- Email: [email protected]
Electronic spectra and (hyper)polarizabilities of ketocyanine dye complexes with metal ions
Journal of Molecular Structure 1033 (2013) 236–242
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Electronic spectra and (hyper)polarizabilities of ketocyanine dye complexes with metal ions Mousumi Das a,⇑, Sanjib Kr Sardar a, Sanjib Bagchi b,⇑ a b
Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741 252, Nadia, West Bengal, India Department of Chemistry and Bio Chemistry, Presidency University, 86/1 College Street, Kolkata 700 073, West Bengal, India
h i g h l i g h t s " We carried out TDDFT calculation on ketocyanine and its metal ion complexes. " Theoretical calculations are in good agreement with experimental observation. " Calculation of first static hyperpolarizabilities are reported for metal-ion complexes. " Hyperpolarizability suggests promising NLO application for above complexes.
a r t i c l e
i n f o
Article history: Received 27 July 2012 Received in revised form 17 October 2012 Accepted 17 October 2012 Available online 26 October 2012 Keywords: Ketocyanine Absorption spectra Time-dependent DFT Nonlinear optical responses Hyperpolarizability
a b s t r a c t The interaction of a ketocyanine dye, an example of donor–acceptor–donor (D–A–D) chromophore, and its parent merocyanine (D–A) dye with metal ions have been investigated using density functional theory. Time dependent density functional theory (TDDFT) is employed to target the lowest singlet excited dipole allowed states of optimized structures of dye–metal ion complexes in acetonitrile solution. The calculated excitation energies are in good agreement with absorption spectra obtained in experiment. We have also calculated the linear and static first hyperpolarizabilities of those dye–ion complexes and found these complexes to be promising in the field of nonlinear optoelectronics. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction N-substituted derivatives of 2,5-bis [propylene] cyclopentanone, commonly known as ketocyanine dyes, are characterized by solvent sensitive absorption and fluorescence properties [1,2]. Due to the presence of electron donor (amino) and electron acceptor (carbonyl) groups in the molecule, the electronic transition is associated with intramolecular charge transfer (ICT) and as such the optical response of the molecule is very much sensitive to the microenvironment. Electronic spectroscopic and photophysical properties of these molecules have been extensively studied in various media in recent years [3–14]. These compounds are also used as laser dyes and have industrial application in polymer imaging system [15–17]. Recent experimental observations suggest a significant change in absorption spectra due to the formation of complex involving the dye and metal ions [18–22]. It has also been observed that interactions of these dyes with cations in acetonitrile solution cause ⇑ Corresponding authors. E-mail address: [email protected] (M. Das). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.10.042
a substantial shift of absorption and fluorescence maxima indicating the formation of dye-cation complexes in the ground and excited state. In recent times such sensitivity of spectral features have also been investigated theoretically based on density functional and time-dependent density functional theory calculations for interactions of ketocyanine dyes with Li+, Mg2+ ions taking into account of implicit and explicit solvent effect [23–25]. The objective of our present work is to study the lowest singlet dipole allowed excited state of complexes involving metal ions and a symmetric ketocyanine dye having D–A–D system of chromophores using time-dependent density functional theory and to compare the corresponding excitation energies with electronic absorption spectra observed in experiment. Work has also been done using the D–A counterpart, the parent merocyanine dye, in place of the ketocyanine dye. The symmetric ketocyanine (KD1) and its parent merocyanine dye (DN1) are shown schematically in Fig. 1. In recent times, a considerable amount of research attention has been focused on the design of molecule-based materials with large nonlinear optical (NLO) responses [26–28]. Very recently, linear and first hyperpolarizabilities of two non-centrosymmetric yet symmetric ketocyanines
237
M. Das et al. / Journal of Molecular Structure 1033 (2013) 236–242
Fig. 1. Schematic diagrams of the studied D–A–D chromophore, ketocyanine (KD1) (A) and its parent D–A chromophore, merocyanine (DN1) (B).
have been calculated by Ponterini et al. on the basis of essential three state model showing substantial vectorial components of first hyperpolarizability which are quite comparable with their parent merocyanines and thus proved to possess significant nonlinear optical properties [29]. In this context their very recent linear and first hyperpolarizability calculation based on three-state-model and TDDFT-SOS analysis of a penta-heptamethine ketocyanine is quite important [30]. As large hyperpolarizabilites are the key features of strong NLO responses, we are interested in studying the static linear and first hyperpolarizabilities of the above dyes (KD1, DN1) and dye–metal ion complexes in view of their promising applicability in the field of nonlinear photonics.
