Solvatochromic behavior and electronic structure of some symmetric 2-aminophenol Schiff base derivatives

Journal of Molecular Liquids 199 (2014) 57–66

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Solvatochromic behavior and electronic structure of some symmetric 2-aminophenol Schiff base derivatives Yadigar Gülseven Sıdır a,⁎, İsa Sıdır a, Halil Berber b, Gülşen Türkoğlu b a b

Bitlis Eren University, Faculty of Arts & Science, Department of Physics, 13000 Bitlis, Turkey Anadolu University, Faculty of Science, Department of Chemistry, 26470 Eskişehir, Turkey

a r t i c l e

i n f o

Article history: Received 28 April 2014 Received in revised form 11 August 2014 Accepted 16 August 2014 Available online xxxx Keywords: Symmetric Schiff bases Solvatochromism Electronic structure Kamlet–Taft parameters Catalan parameters Reichardt–Dimroth parameter

a b s t r a c t The solvatochromic behavior and electronic structure of four symmetric 2-aminophenol Schiff base derivatives were investigated by using electronic absorption spectra in fourteen different spectroscopic grade solvents. The electronic transitions of these molecules have been interpreted. The electronic transition mechanisms and properties are investigated with four different linear solvation energy relationship (LSER) methods, which use different parameters such as Kamlet–Taft parameter (dielectric function f(ε) = (ε − 1) / (ε + 2), refractive index function f(n) = (n2 − 1) / (n2 + 1), hydrogen bond acceptor capacity (β) and hydrogen bond donor capacity(α)), Catalan parameters (polarity/polarizability (SPP), acidity of solvents (SA) and basicity of solvents (SB)), Marcus optical dielectric function and Reichardt–Dimroth ET solvent parameter. Some electronic parameters, such as EHOMO, ELUMO, EHOMO − 1, ELUMO + 1, dipole moment, electron affinity, electronegativity and ionization potential, MEP (molecular electrostatics potential) and SAS (solvent accessibility surface) were calculated by using B3LYP/6-31G(d,p) method. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Schiff base compounds having imine groups (CH–N) between two benzene rings and π-conjugate system can be used to design various molecular electronic devices because of having specific electronic structure process like ESIPT (excited state intra/inter-molecular proton transfer). This process occurs both in solvent medium and solid state phase. ESIPT eventuates to H-atom transfer from hydroxyl O atom to imines N atom in solid state [1–3]. In addition, the formation of ESIPT process leads Schiff base derivatives to have photochromism and thermochromism properties. These compounds have important characteristic properties and many application areas such as photochromic, thermochromic, optical sensor, molecular memory retention, measure and control of radiation intensity in optic computers and display systems [4–9]. These compounds are known to be one of the most important classes used for the synthesis of novel optical and conductor materials. Moreover, compounds including imine group are still an important research topic due to having various applications in optical communication, electronic, optoelectronic and photonic areas [10]. Many Schiff base compounds have been found to have liquid crystal properties in recent years [11,12]. It is reported that these compounds can be used as non-linear optic material due to exhibiting positive solvatochromism [13]. The Schiff base accommodates different metal centers involving various coordination modes thereby allowing ⁎ Corresponding author. Tel.: +90 434 2285170; fax: +90 434 228 51 71. E-mail address: [email protected] (Y. Gülseven Sıdır).

http://dx.doi.org/10.1016/j.molliq.2014.08.018 0167-7322/© 2014 Elsevier B.V. All rights reserved.

successful synthesis of homo- and hetero-metallic complexes with varied stereochemistry [14]. The spectral behavior of any organic compound is considerably related to its structure in both the ground and excited states. The investigation of solvent–solute interactions is very important, when the solvent medium can substantially influence the chemical and physical properties of the solute. Solvatochromic behavior of molecules born out from the solvation of ground and excited states of the lightabsorbing molecule, thus, provides a convenient tool to study the photophysical and related different properties [15–18]. These compounds are very usable in the design of new different electronic devices because symmetric Schiff base molecules have specific electronic structure. Thus, in this study, we have researched the solvatochromic behavior and electronic structure of four symmetric 2-aminophenol Schiff base derivatives. The electronic transitions of molecules were measured in fourteen solvents with various polarities. The solvatochromic behavior of investigated molecules was determined by using Kamlet–Taft Parameter (dielectric function f(ε) = (ε − 1) / (ε + 2), refractive index function f(n) = (n2 − 1) / (n2 + 1), hydrogen bond acceptor capacity (β) and hydrogen bond donor capacity(α)), Catalan parameters (polarity/ polarizability (SPP), acidity of solvents (SA) and basicity of solvents (SB)), Marcus optical dielectric function and Reichardt–Dimroth ET solvent parameter. DFT calculations on some electronic parameters in the ground state of the symmetric Schiff base derivatives were performed and discussed. In addition, solvent accessibility surface (SAS) and molecular electrostatic potential (MEP) were depicted and evaluated.

