Journal of Molecular Liquids 244 (2017) 453–463
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A combined experimental and computational investigation of solvatochromism of nonpolar laser dyes: Evaluation of ground and singlet excited-state dipole moments G.H. Pujar a, M.N. Wari a, B. Steffi c, H. Varsha c, B. Kavita c, C. Yohannan Panicker b, C. Santhosh c, Ajeetkumar Patil d, Sanjeev R. Inamdar a,⁎ a
Laser Spectroscopy Programme, Department of Physics and UGC-CPEPA, Karnatak University, Dharwad 580003, India Department of Physics, Fatima Mata National College, Kollam 691 001, India c Centre for Atomic and Molecular Physics, Manipal University, Manipal, India d BioSyM, Singapore-MIT Alliance for Research and Technology (SMART), Singapore 138602, Singapore b
a r t i c l e
i n f o
Article history: Received 25 July 2017 Received in revised form 18 August 2017 Accepted 22 August 2017 Available online 24 August 2017 Keywords: Exalite laser dyes ZINDO-PCM Natural bond orbital Dipole moments Solvatochromism DFT and TD-DFT
a b s t r a c t In the present work, the solvatochromism of two large nonpolar laser dyes Exalite 384 (E384) and Exalite 416 (E416) has been studied both experimentally and computationally. The steady state absorption and fluorescence spectra have been measured in a series of polar protic, polar aprotic and non-polar/weakly polar solvents to investigate their solvatochromism and determine dipole moments. Various solvent correlation techniques, like Lippert–Mataga, Bilot–Kawski, Kawski–Chamma–Viallet, Bakhshiev and Reichardt methods were used to evaluate the singlet excited and ground state dipole moments. Kamlet–Taft and Catalan solvent parameters were used by means of multiple linear regression (MLR) method to analyze specific and non-specific solute-solvent interactions. Computational studies were carried out to optimize ground and excited state geometries using density functional theory (DFT) and time-dependent density functional theory (TD-DFT), respectively, in vacuum. This study also extends to evaluate the electronic transition energies from ground to first electronic excited state of solvated laser dyes employing semiempirical wave function model. In particular, the semiempirical method ZINDO has been combined with integral equation formalism of polarizable continuum model (IEF-PCM) to calculate solute-solvent interaction potential which is comparatively studied with ET(30) polarity scale along with experimental. The intramolecular charge transfer and hybridization is demonstrated by natural bond orbital analysis (NBO). The ground (μg) and excited state dipole moments (μe) of these dyes computed and those determined experimentally are compared and the results are discussed. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The organic probes with poly-p-phenylenes have generated great interest in the field of optics through the last couple of decades due to their spectroscopic properties, absorption and photoluminescence, promising nonlinear optical coefficients, and lasing abilities [1– 6].These explorations have led to their use as laser dyes, fluorescent dyes and for other spectroscopic applications. Several laser dyes having p-phenylene as subunit are commercially available from Exciton Chemical Co., USA. These dyes are highly fluorescent and are used as laser dyes mainly in the UV and blue region [7–9]. The exalite series dyes have been known as one of the successful dye laser sources and caught considerable interest in applications ranging from dermatology to molecular spectroscopy [10] as well as in single crystal analysis [11]. In ⁎ Corresponding author. E-mail address:
[email protected] (S.R. Inamdar).
http://dx.doi.org/10.1016/j.molliq.2017.08.078 0167-7322/© 2017 Elsevier B.V. All rights reserved.
particular the titled compounds, exalite 384 (E384) and exalite 416 (E416) are widely used as fluorescent laser dyes in solution and exhibit efficient photophysical properties (absorption/fluorescence‚ λmax values of E384 are 324/378 nm and of E416 are 353/413 nm in 1,4-dioxane, respectively [12,13]). They have high absorption coefficients at 355 nm and excellent operating lifetimes in 1,4-dioxane solvent, making them potential candidates for pumping with the third harmonic of a Nd:YAG laser as well as under XeCl (308 nm) pumping. E384 and E416 are class of para-quaterphenylenes and their molecular structures were optimized to maximize the performance of the compounds as fluorescent dyes. There are four and six aromatic rings in E384 and E416 dyes, respectively (Fig. 1). The effects of these substituted rings have reflected on the optical properties [12,13]. The determination of dipole moments of laser dyes play an important role in research as it provides an insight into electron density, charge distribution around the probes, electronic and geometrical structure of a dye in bulk solution. In addition, the knowledge of excited state
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Fig. 1. Molecular structures of (A) E384 and (B) E416 employed in this study.
