91; DMTTC; aprotic solvents

ð8Þ

Fig. 4a shows that the dependence of the absorption energy, νa, on the solvent polarity scale EN T gives two slopes for each compound that reflects the sensitivity of each of them towards solvent polarity. Slope of Eqs. (5)–(8) show similar dependence of the absorption energy on the normalized Reichardt's polarity scale in protic and aprotic solvents for both compounds. As have been seen from Fig. 2 and Table 1 that the changes of the fluorescence emission maxima are much more pronounced than that of the absorption spectra for both compounds. It has been found that the fluorescence emission maxima of MTTC and DMTTC in ethanol and acetonitrile are about the same despite the large difference in the dielectric constant between both solvents. Such similarity in the fluorescence emission maxima indicates that the emissive intramolecular charge transfer state is significantly stabilized in protic solvents and

ð13Þ

ð14Þ

ð15Þ

ð16Þ

Correlation of the Stokes shift with EN T was proposed by Reichardt and developed by Ravi et al. [74] according to the following Equation: Δμ 2 a3B Δν ¼ 11; 307:6 Δμ 2B a3

! EN T þ constant

ð17Þ

where Δμ = (μe − μg) is the dipole moment difference between the excited state dipole moment and ground state dipole moment, a is the Onsager cavity radius, ΔμB and aB are the corresponding parameters for the betaine dye with 9.0 D and 6.2 Å, respectively [72]. Dipole moment change (Δμ) can be calculated from Eq. (18) using the slope obtained from Eq. (17) [74]: Δμ ¼ μ e −μ g ¼

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 81 S 3 11; 307:6 6:2 a

ð18Þ

Calculation of the dipole moment change (Δμ) using Eq. (18) and slopes, S, of Eqs. (13)–(16) results in a value of 15.5 ± 5.0 D in protic solvents and 21.7 ± 3.0 D in aprotic solvents for MTTC and 14.0 ± 2.0 D in protic solvents and 6.0 ± 3.0 D in aprotic solvents for DMTTC. Such large difference in the dipole moment changes for both compounds reflect that the charge redistribution in the excited singlet state is quite

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Fig. 5. Plot of (a) the fluorescence quantum yield, Φf, (b) the excited state lifetime, τ, (c) radiative rate constant, kr, and (d) non-radiative rate constant, knr, versus the solvent polarity parameter EN T.

different in aprotic compared to that in protic solvents which is being due to the importance of the hydrogen bonding interactions in the excited state [48,54]. Therefore, it has been found that the dependence of the photophysical properties on the different solvent polarity parameters is better explained when the solvents are divided into aprotic and protic solvents as can be seen in Figs. 3–5. Solvent effect on the fluorescence quantum yields, Φf, was also studied. It has been found that dependence of Φf on the solvent polarity function Δf is very scattered while its dependence on EN T was much pronounced as shown in Fig. 5a. The fluorescence quantum yield of DMTTC and MTTC were found to be approximately the same in protic solvents for each compound. In aprotic solvents, the fluorescence quantum yield was found to increase in a non-linear way with increasing EN T values with maximum values reported in DMSO for both compounds. In protic solvents, the dependence of the fluorescence quantum yield, Φf, was not changed with increasing EN T values. Fluorescence quantum yield measured in water for both compounds were the lowest due to the strong hydrogen bonding interactions leading to increased non radiative rate constants. The excited singlet state lifetime, τ, has shown similar trend on both Δf and EN T solvent polarity parameters, where its dependence on Δf is very scattered while its dependence on EN T is about the same in protic solvents and increases in aprotic solvents with increasing EN T values up to the maximum value recorded in DMSO (Fig. 5b). The lifetime of MTTC and DMTTC in water was fitted by biexponential function due to decay of the charge transfer state and that strongly influenced by hydrogen bonding interactions. It has also been found that radiative rate constant, kr (=Φf / τ), for both MTTC and DMMTC was independent on both solvent polarity parameters Δf and EN T as shown in Fig. 5c, while the non-radiative rate constant, knr (=(1 − Φf) / τ), show clear dependence on EN T , as it decreases with increasing values of EN T as shown in Fig. 5d. Calculated rate constants listed in Table 1, shows that the nonradiative decay rate constants are much larger than the radiative rate constants, indicating that the nonradiative transition is the main pathway for the excited-state deactivation. Fig. 5d show that the