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Y. Gülseven Sıdır et al. / Journal of Molecular Liquids 199 (2014) 57–66

SB1

SB2

SB3

SB4 Fig. 1. Molecular structures of the studied SSBs.

potential etc.), solvent accessibility surface (SAS), molecular electrostatic potentials (MEPs) were obtained.

2. Materials and methods 2.1. Experimental section

2.3. Statistical methods Synthesis and purification of the studied symmetric Schiff bases (SSB) are performed as given in the references [19,20]. The molecular structures of these molecules are depicted in Fig. 1. Moreover, IUPAC names are listed in Table 1. All of the used organic solvents purchased from Sigma & Aldrich are spectroscopic grade. The solutions have been prepared with the concentration of 10− 5 M. The UV–visible spectra of the prepared solutions are recorded using Shimadzu UV2101 Pc series spectrometer in 1 cm quartz cell. 2.2. Quantum chemical calculations Firstly, energy optimization was performed with AM1 [21]. method after molecules were drawn with ChemOffice03. Secondly, input files of molecules have been established in GaussView5 software [22]. In the last step, input files were created by using B3LYP/6-31G(d,p) [23–25] level of theory and transferred to Gaussian09W packet program [26] for calculation. Quantum chemical calculations were completed and some electronic parameters (EHOMO, ELUMO, EHOMO − 1, ELUMO + 1, dipole moment, electron affinity, electronegativity and ionization

LSERs (linear solvation energy relationships), which effects the solvent polarity on the spectral features of solute, is used for determining to coefficients obtained from Kamlet–Taft [27] parameters with Eq. (1) and Catalan parameters [28] with Eq. (2) using multiple linear regression analysis (MLRA). νmax ¼ C0 þ C1 : f ðnÞ þ C2 : f ðεÞ þ C3 :β þ C4  α

ð1Þ

νmax ¼ C5 þ C6  SPP þ C7  SA þ C8  SB:

ð2Þ

In these equations, νmax is defined as the maximum absorption band. In Eq. (1), dielectric function is characterized as f(ε) = (ε − 1) / (ε + 2), refractive index function as f(n) = (n2 − 1) / (n2 + 1) and where, β and α are Kamlet–Taft parameters. In Eq. (2), polarity/polarizability is characterized as SPP, acidity of solvents as SA and basicity of solvents as SB. C0 and C5 coefficients describe the maximum absorption band in gaseous phase for Kamlet–Taft and Catalan solvatochromism, respectively.

Table 1 IUPAC names of the studied Schiff bases. Molecule

IUPAC name

SB1 SB2 SB3 SB4

2,2′-(1E,1′E)-(2,2′-(ethane-1,2-diylbis(oxy))bis(2,1-phenylene))bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)diphenol 2,2′-(1E,1′E)-(2,2′-(propane-1,3-diylbis(oxy))bis(2,1-phenylene))bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)diphenol 2,2′-(1E,1′E)-(2,2′-(propane-1,3-diylbis(oxy))bis(2,1-phenylene))bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(4-chlorophenol) 2,2′-(1E,1′E)-(2,2′-(propane-1,3-diylbis(oxy))bis(2,1-phenylene))bis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(4-bromophenol)

Y. Gülseven Sıdır et al. / Journal of Molecular Liquids 199 (2014) 57–66

1,25

1,75

1,4-Dioxane CCl4

SB1 1,00

0,50

1,4-Dioxane CCl4 CHCl3

1,25

Abs.

Abs.

0,75

SB2

1,50

CHCl3 Ethylacetate Ethanol Methanol 1-Butanol THF

59

Ethylacetate Ethanol Methanol 1-Butanol THF

1,00 0,75 0,50

0,25

1,75

300

350 400 Wavelength (nm)

450

SB3

1,75

1,4-Dioxane CCl4

400

450

1,00 0,75

500

CHCl3 Ethylacetate Ethanol Methanol 1-Butanol THF

0,75 0,50

0,25

0,25

450

1,4-Dioxane CCl4

1,00

0,50

350 400 Wavelength (nm)

SB4

1,25

Ethylacetate Ethanol Methanol 1-Butanol THF

300

350

1,50

CHCl3

1,25

0,00 250

300

Wavelength (nm)

1,50

Abs.

0,00 250

500

Abs.

0,00 250

0,25

0,00 250

500

300

350 400 Wavelength (nm)

450

500

Fig. 2. The UV–visible spectra in different solvents of the studied SSBs.