2. Theoretical and computational approach 2.1. Optimized structures in the ground state The molecular structures of pure ketocyanine (KD1) and its parent merocyanine (DN1) dye as well as corresponding dyecation complexes are fully optimized using density functional theory (DFT) implemented in Gaussian 09 software [31]. Becke’s three-parameter exchange functional combined with LYP correlation functional (B3LYP) is employed in all calculation. The basis set 6-311G++(d,p) are used to study the free merocyanine and its metal ion complexes where as relatively economical 6-311G(d,p) was used to study the free ketocyanine dye and its metal ion complexes. In case of merocyanine (DN1) dye, we have also performed ground state geometry optimization using relatively economical basis set 6-311G(d,p) for comparison with the results obtained from 6-311G++(d,p) basis set. The metal ions considered in our calculations are Li+, Ca2+ and Mg2+. At all optimized structures of merocyanine dye and their complexes obtained from DFT/6-311G++(d,p) (DFT/6-311G(d,p)) level of theory, excitation spectra were calculated in 6-311G++(d,p) (6-311G(d,p)) basis with B3LYP functional using time-dependent density functional theory (TDDFT) and comparatively economical 6-311G(d,p) basis set is used for ketocyanine dye (KD1) and their complexes with same B3LYP functional in TDDFT calculation. The effect of solvent on excitation spectra of the above TDDFT calculations had been incorporated considering modeling the solvent implicitly. The above implicit modeling termed as Polarizable Continuum Model (PCM) implemented in Gaussian 09 and default values for acetonitrile solution were taken as the parameters of the solvent. In calculation the default values of dielectric constants
of acetonitrile solvent are taken as 36.64 at static constant frequency and 1.806 at infinite frequency. 2.2. Molecular polarizabilities The molecular polarizability is defined by its response to applied electric field e. The perturbed system Hamiltonian is given ! ! by H0 ¼ M ~ e where M stands for the induced dipole moment and this will modify the system energy. Under the influence of static electric field, the modified energy can be expanded as a Taylor series.
! 1 XX @ 2 E Eð~ eÞ ¼ Eð0Þ þ ei þ ei ej @ ei 0 2 i j @ ei @ ej i 0 ! 1 XXX @3E þ ei ej ek þ . . . 6 i j k @ ei @ ej @ ek X @E
ð1Þ
0
The energy derivatives taken at zero electric field provide the static response properties of the system with respect to the applied electric field. The measure of interaction is attributed to first, second and higher order properties of the system based on the degree of differentiation of the energy. The single energy derivatives are the dipole moments (li) and higher order terms introduce static linear polarizabilities (aij), the first order hyperpolarizability (bijk) respectively and are defined as,
li
@E ¼ ; @ ei 0
@2E aij ¼ @ ei @ ej
! ; 0
bijk
@3E ¼ @ ei @ ej @ ek
! ð2Þ 0
However the above expressions for linear and first hyperpolarizabilities are not used explicitly for calculation whereas the individual tensor components of static linear polarizability and first hyperpolarzabilities have been calculated within the framework of density functional theory (DFT) using static finite field approach implemented in Gaussian 09 at zero frequency respectively. The step size in the finite electric field is used as 0.001 atomic unit in polarizability calculation. Along with its vector components, we report the average linear polarizability (aavg) for DN1 and KD1. The average polarizability is defined as,
1 3
aavg ¼ ðaXX þ aYY þ aZZ Þ
ð3Þ
In general, the experimental accessible vector component of first hyperpolarizability tensor is along the direction of the ground state dipole moment. The molecular long axis is taken along X-axis