dipole moments is often useful to design new molecules and nonlinear optical materials [14]. Generally, the electronic spectra of organic compounds and laser dyes are affected by their immediate environment. There are several major factors which influence the electronic spectra. Among the major environmental factors, particular importance is given to solvent effects. The ground and excited states dipole moments of a probe depend on the distribution of electrons in their respective states. A change of organic solvent is usually characterized by a change in dielectric constant, polarizability and polarity of the surrounding medium. Hence, the solvents affect the ground and excited states dipole in many ways. The solvent effects analysis is very important in understanding of the excited state behavior of the probe. Several researchers and also our group [15–29] have reported on photophysical and chemical characteristics of various chromophores as a function of refractive index, polarizability and polarity of the solvents, concentration, viscosity, pH, etc. The excitation and emission spectra of a dye are generally affected by the solvents and depend on the type of interaction between solvent and solute. From these effects, it is possible to draw information regarding the behavior of the excited state as compared to the ground state. The spectral transitions (π* ← n, π* ← π, etc.) can be identified using the spectral shift data as function of solvent polarity. The π* ← n and π* ← π bands shows blue and red shift, respectively, as function of increasing solvent polarity. The well known term solvent polarity is closely related to solvation capability also known as solvation power. Though the solvent polarity seems qualitatively easy but difficulty arises while expressing precisely and quantitatively. Series of attempts have been made to explain the solvent polarity quantitatively based on individual macroscopic solvent properties such as refractive index, dipole moment and relative permittivity. Such parameters are found to be inadequate in generalizing the solvent polarity parameter. Dimroth and Reichardt [16] have successfully proposed the empirical solvent polarity parameter, ET(30), which is based on the transition energy of the dissolved pyridinium N-phenolate betaine dye. The ET(30) values determined for N360 organic solvents [16] and therefore it is the most famous and extensively used polarity scales to understand the solvatochromic shift behavior. This solvatochromic shifts technique is imperative in the evaluation of first electronically excited state dipole moment of solute probes and widely accepted among the various available methods due to its high linear correlation between solvent polarity functions and spectroscopic parameters. The traditional theories underlying this solvatochromic shift method have recently seen further refinement that helps extract more useful information on orientation broadening of absorption spectra, electronic transitions, electrostatic properties of a solute via solvatochromic shift [30]. Some modern and novel theories describing solvatochromic behavior have also been developed to estimate dipole moments in the excited states. For example, Vladimir Pavlovich [31] has concisely reviewed the current research trend on solvatochromism and non-radiative decay of intramolecular charge transfer states based on classical theory of fluctuations and thermodynamic methods. There are several reports available on solvatochromic shift technique for the evaluation of dipole moments (ground and singlet excited state) of various laser dyes [18–26]. However, there are no reports in literature for the estimation of ground state and singlet excited state
dipole moment values of E384 and E416 in a series of non-polar/weakly polar, polar protic and aprotic solvents. So far, the solvatochromic behavior and dipole moments of the exalite laser dyes have been estimated in polar protic and aprotic solvents [27–29]. Thus, it would be intriguing to measure dipole moments in nonpolar/weakly polar solvents also as no reports found in the literature. The present investigation concerns about the solvents effect on electronic absorption and emission spectra of E384 and E416 laser dyes in various polar protic and aprotic as well as in nonpolar/weakly polar solvents. The experimental dipole moments and change in dipole moment were estimated for the ground and excited state by various solvent correlation techniques using solvatochromic spectral shift method and hence to elicit the effect of solvents in their estimation. In addition, the ground dipole moments are calculated by DFT while excited state by TD-DFT formalism. Discussion ends with solvatochromic analysis computed according to AM1-PCM for ground state and ZINDO-PCM for excited states. The computational studies were also extended to elucidate the intramolecular charge transfer and hybridization using NBO analysis on laser dyes E384 and E416. From this study, it is defensible that excited state dipole moments of short lived species such as exalite laser dyes are often useful in designing and developing of nonlinear optical materials which allow photochemical transformations. 2. Materials and methods 2.1. Materials The commercially available laser dyes, exalite 384 and exalite 416 (see Fig. 1) were procured from Exciton Chemical Co., USA and were used as received. Spectroscopic grade solvents (HPLC grade, Fluka) used in the study were of highest available purity. 2.2. Instrumentation The steady-state absorption spectra were measured with dualbeam UV–Vis-NIR spectrophotometer (JASCO Model V-670). Photoluminescence (PL) spectra were recorded employing spectrofluorometer (JY Horiba, model Floromax-4) using integration time of 1 s nm− 1, resolution of 1 nm, an excitation slit width of 1.5 nm and emission slit width of 1.5 nm. All spectra were collected at room temperature using quartz cuvettes with 1 cm optical path length. The concentration of the solutions was kept quite low (~ 10 μM) in these measurements in order to avoid self-absorption and aggregation in absorption and PL, respectively. Linear fit was done by using origin 8.0 pro software and MLR regressions was done using Microsoft excel 2007. 2.3. Theoretical background for the evolution of dipole moments and multiple linear regression analysis The dipole moments of both electronically excited and ground states can be estimated using refractive index (n), bulk dielectric constant (ε) of the solvent and solvatochromic shifts. Bilot and Kawski [32–33] reported quantum chemical calculations to compute the experimental
G.H. Pujar et al. / Journal of Molecular Liquids 244 (2017) 453–463
value of ground (μg) and singlet excited state dipole moments (μe) which involved two solvatochromism expressions; f ¼ m1 FðnεÞ þ constant a −υ υ
ð1Þ
f ¼ −m2 ½ FðnεÞ þ 2 gðnÞ þ constant a þ υ υ
ð2Þ
þ1 ε−1 where, FðnεÞ ¼ 2n ½ − nn2−1 and gðnÞ ¼ 32 ½ þ2 n2 þ2 εþ2 2
2
n4 −1 2 ðn2 þ2Þ
1
m ¼
2
m ¼
2 2 μ e −μ g
ð3Þ
3
hca
2 μ 2e −μ 2g hca
ð4Þ
3
where, c is the velocity of light, h is the Planck's constant, a is the Onsager radius and μg and μe are the ground and excited state dipole moments of the probe. If the symmetry of molecules remains unchanged in the electronic states the ground and singlet excited state dipole moments were found to be parallel [33]. In such cases, the values of μg and μe from Eqs. (3) and (4) can be given as follows. 3
hca 2 m1
m1 þ m2 μe ¼ 2
hca 2 m1
3
ð5Þ !1=2 ð6Þ
When the dipole moments in ground and excited state are not parallel to one another, the angle (φ) between them can be estimated using Eq. (7) [34]. cosϕ ¼
m1 ¼ m2 ¼
m3 ¼
2 2 μ e −μ g hca
m1 1 μ 2g þ μ 2e − 2 μ 2e −μ 2g 2 μ gμ e m
ð7Þ
The expressions given by Lippert-Mataga (Eq. (8)) [35–36], Bakhshiev (Eq. (9)) [37] and Kawski-Chamma-Viallet (Eq. (10)) [38– 39] can also be used to evaluate the experimental singlet excited state dipole moment. a −υ f ¼ m1 F1 ðεnÞ þ constant υ
ð8Þ
f ¼ m2 F2 ðεnÞ þ constant a −υ υ
ð9Þ
f a þ υ υ ¼ −m3 F3 ðεnÞ þ constant 2
ð10Þ
f are absorption and fluorescence maxima a and υ where, υ wavenumbers in cm−1 respectively, m1, m2, m3 are slopes obtained from linear relationships and F1, F2 and F3 are solvent polarity functions as expressed in Eqs. (11)–(13).