The C1 expounds the orientation induction interaction between solute/ solvent molecules, while C2 symbolizes the dispersion polarization interactions between them. However, C3 and C4 coefficients represent the contributions from hydrogen bond acceptor/donor capacity.

The C 5 coefficient in Catalan solvatochromism indicates the effect of the maximum absorption band shifts due to solvent polarity/ polarizability (SPP) parameter. The C6 coefficient derived from Catalan solvatochromism has been defined as the effect of the solvent acidity

1,25

0,75 2-Propanol Solvent

0,50

SB1 SB2 SB3 SB4

DMSO Solvent

SB1 SB2 SB3 SB4

1,00

Abs.

Abs.

0,75

0,50

0,25 0,25

0,00 250

300

350

400

Wavelength (nm)

450

500

550

0,00 250

300

350

400

Wavelength (nm)

Fig. 3. The UV spectra in 2-propanol and DMSO of the investigated SSB derivatives.

450

500

357(8.26) 456(1.20) 360(5.86) 351(9.41) 462(0.82) 359(8.50) 461(0.83) 275(6.61) 360(8.18)

359(7.52)

271(4.94) 350(5.63)

274 (6.35) 361 (7.97)

230(1.96) 234(1.96) 270(1.18) 361(0.97)

363(8.46)

271(6.69)

359(8.48)

282(9.07) 235(16.17) 271(7.01)

361(9.93)

363(8.22)

362(15.02) 465(0.62)

272 (7.29) 267(12.36) 270(7.08)

217(9.69) 235(10.23) 271(9.69)

278(5.27) 359(5.90) 467(0.52) 271(7.02) 360(8.51) 461(0.83) 271(7.28) 359(9.03) 362(8.62) 272(3.89) 359(4.51) 457(0.39) 274(6.86) 360(8.95) 454(0.27) 271(4.37) 359(5.55) 274(7.63) 359(9.64) SB4

SB2

SB3

272(6.35) 350(7.54)

356(8.47)

270(7.49) 351(8.73) 348(9.43)

269(9.04) 347(9.72) 348(8.54) 270(8.76) 348(9.67)

273(7.61) 356(8.708) 458(1.10) 231(18.29) 269(12.56) 361(15.63) 470(0.59) 362(8.08)

235(17.48) 270(6.70) 288(5.68) 360(9.39) 214 (14.87) 233(15.75) 270(6.76) 360(8.14)

271(7.92) 345(8.71) 452(0.60) 229(14.72) 271(7.48) 348(8.75) 449(0.67) 235(16.17) 270(8.05) 348(8.75) 274(8.18) 351(10.66)

228(9.05) 254(8.41)

270(5.39) 348(6.05) 446(0.34) 231(9.23)

267(9.13) 352(9.73)

248(7.64)

269(6.96) 346(7.46) 267(9.73) 350(9.61) 272(6.18) 350(7.16)

271(7.03) 347(7.97) 446(0.96) 230(19.31) 269(12.56) 361(15.63) 465(0.59)

234(12.53) 269(6.96) 288(5.36) 351(8.72) 212(15.92) 230(13.12) 269(7.61) 349(8.68)

281(7.28) 336(7.44) 450(0.47) 233(14.62)

THF n-Hexane

233(3.17) 257(1.96) 271(1.41) 340(1.65) 236 (1.56)

1-butanol Etylacetate

Ethanol

352(8.18)

269.5(9.095) 348.5(9.65) 349(8.40)

CHCl3

Figs. 2–4 show electronic absorption spectra of SSB derivatives in different solvents. The maximum wavelengths (nm) and molar extinction coefficients (εmax = 104 M−1 cm−1) observed in the UV–visible spectra are listed in Table 2. We can see from Table 2 and Figs. 2–4 that the absorption spectra of SSB derivatives exhibit two main electronic bands in all of solvents. Observed two main electronic bands are located in the range of 267–274 nm (4.6436–4.5250 eV) (Band A) and 336–362 nm (3.6900–3.4250 eV) (Band B). The absorption band A is π–π* (S0 → S1) electronic transition attributed to growth out from phenyl rings. This band does not change depending on solvent polarity, but it is sensitive on type and positions of substituents located on phenyl rings. The electronic band B, which shows intensive absorption peak, is because of π–π* electronic transition (S0 → S1) to come from Schiff base system. This band is ranging with the change of solvent polarity. As can be seen from Table 1, this band is partially red shifting while solvent polarity increases. Additionally, electronic absorption spectra of some SSB derivatives designate different electronic transitions in the range of 212–248 nm (5.8483–4.9994 eV) (A* band) which are observed at higher energy comparison to A absorption band. The A* band is to be composing due to the conjugation between Schiff base and phenyl ring system. The electronic absorption spectra in polar protic solvents (2-propanol, ethanol, 1-butanol and methanol) have specially came into being in the range of 446 nm (2.7799 eV)–467 nm (2.6549 eV), which is correspond to n–π* electronic transition, due to strong intramolecular hydrogen bonding between hydroxyl group in ortho-position of phenyl ring and azomethine group (\CH = N\). It is early reported that this transition is resultant of keto-amine tautomerism being formed in SBB derivatives, in that, has been named as ESIPT process, also [20].

