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Journal of Molecular Structure 1102 (2015) 11e17 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc Electronic spectra and hyperpolarizabilities of structurally similar donoreacceptor dyes. A density functional theory analysis Amrita Sarkar a, Mousumi Das a, **, Sanjib Bagchi b, * a b Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, West Bengal, India Department of Chemistry and Biochemistry, Presidency University, 86/1, College Street, Kolkata 700 073, India a r t i c l e i n f o a b s t r a c t Article history: Received 27 May 2015 Received in revised form 11 August 2015 Accepted 12 August 2015 Available online 18 August 2015 Studies with density functional theory (DFT) have been done to reinforce our previous experimental findings involving the solvatochromism and the effect of protonation and for three structurally similar donoreacceptor dyes exhibiting intramolecular charge transfer transition. These dyes have similar donor (indole N/amino N) site and similar carbonyl O as the acceptor centre. The dye with an amino N donor site and indanone O as the acceptor centre has the lowest value of the energy gap between HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) and highest percentage of charge transfer from the N to the O centre. Time dependent density functional theory (TDDFT) has been used to calculate the excitation energy to the lowest singlet excited dipole-allowed states of the dyes. Effect of solvation on excitation energy has been studied by the use of polarisable continuum model (PCM). Computational results indicate that the excitation energy of these dyes is sensitive to solvent polarity and exhibits a red shift as polarity increases. The calculated excitation energies are in good agreement with the values of absorption maximum of these dyes in different solvents obtained in experiment. Studies on protonation of the dyes show that the carbonyl O to be the most favourable site of protonation for all the three dyes. Calculations of linear and first hyperpolarizabilities indicate these dyes to be suitable candidates for possible non-linear optical application. © 2015 Elsevier B.V. All rights reserved. Keywords: Intramolecular charge transfer Protonation Time dependent density functional theory Polarisable continuum model Hyperpolarizability 1. Introduction Optical response of a molecule containing electron donor and acceptor centres is a topic of current interest. Ketocyanine dyes and dyes structurally akin to them are a subject of extensive investigation [1e5]. This class of donoreacceptor type of dyes are known to exhibit intramolecular charge transfer (ICT) transition, with the charge being transferred from the amino N donor centre to the carbonyl O acceptor centre [6e8] and the ICT band shows significant solvent sensitivity. As a result, such compounds are found to be promising candidates as micropolarity reporters [9e11]. Solvation characteristics of these compounds have thus been studied in various homogeneous and microheterogeneous media where the dyes can act as marker probes for establishing the microenvironment of a system [9e12]. They have also been reported as * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Das), [email protected] (S. Bagchi). http://dx.doi.org/10.1016/j.molstruc.2015.08.027 0022-2860/© 2015 Elsevier B.V. All rights reserved. microviscosity sensors and have been used to study the environmental rigidity of a medium [2,12,13]. Ketocyanine and merocyanine dyes also act as pH sensors and show reversible colour change with the addition of acids [10,13,14]. Experimental observations have revealed that the carbonyl oxygen possibly undergoes protonation in presence of a strong acid [5,13,14] with the generation of a protonated enol type of species which has absorption at higher wavelength relative to the neutral dye. The objective of the present work is to provide a DFT analysis of the solvent and pH sensitivity of these dyes. The donoreacceptor dyes, N1, N2 and DN2 shown in Fig. 1, have been used in the present study. Of the three dyes, N1 and N2 have a similar donor indole N site while N2 and DN2 have a similar indanone O as the acceptor centre. Studies on solvatochromism and protonation have extensively been done on these dyes; some preliminary calculations at the DFT level in the gas phase using 6-31G basis set and B3LYP functional has also been performed on these dyes [13,14]. But 