nonradiative rate constants for DMTTC in protic solvents are comparable to those in nonpolar aprotic solvents and decreases in polar aprotic solvents with the increase in values EN T . On the other hand, the nonradiative rate constant shows a linear dependence with EN T values and decreases with the increase in EN T values with a slope higher in aprotic solvents than in protic solvents. The dependence of the spectroscopic properties on the solvent polarity scale given by Reichardt, EN T , as presented in Figs. 4 and 5 show two distinct groups of plots, one for aprotic solvents and the other for protic solvents with water as an exception in all cases. This implies that hydrogen-bonding interactions play an important contribution to the photophysical properties in addition to dipole-dipole interactions. In addition, the plots reveal that in most cases both compounds are more sensitive to polarity changes in aprotic solvents than in protic solvents and MTTC is always more sensitive than DMTTC. The specific interactions induced by protic solvents induce an efficient vibronic coupling with the excited states of both compounds, reducing the fluorescence quantum yield and increasing the rate of internal conversion with the lowest value reported in water. Therefore, in order to examine solute-solvent interactions in more details and to assess which solvent parameters have the important impact on the studied photophysical properties of MTTC and DMTTC, three multiple-linear solvation energy correlations according to Kamlet-Taft (Eq. (19)), Catalán (Eq. (20)), and Laurence et al. (Eq. (21)) were employed and results were shown in Figs. 6–8 and collected in Table 2. In Kamlet-Taft approach [42], the solvent dipolarity/polarizability measures the ability of the solvent to stabilize a charge or dipole through nonspecific dielectric interactions (π*), the solvent's hydrogen-bond donor strength is given by (α), and hydrogen-bond acceptor strength is given by (β), are correlated using the following equation: A ¼ A0 þ pπ  þaα þ bβ

ð19Þ

where A is the solvent dependant property, A0 is the solvent dependant property in a reference solvent, and (p, a and b) are the susceptibility constants.

A.M. Dappour et al. / Journal of Molecular Liquids 286 (2019) 110856

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Fig. 7. Percentage contribution of solvent parameters obtained from Kamlet-Taft analysis (Eq. (19)) for different spectroscopic properties of MTTC and DMTTC.

Fig. 6. Kamlet-Taft linear solvation energy relationship plot of the calculated values of the absorption energy, νa, emission energy, νf, Stokes's shift, Δν, radiative rate constant, kr, and non-radiative rate constant, knr, against their corresponding experimental data.

Determination of the susceptibility constants, p, a and b, is the key step in the solvatochromic analysis as it provides the weight of each solvent parameter contribution to the spectral property. Fit of the multiple regression analysis on the absorption energy, emission energy, Stokes shift, radiative and non-radiative rate constants using Eq. (19) with solvent parameters collected in Table 2 gives good results with correlation coefficient, R2, ≥0.90 in all cases (as shown in Table 2). Fig. 6 shows the calculated values against the corresponding experimental data using Eq. (19). In addition, more quantitative measure of the impact of the individual properties of the solvent are standardized coefficients. These factors allow determining the percentage share of the specific and nonspecific parameters on the measured solvent dependent photophysical properties, and the obtained coefficients are presented in Fig. 7. The linear solvation energy relationship for the absorption energy, νa, dependence on π*, α and β parameters shows that the contribution of the three parameters are about the same for both MTTC and DMTTC with the specific interactions play the major contribution to the solvent dependent property. The solvent's hydrogen bond acceptor strength, β, contribution is about 47.0 ± 3.0%, while the solvent's hydrogen bond donor strength, α, contributes about 34.0 ± 2.0%. The contribution of the non-specific interaction, π*, was found to be the lowest with a percentage about 19.0 ± 2.0% for both compounds (Fig. 7). On the other hand, emission energy, νf, dependence on π*, α and β parameters show that the polarizability of the solvent has a contribution is about 69.0 ± 4.0% for MTTC and about 76.0 ± 5.0% for DMTTC, while the solvent's hydrogen bond donor strength has equal contribution of about 19.0 ± 3.0%. In addition, the dependence of the Stokes shift, Δν, on these parameters shows that the polarizability of the solvent contribution is about 42.0 ± 3.0%, while the specific interactions α and β have equal contributions of about 30.0% each for MTTC. The three parameters show non-major contribution of any of them on the solvent dependent property for DMTTC (Fig. 7). The dependence of the nonradiative rate constant, knr, of MTTC on the Kamlet-Taft parameters shows that the non-specific interactions dominates with 64.0 ± 5.0% while the solvent's hydrogen-bond donor strength has a contribution of 23.0 ± 2.0% and the solvent's hydrogenbond acceptor strength has a low contribution of 13.0 ± 2.0%. The dependence of the nonradiative rate constant, knr, of DMTTC on π* is slightly higher than that of MTTC with a contribution of 73.0 ± 4.0% and about 25.0% effect of the solvent's hydrogen-bond acceptor strength. Solvent's hydrogen bond-donor strength was found to have no contribution on knr in the case of DMTTC. The dependence of the radiative rate constants, kr, of MTTC and DMTTC on Kamlet-Taft parameter are about the same in both cases.