CCl4

3.1. Electronic absorption spectra

2-Propanol

3. Result and discussion

DMSO

on the maximum electronic absorption band, while the C7 coefficient derived from Catalan solvatochromism has been defined as the effect of the solvent basicity on maximum electronic absorption band. The MLRA is performed with using SPSS 15.0 program. All the same, we have correlated to Marcus optical dielectric solvent function [29] (1 − εopt) / (2εopt + 1) versus maximum electronic transition energy of SSB molecules. Likewise, the maximum electronic transition energy of SSB molecules have been linearly correlated with Reichardt–Dimroth solvent ET parameter [30,31].

Table 2 The maximum wavelengths (nm) and molar extinction coefficients (εmax = 104 M−1 cm−1) of UV–visible spectra of the investigated Schiff bases.

Fig. 4. The UV spectra in n-hexane of the investigated SSB derivatives.

233(12.01)

500

1,4-Dioxane

450

Acetone

350 400 Wavelength (nm)

DMF

300

Molecule

0,00 250

229(7.20)

0,25

235(15.47) 270(9.26) 287(6.82) 347(10.77)

Toluen

0,50

SB1

Abs.

0,75

270(7.62) 346(8.42) 447(0.62) 233(11.60)

SB1 SB2 SB3 SB4

271(4.48) 347(4.73)

n-Hexane solvent

Methanol

1,00

271(8.35) 344(8.77) 444(1.08) 228(12.14)

Y. Gülseven Sıdır et al. / Journal of Molecular Liquids 199 (2014) 57–66 228(14.59)

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Y. Gülseven Sıdır et al. / Journal of Molecular Liquids 199 (2014) 57–66

61

Fig. 5. Resonance structures of SB1 and SB2 occurred via ESIPT mechanism under excitation.

3.2. Effect of substituents on UV–visible spectra The SB2, SB3 and SB4 molecules have the same molecular skeleton with different substituents located on the phenolic ring. Although there is not any substituent linked to phenolic structure of SB2 molecules, SB3 and SB4 have \Cl and \Br substituents in meta-position of phenolic structure, respectively. So, spectroscopic properties and molecular structure of these two molecules are affected by these substituents. If SB2 molecule is

considered as model for other two molecules, it is expected that SB3 and SB4 are affected by the substituents in proportion with substituent constants. The substituent constants are ρm-Cl = 0.37, ρp-Cl = 0.23, ρm-Br = 0.39 and ρp-Br = 0.23, thus, SB4 is the most affected one by the substituent among the studied molecules [32]. According to experimental results (Table 2, Figs. 2–4), π–π* electronic transition exhibiting around 270 nm has not been nearly affected by solvent medium. But, the n–π* transition observed around 350 nm region has been affected by the solvent medium.

Table 3 LSER coefficients for Kamlet–Taft parameters and statistical parameters for π–π⁎ electronic transition of SSBs. Molecules

C0 (cm−1)

C1

C2

C3

C4

R2

R

F

P

Number of solvent

SB1 SB2 SB3 SB4

29,964.57 29,447.62 28,801.43 28,733.04

−3462.93 −2765.15 −3327.03 −3386.41

−254.49 −15.84 212.68 255.20

164.32 127.47 5.65 −74.82

79.49 141.23 −99.69 −55.20

0.814 0.737 0.720 0.848

0.902 0.859 0.849 0.921

7.648 5.616 5.793 9.773

0.011 0.019 0.014 0.005

12 (except for 1-butanol and n-hexane) 13 (except for CHCl3) 14 12 (except for 1,4-dioxane and 1-butanol)

C0: Wavenumber in gaseous phase of molecule. The C1, C2, C3 and C4 derived from LSER are defined to contributions of SPP, SA and SB respectively. The R2, R, F and P are determined as regression coefficient, correlation coefficient, F statistic value and predictions, respectively.

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Table 4 The Kamlet–Taft parameters for used solvents and the observed and calculated B band (π–π* electronic transitions) in UV–visible spectra of investigated SSBs. Solvents

DMSO DMF Methanol Ethanol Acetone 2-Propanol 1-Butanol THF Ethyl acetate Chloroform Toluene CCl4 1,4-Dioxane n-Hexane

νCal.