6-31G basis set is known to be inadequate for the calculation of the spectra. In the present work a DFT analysis has been presented using adequate basis set and different functionals. Structures of the dyes have been optimised at 12 A. Sarkar et al. / Journal of Molecular Structure 1102 (2015) 11e17 Fig. 1. Optimised structure of the dyes. the level of DFT. The absorption maxima (lmax) have been calculated using TDDFT and also the effect of solvent has been incorporated using polarisable continuum model (PCM). The results obtained from theoretical findings have been suitably compared to our previously obtained experimental observations [13,14]. Among three dyes studied, N1 and N2 have an indole moiety present in their molecular structure. Protonation of indole nucleus has been a topic of interest from both theoretical and synthetic point of view [15,16]. Indole chromophores serve as model to study complex photophysics of amino acid tryptophan [17]. Therefore the effect of pH variation is an important study for indoles. Several reactions of indole such as dimerisation, trimerisation and hydrogenation are acid catalysed [18e20]. Recent theoretical work on the effect of protonation at different centres of indole indicates that carbon atom at the 3 position of indole (C3) to be a favourable position for protonation [17]. Since the C3 in N1 and N2 is attached to a conjugated system, it is interesting to see the effect of conjugation on the protonation of the indole moiety in the dyes. It is worth studying whether carbonyl O centre or the indole moiety is the energetically favourable site of protonation in N1 and N2. This point has also been addressed in the present work. Apart from the theoretical investigation on lowest singlet excited states in these dyes, their non-linear optical responses under the influence of highly intense laser radiation is also worth studying. A considerable of amount of research is in progress in designing molecule based materials with large non-linear optical responses (NLO) [21e25]. Previous calculation on donoreacceptor merocyanines and its metal ion complexes indicate that they possess significant first hyperpolarizability and are thus promising in field of NLO [26]. Ketocyanine and merocyanine compounds have also been used as laser dyes and have found application in polymer imaging systems [27,28]. In this present work the linear and first hyperpolarizabilities of above dyes have also been carried out. 2. Theoretical and computational methods 2.1. Optimised geometry and solvation property The ground state molecular geometries of the three structurally similar donor acceptor dyes N1, N2 and DN2 and their corresponding protonated forms have been fully optimised using density function theory (DFT) implemented in Gaussian-09 software [29]. We have used both exchange correlation functional B3LYP and dispersion-corrected density functional wB97XD for our calculation and 6-311þþG(d,p) basis set has been employed for the study. Fig. 1 shows the optimised geometries of these dyes and the X and Y axes are chosen for the calculation. Nearly planar optimised molecular geometries have been obtained for all the three dyes. To calculate the excitation energies, TDDFT has been performed using these molecules optimised in gas phase. The effect of soluteesolvent interactions on the excitation energy has been incorporated modelling the solvent implicitly. This implicit modelling termed polarisable continuum model (PCM) implemented in Gaussian-09 software have been used [30,31]. The neat solvents studied are water, methanol, ethanol, acetone, acetonitrile, dichloromethane, choloroform, dimethyl sulfoxide, tetrahydrofuran and cyclohexane. Default values of dielectric constants for these above solvents have been used in the calculations. 2.2. Molecular polarisabilities The polarisabilities of a molecule are defined as a response to m Þ. the applied electric field ð! ε Þ that induces a dipole moment ð! The perturbed Hamiltonian under the influence of electric field, is m! given by H 0 ¼ ! ε . The modified energy of the system can be expressed in a Taylor series as follows: X vE 1 X X v2 E ε þ vεi 0 i 2 vεi vεj i i j ! 