ð14Þ
3
2 μ 2e −μ 2g hca
ð15Þ
3
Another method proposed by Reichardt [16,40] generally gives better correlations of the spectral shifts with the empirical microscopic solvent polarity parameter (EN T ) than the traditionally adopted technique based on bulk solvent polarity functions involving dielectric constant (ε) and refractive index (n). Hence, this method is also used to evaluate the excited state dipole moments. It minimizes the errors associated with the estimation of Onsager cavity radius ‘a’. The correlation between spectral shifts with EN T is given by the following equation; a −υ f ¼ m4 ETN þ constant υ " with m4 ¼ 11307:6
!1=2
m2 −m1 2
μg ¼
a −υ f ) vs. F1, F2 and (υ A þ υ F)/2 vs. F3 give linear plots The plots of (υ with the corresponding slopes m1, m2 and m3, as shown in the following equations.
are solvent polar-
ity functions with n; refractive index, ε; permittivity of various solvents and the variables m1 and m2 are slopes of linear graphs obtained from Eqs. (1) and (2), respectively.
455
Δμ Δμ B
ð16Þ
2 3 # aB a
ð17Þ
where ΔμB and Δμ are the changes in dipole moments of the probe on excitation and ‘aB’ and ‘a’ are the Onsager radius of betaine dye and of the molecule of interest, respectively. The reported values for ΔμB and ‘a’ are 9D and 6.2 Ǻ, respectively. Using these values, the difference between ground and excited state dipole moments can be estimated by using the Eq. (18); " Δμ ¼ μ e −μ g ¼
m4 81
ð6:2=aÞ3 11307:6
#1=2 ð18Þ
where m4 is the slope of the linear plot of EN T vs. Stokes shift. Kamlet and co-workers [41–43], proposed the multiple linear regression (MLR) method based on linear solvation energy relationship to study the solute-solvent interactions of the probe as a function of solvent polarity. MLR method shows relationship between Stoke's shift and absorption transition energy with an index of the solvents polarity/dipolarizability. This method is a measure of the ability of solvents to stabilize a charge/dipole through indices of the solvent's hydrogen bond donor (hbd) strength (α) [42], hydrogen bond acceptor (hba) strength (β) [43] and non-specific dielectric interactions (π*) [44], according to Eq. (19). y ¼ y0 þ aα þ bβ þ cπ
ð19Þ
where, y; spectroscopic property under consideration, y0; spectroscopic property in gas phase; α; measure of solvents acidity (hbd), β; measure of basiscity (hba) and π*; measure of polarity/dipolarizability scale of solvents. a, b and c are regression coefficients. Catalan solvent polarity/dipolarizability parameters [45] were used for MLR analysis to quantify the solvent effects on absorption, emission and Stoke's shift energies.
F1 ðε; nÞ ¼
ε−1 n2 −1 − 2ε þ 1 2n2 þ 1
ð11Þ
F2 ðε; nÞ ¼
2n2 þ 1 ε−1 n2 −1 − 2 2 n þ2 εþ2 n þ2
ð12Þ
y ¼ y0 þ dSA þ eSB þ fSP
ð13Þ
where, SA, SB and SP are scales that describe acidity, basicity and polarizibility/dipolarity of the solvents, respectively. d, e and f are regression coefficients.
" F3 ðε; nÞ ¼
# 3 n4 −1 2n2 þ 1 ε−1 n2 −1 þ − 2 2ðn2 þ 2Þ ε þ 2 n2 þ 2 2ðn2 þ 2Þ
ð20Þ
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Fig. 2. Normalized absorption and emission spectra of (A) E384 and (B) E416 in polar protic solvents at 300 K.
solvatochromism, the optimized ground geometries of the probes in vacuum as well as in all the studied solvents were optimized using the AM1 semi empirical method and AM1/IEF-PCM, respectively. The transition energies or electronic absorption spectra were obtained using ZINDO and ZINDO/IEF-PCM for isolated and solvated system, respectively.
2.4. Computational details The ground and excited state geometries of poly-p-phenylenes (E384 and E416) molecules were optimized employing DFT and TDDFT at hybrid functional B3LYP/6-311G(d) level in vacuum using Gaussian 09 W software package [46]. With a view to explore the
3. Results and discussion In order to estimate the effect of the solvent environment nature on laser dyes, the spectral characteristics were analyzed in various solvents drawn from three groups: polar protic and/or proton donor (alcohols), non-polar or weakly polar (alkanes, cyclopentane, cyclohexane, isopentane) and polar aprotic and/or electron donor (acetone, acetonitrile, DMF, DMSO,). 3.1. Effects of solvent on absorption and fluorescence spectra The knowledge of photophysical characteristics of exalite laser dyes in various solvents of different solvent polarities helps in understanding their applicability as laser dyes. Fig. 2 (A & B) shows the typical electronic absorption and photoluminescence (PL) spectra of E384 and E416 in polar protic Table 1 Absorption (λa) and fluorescence maxima (λf) in nm; wave numbers (ῡ) in cm−1 and spectral shift data of E384 and E416 in various solvents at 300 K. Solvents
λa
a υ
λf
f a −υ υ
f υ
f a þ υ υ
a þ υ f) / 2 (υ
E384
E416
E384
E416
E384
E416
E384
E416
E384
E416
E384
E416
E384
E416
326 326 327 327 327 328 328 328 328 328
349 350 350 351 351 352 352 351 352 351
378 379 380 381 381 380 381 382 381 381
407 412 411 414 413 414 414 414 414 413
30,675 30,675 30,581 30,581 30,581 30,488 30,488 30,488 30,488 30,488
28,653 28,571 28,571 28,490 28,490 28,409 28,409 28,490 28,409 28,490
26,455 26,385 26,316 26,247 26,247 26,316 26,247 26,178 26,247 26,247
24,570 24,272 24,331 24,155 24,213 24,155 24,155 24,155 24,155 24,213
4220 4290 4265 4334 4334 4172 4241 4310 4241 4241
4083 4300 4241 4335 4277 4255 4255 4335 4255 4277
57,130 57,060 56,897 56,828 56,828 56,804 56,735 56,666 56,735 56,735
53,223 52,843 52,902 52,645 52,703 52,564 52,564 52,645 52,564 52,703
28,565 28,530 28,448 28,414 28,414 28,402 28,367 28,333 28,367 28,367
26,612 26,422 26,451 26,322 26,352 26,282 26,282 26,322 26,282 26,352
Nonpolar/weakly polar solvents 11. Pentane 325 346 12. Hexane 326 346 13. Heptane 326 349 14. Octane 326 349 15. Nonane 326 350 16. Decane 327 350 17. Dodecane 327 350 18. Hexadecane 324 347 19. Cyclopentane 326 351 20. Cyclohexane 328 350 21. Isopentane 327 347