νExp.

Kamlet–Taft solvent parameters β

α

f(ε)

f(n)

SB1

SB2

SB3

SB4

SB1

SB2

SB3

SB4

0.76 0.69 0.66 0.75 0.48 0.84 0.84 0.55 0.45 0 0.11 0 0.37 0

0 0 0.98 0.86 0.08 0.76 0.84 0 0 0.44 0 0 0 0

0.938 0.922 0.913 0.887 0.867 0.863 0.846 0.686 0.625 0.559 0.315 0.292 0.287 0.226

0.372 0.343 0.276 0.299 0.297 0.309 0.323 0.328 0.306 0.352 0.382 0.361 0.338 0.308

28,571 28,735 29,069 28,901 28,735 28,818 29,761 28,818 28,901 28,694 28,653 28,571 28,818 29,411

28,490 28,571 28,818 28,735 28,735 28,735 28,985 28,490 28,735 27,700 28,409 28,409 28,490 28,653

27,777 27,855 28,089 27,778 28,089 27,855 27,855 27,777 27,855 27,700 27,624 27,624 27,855 27,777

27,700 27,777 28,011 27,855 27,855 27,700 28,490 27,700 27,855 27,624 27,548 27,548 28,571 27,777

28,560 28,653 28,960 28,894 28,799 28,871 – 28,741 28,818 28,635 28,576 28,638 28,780 –

28,499 28,571 28,890 28,823 28,684 28,792 28,764 28,597 28,647 28,398 28,443 28,554 28,592

27,765 27,858 27,981 27,913 27,991 27,884 27,824 27,856 27,917 27,702 27,595 27,661 27,738 27,824

27,653 27,753 27,926 27,843 27,907 27,800 – 27,753 27,821 27,656 27,508 27,583 – 27,747

Table 5 LSER coefficients for Catalan solvatochromism and statistical parameters for π–π⁎ electronic transition of SSBs. Molecules

C5 (cm−1)

C6

C7

C8

R2

R

F

P

Number of solvent

SB1 SB2 SB3 SB4

29,033.72 29,430.84 27,835.59 28,214.728

−574.82 −1917.76 −269.96 −1052.83

417.01 −222.20 −182.91 222.41

347.70 1464.53 385.17 1029.79

0.623 0.678 0.688 0.747

0.789 0.823 0.829 0.864

4.404 6.312 5.870 5.902

0.042 0.014 0.020 0.032

12 (except for 1-butanol and n-hexane) 13 (except for methanol) 12 (except for acetone and methanol) 10 (except for DMSO, 2-propanol, THF and 1,4-dioxane)

C0: Wavenumber in gaseous phase of molecule. The C5, C6, C7 and C8 derived from LSER are defined to contributions of SPP, SA and SB respectively. The R2, R, F and P are determined as regression coefficient, correlation coefficient, F statistic value and predictions, respectively.

Differences between maximum absorption bands in solvent medium are determined about 10 nm as indicated below equations: Δλ ¼ λSB3 −λSB2  10 nm

Δλ ¼ λSB4 −λSB2  10 nm: The wavelength differences (Δλ) are positive values, thus electron distributions of SB2 molecules are relatively larger in comparison with the electron distributions of SB3 and SB4 due to the nature of \Cl and \Br substituents linked to phenolic ring molecules. The differences in wavelengths come from mesomerically electron donating nature of SB3 and SB4 molecules. We observe that the formation of keto-amino tautomerism is over 400 nm wavelength, while enol–imino form appeared in the range of 270–350 nm wavelength. These absorption band evidences the formation of keto-enol tautomerism, appears in methanol, ethanol and 1-butanol solvents for SB1 molecule; methanol, ethanol, 2-

propanol and CHCl3 solvents for SB2 molecule; DMSO, methanol, ethanol, 1-butanol and 2-propanol solvents for SB3 molecule, and methanol, ethanol, 1-butanol, 2-propanol and CHCl3 solvents for SB4 molecule. Fig. 5 depicts the possible resonance structure along with excited state intramolecular proton transfer (ESIPT) mechanism of the studied molecules.

3.3. Solvatochromic behaviors The statistical parameters for π–π⁎ electronic transitions with the highest optical density of symmetric Schiff bases are listed in Tables 3 and 5. Derived LSER models by using both Kamlet–Taft parameters and Catalan parameters capable of determining the interactions between solute and solvent molecules are satisfactory according to statistical parameters (R2, R, P and F). The Catalan parameters of used solvents and the observed and calculated B band (π–π* electronic transitions) in UV–visible spectra of investigated SSBs are listed in Table 6,

Table 6 Catalan parameters for the used solvents and the observed and calculated B band (π–π* electronic transitions) in UV–visible spectra of the investigated SSBs. Solvents

DMSO DMF Methanol Ethanol Acetone 2-Propanol 1-Butanol THF Ethyl acetate Chloroform Toluene CCl4 1,4-Dioxane n-Hexane

νCal.