1X X X v3 E þ εi εj εk þ / 6 i vεi vεj vεk j Eð! ε Þ ¼ Eð0Þ þ k ! εi εj 0 (1) 0 The energy derivatives at zero electric field provide the static responses of the system with respect to the applied electric field. . The single derivatives of energy are the dipole moments (mi), and higher order terms introduce static linear polarisability (aij), the first hyperpolarizability (bijk) respectively and are expressed as, ! ! vE v2 E v3 E mi ¼ ; aij ¼ ; bijk ¼ vεi 0 vεi vεj vεi vεj vεk 0 (2) 0 In our study, we have calculated the individual tensor components of static linear polarisability and first hyperpolarizabilities within the framework of DFT using static finite field approach implemented in Gaussian-09 at zero frequency [29]. The tensor components of linear and first hyperpolarizabilities have been calculated on the optimised geometries of the dyes. Along with the tensor components, the average values of linear polarisability (aavg) are reported as, aavg ¼ 1 axx þ ayy þ azz 3 (3) The first hyperpolarizability is a third rank tensor with 27 components that can be reduced to 10 unique components due to Kleinman symmetry [32]. Various scalar measures of the tensor components have been computed in literature for the sake of comparison with experiment. The average of first hyperpolarizability can be computed as [33], 1 2 〈b〉 ¼ b2x þ b2y þ b2z ; bi ¼ biii þ 1 X b þ bjij þ bjji 3 j1;3 ijj (4) A. Sarkar et al. / Journal of Molecular Structure 1102 (2015) 11e17 Similarly in experiment, the accessible tensor component of first hyperpolarizability tensor is along the direction of the ground state dipole moment. So we have also computed the strength of hyperpolarizability (bm) in the direction of molecular ground state dipole moment (m) defined as, bm ¼ X mi bi jmj i1;3 (5) These two average values of static first hyperpolarizabilities for the dyes N1, N2 and DN2 will be addressed. The calculated large average values of the first hyperpolarizabilities measure the nonlinear responses of the above dyes. 3. Results and discussion 3.1. Effect of solvent on optical absorption of the dyes The Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) of these dyes have been shown in Fig. 2. The HOMO of these dyes are characterised by a large electron density on the indole group (phenyl group containing the N centre in DN2) relative to the phenyl group at the other side of the molecule. On the other hand, the relative electron density on the phenyl group at the other side is seen to increase in the LUMO of the dyes. This suggests that the HOMO/LUMO transition within the dye is associated with an intramolecular charge transfer (ICT). The ICT nature of the S0eS1 transition has also been established from solvatochromism exhibited by the dyes [13,14]. The difference in the energy of the HOMO and the LUMO for the dyes comes in the order N1 (0.215 a.u.) ~ N2 (0.213 a.u.) > DN2 (0.202 a.u.). Thus the HOMOeLUMO energy gap is lowest for DN2. The comparatively higher HOMOeLUMO gap for N1 and N2 is possibly due to their similar molecular structure with respect to the presence of an aromatic indole moiety in both the dyes. Values of ground state dipole moment (m0) for N1, N2 and DN2 have been obtained as 6.3, 6.8 and 6.5 Debye respectively. Values of m1, the dipole moment of the first singlet excited state have been experimentally obtained and reported to be 10.3 D, 17.8 D and 18.7 D for N1, N2 and DN2 respectively in our previous studies [13,14]. Thus, the extent of charge transfer upon excitation from the amino N to the carbonyl O is found to be 12.8% (N1), 34.6% (N2) and 38.9% (DN2). The percentage of charge transfer in N1 is much smaller compared to that in N2 and DN2. This is probably because there is a possibility of free rotation of the carbonyl centre containing the 13 phenyl ring about the ‘CeC’ bond shown in Fig. 1 which is strategically restricted in N2 due to the presence of an indanone moiety at its acceptor centre. This bond rotation partially ruptures the effective conjugation between the N and O centres in N1 which is unperturbed in N2. Our previous experimental findings support the view. As reported in our previous studies, these dyes are characterised by ICT bands and the absorption maximum for all these dyes exhibits solvent sensitivity, showing a bathochromic shift with increase in solvent polarity [13,14]. We carried out TDDFT calculations on the optimised geometries. The absorption maxima for lowest singlet excitation (lmax) for pure solvents for these dyes have been calculated using polarisable continuum model (PCM) with B3LYP and wB97XD functionals. Fig. 3 shows a comparison of the plots of theoretical and experimental absorption maxima (lmax) values for