378 378 378 379 380 380 380 381 380 380 377
409 410 410 410 412 412 412 412 412 411 414
30,769 30,675 30,675 30,675 30,675 30,581 30,581 30,864 30,675 30,488 30,581
28,902 28,902 28,653 28,653 28,571 28,571 28,571 28,818 28,490 28,571 28,818
26,455 26,455 26,455 25,707 26,316 26,316 26,316 26,247 26,316 26,316 26,525
24,450 24,390 24,390 24,390 24,272 24,272 24,272 24,272 24,272 24,331 24,155
4314 4220 4220 4290 4359 4265 4265 4617 4359 4172 4056
4452 4511 4263 4263 4300 4300 4300 4547 4218 4241 4664
57,224 57,130 57,130 56,382 56,991 56,897 56,897 57,111 56,991 56,804 57,106
53,352 53,292 53,044 53,044 52,843 52,843 52,843 53,090 52,762 52,902 52,973
28,612 28,565 28,565 28,191 28,495 28,448 28,448 28,555 28,495 28,402 28,553
26,676 26,646 26,522 26,522 26,422 26,422 26,422 26,545 26,381 26,451 26,487
Polar aprotic solvents 22. Acetone 23. Acetonitrile 24. DMF 25. DMSO
380 379 382 385
414 413 418 421
30,675 30,488 30,488 30,395
28,818 28,490 28,249 28,169
26,316 26,385 26,178 25,974
24,155 24,213 23,923 23,753
4359 4103 4310 4421
4664 4277 4325 4416
56,991 56,873 56,666 56,369
52,973 52,703 52,172 51,922
28,495 28,437 28,333 28,185
26,487 26,352 26,086 25,961
Polar protic solvents 1. Methanol 2. Ethanol 3. Propanol 4. Butanol 5. Pentanol 6. Hexanol 7. Heptnol 8. Octanol 9. Nonaol 10. Decanol
326 328 328 329
350 351 354 355
G.H. Pujar et al. / Journal of Molecular Liquids 244 (2017) 453–463
457
solvents, respectively (see Fig. S1–S2, ESI, for nonpolar/weakly polar solvents and polar aprotic solvents). The spectroscopic data (absorption, emission maxima in nm, wave numbers in cm−1 and spectral shifts) of E384 and E416 in various selected solvents are tabulated in Table 1. All the calculated solvent polarity parameters and functions for selected solvents are listed in Table S1, ESI. The solvent parameters like n, ε, Kamlet-Taft parameters (α, β and π*), Reichardt parameter (EN T ) and Catalan parameters (SP, SA and SB) are referred from the literature [16,47–48]. From Table 1, it is noticed that both E384 and E416 laser dyes exhibit small magnitude of shift (~5 and 9 nm for E384 and E416, respectively) among the selected solvents in the excitation spectra. However, a slightly larger shift (~8 and 14 nm for E384 and E416, respectively) is observed in the emission spectra compared to excitation spectra. This implies that the ground state energy is less affected by the polarity of solvents as compared to the excited state. It means, both laser dyes are more stabilized in the excited state than the ground state and hence, the expected value for ground state dipole moment would be lesser compared to excited state. The values of Stokes shift were observed to be 4220 to 4421 cm−1 (in case of E384) and 4083 to 4416 cm−1 (in E416) for polar protic solvent (methanol) and polar aprotic solvent (DMSO), respectively. These observations reveal that both laser
Fig. 3. Linear plots of Bilot-Kawski (a–b), Lippert–Mataga (c), Bakhshiev (d), Kawski–Chamma–Viallet (e) and Reichardt (f) correlation methods in polar protic and aprotic solvents (left column) and nonpolar/weakly polar solvents (right column) for Exalite 384 laser dye.
458
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dyes might be influenced by solvent parameters like hydrogen bond acceptor/donor strength, solvent polarity/depolarizibility. In addition, it is noticed that the observed shifts for E416 are higher compared to E384 molecule due to their considerable structural differences and relative polar nature. 3.2. Solvatochromism of laser dyes: an experimental and theoretical calculations of dipole moments Generally, the shifts in the spectra are due to the solute-solvent interactions, which include general and/or specific type of interactions. In general type of solvent effects, the refractive index and dielectric constant of solvents play a significant rule for the spectral shifts (Stokes shift) where as in specific type, shifts are associated with hydrogen bonding, n-donor and π-donor of the solvents. The solvatochromism of laser dyes can be studied using correlation between spectral shifts and solvent polarity as given by Bilot–Kawski (Eqs. (1) and (2)), Lippert–Mataga (Eq. (8)), Bakhshiev (Eq. f ) vs. F(ε,n), (ῡa + ῡf) vs. F(ε,n) + 2 g (n), a −υ (9)), Kawski–Chamma–Viallet (Eq. (10)) and Reichardt linear correlations (Eq. (16)). The graphs of (υ a þ υ f )/2 vs. F3(ε,n) were plotted for both dyes E384 and E416 and the typical linear plots are shown in the (ῡa − ῡf) vs. F1(ε,n), F2(ε,n), EN T and (υ Fig. 3 (a–f) for E384 probe. The linear plot data for all solvent correlation methods are tabulated in the Table 2. Better correlation coefficients (R2) are obtained using maximum number of solvents to get the best linearity for all the methods (Note that the circle points in red color were not taken into account for regression). Theoretically the dipole moments were estimated by Gaussian 09 package and for experimental determination, solvent correlation methods are used. The slopes (m1 and m2) of the Bilot–Kawski correlations are used to estimate the ground and singlet excited state dipole moments (μg and μe) by applying Eqs. (5) and (6). The magnitudes of μe are also obtained from the slopes (m1, m2, m3 and m4) of Lippert–Mataga, Bakhshiev, Kawski– Chamma–Viallet and Reichardt correlations using Eqs. (14), (15) and (18). The Onsager cavity radius of E384 and E416 is calculated using Edward's atomic increment method [49]. All these results are tabulated in Table 3. Using G09 software, the ground state dipole moments (μg) are found to be 0.7175 and 1.2316 D for E384 and E416, respectively (see Table 3). From the Bilot-Kawski method, obtained values of μg are 1.6033 and 2.5532 D (in polar protic and aprotic solvents) and 1.3433 and 2.1582 D (in nonpolar/weakly polar solvents) for E384 and E416, respectively. As can be seen from the Table 3, the singlet excited state