238
M. Das et al. / Journal of Molecular Structure 1033 (2013) 236–242
therefore X-component of linear (a) and first hyperpolarizabilities (b) are contributing most for merocyanine (DN1) molecule as its ground state dipole moment coincides along molecular X axis. According to choice of Cartesian coordinate axes for symmetrical ketocyanine (KD1) (c2v molecular symmetry), the dipole moment is aligned along the Y axis, so Y-component of first hyperpolarizability will be most important towards NLO responses. The first hyperpolarizability is a third rank tensor with 27 components that can be reduced to 10 components due to Kleinman symmetry [32]. Computational method mostly gives important hyperpolarizability components that would be interesting from the experimental point of view. 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,
1 hbi ¼ b2X þ b2Y þ b2Z 2 ;
bi ¼
1X ðb þ bjij þ bjji Þ 3 j¼1;3 ijj
ð4Þ
To interpret electric field induced second harmonic generation (EFISH) measured in experiment, the hyperpolarizability average required to compute is bl, known as the strength of hyperpolarizability in the direction of molecular ground state dipole moment and defined as,
bl ¼
X
li bi =jlj
ð5Þ
i¼1;3
We will address those two average values of static first hyperpolarizabilities for isolated and metal ion complexes of merocyanine and ketocyanine molecules. 3. Results and discussion 3.1. Optimized geometry in the ground state and optical absorption The optimized geometries of the above dyes (DN1, KD1) and their corresponding metal-ion complexes are shown in Figs. 2 and 3 respectively. The dyes and corresponding dye–metal ion complexes are planar as C–N–C–C dihedral angles are almost zero. KD1 has a symmetry plane passing through the carbonyl group with a dipole moment 5.25 D along the carbonyl axis. The effective dipole moment of the parent DN1 molecule is 6.74 D. As KD1 and DN1 have both oxygen and nitrogen centers, the metal ion can bind on either of these two centers. Earlier work has shown that metal ion binding on the oxygen of KD1 was to be lower in energy than
that binding on the nitrogen by nearly 20 kcal mol1 [22]. The cation-oxygen distances were obtained in vacuum and also in solvent for both DN1 and KD1 dye–metal ion complexes. The geometry optimizations have been done employing polarization continuum model implemented in Gaussian to incorporate the effect of solvent to obtain the cation-oxygen distance in solvent. In case of DN1 dye–metal complexes, the Li+–O, Ca2+–O and Mg2+–O distances were obtained to be 1.67 (1.85), 1.97 (2.22), and 1.92 (1.91) Å respectively in vacuum (solvent). Similarly for KD1 dye– metal complexes, Li+–O, Ca2+–O and Mg2+–O distances were obtained as 1.65 (1.81), 1.96 (2.17), and 1.85 (1.87) Å respectively in vacuum (solvent). For both DN1 and KD1 dye–metal complexes the Li+–O, Ca2+–O distances are expected to increase in presence of acetonitrile solvent as the polarization of the solvent stabilizes the free cations better than complex. So the interaction strength between the metal cation and the molecule become weaker hence cation-dye distance increases. On the other hand Mg2+–O distance shows almost no change in solvent medium for both dye–metal complexes. However for DN1–Mg2+ complex in vacuum Mg2+ cation inclines more towards the carbonyl bond and the angle associated with carbonyl bond (
Fig. 2. Optimized structures in the ground state of the merocyanine dye (DN1) (A), Li+–DN1 complex (B), Ca2+–DN1 complex (C), Mg2+–DN1 complex (D).
M. Das et al. / Journal of Molecular Structure 1033 (2013) 236–242
239
Fig. 3. Optimized structures in the ground state of the ketocyanine dye (KD1) (A), Li+–KD1 complex (B), Ca2+–KD1 complex (C), Mg2+–KD1 complex (D).