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A.M. Dappour et al. / Journal of Molecular Liquids 286 (2019) 110856

The relative contributions of specific and non-specific interactions on the photophysical properties of both MTTC and DMMTC are similar and very pronounced. Specific interactions have major contributions on νa, Δν and kr, while the non-specific interactions dominate in case of νf and knr. The dependence of the excited state properties on the polarizability factor and the calculated dipole moment change support the intramolecular charge transfer (ICT) nature of these excited states. Fig. 6 shows the plot of the calculated νa, νf, Δν, kr and knr versus the experimentally obtained data in all solvents. However, results of the multiple regression analysis according to Kamlet-Taft Eq. (19) show that the non-specific contribution to the emission energy, νf, and the nonradiative rate constant, knr, is higher for DMTTC than for MTTC despite the higher dipole moment changes for the latter as calculated from the dependence on EN T parameter. Recently, Catalán [43,44] improved Kamlet-Taft equation by splitting the polarizability parameter (π*), into solvent polarizability (SP) and dipolarity (SdP) parameters according to the following equation: A ¼ A0 þ sSP þ dSdP þ aSA þ bSB

Fig. 8. Catalán's linear solvation energy relationship plot of the calculated values of the absorption energy, νa, emission energy, νf, Stokes's shift, Δν, radiative rate constant, kr, and non-radiative rate constant, knr, against their corresponding experimental data.

ð20Þ

where SP is the solvent polarizability, SdP is the solvent dipolarity, SA is the solvent's hydrogen bond donor strength and SB is the solvent's hydrogen bond acceptor strength. Multiple regression analysis according to Catalan's Eq. (20) gives better statistics than Kamlet-Taft model as seen from the slightly higher values of R2 and also is consistent with higher dipole moment change (Δμ) of MTTC. Plots of the obtained calculated data according to Eq. (20) versus the corresponding experimental data are shown in Fig. 8. It can be observed from Table 2 that the total non-specific interactions dominate the dependence of νf and knr on Catalan's parameters (with higher contribution in case of MTTC than for DMTTC), while the specific interactions dominate the dependence of νa, Δν and kr on Catalan's parameters. The multiple regression analysis for the absorption energy dependence, νa, on SP, dSP, SA and SB parameters shows that the solvent's polarizability parameter (SP) has a contribution of 24.0 ± 2.0% for MTTC and about 10.0 ± 2.0% higher in case of DMTTC and no contribution from the solvent's dipolarity, SdP, in both cases. The solvent's hydrogen bond donor strength, SA, has similar contribution of about 40.0 ± 4.0% of the absorption energy, while the solvent's hydrogen bond acceptor strength, SB, has lower contributions with 34.0 ± 3.0% in case of MTTC and 27.0 ± 3.0% in case of DMTTC. Therefore, the specific interactions control the absorption energy with 75.0 ± 6.0% in case of MTTC and 65.0 ± 5.0% in case of DMTTC (Fig. 9). The linear solvation energy relationship for the emission energy, νf, dependence on Catalán's solvent parameters SP, dSP, SA, and SB is dissimilar to that of the absorption energy. In case of the emission energy, νf, the non-specific interactions contribute with 78.0 ± 7.0% shared equally between the solvent's polarizability, SP, and the solvent's dipolarizability, SdP in case of MTTC. Effect of the specific interactions on the emission energy, νf, is mainly from the solvent's hydrogen bond donor strength, SA, while the solvent's hydrogen bond acceptor strength has no contribution in case of MTTC. It has been found that the specific and non-specific interactions have about equal contributions to the emission energy, νf, in case of DMTTC with the solvent's polarity, SP, and solvent's hydrogen bond donor strength, SA, plays the major control in each case. The Stokes shift, Δν, dependence on Catalan's parameters shows that the specific and non-specific interactions are equal for MTTC and DMTTC with higher contribution from the non-specific interactions and major contribution from the solvent's hydrogen bond donor strength in both cases. On the other hand, the solvent's dipolarity, SdP, contribution is higher in case of MTTC, while the solvent's polarizability is higher in case of DMTTC. The dependence of the nonradiative rate constant, knr, on the nonspecific interactions is very high and consistent with the observed Δμ (vide supra) being higher in case of MTTC than in case of DMTTC with major contribution from the solvent's dipolarity, SdP, in case of MTTC and solvent's

A.M. Dappour et al. / Journal of Molecular Liquids 286 (2019) 110856

9

Table 2 Values obtained for the susceptibility constants using the multiple regression analysis of Eqs. 19–21. Kamlet and Taft νa νf Δν knr/108 s−1 kr/108 s−1

MTTC DMTTC MTTC DMTTC MTTC DMTTC MTTC DMTTC MTTC DMTTC

Catalàn νa νf Δν knr/108 s−1 kr/108 s−1

MTTC DMTTC MTTC DMTTC MTTC DMTTC MTTC DMTTC MTTC DMTTC

Laurence νa νf Δν knr/108 s−1 kr/108 s−1

MTTC DMTTC MTTC DMTTC MTTC DMTTC MTTC DMTTC MTTC DMTTC

A0

p

a

b

R2

26,871.8 (±303.2) 27,566.6 (±229.6) 22,621.8 (±122.0) 23,573.3 (±123.6) 4082.5 (±411.5) 4179.7 (±588.0) 42.2 (±2.4) 56.9 (±2.4) 2.93(±0.2) 4.61E+08 (±0.5)