dipole moments (μe) are larger than that of ground state (μg) for both laser dyes which confirms that the poly-p-phenylenes compounds are more polarized upon excitation. The DFT and TD-DFT computations also predict the same trend. This higher polarizibility of excited state might be due to irregularity of charge distribution between the electronic states, charge transfer, the nature of geometrical changes (like twisting, changes in the planarity, etc.), larger solute-solvent interaction in the singlet excited state and/or intermolecular bonding with solvents. The variation in the values of excited state dipole moments is observed for different solvent correlation methods which are due to the assumptions involved in those methods. From microscopic solvent polarity correlation method, the difference between ground and excited state dipole moments (Δμ) were obtained by using a −υ f ) on EN the slope m4 in Eq. (18) and tabulated in Table 3. The linear dependence of (υ T implies the possibility of occurrence of general type of solutesolvent interactions (Fig. 3f). The ratio of excited state to ground state dipole moment was calculated using the values obtained from Bilot-Kawski method and found to be 2.62 and 1.96 (in polar protic and aprotic solvents) for E384 and E416, respectively and 2.84 and 2.17 (in nonpolar/weakly polar solvents). The dipole moment difference of two electronic states is found to be positive (i.e., Δμ N 0) and the ratio is also greater than unity (i.e., μe/μg N 1) for both dyes. The same is estimated from AM1/PCM and ZINDO/PCM computations (see Table 4) for all class of solvents. This confirms that excited states are more polar than ground state. Based on dipole moment values obtained from Bilot-Kawski solvent correlation, the dipole moment angle between two electronic states is calculated using Eq. (7) and found to be ~4–6° for E384 and 2–3° for E416, implying that electronic states are almost parallel to each other. It may be noted from both experimentally and computationally that the measured values of μg, μe and Δμ for E416 are higher compared to E384 which can be attributed to the structural/geometrical difference and relative polar nature between the two laser dyes. Note that the compound E416 has six aromatic rings along with two extended oxygen atoms which make it relatively more dipolar whereas E384 has only four aromatic rings (Fig. 1). Upon comparing both electronic state dipole moment values, it is noticed that for both theoretical and experimental values of ground and excited state dipole moments correlate well for laser dyes E384 and E416. That is, experimentally, the magnitudes of μe are higher than μg for both dyes as also observed theoretically from Gaussian 09. 3.3. Multiple linear regression analysis (MLR) The MLR can provide information about different types of interactions (including general and specific type) between solute and solvent. The general solvent effects result from the dielectric constant (ε) and refractive index (n) of the solvents. These constants significantly affect the dipole moments of probes and freedom of electrons motion in the solvent molecules. Specific solvent effects are referred to specific chemical interactions
Table 2 Linear plot data for E384 and E416 obtained from various correlation methods. Method
Bilot-Kawski
Solvents
Polar protic and aprotic Nonpolar/weakly polar
Lippert-Mataga Bakhshiev Kawski-Chamma-Viallet Reichardt
Polar protic and aprotic Nonpolar/weakly polar Polar protic and aprotic Nonpolar/weakly polar Polar protic and aprotic Nonpolar/weakly polar Polar protic and aprotic Nonpolar/weakly polar
Slope
1
m m2 m1 m2 m1 m1 m2 m2 m3 m3 m4 m4
Intercept
Correlation coefficient (R2)
No. of data points
E384
E416
E384
E416
E384
E416
E384
E416
563.3 1258.6 512.6 1068.3 1941.6 1494.0 1738.5 1270.9 4186.9 3368.0 1217.1 1100.9
371.8 1143.9 395.4 1023.8 1280.3 1102.0 1199.5 1089.9 3100.2 2613.1 1001.8 500.1
3910.0 55,771.1 4314.9 57,189.5 4124.6 4285.0 4101.3 4337.6 28,723.1 29,670.4 4130.6 4029.2
4102.7 52,256.1 4375.2 53,028.3 4206.1 4342.4 4160.6 4321.9 27,596.9 28,174.2 4137.3 4214.1
0.80 0.83 0.82 0.80 0.89 0.81 0.83 0.81 0.86 0.84 0.77 0.79
0.88 0.89 0.86 0.85 0.88 084 0.92 0.85 0.87 0.88 0.79 0.78
08:14 08:14 05:09 05:09 08:13 06:09 07:13 07:09 10:14 08:10 08:10 05:08
08:14 08:14 06:10 06:10 09:14 06:09 08:14 06:09 09:14 07:10 11:14 04:07
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459
Table 3 Dipole moments in two electronic states (‘μ’ in Debye), angle between dipole moments (‘φ’ in degree) and Onsager radius (‘a’ in Ǻ) of E384 and E416 laser dyes. Laser dye
μga
μeb
Solvent
aj
μgc
μ ec
μed
μee
μef
μeg
Δμh
(μe/μg)
φi
E384
0.7175
1.5163
4.94
E416
1.2316
2.5542
Polar protic and aprotic Nonpolar/weakly polar Polar protic and aprotic Nonpolar/weakly polar
1.6033 1.3433 2.5532 2.1582
4.2013 3.8216 5.0126 4.6914
6.4266 5.5742 7.1168 6.3621
6.1674 5.2455 6.9704 6.3688
7.2620 6.4930 7.5464 6.8676
3.7034 3.3405 4.7731 3.7300
2.1000 1.0997 2.2199 1.5685
2.62 2.84 1.96 2.17
3.97 5.92 2.99 2.28
a b c d e f g h i j
5.47
Ground state dipole moment computed at B3LYP/6-311G(d) level using G09 package in vacuum. Excited state dipole moment calculated at TD-DFT/B3LYP/6-311G(d) level using G09 package in vacuum. Ground and excited state dipole moments obtained using Bilot–Kawski method (Eqs. (5) and (6)). Excited state dipole moment calculated using Lippert Mataga method (Eq. (14)). Excited state dipole moment obtained using Bakhshiev method (Eq. (14)). Excited state dipole moment calculated using Kawski–Chamma–Viallet method (Eq. (15)). Excited state dipole moment calculated using Reichardt method. Difference between ground and excited state dipole moments is calculated using (Eq. (18)). Angle between the ground and excited state dipole moments obtained using Eq. (7). Onsager radius of E384 and E416 calculated using Edward's atomic increment method.