Table 1 The ground state optimized C–O bond lengths in carbonyl group along with carbon– carbon bond lengths involved in conjugation for pure merocyanine (DN1) and its metal ion complexes. All distances are in Å. Metal ion
C5–O
C1–C2
C2–C3
C3–C4
C4–C5
Pure Li+ Ca2+ Mg2+
1.22 1.27 1.33 1.31
1.48 1.42 1.39 1.39
1.35 1.37 1.40 1.41
1.43 1.40 1.38 1.37
1.36 1.38 1.40 1.41
Table 2 The ground state optimized C–O bond lengths in carbonyl group along with carbon– carbon bond lengths involved in conjugation for pure ketocyanine (KD1) and its metal ion complexes. All bond lengths are in Å. Metal ion
C5–O
C1–C2
C2–C3
C3–C4
C4–C5
Pure Li+ Ca2+ Mg2+
1.22 1.28 1.33 1.30
1.36 1.37 1.38 1.39
1.43 1.41 1.39 1.39
1.35 1.37 1.38 1.39
1.48 1.43 1.41 1.42
enolate forms 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 ‘enol’ form increases due to complex formation with metal ions. Position of the band maximum depends on the contribution of the enol form, a larger contribution of enol structure causes a shift of the band towards higher wave length. In an aprotic solvent like acetonitrile the actual structure is closer to the ‘keto’ form; on the other hand, the actual structure is closer to other extreme form namely the enol form in protic solvents like alcohols. A red1 shift of the HOMO to LUMO transition of the dye (equivalently, shortening of the HOMO–LUMO gap) is thus expected on complex formation. The calculated values of wave length corresponding to maximum absorption have been shown in Tables 3 and 4 for DN1 and KD1 respectively. The agreement with experimental results is good. Fig. 5 shows the lowest energy band of both merocyanine (DN1) and ketocyanice (KD1) dyes and their metal complexes in acetonitrile solution observed in experiment. The absorption spectra of the dye–metal ion complexes are characterized by a shoulder at higher wave length. The spectrum can be resolved into two Gaussian components. The lowest singlet transitions are tabulated in Table 3 and in Table 4 for merocyanine 1 For interpretation of color in Figs. 5 and 6, the reader is referred to the web version of this article.
and ketocyanine dye and their metal ion complexes respectively. The corresponding experimental data are shown for comparison. The lowest singlet excitation (S0–S1) with non-negligible intensity in free merocyanine dye (DN1) molecule corresponds to the HOMO to LUMO transition. The calculated excitation energy (440.2 nm) is well comparable with the absorption maximum (445 nm) obtained in experiment for DN1. Similarly for free ketocyanine dye, the calculated lowest singlet excitation energy (503.7 nm) is in very good agreement with absorption maxima obtained in experiment (503 nm). PCM description failed to give good results when applied to an alcohol, e.g., methanol. Thus for the dyes under TDDFT calculation using PCM, the values of wavelength corresponding to maximum absorption came as 489.4 and 439.4 nm for KD1 and DN1 respectively. Values differ widely from the experimental values of 525 and 463 nm respectively for the dyes. This is intelligible in view of the fact that methanol interacts with the dye through specific hydrogen bonding interaction (between carbonyl group and the –OH group of the solvent). This is not taken into account into a continuum model. To take H-bonding into account we optimized the dye–methanol hydrogen bonded structure and that under TDDFT calculation using PCM taking methanol as solvent (static and optical dielectric constant as 32.63 and 1.758 respectively) gave 502.6 and 444.6 nm as the wavelength of maximum absorption for KD1 and DN1 respectively. However the calculated excitation energies due to transition from HOMO to LUMO for Li+–DN1 (457.2 nm), Ca2+–DN1 (463.8 nm) and Mg2+–DN1 (489.1 nm) complexes are different from corresponding experimental observations (494, 492, 500 nm). The excitation energies calculated at TDDFT/6-311G(d,p) level of theory have also been reported in Table 3 for comparison. The HOMO– LUMO energy gap is 3.31 eV for free DN1 molecule and decreases to 2.82 eV (Li+–DN1), 1.33 eV (Ca2+–DN1), 1.30 eV (Mg2+–DN1) on complex formation as expected. On complex formation both HOMO and LUMO energies of free dye molecule decrease and dye complex formation results in the red shift of observed singlet transition in agreement with experimental data. The HOMO to LUMO singlet excitation energies (546.2 nm, 578.9 nm, 576.5 nm) in the complexes with Li+, Ca2+, Mg2+ ions respectively are also in good agreement with experimental data (540 nm, 570 nm, 580 nm) for ketocyanine (KD1). The diagrams of frontier orbitals contributing to the observed singlet transitions in calculation for free ketocyanine dye and its metal ion complexes are shown in Fig. 6. Like parent merocyanine, the HOMO–LUMO energy gap (3.03 eV) for free ketocyanine decreases to 2.32 eV, 0.45 eV, 0.47 eV for its Li+, Ca2+, Mg2+ dye complexes. Although we have correlated the singlet vertical transition energy with the position of the absorption maxima for
240
M. Das et al. / Journal of Molecular Structure 1033 (2013) 236–242
Fig. 4. ‘Keto’ and the zwitterionic ‘enol’ form of KD1 and DN1 dyes.