947.8 (±404.2) 656.1 (±306.1) −1681.8 (±153.0) −1330.8 (±181.6) 2721.2 (±548.5) 3031.8 (±783.8) −34.6 (±3.6) −2.4 (±0.3) −1.32 (±0.3) −2.10E+08 (±0.7)

1438.3 (±289.1) 1350.9 (±219.0) −457.2 (±107.8) −324.6 (±86.9) 2055.3 (±392.4) 3029.9 (±560.7) −12.7 (±2.6) −0.45 (±1.6) −1.03 (±0.2) −1.66E+08 (±0.4)

−2076.2 (±387.4) −1790.7 (±293.4) −309.2 (±127.3) −87.0 (±97.6) −1758.8 (±525.8) −3664.6 (±751.3) −7.2 (±2.9) −8.25 (±1.6) 1.30 (±0.3) 2.53E+08 (±0.6)

0.87 0.90 0.95 0.91 0.90 0.90 0.95 0.93 0.87 0.81

A0

s

d

a

b

R2

27,967.8 (±1245.4) 28,871.8 (±797.9) 23,668.8 (±495.3) 25,173.9 (±1342.0) 4771.3 (±1525.3) 3553.1 (±2001.6) 52.7 (±10.6) 85.4 (±12.7) 4.05 (±1.2) 7.15E+08 (±1.9)

−1080.0 (±1706.5) −1538.1 (±1090.4) −1532.0 (±668.7) −3295.6 (±1834.0) −383.9 (±2067.8) 2089.1 (±2742.7) −20.5 (± 14.2) −59.7 (17.3) −1.68 (±1.7) −3.99 (±2.5)

57.4 (±391.6) −87.5 (±239.6) −1566.1 (±153.4) −732.0 (±402.9) 1458.9 (±459.2) 899.2 (±629.3) −29.5 (±0.3) −13.0 (±4.0) −0.21 (±0.4) −0.25 (± 0.6)

1863.8 (±415.2) 1721.9 (±257.1) −827.3 (±162.6) −2916.1 (±432.5) 2918.1 (±492.4) 4403.6 (±667.3) −0.07 (±3.5) −20.1 (±4.2) −2.1 (±0.4) −3.37E+08 (±0.6)

−1557.6 (±425.3) −1220.7 (±253.2) −40.6 (±155.6) 1172.5 (±425.9) −1191.5 (±465.1) −2735.7 (±683.6) −9.25 (± 3.3) 8.57 (±4.3) 0.58 (±0.4) 1.54E+08 (±0.6)

0.92 0.95 0.97 0.94 0.93 0.95 0.95 0.93 0.88 0.90

A0

di

e

a

b

R2

28,196.2 (±2828.1) 28,595.6 (±2028.1) 23,567.8 (±868.2) 24,281.2 (±599.0) 6574.8 (±569.2) 3369.8 (±5606.4) 57.8 (±14.4) 77.8 (±10.6) 6.5E+08 (±2.6) 11.1E+09 (±4.7)

−1513.5 (±3542.6) −1181.8 (±2540.5) −1304.6 (±1078.0) −1502.5 (±725.8) −2673.2 (±706.1) 1391.4 (±6970.4) −27.8 (±17.9) −36.5 (±12.9) −4.3 (±3.1) −8.0 (±5.7)

−317.0 (±841.7) −245.5 (±603.6) −2173.2 (±264.9) −906.8 (±197.7) 1386.7 (±169.8) 741.3 (±1662.9) −41.7 (±0.4) −19.0 (±3.5) −0.5 (±0.8) −1.41 (±1.4)

1374.1 (±502.7) 1245.1 (±360.5) −412.8 (±159.0) −109.8 (±137.6) −148.7 (±135.5) 3153.4 (±1003.8) −4.3 (±3.4) 1.7 (±2.4) 0.8 (±0.4) −2.69E+08 (±0.8)

−962.8 (±892.1) −989.1 (±639.8) 578.3 (±261.5) 114.1 (±175.6) −320.4 (±173.1) −1692.0 (±1641.0) 9.23 (±4.4) −2.7 (±3.1) 0.6 (±0.7) 2.10E+08 (±1.3)