νExp.

Catalan Parameters SPP

SA

SB

SB1

SB2

SB3

SB4

SB1

SB2

SB3

SB4

1.00 0.95 0.86 0.85 0.88 0.85 0.84 0.84 0.80 0.79 0.66 0.63 0.70 0.52

0.07 0.03 0.61 0.40 0 0.28 0.34 0 0 0.05 0 0 0 0

0.65 0.61 0.55 0.66 0.48 0.76 0.81 0.59 0.54 0.07 0.13 0.04 0.44 0.06

28,571 28,735 29,069 28,901 28,735 28,818 29,761 28,818 28,901 28,694 28,653 28,571 28,818 29,411

28,490 28,571 28,818 28,735 28,735 28,735 28,985 28,490 28,735 27,700 28,409 28,409 28,490 28,653

27,777 27,855 28,089 27,777 28,089 27,855 27,855 27,777 27,855 27,700 27,624 27,624 27,855 27,777

27,700 27,777 28,011 27,855 27,855 27,700 28,490 27,700 27,855 27,624 27,548 27,548 28,571 27,777

28,713 28,711 28,982 28,938 28,692 28,929 28,976 28,757 28,765 28,626 28,701 28,685 28,785 28,754

28,445 28,492 28,451 28,670 28,437 28,858 28,935 28,689 28,700 28,017 28,362 28,283 28,737 28,517

27,835 27,835 27,834 27,835 27,835 27,835 27,835 27,835 27,835 27,833 27,834 27,834 27,835 27,834

27,844 27,848 28,008 28,083 27,776 28,169 28,242 27,941 27,935 27,470 27,656 27,594 27,933 27,725

Y. Gülseven Sıdır et al. / Journal of Molecular Liquids 199 (2014) 57–66

Fig. 6. Marcus solvatochromism for the investigated SSB derivatives.

while Kamlet–Taft parameters of used solvents and the observed and calculated B band (π–π* electronic transitions) in UV–visible spectra of investigated SSBs are listed in Table 4. As can be seen from Tables 3 and 5 that the C0 and C5 coefficients indicate the π–π⁎ electronic transition wavelengths in gas phase of SSB derivatives for both Kamlet–Taft solvatochromism and Catalan solvatochromism. The C1 coefficients have negative sign. Thus, this transition is eventuating to bathochromic effect (red shift) according to solvent polarity and it depends on dispersion–polarization forces. The absolute C1 value is much bigger than absolute C2 value. In other words, these transitions appertain to chromophore of SSB derivatives, which are much more effective on refractive index function (dispersion interaction) in comparison to dielectric function (dipolar interaction). The C3 absolute value, which is Hbond accepting ability, is much bigger than C4 absolute value, which is H-bond donor ability. But, this case is not the same for SB3 molecule. So, SB3 molecule does not exhibit bathochromic shift for π–π⁎ electronic transitions. According to Table 5, the polarity/polarizability of solvent coefficient (SPP parameters) C6 correlates the solvatochromic shifts with the solvent polarity. The C6 coefficient is a negative sign. It means that positive solvatochromic shifts in maximum absorption band of SSB molecules. In other words, maximum absorption band shifts towards lower energy, in that, exposure bathochromic effect. The coefficient controlled the acidity of the solvent, C7 is found as the lowest one except for SB1 molecule and thus, it does not play an important role in absorption spectra. The C8 coefficient represents the basicity of solvent has a negative value suggesting that the absorption band shifts to lower energies with the increasing basicity of the solvent. This means that these effects can be interpreted with the stability in the resonance structure of the chromophores. Possible resonance structure is predominant in the S1 state and the stabilization of the S1 state with the solvent

Fig. 7. The solvatochromism according to Reichardt–Dimroth parameters for the investigated SSB derivatives.