Table 3 Energies E (nm) of lowest singlet transitions, oscillator strengths f of the merocyanine dye molecule (DN1) and corresponding dye–metal ion complexes in acetonitrile solution. The calculated energy values at maximum absorption are compared with experimental observations. H and L stands for HOMO and LUMO orbital. The superscripts a and b indicate calculated energy values for 6311G++(d,p) and 6311G(d,p) basis sets respectively for comparison. Excitation DN1 Li+–DN1 Ca2+–DN1 Mg2+–DN1
H?L H?L H?L H?L
E (calculated) a
440.2 , 457.2a, 463.8a, 489.1a,
b
427.0 450.9b 456.8b 484.9b
E (experiment)
f
445 494 492 500
1.27 1.51 1.68 1.50
Table 4 Energies E (nm) of lowest singlet transitions, oscillator strengths f of the ketocyanine dye (KD1) molecule and corresponding dye–metal ion complexes in acetonitrile solution. The calculated energy values at maximum absorption are compared with experimental observations. H and L stand for HOMO and LUMO orbital respectively.
KD1 Li+–KD1 Ca2+–KD1 Mg2+–KD1
Excitation
E (calculated)
E (experiment)
f
H?L H?L H?L H?L
503.7 546.2 578.9 576.5
503 540 570 580
2.32 2.57 2.71 2.71
both dyes and their corresponding metal ion complexes, we would like to emphasize that in principle they can be compared approximately as the position of absorption maxima depends predominately on the vibronic structure of the absorption band. The exact shape of the absorption spectra can be simulated and compared with experimental data by employing Franck–Condon vibrational analysis performed in earlier studies [25,33].
3.2. Nonlinear polarizabilities At each optimized molecular geometry, static linear and first hyperpolarizability tensor components have been calculated using finite field approach within the framework of density functional theory employing B3LYP exchange–correlation functional using the same basis sets which have been used in the study of excitation spectra. Numerical derivatives have been used to obtain the NLO responses. Tables 5 and 6 summarizes the vector components of linear polarizabilities of merocyanine (DN1) and its metal-ion complexes and same for ketocyanine molecule and its metal ion complexes respectively. The linear polarizability is mostly contributed and dominated by aXX components for DN1 and KD1. We also observed a slight increase in the average value of a from isolated through singly to doubly charged metal ion complexes. Such observation holds good for both DN1 and KD1. The average value of linear polarizability increases about 12% from isolated DN1 to singly charged metal ion DN1 complex whereas for KD1 such increment
Fig. 5. Normalised absorption spectrum of the symmetrical dye (KD1) (A) and its parent unsymmetrical dye (DN1) (B) for the different metal ions in acetonitrile solvent.
is nearly 25%. The maximum increment is nearly 40% from singly to doubly charged metal ion complexes for DN1 and KD1 both. Tables 7 and 8 lists the tensor components of static first hyperpolarizability, average static first hyperpolarizabilty (hbi), vector hyperpolarizability along the dipole moment (bl) for DN1 and KD1 respectively. In case of DN1 and its metal ion complexes, the static first hyperpolarizability is mostly dominated by its tensor component along its molecular axis X. Therefore the vector component along the direction of ground state dipole moment (bl) is smaller than to average static first hyperpolarizabilty (hbi).
241
M. Das et al. / Journal of Molecular Structure 1033 (2013) 236–242
HOMO
LUMO
(A)
HOMO
LUMO
(B)
HOMO
LUMO
(C)
HOMO
LUMO
(D) Fig. 6. Plots of frontier orbitals for free KD1 dye (A), Li+–KD1 complex (B), Ca2+–KD1 complex (C), Mg2+–KD1 complex (D).