0.72 0.81 0.95 0.82 0.96 0.72 0.96 0.9 0.76 0.72

polarizability, SP, in case of DMTTC. The impact of the non-specific interactions on the non-radiative rate constant, knr, is very high in case of MTTC with no contribution from the solvent's hydrogen bond donor strength, SA, while the solvent's hydrogen bond donor strength, SA, contributes more in case of DMTTC. Table 2 shows that the radiative rate constant, kr, depends more on the specific interactions and very weak impact from the solvent's dipolarizability, SdP. In addition, the impact of solvent's hydrogen bond donor strength, SA, is higher than the solvent's hydrogen bond acceptor strength, SB, for MTTC and DMTTC. Laurence et al. [45] have recently modified the linear solvation energy relationship by introducing new parameters to describe both

dispersion and induction interactions, DI, and ES parameter to describe the electrostatic interactions between permanent multipoles of the solute and solvent. The solvent's hydrogen bond acidity, α, and the solvent's hydrogen bond basicity, β, have the same definition as have been described by previous models. A ¼ A0 þ diDI þ eES þ a1 α þ b1 β

ð21Þ

where di, e, a, and b are the corresponding regression coefficients. The contribution of dispersion and induction forces was correlated to the function f(n2D) which is given by:       f n2D ¼ n2D −1 = 2n2D þ 1

ð22Þ

where n2D is the solvent's refractive index. Laurence et al. have chosen the function f(n2D) to describe the sum of dispersion and induction forces exerted by solvents which has given the symbol DI, and is defined for a given solvent by: h     i h     i DI ¼ f n2D s − f n2D g = f n2D CS −f n2D g 2

ð23Þ

where the gas phase was taken as the natural origin of the scale, and carbon disulfide was chosen as the scaling solvent. In simple words, Laurence et al. correlated DI to the solvent's refractive index via the following equation:   DI ¼ 3:817 f n2D s

Fig. 9. Percentage contribution of solvent parameters obtained from Catalán's analysis (Eq. (20)) for different spectroscopic properties of MTTC and DMTTC.

ð24Þ

In addition to the dispersion and induction contribution to the solvent effect, Laurence et al. have separated solute-solvent electrostatic interaction (ES) into two categories; the first is for non‑hydrogen bonding (non-HBD) solvents and the other for hydrogen bonding solvents. In

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A.M. Dappour et al. / Journal of Molecular Liquids 286 (2019) 110856

order to get dimensionless ES electrostatic parameter for non-HBD solvents, they have used the ΔET(30) value of DMSO (14.01 kcal mol−1) as the scaling factor and define ES as ES ¼ ΔET ð30Þ=14:01

ð25Þ

and    ΔET ð30Þ ¼ ET ð30Þ− 30:06 þ 4:446 f n2D

ð26Þ

where ET(30) is the molar electronic energy of the S0 → S1 transition of betaine dye 30, ΔET(30) is the displacement of other solvents from this “alkane line” which is obtained from the plot of ET(30) versus f(n2D) and f (n2D) = (n2D − 1)/(2n2D + 1). On the other hand, ET(30) values in case of HBD solvents has a contribution from the hydrogen bonding effect and therefore cannot be used directly for the determination of the ES values. In this case; the non-specific solvent effect must be taken into consideration in the calculation of ES. Eqs. (25) and (26) can be rephrased to have the form: h   i ES ¼ ET ð30Þnsp − 30:06 þ 4:446 f n2D =14:01

ð27Þ

and ET ð30Þnsp ¼ 0:693ET ðPCM−TD−DFTÞ þ 1:31

ð28Þ

where ET(30)nsp can be calculated within the time-dependent density functional theory (TD-DFT) framework, using a polarizable continuum solvation model (PCM) [35]. Solvent's hydrogen bond acidity (α) parameter and hydrogen bond basicity (β) parameter were determined also by Laurence et al. and were found to be on the same scale as previously given by Tamlet-Taft and Catalan. Laurence Eq. (21) was also used to fit the photophysical properties given in Table 1, namely the absorption energy, emission energy, Stokes shift, and the radiative and non-radiative rate constants (Fig. 10). Results of Laurence equation fitting was collected in Table 2. The obtained regression coefficients clearly show that the non-specific interactions (DI + ES) contributions are always higher than specific interactions for all photophysical properties except the absorption energy, νa, for MTTC and DMTTC and Δν for DMTTC. The clear dependence on DI and ES is consistent with the dependence on Δf (Eq. (4)) and EN T (vide supra) (Fig. 11).

Fig. 10. Laurence's linear solvation energy relationship plot of the calculated values of the absorption energy, νa, emission energy, νf, Stokes's shift, Δν, radiative rate constant, kr, and non-radiative rate constant, knr, against their corresponding experimental data.

Fig. 11. Percentage contribution of solvent parameters obtained from Laurence's analysis (Eq. (21)) for different spectroscopic properties of MTTC and DMTTC.