(hydrogen bonding and complexation) between the solute and solvent molecules. From the existing reports, it is confirmed that the shape, position and intensity of absorption spectra can elucidate the spectral shifts along with nature of solute and solvent. Such spectral shifts are correlated with f and υ a −υ f are correlated with α, β and π* as given by Eq. (19) using a, υ Kamlet–Taft parameters (α, β and π*). The spectroscopic parameters like υ MLR analysis. This analysis gives more information about solvatochromic behavior during electronic transitions between ground and excited states based on solute and solvent interactions. In the present study, Kamlet–Taft correlation method is used for polar protic solvents (methanol-decanol; except nonanol) for both laser dyes E384 and E416 using Eq. (21) leading to:
E384
υa ¼ 30061 þ 501 α−459 β þ 802 π ; r ¼ 0:96 υ f ¼ 25773 þ 350 α−75 β þ 625 π ; r ¼ 0:79
E416
υa ¼ 28275 þ 22 α−199 β þ 818 π ; r ¼ 0:87 υ f ¼ 23567−104 α þ 299 β þ 1049 π ; r ¼ 0:82
ð21Þ
From these equations, it is noticed that coefficient, c of π* is appreciably higher than the other coefficients, a and b of hbd, hydrogen bond donor strength (α) and hba, hydrogen bond acceptor strength (β) for both compounds. Hence, the effect of π* (polarity/dipolarizability scale of solvents) influences solute and solvent interactions in a major way. In other words, the absorption and fluorescence spectral shifts are controlled by non-
Table 4 Comparison of experimental and calculated electronic transition energies (in eV), solvent polarity scale ET(30) along with the two electronic states dipole moments (in D). Solvents
ET(30)/(kcal/mol)
Eexp (eV)
Etheory (eV)a
μgb
μea
Δμ
μe/μg
Ex-384
Ex-416
Ex-384
Ex-416
Ex-384
Ex-416
Ex-384
Ex-416
Ex-384
Ex-416
Ex-384
Ex-416
3.804 3.804 3.792 3.792 3.792 3.780 3.780 3.780 3.780 3.780
3.553 3.543 3.543 3.533 3.533 3.523 3.523 3.533 3.523 3.533
3.6140 3.6120 3.6070 3.6064 3.6053 3.6037 3.6031 3.6028 3.6027 3.6026
3.4199 3.4182 3.4170 3.4145 3.4142 3.4181 3.4177 3.4139 3.4141 3.4143
1.0825 1.0636 1.0475 1.0312 1.0164 0.9934 0.9801 0.9603 0.9390 0.9168
1.3925 1.3833 1.3750 1.3666 1.3590 1.3485 1.3398 1.3293 1.2920 1.2805
1.6797 1.6670 1.6561 1.6449 1.6349 1.6188 1.6094 1.5953 1.5799 1.5637
1.6983 1.6924 1.6870 1.6814 1.6763 1.6009 1.6635 1.6564 1.6447 1.6375
0.5972 0.6034 0.6086 0.6137 0.6185 0.6254 0.6293 0.6350 0.6409 0.6469
0.3058 0.3091 0.3120 0.3148 0.3173 0.2524 0.3237 0.3271 0.3527 0.3570
1.5516 1.5673 1.5810 1.5951 1.6085 1.6295 1.6420 1.6612 1.6825 1.7056
1.2196 1.2234 1.2269 1.2303 1.2334 1.1871 1.2416 1.2460 1.2729 1.2787
Nonpolar/weakly polar solvents 11. Pentane 31.0 12. Hexane 31.0 13. Heptane 31.1 14. Octane 31.1 15. Nonane 31.0 16. Decane 31.0 17. Dodecane 31.1 18. Hexadecane – 19. Cyclopentane – 20. Cyclohexane 30.9 21. Isopentane 30.9
3.815 3.804 3.804 3.804 3.804 3.792 3.792 3.827 3.804 3.780 3.792
3.584 3.584 3.553 3.553 3.543 3.543 3.543 3.573 3.533 3.543 3.573
3.6149 3.6126 3.6119 3.6110 3.6097 3.6081 3.6080 3.6111 3.6085 3.6051 3.6030
3.4334 3.4325 3.4301 3.4295 3.4288 3.4287 3.4280 3.4283 3.4232 3.4251 3.4318
0.6298 0.6346 0.6377 0.6407 0.6427 0.6452 0.6474 0.6508 0.6428 0.6484 0.6354
1.0960 1.0998 1.1022 1.1046 1.1063 1.1082 1.1099 1.1126 1.1063 1.1107 1.1005
1.3177 1.3226 1.3258 1.3290 1.3310 1.3336 1.3358 1.3392 1.3311 1.3368 1.3235
1.4540 1.4566 1.4583 1.4600 1.4611 1.4624 1.4636 1.4654 1.4611 1.4641 1.4571
0.6879 0.6880 0.6881 0.6883 0.6883 0.6884 0.6884 0.6884 0.6883 0.6884 0.6881
0.3580 0.3568 0.3561 0.3554 0.3548 0.3542 0.3537 0.3528 0.3548 0.3534 0.3566
2.0922 2.0841 2.0790 2.0743 2.0709 2.0669 2.0633 2.0577 2.0707 2.0616 2.0829
1.3266 1.3244 1.3230 1.3217 1.3207 1.3196 1.3186 1.3170 1.3207 1.3181 1.3240
Polar aprotic solvents 22. Acetone 23. Acetonitrile 24. DMF 25. DMSO
3.803 3.780 3.780 3.769
3.542 3.533 3.503 3.493
3.6105 3.6097 3.6094 3.6018
3.4226 3.4211 3.4146 3.4136
1.0474 1.0879 1.0903 1.1018
1.3437 1.3619 1.3629 1.3680
1.6160 1.6267 1.6273 1.6303
1.6560 1.6833 1.6848 1.6925
0.6086 0.5954 0.5945 0.5907
0.3123 0.3214 0.3219 0.3245
1.5810 1.5473 1.5453 1.5361
1.2324 1.2359 1.2361 1.2372
Polar protic solvents 1. Methanol 2. Ethanol 3. Propanol 4. Butanol 5. Pentanol 6. Hexanol 7. Heptnol 8. Octanol 9. Nonaol 10. Decanol
a b
55.4 51.9 50.7 49.7 49.1 48.8 48.5 48.1 47.8 47.7
42.2 45.6 43.2 45.1
Obtained using ZINDO/IEF-PCM. Obtained employing AM1/IEF-PCM from G09 package.