above dyes using both the functionals. The calculated absorption maxima using B3LYP functional show better agreement (as evident from the R2 values) with experimental results as compared to wB97XD functional. The difference between the theoretical and experimental lmax values have been shown as a function of solvent polarity in Fig. 4. Again, the use of the B3LYP functional is seen to give better result. Both experimental and theoretical values (using B3LYP functional) of lmax for these dyes have been listed in Table 1. Only the non-specific soluteesolvent interaction is taken care of in PCM, the effects of specific soluteesolvent interaction (hydrogen bonding), as present in a polar protic solvent is not considered. In order to include the effect of specific interaction, we have adopted a method described earlier by our group [26]. For the solvents where hydrogen bonding is important, namely, water, methanol, ethanol, we have initially optimised the dyeesolvent complex (solvent molecule placed at the carbonyl oxygen centre of the dyes (Fig. S1 in Supporting information)) prior to performing TDDFT calculation using the same polar protic solvent e.g., water, methanol and ethanol. This takes into account the possible hydrogen bond donation interaction of the solvent with the dyes besides the nonspecific interactions. Theoretically calculated values of lmax listed in Table 1 for the dyes in protic solvents, namely, water, methanol and ethanol are those obtained after the incorporation of the effect of hydrogen bonding. This supports that the dyes interact with polar protic solvents through both specific hydrogen bonding and nonspecific dielectric interactions. For all the three dyes both experimental and theoretical studies conform to the solvent sensitivity of absorption maximum. Out of the above three dyes, the calculated values of lmax in N2 are found to be very close to the experimental absorption maxima. Fig. 2. Relative charge distribution in the HOMO and LUMO of the dyes N1 ((a) and (b) respectively), N2 ((c) and (d) respectively) and DN2 ((e) and (f) respectively). 14 A. Sarkar et al. / Journal of Molecular Structure 1102 (2015) 11e17 Fig. 3. Plots of theoretical versus experimental lmax values using both B3LYP ( ) and wB97XD ( ) functionals for the three dyes. Numbers refer to the serial number of solvents in Table 1. This is probably due to the fact that the free rotations of the carbonyl centre about C]C bond is restricted in N2 and in DN2 due to the presence of indanone moiety in the acceptor centre. Similarly the presence of indole group at the donor centre in N2 provides restricted structural relaxation as compared to DN2. This may be the reason optimised theoretical structure of N2 proves to be a close approximation to experimental structure whereas they may be different in experiment for N1 and N2. These dyes being hydrophobic have been found to be poorly soluble in water. Hence it is difficult to obtain the value of lmax in pure water, which is rather obtained by extrapolating the results obtained from studies of the dyes carried out in aqueous mixed binary solvent systems. Theoretical calculations, on the other hand appropriately provide a means to obtain the value of lmax for such hydrophobic dyes in water and the values obtained are found to be comparable to the experimentally obtained values. 3.2. Effect of protonation Experimental studies involving the effect of strong acid e.g., hydrochloric acid to the solution of any of these dyes in acetonitrile or methanol indicate the generation of protonated form of the dye having an absorption maximum at a longer wavelength compared to that of the neutral dye [13,14]. Protonation has been suggested to take place at the electron rich carbonyl O centre of the dye rather than at the electron deficient amino N centre. In the present work, this has been theoretically looked into by considering protonation at both the centres in DN2. The protonation at O centre (E ¼ 864.84 a.u.) is energetically more favourable compared to protonation at N centre (E ¼ 864.79 a.u.). All the dyes are characterised by alternate single and double bonded conjugation between the amino N and the carbonyl O in their neutral form as represented in Fig. 5(a). Protonation at O centre of these dyes leads to a change in bond length values for bonds between the donor and the acceptor centre which can be represented by the generalised structure Fig. 5(a). The bond