A.M. Dappour et al. / Journal of Molecular Liquids 286 (2019) 110856

4. Conclusion The excited state photophysical properties of two methoxyphenyl bithiophene carbonitrile derivatives MTTC and DMTTC were studied and were found to be very sensitive to the solvent properties. Dependence of the photophysical properties on the solvent polarity parameters, EN T , and the reaction field factor, Δf, were found to be best observed when the solvents were classified to protic and aprotic solvents. Dipole moment changes, Δμ, calculated from the dependence on the Stokes shift on Δf were found to be 15.2 ± 3.0 D and 8.8 ± 2.0 D for MTTC and 13.6 ± 3.0 D and 4.8 ± 2.0 D for DMTTC in protic and aprotic solvents, while the dependence of the Stokes shift on EN T results in values of 15.5 ± 2.0 D and 21.7 ± 3.0 D for MTTC and 14.0 ± 2.0 D and 6.0 ± 3.0 D for DMTTC in protic solvents and aprotic solvents, respectively. Quantification of specific and non-specific solvent contribution towards the observed solvatochromic behavior is estimated using multiparametric Kamlet-Taft, Catalàn and Laurence solute-solvent interaction models. Non-specific interactions were found to have major contributions on the emission energy, νf, and non-radiative rate constants, knr, while the specific interactions have the major contributions on other photophysical properties such as absorption energy, νa, Stokes shift, Δν, and the radiative rate constants, kr, as elucidated by KamletTaft and Catalan models. On the other hand, application of the newly developed Laurence model on the photophysical properties of MTTC and DMTTC have shown that specific interactions are only important in case of absorption energy, νa. References [1] Y. Dong, A. Bolduc, N. McGregor, W.G. Skene, Org. Lett. 13 (2011) 1844–1847. [2] J. Roncali, Chem. Rev. 97 (1997) 173–206. [3] Y. Li, J. Hu, G. He, H. Zhu, X. Wang, Q. Guo, A. Xia, Y. Lin, J. Wang, X. Zhan, J. Phys. Chem. C 120 (2016) 13922–13930. [4] L. Beverina, J. Fu, A. Leclercq, E. Zojer, P. Pacher, S. Barlow, E.W. Van Stryland, D.J. Hagan, J.L. Bredas, S.R. Marder, J. Am. Chem. Soc. 127 (2005) 7282–7283. [5] W.J. Yang, D.Y. Kim, M.-Y. Jeong, H.M. Kim, S.-J. Jeon, B.R. Cho, Chem. Commun. (2003) 2618–2619. [6] S.J. Chung, M. Rumi, V. Alain, S. Barlow, J.W. Perry, S.R. Marder, J. Am. Chem. Soc. 127 (2005) 10844–10845. [7] M. Albota, D. Beljonne, J.-L. Br8das, J.E. Ehrlich, J.-Y. Fu, A.A. Heikal, S.E. Hess, T. Kogej, M.D. Levin, S.R. Marder, D. McCord-Maughon, J.W. Perry, H. Rçckel, M. Rumi, G. Subramaniam, W.W. Webb, X.-L. Wu, C. Xu, Science 281 (1998) 1653–1656. [8] C. He, Q. He, Y. He, Y. Li, F. Bai, C. Yang, Y. Ding, L. Wang, J. Ye, Sol. Energy Mater. Sol. Cells 90 (2006) 1815–1827. [9] X.H. Zhang, B.J. Chen, X.Q. Lin, O.Y. Wong, C.S. Lee, H.L. Kwong, S.T. Lee, S.K. Wu, Chem. Mater. 13 (2001) 1565–1569. [10] S. Roquet, A. Cravino, P. Leriche, O. Alévêque, P. Frére, J. Roncali, J. Am. Chem. Soc. 128 (2006) 3459–3466. [11] O.P. Varnavski, J.C. Ostrowski, L. Sukhomlinova, R.J. Twieg, G.C. Bazan, T. Goodson, J. Am. Chem. Soc. 124 (2002) 1736–1743. [12] N. Sarkar, K. Das, A. Datta, S. Das, K. Bhattacharyya, J. Phys. Chem. 100 (1996) 10523–10527. [13] V.V. Meshkovaya, A.V. Yudashkin, Y.N. Klimochkin, H. Meier, Dyes Pigments 113 (2015) 435–446Angew. Chem. Int. Ed. 44 (2015) 2482–2506. [14] R.A.J. Janssen, L. Smilowitz, N.S. Sariciftci, D. Moses, J. Chem. Phys. 101 (1994) 1787–1798. [15] M. Theander, O. Inganäs, W. Mammo, T. Olinga, M. Svensson, M.R. Andersson, J. Phys. Chem. B 103 (1999) 7771–7780. [16] M.R. Andersson, O. Thomas, W. Mammo, M. Svensson, M. Theander, O. Inganäs, J. Mater. Chem. 9 (1999) 1933–1940. [17] H. Chosrovian, S. Rentsch, D. Grebner, D.U. Dahm, E. Birckner, H. Naarmann, Synth. Met. 60 (1993) 23–28. [18] M. Granström, M.G. Harrison, R.H. Friend, in: D. Fichou (Ed.), Handbook of Oligoand Olythiophene, Wiley-VCH, New York, 1999 , (ch. 8). [19] G. Lanzani, G. Cerullo, S. Stagira, S. De Silvestri, J. Photochem. Photobiol. A Chem. 144 (2001) 13–19. [20] R.S. Becker, J.S. de Melo, A.L. Macanita, F. Elisei, Pure Appl. Chem. 67 (1995) 9–16. [21] R.S. Becker, J.S. de Melo, A.L. Macanita, F. Elisei, J. Phys. Chem. 100 (1996) 18683–18695. [22] J.S. de Melo, L.M. Silva, L.G. Arnaut, R.S. Becker, J. Chem. Phys. 111 (1999) 5427–5433. [23] J. Pina, H.D. Burrows, R.S. Becker, F.B. Dias, A.L. Maҫanita, J. Seixas de Melo, J. Phys. Chem. B 110 (2006) 6499–6505. [24] J.S. de Melo, H.D. Burrows, M. Svensson, M.R. Andersson, A.P. Monkman, J. Chem. Phys. 118 (2003) 1550–1556. [25] S. Rentsch, J.P. Yang, W. Paa, E. Birckner, J. Schiedt, R. Weinkauf, Phys. Chem. Chem. Phys. 1 (1999) 1707–1714.