Table 5 The linear polarizabilitiy (1024 cm3) of parent merocyanine (DN1) and its metal ion complexes.
DN1 Li+–DN1 Ca2+–DN1 Mg2+–DN1
aXX
aYY
aZZ
h ai
85.34 101.64 113.83 152.05
36.40 37.41 41.52 44.10
19.37 18.88 18.75 21.07
47.04 52.64 58.04 72.39
Table 6 The linear polarizabilitiy (1024 cm3) of ketocyanine (KD1) and its metal ion complexes.
KD1 Li+–KD1 Ca2+–KD1 Mg2+–KD1
aXX
aYY
aZZ
h ai
151.31 205.81 257.85 293.39
46.12 47.62 121.70 93.21
21.97 22.16 24.11 25.90
73.14 91.86 134.56 137.50
The merocyanine D–A chromophore (DN1) and also its metal ion complexes show substantial first hyperpolarizability and thus can be considered to be good candidates for nonlinear optics
Table 7 The first hyperpolarizabilitiy (1030 esu1 cm5) of parent merocyanine (DN1) and its metal ion complexes.
DN1 Li+–DN1 Ca2+–DN1 Mg2+–DN1
bx
by
bz
hbi
bl
62.47 86.11 80.73 620.14
19.67 30.94 37.97 144.18
0.01 0.048 0.09 76.49
65.49 91.50 89.22 641.26
57.67 65.15 78.57 631.36
Table 8 The first hyperpolarizabilitiy (1030 esu1 cm5) of ketocyanine (KD1) and its metal ion complexes.
KD1 Li+–KD1 Ca2+–KD1 Mg2+–KD1
bx
by
bz
hbi
bl
0.04 0.08 0.01 0.13
47.46 87.18 332.33 167.56
0.05 0.02 0.17 0.06
47.46 87.18 332.33 167.56
47.29 87.03 332.23 167.07
applications. Table 7 exhibits Mg2+–DN1 complex would be most promising among them having comparatively larger values of static first hyperpolarizability. On the other hand for symmetric (C2v)
242
M. Das et al. / Journal of Molecular Structure 1033 (2013) 236–242
ketocyanine (KD1) with D–A–D chromophore and its metal ion complexes the ground state dipole moment is aligned along the carbonyl group or Y axis. Therefore the average static first hyperpolarizability (hbi) is nearly same as bl. The most significant observation is that the studied ketocyanine and its metal ion complexes possess significant first hyperpolarizability and the order of magnitude of hbi and bl are well comparable with those calculated for their parent merocyanine (DN1) with D–A chromophore. So KD1 dye and its metal ion complexes studied also possess significant nonlinear optical properties. Such results is unexpected in view of the fact that symmetric ketocyanines are quasi linear and would have an affinity with centrosymmetric D–A–D chromophore that have vanishing first hyperpolarizabilities. Earlier work on other symmetric ketocyanines has shown similar observation as the calculated first hyperpolarizabilties are quite significant in agreement with our calculations [29]. 4. Conclusions In our paper we have studied linear and nonlinear optical properties of symmetric ketocyanine, an example of D–A–D chromophore and its parent merocyanine (D–A) dyes. Besides isolated molecule we were also interested in studying the linear and nonlinear optical responses of metal ion complexes of those dyes. The electronic absorption spectra have been investigated for DN1 and KD1 dyes and their complexes within the framework of time-dependent density functional theory considering the effect from acetonitrile solvent using Polarizable Continuum Model. The calculated absorption maxima are well compared with experimental observations. We have also calculated the static linear and nonlinear optical responses of above dyes and their metal ion complexes using finite field approach based on density functional theory. The calculated static first hyperpolarizailities are quite significant for DN1 and its metal complexes and are