63

basicity would be more important than that of S0 state. The increase of solvent basicity gives rise to absorption peaks shift to longer wavelengths. Consequently, the energy difference between S1 (excited state (LUMO)) and S0 (ground state (HOMO)) of molecules reduces. According to Catalan solvatochromism, we observe |C7| N |C8|. Thus, maximum absorbance band is affected by basicity much more compared to acidity of using solvents. There are linear correlations between maximum absorption energy and Marcus optical dielectric solvent functions. The correlation coefficients have been found as 0.7416 for SB1 molecule, 0.8592 for SB2 molecule, 0.7753 for SB3 molecule and 0.7558 for SB4 molecule, respectively. So, we observed that the transition dipole moments are associated with absorption spectra and direction of excited state dipole moment which is opposite to that of ground state dipole moment as can be seen from Fig. 6. Fig. 7 depicts to linear correlation of maximum absorption energy with Reichardt–Dimroth solvent ET parameter. According to Fig. 7, dielectric solvent solute interactions are responsible for bathochromic shift happens in maximum absorption band as dependent to increasing solvent polarity. The linear correlation coefficients between maximum electronic transition energy and Reichardt–Dimroth solvent ET parameter are determined as 0.74, 0.70, 0.55 and 0.64 for SB1, SB2, SB3 and SB4, respectively. 3.4. Solvent accessibility surface, molecular electrostatic potential, electronic properties and frontier molecular orbital analysis The solvent accessible surfaces (SAS) of investigated molecules are depicted in Fig. 8. SAS, which shows the interaction sites with solvent, is very usable to explain specific solute–solvent interactions. Red regions indicate interaction zones of solvent with oxygen atom of \OH group in phenolic moiety. Dark red regions show the interaction zones of solvent with bromine atom. Blue regions are interaction surfaces of solvents with nitrogen atom of Schiff bases. Green regions show zones to interact with solvents of Cl atoms. Gray regions as seen as SAS plots are entering places to interact with solvents of benzene rings, hydrocarbon moiety and hydrogen atoms. It is clear that the interactions between SSBs and solvent have generally been controlled by oxygen, nitrogen, bromine and chlorine atoms. These interactions correspond to specific solvent–solution interactions. MEP surfaces of the studied compounds are illustrated in Fig. 9. MEP gives information about electrostatic potential of a molecule and it provides a visual method to understand relative polarity. The color scheme for the MEP surface is red-electron rich or partially negative charge; blue-electron deficient region; and yellow-slightly electron rich, respectively. As seen in Fig. 9, negative regions, where come into existence nucleophilic attack, are oxygen atoms in OH moiety, and nitrogen atoms in Schiff bases and, chlorine and bromine atoms in meta-position of benzene ring. Conversely, the regions having positive potential where electrophilic attacks to molecule happens, are hydrogen and carbon atoms in hydrocarbon moiety and benzene rings. SB3 molecule has relatively larger irregular charge distribution which supports in comments of dipole moment. The four different molecular orbitals, which is HOMO, HOMO-1, LUMO and LUMO + 1 of SSB molecule were calculated and listed in Table 7. The HOMO and LUMO plots for every SSB compound are shown in Fig. 10. ΔE HOMO–LUMO is a very important parameter because it determines electron transfer properties. The HOMO– LUMO transition implies that intramolecular charge transfer takes place within the molecule [33]. ΔE HOMO–LUMO for SB molecules is ordered as ΔESB1 N ΔESB4 N ΔESB3 N ΔESB2 . We can say from Fig. 10 that HOMO is localized on all molecular structure for SB1 molecule due to molecular geometrical structure, while LUMO is localized on benzene ring and Schiff base group. While the HOMO of SB2 molecule is the distribution on benzene rings and Schiff bases, the LUMO of this molecule is distributed on the same zone. For SB3 molecule, HOMO is localized only on benzene ring, Schiff bases and Cl atom,

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SB1

SB2

SB3

SB4 Fig. 8. Solvent accessibility surface (SAS) of the investigated SSB derivatives.

when LUMO is localized only on benzene ring and Schiff base. As seen in Fig. 10, the distributions of HOMO and LUMO for SB4 are located on the same region. Dipole moment reflects the molecular charge distribution and is given as a vector in three dimensions. Therefore, it can be used as descriptor to depict the charge movement across the molecule. The direction of the dipole moment vector in a molecule depends on the centers of positive and negative charges. The μSB2 molecule has the highest dipole moment, in that, it has the highest charge distribution among the other molecules. This is attributed to inductive and steric effects on SB2 molecule. Afterwards, the order of dipole moments for the other compounds is μSB3 N μSB1 N μSB4. SB3 molecule includes Cl atom in meta-position while SB4 molecule has Br atom in meta-position. Electronegativity of Cl atom is bigger than Br atom. So, dipole moment of SB3 molecule is bigger than SB4 molecules. The dipole moment in a molecule is an important property that is mainly used to study the intermolecular interactions involving the non-bonded type dipole–dipole interactions. The slightly lower orbital energy gap and high dipole moment of SB2 molecule show its higher activity as compared to other molecules. The solvatochromic shifts exhibit characteristic of a large dipole moment. The calculated mean polarizabilities of SSBs derivatives are order as below:

electric field of light. Thus, molecular electronic charge distributions have been rearranged by interaction with electric field of light. Global electronegativity values of investigated molecules are given in Table 7. According to Table 7, the SB3 molecule has the bigger electronegativity. Thus, electronegativity of SB3 molecule carrying \Cl atom in meta-position has affected on the entire of molecular structure. Large electronegativity increases the interaction level of molecule with medium [34]. If a molecule has bigger electron affinity and ionization potential values, it has unstable molecular structure. Thus, molecule must come in different interactions, which is hydrogen bonding or other different interactions (van der Waals interactions). SB3 molecule has the higher electron affinity and ionization potential among the investigated molecules in this study. Molecular softness is a measure of molecular reactivity, while molecular hardness is a measure of molecular stability [34]. We can say from Table 7 that the lowest active molecules are SB1 and SB3, whereas the bigger stable one is SB1 molecule. Resultantly, SB3 molecule is the most active molecule.

αSB4 NαSB3 NαSB2 NαSB1 :

• We have observed the four electronic bands in electronic absorption spectra. First band is in the range of 267–274 nm due to π–π*electronic transition in phenyl rings, while the second band is observed in the range of 336–362 nm due to π–π* electronic transition in Schiff base

Polarizability is proportional with molecular volume. The bigger molecular polarizability means the more interaction with the

4. Conclusions In handle paper, final findings are listed in conclusions as follows:

Y. Gülseven Sıdır et al. / Journal of Molecular Liquids 199 (2014) 57–66

65

Fig. 9. Molecular electrostatic potentials (MEPs) of the investigated SSB derivatives.

system. Electronic transition bands at 212–248 nm are observed as third band. This band comes into existence in a result of conjugations between Schiff bases and phenyl ring system. Fourth electronic bands

Table 7 Some electronic parameters of SSB derivatives. Parameters/molecules

SB1

SB2

SB3

SB4

HOMO (eV) LUMO (eV) HOMO-1 (eV) LUMO + 1 (eV) ΔEHOMO–LUMO (eV) Electronegativity (eV) Ionization potential (eV) Electron affinity (eV) Molecular hardness (eV) Molecular softness (eV)

−5.40 −1.50 −5.55 −1.37 −3.90 3.45 5.40 1.50 1.95 0.51

−5.23 −1.67 −5.74 −1.23 −3.56 3.45 5.23 1.67 1.78 0.56

−5.69 −1.99 −5.80 −1.65 −3.71 3.84 5.69 1.99 1.85 0.54

−5.64 −1.74 −5.79 −1.69 −3.89 3.69 5.64 1.74 1.95 0.51

Dipole moment (μ) (Debye) μx (Debye) μy (Debye) μz (Debye) μtotal (Debye)

−2.27 0.93 −1.96 3.14

−4.54 −3.05 −1.72 5.74

0.09 −0.16 3.84 3.84

0.30 −2.51 −1.73 3.06

549.77 18.55 583.44 −186.74 −1.19 522.29 551.83

704.88 −147.35 528.73 16.69 41.02 454.06 562.56

584.61 −193.69 669.84 88.80 16.67 571.61 608.69

832.10 −140.16 686.28 60.53 −6.43 370.80 629.73

Polarizability (α) αxx αxy αyy αxz αyz αzz α

•

•

•

•

•

have been observed at 446–467 nm, which comes into existence corresponding to tautomeric forms. In this paper, we have analyzed the absorption spectra of SSB derivatives in different solvent media. We have observed red shift in the maximum absorption band with increasing solvent polarity. So, the investigated molecules can be considered as candidate for non-linear optic materials. According to the electronic absorption spectra of SBB derivatives, these compounds possess keto-amine tautomer form in solvent medium which process is named as excited state intra-molecular proton transfer (ESIPT). According to result of Kamlet–Taft solvatochromism, dispersion interactions in accordance with dipolar interaction, which is a non-specific solute–solvent interaction, are more effect in electronic absorption spectra, and hydrogen bond acceptor capacity in accordance with hydrogen bond donor capacity, which is a specific solute–solvent interaction, are more affected in electronic spectra. The occurrence of positive solvatochromism in electronic spectra is supported by Kamlet–Taft solvatochromism, also. We have observed positive solvatochromism in Catalan solvatochromism (with C6 coefficient), again. Basicity of solvents has been found more effective on solvatochromism than acidity of solvents. The SB3 is found as the most active molecule in solvent medium.

Acknowledgment This study is supported by Bitlis Eren University, Scientific and Technological Application and Research Center.

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SB1

LUMO

HOMO

SB2

LUMO

HOMO

SB3

LUMO

HOMO

SB4

LUMO

HOMO

Fig. 10. The HOMO and LUMO plots of the investigated SSB derivatives.

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