length alternation gets modified significantly upon protonation and the values of bond lengths for the neutral and the O-protonated dyes have been listed in Table 2. Fig. 5(b) shows the variation of relative bond length values in N1 upon protonation at O centre. The presence of proton causes a decrease in the bond length of single bonds (NeC2, C3eC4 and C5eC6) whereas an increase in the same for double bonds (C2eC3, C4eC5 and C6eO) is observed. Such an increase in the carbonyl bond length (C6eO) has also been found when the dye forms complex with metal ions [26,34]. Structure of such dyes can be represented as a resonance hybrid of two valence bond structures, namely, the ‘neutral’ and the valence bond charge transfer (VB-CT) form of the dyes [35,36]. Contribution of the latter form will modify the bond length alteration in the way discussed above. The observed change in the bond length indicates that the percent contribution of the ‘VB-CT’ form increases due to protonation at the carbonyl O centre of the dyes. Similar observations have been reported for similar dyes undergoing complexation with metal ions at the carbonyl O centre [24,26]. A. Sarkar et al. / Journal of Molecular Structure 1102 (2015) 11e17 15 Fig. 4. Plot of the difference between the theoretical and experimental lmax values as a function of solvent polarity parameter, ET(30), for the three dyes using both B3LYP ( ) and wB97XD ( ) functionals. Numbers refer to the serial number of solvents in Table 1. Among three dyes studied, N1 and N2 consist of an indole moiety with a conjugated substituent at C3 position of the indole. Alata and co-workers have recently studied the protonation of indole in gas phase both theoretically and experimentally and has reported C3 protonation to be energetically most favourable [17]. Similar results for protonation of indole have been obtained as shown in Fig. 6. The effect of conjugated substituent at C3 on protonation at various positions of indole moiety of N1 and N2 has been investigated. From Fig. 6 it is evident that protonation at carbonyl O is energetically much more favourable compared to protonation at any position of the indole moiety in N1 and N2. Protonation at C3 position of either N1 or N2 is energetically most unfavourable. This is rationalisable in view of the fact that protonation at this position completely ruptures the conjugation between N to O (since the sp2 C3 atom is converted to sp3 centre). On Table 1 List of theoretically calculated (PCM) values for lmax (nm) in neat solvents using B3LYP functional. The values in the parenthesis correspond to those determined experimentally [13,14]. All the oscillator strength values range between 0.7 f 1.1. Sl. No. Solvents lmax (nm) N1 1 2 3 4 5 6 7 8 9 10 Water Methanol Ethanol Acetone Acetonitrile Tetrahydrofuran Dichloromethane Chloroform Dimethylsulfoxide Cyclohexane 415 414 414 407 408 406 407 405 410 398 N2 (408) (391) (389) (378) (379) (377) (382) (382) (392) (359) 417 417 417 408 408 406 407 405 410 399 DN2 (420) (412) (414) (403) (403) (405) (404) (407) (414) (391) 421 420 421 419 419 417 419 416 421 409 (460) (441) (439) (424) (428) (421) (427) (428) (438) (407) comparing with the relative energy values for protonation of indole, protonation at any position (except C3) of the indole group of N1 and N2 is almost unaffected due to the presence of substitution at C3 position (Fig. 6). This indicates that it is more important to maintain the conjugation between N and O than the aromaticity of indole moiety in N1 and N2. 3.3. Molecular (hyper)polarisabilities On each optimised structures of N1, N2 and DN2, we have also calculated static linear polarisabilities and first hyperpolarizabilities of above donoreacceptor dyes to investigate possible application of these systems in NLO application. We have used 6311þþG(d,p) basis set and B3LYP exchange correlation functional in (hyper)polarisability calculation. The calculation has also been done for wB97XD exchange correlation functional for comparison. We have not considered the effect of solvent in hyperpolarizability calculation. The tensor components of linear polarisabilities, average linear polarisability and first hyperpolarizability components along three cartesian axes have been listed in Table 3 for B3LYP functional only. Use of wB97XD exchange correlation functional gives similar results. It is found that the linear polarisability components are mostly contributed from