11

[26] W. Zhu, Y. Jiang, Phys. Chem. Chem. Phys. 2 (2000) 47–52. [27] M.C.R. Delgado, V. Hernández, J. Casado, J.T.L. Navarrete, J. Raimundo, P. Blanchard, J. Roncali, Chem. Eur. J. 9 (2003) 3670–3682. [28] J. Casado, V. Hernández, O. Kim, J. Lehn, J.T.L. Navarrete, S.D. Ledesma, R.P. Ortiz, M.C.R. Delgado, Y. Vida, E. Pérez-Inestrosa, Chem. Eur. J. 10 (2004) 3805–3816. [29] E. Ortí, P.M. Viruela, R. Viruela, F. Effenberger, V. Hernández, J.T.L. Navarrete, J. Phys. Chem. A 109 (2005) 8724–8731. [30] J. Lipiński, W. Bartkowiak, Chem. Phys. 245 (1999) 263–276. [31] H. Meier, B. Mühling, H. Kolshorn, Eur. J. Org. Chem. (2004) 1033–1042. [32] [a] A.S.I. Amer, A.M.M. Alazaly, A.A. Abdel-Shafi, J. Photochem. Photobiol. A Chem. 369 (2019) 202–211; [b] A.A. Abdel-Shafi, Spectrochim. Acta A 66 (2007) 1228–1236. [33] F. Effenberger, F. Würthner, F. Steybe, J. Organomet. Chem. 60 (1995) 2082–2091. [34] F. Effenberger, F. Würthner, Angew. Chem. Int. Ed. Eng. 32 (1993) 719–721. [35] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, VCH, Weinheim, 2003. [36] M. Danko, A. Andicsová, P. Hrdlovič, D. Račko, D. Végh, Photochem. Photobiol. Sci. 12 (2013) 1210–1219. [37] V. Lukeš, M. Danko, A. Andicsová, P. Hrdlovič, D. Végh, Synth. Met. 165 (2013) 17–26. [38] P.K. Nandi, K.K. Das, S.P. Bhattacharyya, J. Mol. Struct. (THEOCHEM) 466 (1999) 155–164. [39] S. Meng, J. Ma, J. Phys. Chem. B 112 (2008) 4313–4322. [40] A. Bolduc, Y. Dong, A. Guérin, W.G. Skene, Phys. Chem. Chem. Phys. 14 (2012) 6946–6956. [41] W. Rettig, V. Kharlanov, F. Effenberger, F. Steybe, Chem. Phys. Lett. 404 (2005) 272–278. [42] M.J. Kamlet, J.L.M. Abboud, M.H. Abraham, R.W. Taft, J. Organomet. Chem. 48 (1983) 2877–2887. [43] J. Catalán, J. Phys. Chem. B 113 (2009) 5951–5960. [44] J.C. del Valle, F. Garcia Blanco, J. Catalán, J. Phys. Chem. B 119 (2015) 4683–4692. [45] C. Laurence, J. Legros, A. Chantzis, A. Planchat, D. Jacquemin, J. Phys. Chem. B 119 (2015) 3174–3184. [46] M.A. Ismail, R.K. Arafa, M.M. Youssef, W.M. El-Sayed, Drug Des. Devel. Ther. 8 (2014) 1659–1672. [47] W.H. Melhuish, S.R. Meech, D. Phillips, J. Phys. Chem. 65 (1961) 229–235J. Photochem. 23 (1961) 193–217. [48] O.F. Mohammed, J. Phys. Chem. A 114 (2010) 11576–11582. [49] M. Viard, J. Gallay, M. Vincent, O. Meyer, B. Robert, M. Paternostre, Biophys. J. 73 (1997) 2221–2234. [50] W. Nowak, P. Adamczak, A. Balter, A. Sygula, J. Mol. Struct. (THEOCHEM) 193 (1986) 13–23. [51] A.B.J. Parusel, W. Nowak, St. Grimme, G. Köhler, J. Phys. Chem. A 102 (1998) 7149–7156. [52] M. Brozis, K.A. Kozyra, V.I. Tomin, J. Heldt, J. Appl. Spectrosc. 