460
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Fig. 4. Ground state Optimized structure of E384 (a) and E416 (b) obtained at DFT/B3LYP/6-311G(d) level of theory in vacuum. The arrow indicates the direction of dipole moment.
specific interactions resulting from polarity and/or dipolarizability of solvents. However, in E384 molecule, the values of hbd (α) are higher than hba (β) which implies that hbd influence is more than hba. In E416 dye, hba influences more compared to hbd. Furthermore, MLR equation formed by Catalan correlates spectral parameters (absorption and fluorescence maxima wavenumbers) with SA, SB, a) and fluorescence maxima and SP as presented in Eq. (20). Using this equation, the regression coefficients were evaluated for absorption maxima (υ f ) using protic solvents (methanol-decanol; except pentanol and decanol) for both laser dyes E384 and E416 and shown in Eq. (22). (υ E384 E416
υa ¼ 30312 þ 72 SA þ 1134 SB−250 SP; r ¼ 0:97 υa ¼ 34101−1983SA−4862SB−2857SP; r ¼ 0:90 υ f ¼ 22796 þ 53 SA þ 6042 SB þ 1711 SP; r ¼ 0:86 υ f ¼ 31552−1912SA−7644SB−3905SP; r ¼ 0:92
ð22Þ
According to the above Catalan MLR equations, it is noticed that the coefficients of SB (e) and SP (f) are higher than those of SA (d) which indicates that both laser dyes E384 and E416 are more sensitive to polarizibility and/or dipolarizability of solvents, less sensitive to hydrogen bonding effect and basicity of solvents affects the absorption maxima. 3.4. Computational studies Optimized ground state molecular structures of E384 and E416 are shown in Fig. 4. The arrow head gives the direction of dipole moment of the molecules.
Fig. 5. Comparison of experimental transition energies of (a) E384 and (b) E416 with calculated ZINDO-PCM on ET(30) scale in polar protic solvents (methanol-decanol).
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461
Table 5A Second-order perturbation theory analysis of Fock matrix in NBO basis corresponding to the intra molecular bonds of the title compound E416. Donor(i)
Type
ED/e
Acceptor(j)
Type
ED/e
E(2)a
E(j)–E(i)b
F(i,j)c
C52-C53
σ
1.98056
C52-C53
π
1.65825
C95-C96
σ
1.98055
C95-C96 LPO58 LPO58 LPO104 LPO104
π σ π σ π
1.65851 1.96363 1.83987 1.96360 1.83990
C48–C53 C51–C52 C48–C49 C50–C51 C94–C95 C96–C97 C94–C99 C52–C53 C52–C53 C95–C96 C95–C96
σ* σ* π* π* σ* σ* π* σ* π* σ* π*
0.02258 0.01351 0.31361 0.36730 0.01350 0.02254 0.36762 0.02851 0.39641 0.02851 0.39645
3.65 2.74 16.73 21.56 2.74 3.65 21.53 7.47 30.65 7.48 30.65
1.27 1.29 0.29 0.30 1.29 1.27 0.30 1.11 0.34 1.11 0.34
0.061 0.053 0.063 0.072 0.053 0.061 0.072 0.081 0.097 0.081 0.097
a b c
E(2) means energy difference of hyper-conjugative interactions (stabilization energy in kJ/mol). Energy difference (a.u.) between donor and acceptor i and j NBO orbitals. F(i,j) is the Fock matrix elements (a.u.) between i and j NBO orbitals.
3.4.1. Solvatochromism analysis The combination of ZINDO-PCM methods have been successfully implemented to explain the solvatochromic behavior particularly in approximating the excitation energies of molecules theoretically. Although TD-DFT and time dependent Hartree-Fock (TD-HF) quantum mechanical level calculations provide accurate results but they limit the size of the systems which can be treated and very slower in the calculation. Large sized molecules, as considered in the present study, are better approximated by semiempirical method, ZINDO combined with IEF-PCM. The detailed computational formalism behind the ZINDO-PCM method has been presented in some earlier reports ([50–51] and references therein). The solvatochromic effect on electronic transition energies have been computed and analyzed by ZINDO-PCM formalism in all the kind of solvents. The predicted electronic excitation energies by semiempirical ZINDO method is most popular due to its faster calculation over formal TDDFT. Herein, the ZINDO-PCM calculations on laser dyes E384 and E416 are presented. The transition energies and their dependence on solvent polarity scale, ET(30), are studied and compared with experimental data. Fig. 5 shows the typical E vs. ET(30) plot for E384 and E416 dyes in polar protic solvents and corresponding data are tabulated in Table 4. The class of polar protic solvents (methanol-decanol) have strong tendency to interact with probes. The experimental data reveals a blue shift of the transition energy of E384 and E416 with the solvent polarity. It is interesting to note that the same trend is reproduced by ZINDO-PCM calculation. Table 4 also show that the maximum difference among the experimental and calculated data for E384 and E416 in protic solvents is found be 0.18 and 0.08 eV, respectively being very close to each other. The relative polar nature of E416 as compared to E384 may be the reason for lesser difference. In case of polar aprotic solvents, a red shift is observed with increasing polarity (acetone-DMSO) as computationally backed by ZINDO-PCM for E384 and E416 probes having differences 0.19 and 0.11 eV, respectively. In the other classes of solvents (nonpolar/weakly polar solvents), bathochromic shift is observed when solvents changing from pentane to isopentane (Table 4). This shift is attributed to the dispersion forces which cause the solvation of the solutes. Despite of bit nonlinearity in experimental data, the ZINDO-PCM analyzed data are well correlated with maximum differences of 0.21 for E384 and 0.15 eV for E416 dye. It is noticed that the maximum differences for nonpolar solvents are slightly higher than other class of solvents, probably due to weak interaction between solutes and nonpolar solvents. However, in conclusion, the obtained experimental transition energies trend is well reproduced for all the classes of solvents. 3.4.2. Natural bond orbital (NBO) analysis The NBO computations were performed using NBO 3.1 program [52] as implemented in the Gaussian09 package at the DFT/B3LYP level in order to understand various second-order interactions between the filled orbitals of one subsystem and vacant orbitals of another subsystem, which is a measure of the intermolecular delocalization or hyper conjugation. NBO analysis provide an efficient method for studying interesting features of molecular structure besides they give strong insight in the intra and inter molecular bonding and interaction among bonds, and also provides a convenient basis for investigation charge transfer or conjugative interactions in molecular system [53]. Another useful aspect of NBO method is that it gives information about interactions in both filled and virtual orbital spaces that could enhance the analysis of intra and intermolecular interactions. The Table 5B NBO results showing the formation of Lewis and non-Lewis orbitals in E416 molecule. Bond (A–B)
ED/energy (a.u.)