molecular X-axis. The tensor components of the linear polarisabilities are comparable for three systems. On the other hand the molecular X-axis hyperpolarizability component (bx) of these dyes contributes most towards its average value (<b>) as compared to other two components (by and bz). Table 3 also suggests that average first hyperpolarizabilities of N1, N2 and DN2 are quite large and they could find suitable application in nonlinear optoelectronics. Calculation also reveals that average static hyperpolarizability 16 A. Sarkar et al. / Journal of Molecular Structure 1102 (2015) 11e17 Fig. 5. (a). Generalised representation of the conjugation between amino N and carbonyl O of the three dyes. (b): Relative variation of bond lengths upon protonation at O centre of N1. Table 2 Values of bond length (Å) for neutral and protonated dyes. Dyes NeC2 C2eC3 C3eC4 C4eC5 C5eC6 C6eO N1 N1eHþ N2 N2eHþ DN2 DN2eHþ 1.36914 1.33643 1.37037 1.33930 1.37345 1.32524 1.38566 1.41469 1.38811 1.41381 1.36089 1.39645 1.43949 1.39808 1.43834 1.40117 1.43072 1.38746 1.35183 1.39017 1.35079 1.38620 1.35413 1.39684 1.47439 1.39821 1.48940 1.41206 1.48181 1.39320 1.22702 1.33457 1.22050 1.32175 1.22315 1.33043 values (<b>, bm) of DN2 molecule is found to be largest among them as this donoreacceptor molecule exhibits lowest HOMOeLUMO energy gap which facilitates the intramolecular charge transfer. Individual tensor components of (hyper)polarisabilities are found to be large for B3LYP exchange correlation functional as compared to wB97XD. But in both cases, the hyperpolarizability tensor components of these dyes are found to be large, so they could be very promising in nonlinear technology. 4. Conclusion Fig. 6. Variation of relative energy values for protonation at different positions in free indole (C) and indole moieties present in N1 ( ) and N2 ( ). The present work deals with theoretical study of photophysical properties and solvation characteristics of three dyes namely, N1, N2 and DN2, using density functional theory. Quantum chemical calculations suggest the possibility of an intramolecular charge transfer transition from the donor amino N to the carbonyl O centre of the dyes as seen from our previous experimental studies of the dyes in neat solvents exhibiting solvent sensitivity of the absorption peak maximum. Theoretically calculated values of HOMOeLUMO energy gap for the three dyes suggests that the value is minimum for DN2 which is correlated to the highest percentage charge transfer between carbonyl O and amino N within DN2. Although computational studies predict similar values of HOMOeLUMO gap for structurally similar dyes N1 and N2, experimental studies indicate that the extent of charge transfer is lower in N1 compared to N2. This is possibly due to the CeC bond rotation containing the acceptor centre which is possible in N1 but is restricted in case of N2 under experimental conditions. Protonation studies indicate energetically more favourable attack at the O centre compared to any position within the indole group. Linear and static first hyperpolarizability calculation on above Table 3 The linear polarisability (a.u.) and first hyperpolarizability tensor components (a.u.) of dyes using B3LYP functional. Dyes axx ayy azz <a> bx by bz <b> bm N1 N2 DN2 416.11 426.95 446.38 237.36 250.65 254.66 119.73 120.26 148.21 257.73 265.95 283.08 5234.20 5143.19 6282.25 2487.80 1246.40 595.13 71.51 44.62 168.35 5795.78 5292.15 6377.88 4342.12 2799.44 5780.67 A. Sarkar et al. / Journal of Molecular Structure 1102 (2015) 11e17 donoreacceptor dyes suggests that they possess significant values of non-linear optical coefficients and DN2 molecule is found to have larger average first hyperpolarizability as compared to N1 and N2. This is possibly due to the greater extent of charge transfer from the amino N centre to the carbonyl O in DN2 compared to that of N1 and N2 as seen previously. Also, similar donor N centre in N1 and N2 gives a comparable value of average first hyperpolarizability of both the dyes. These results imply the above dyes are suitable candidates for possible NLO application. Acknowledgement AS thank CSIR (India) for senior research fellowship. MD acknowledges financial support from Department of Science and Technology, Government of India (SB/FTP/CS-164/2013) and SB is grateful to UGC, India (F. 6 - 19/2011 (SA-II)) for an emeritus fellowship. 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