69 (2002) 480–483. [53] K.A. Kozyra, J.R. Heldt, J. Heldt, M. Engelke, H.A. Diehl, Z. Naturforsch. 58a (2003) 581–588. [54] M. Jόzefowicz, K.A. Kozyra, J.R. Heldt, J. Heldt, Chem. Phys. 320 (2005) 45–53. [55] O.F. Mohammed, O. Kwon, C.M. Othon, A.H. Zewail, Angew. Chem. Int. Ed. 48 (2009) 6251–6256. [56] H. Song, K. Wang, Z. Kuang, Y.S. Zhao, Q. Guo, A. Xia, Phys. Chem. Chem. Phys. 21 (2019) 3894–3902. [57] F. Chen, W. Zhang, Z. Liu, L. Meng, B. Bai, H. Wang, M. Li, RSC Adv. 9 (2019) 1–10. [58] W.M. Kwok, C. Ma, M.W. George, D.C. Grills, P. Matousek, A.W. Parker, D. Phillips, W.T. Toner, M. Towrie, Photochem. Photobiol. Sci. 6 (2007) 987–994. [59] H. Zhu, X. Wang, R. Ma, Z. Kuang, Q. Guo, A. Xia, ChemPhysChem 17 (2016) 3245–3251. [60] F. Chen, W. Zhang, T. Tian, B. Bai, H. Wang, M. Li, J. Phys. Chem. A 121 (2017) 8399–8407. [61] W. Kwok, M.W. George, D.C. Grills, C. Ma, P. Matousek, A.W. Parker, D. Phillips, W.T. Toner, M. Towrie, Angew. Chem. Int. Ed. 42 (2003) 1826–1830. [62] [a] N. Mataga, Y. Kaifu, M. Koizumi, Bull. Chem. Soc. Jpn. 29 (1956) 465–470; [b] E.Z. Lippert, Elektrochem. 61 (1957) 962–975; [c] N. Mataga, Bull. Chem. Soc. Jpn. 36 (1963) 654–659; [d] E.Z. Lippert, Naturforsch. A 10 (1955) 541–545; [e] N. Mataga, Y. Kaifu, M. Koizumi, Bull. Chem. Soc. Jpn. 28 (1955) 690–691. [63] G. Angulo, G. Grampp, A. Rosspeintner, Spectrochim. Acta A 65 (2006) 727–731. [64] [a] J. Mingli, M. Xiaonan, Y. Linyin, W. Haifeng, G. Qianjin, W. Xuefei, W.Y. Ingying, Z. Xiaowei, X. Andong, J. Phys. Chem. A 114 (2010) 7345–7352; [b] W. Verbouwe, L. Viaene, M. Van der Auweraer, F.C. De Schryver, H. Masuhara, R. Pansu, J. Faure, J. Phys. Chem. A 101 (1997) 8157–8165; [c] K. Guzow, M. Milewska, W. Wiczk, Spectrochim. Acta A 61 (2005) 1133–1140. [65] A.A. Abdel-Shafi, M.A. Ismail, S.S. Al-Shihry, J. Photochem. Photobiol. A Chem. 316 (2016) 52–61 (and references therein). [66] Z.R. Grabowski, K. Rotkiewicz, W. Rettig, Chem. Rev. 103 (2003) 3899−4032. [67] P.F. Barbara, W. Jarzeba, Acc. Chem. Res. 21 (1988) 195−199. [68] J. Hicks, M. Vandersall, E. Sitzmann, K. Eisenthal, Chem. Phys. Lett. 135 (1987) 413−420. [69] M. Maroncelli, J. MacInnis, G.R. Fleming, Science 243 (1989) 1674−1681. [70] H. Heitele, Angew. Chem. Int. Ed. Eng. 32 (1993) 359−377. [71] M. Maroncelli, E. W. Castner, B. Bagchi, G. R. Fleming, Faraday Discuss. Chem. Soc. 85 (1988) 199–210. [72] C. Reichardt, Chem. Rev. 94 (1994) 2319–2358. [73] G. Wypych (Ed.), Handbook of Solvents, Volume 1, Properties, 3rd editionChemTec Publishing, Canada, 2019. [74] M. Ravi, T. Soujanya, A. Samanta, T.P. Radhakrishnan, J. Chem. Soc. Faraday Trans. 91 (1995) 2739–2742.