EDA %
EDB %
NBO
s%
p%
σC52–C53 – πC52–C53 – σC95–C96 – π C95–C96 – n1O58 – n2O58 – n1O104 – n2O104 –
1.98056 −0.69895 1.65825 −0.24866 1.98055 −0.69867 1.65851 −0.24840 1.96363 −0.54013 1.83987 −0.30337 1.96360 −0.53973 1.83990 −0.30308
49.94 – 53.61 – 49.95 – 53.61 – – – – – – – – –
50.06 – 46.39 – 50.05 – 46.39 – – – – – – – – –
0.7067 (sp1.93) C+ 0.7075 (sp1.62) C 0.7322 (sp1.00) C+ 0.6811 (sp1.00) C 0.7067 (sp1.93) C+ 0.7075 (sp1.62) C 0.7322 (sp1.00) C+ 0.6811 (sp1.00) C sp1.60 – sp1.00 – sp1.60 – sp1.00 –
34.06 38.20 0.00 0.00 34.06 38.20 0.00 0.00 38.47 – 0.00 – 38.45 – 0.00 –
65.89 61.77 100.0 100.0 65.89 61.76 100.0 100.0 61.47 – 100.0 – 61.49 – 100.0 –
462
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second order Fock matrix was carried out to evaluate the donor–acceptor interactions in the NBO analysis [54]. For each acceptor (j) and donor (i) the stabilization energy (E2) associated with the delocalization i → j is determined as Eð2Þ ¼ ΔEij ¼ qi
2 F i; j E j −Ei
ð23Þ
where qi is the donor orbital occupancy, Ei, Ej are diagonal elements (orbital energies) and F(i, j) is the off-diagonal NBO Fock matrix element. The perturbation energies of significant donor-acceptor interactions for E416 compound are presented in Table 5A. The larger the E(2) value, the intensive is the interaction between electron donors and electron acceptors. The strong interaction n2O104 → π*(C95–C96) has the highest E(2) value 30.65 kcal/mol and a very strong interaction has been in n2O58 → π*(C52–C53) with an energy of 30.65 kcal/mol. Table 5B gives the occupancy of electrons and p-character [55] in significant NBO natural atomic hybrid orbitals. Almost100% p-character was observed in π bonding of C52–C53, C95–C96 and the lone pairs of n2O58 and n2O104. In E384, the strong interactions are: π(C13–C14) → π*(C15–C16), π(C2–C3) → π*(C1–C6), π(C4–C5) → π*(C1–C6), π(C21–C22) → π*(C20–C25), with E(2) values 22.33, 20.97, 20.91, 20.89 kJ/mol and with electron densities 0.39025, 0.34390, 0.34390, 0.37681e respectively. Almost 100% p-character was observed in all the π bonding of C1–C6, C2–C3, C4–C5, C11–C12, C21–C22, C29–C30, C31–C32 and C33–C34. The results for E384 are tabulated in Tables S2A and B, ESI. Note: The symbols and labels appeared in the NBO analysis were assigned according to the optimized geometries as shown in the Fig. 4. 4. Concluding remarks In this work, the electronic ground and singlet excited states dipole moments of E384 and E416 laser dyes have been estimated employing various correlation techniques and compared with those from computational studies. The detailed solvatochromism of these probes was studied comprehensively. From both experimental and computational results, it is found that both these dyes possess lesser dipole moments in ground state than in the excited state. Further, in comparison with various solvent correlation techniques, the Bilot–Kawski and Reichardt methods predict better solute-solvent correlation for the solvatochromic shift. The dipole moments estimated employing these methods are approximately in close agreement with those dipole moments computed using G09 software. The type of interactions between solutes and solvents were estimated by multiple linear regression analysis employing Kamlet-Abboud-Taft and Catalan solvent parameters which reveal that general type/nonspecific interaction of solvents such as polarizibility/dipolarizability influences in a major way. The ZINDOPCM model was adopted to study the solvation potential and/or transition energies as a function of solvent polarity scale, ET(30) which confirms the validity and approximation of the model. Indeed, the experimental data follows the same trend as predicted by this model. The observed differences are below the 0.21 eV in all the solvents studied. Thus the ZINDO-PCM model is a promising method to study the electronic absorption transitions in solvents which can explore solvatochromism of solutes. The intramolecular charge transfer and hybridization in titled probes were interpreted by using NBO analysis. Almost100% p-character is observed for both laser dyes. Acknowledgment The authors gratefully acknowledge the financial support from the University Grants Commission (UGC), New Delhi, India under Centre with Potential for excellence [CPEPA; Grant No. 8-2/2008(NS/PE)]. MNW thanks the UGC for a fellowship under RFSMS scheme and GHP thanks CPEPA for a JRF. The authors thank the University Scientific Instruments Centre, Karnatak University, Dharwad for providing the fluorescence lifetime data. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2017.08.078. References [1] M. Rinke, H. Güsten, H.J. Ache, Photophysical properties and laser performance of photostable UV laser dyes. Part 1. Substituted p-quaterphenyls, J. Phys. Chem. 90 (1986) 2661–2665, http://dx.doi.org/10.1021/j100403a020.
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