considered to be promising for nonlinear optical applications. Interestingly the D–A–D symmetric ketocyanine dye and their metal ion complexes feature same order of magnitude of first hyperpolarizabities as that of their parent merocyanines in spite of the fact that quasi linear ketocyanines tend to have affinity with centrosymmetric D–A–D chromophores which have vanishing first hyperpolarizabilities. Our calculation on first hyperpolarizabilities strongly suggests that both isolated and metal ion complexes of merocyanine and ketocyanine studied above would find significant application in the field of nonlinear optoelectronics. It is worthwhile to mention that the practical application of NLO usually based on solid state devices. Although the dyes KD1 and DN1 can be isolated in the crystalline form, their complexes with the metal ions could not be isolated in crystalline form yet. In view of the enhanced NLO activity of the metal ion complexes of these dyes, isolation of these complexes would be an interesting future study. Acknowledgements MD acknowledges financial support from Department of Science and Technology, Government of India through a SERC
Fast-Track Grant SR/FT/CS-003/2010 and SKS and SB acknowledge financial support from UGC, India. References [1] (a) M.A. Kessler, O.S. Wolfbeis, Spectrochim. Acta Part A 47 (1991) 187; (b) P.K. Das, R. Pramanik, D. Banerjee, S. Bagchi, Spectrochim. Acta 56A (2000) 2763; (c) M. Shannigrahi, R. Pramanik, S. Bagchi, Spectrochim. Acta 59A (2003) 2921. [2] C. Reichardt, Chem. Rev. 94 (1994) 2319. [3] D. Banerjee, A.K. Laha, S. Bagchi, J. Photochem. Photobiol. A 85 (1995) 153. [4] D. Banerjee, S. Mondal, S. Ghosh, S. Bagchi, J. Photochem. Photobiol. A 90 (1995) 171. [5] D. Banerjee, S. Bagchi, J. Photochem. Photobiol. A 101 (1996) 57. [6] M. Shannigrahi, S. Bagchi, J. Photochem. Photobiol. A 168 (2004) 133. [7] M. Shannigrahi, S. Bagchi, J. Phys. Chem. B 108 (2004) 17703. [8] N. Marcotte, S. Fery-Forgues, J. Photochem. Photobiol. A 130 (2000) 13. [9] A.O. Doroshenko, V.G. Pivovarenko, J. Photochem. Photobiol. A 156 (2003) 5. [10] A.O. Doroshenko, M.D. Bilokin, V.G. Pivovarenko, J. Photochem. Photobiol. A 163 (2004) 95. [11] V.G. Pivovarenko, A.V. Klueva, A.O. Doroshenko, A.P. Demchenko, Chem. Phys. Lett. 325 (2000) 389. [12] A.O. Doroshenko, A.V. Grigorovich, E.A. Posokhov, V.G. Pivovarenko, A.P. Demchenko, J. Mol. Eng. 8 (1999) 199. [13] A.O. Doroshenko, L.B. Sychevskaya, A.V. Grygorovych, V.G. Pivovarenko, J. Fluoresc. 12 (2002) 451. [14] K. Rurack, M.L. Dekhtyar, J.L. Bricks, U. Resch-Genger, W. Rettig, J. Phys. Chem. A 103 (1999) 9626. [15] M.V. Barnabas, A. Liu, A.D. Trifanac, V.V. Krougaug, C.T. Chang, J. Phys. Chem. 96 (1992) 212. [16] W.J. Chambers, D.F. Eaton, J. Imaging Sci. 13 (1986) 230. [17] M.D. Baun, C.P. Henry, Ger. Offen. 2,133,315, January 13, 1972 (U.S. Patent Appl. 53,686, July 09, 1970). [18] N. Ray, J. Basu, M. Shannigrahi, S. Bagchi, Chem. Phys. Lett. 404 (2005) 63. [19] J. Basu, M. Shannigrahi, S. Bagchi, J. Phys. Chem. A 101 (2006) 9051. [20] J. Basu, M. Shannigrahi, S. Bagchi, Chem. Phys. Lett. 431 (2006) 278. [21] J. Basu, M. Shannigrahi, S. Bagchi, J. Phys. Chem. A 111 (2007) 7066. [22] S.K. Sardar, K. Srikanth, S. Bagchi, J. Phys. Chem. A 114 (2010) 10388. [23] A. Eilmes, J. Mol. Struct. Theochem. 915 (2009) 141–148. [24] A. Eilmes, Theor. Chem. Acc. 127 (2010) 743. [25] A. Eilmes, Comp. Theor. Chem. 972 (2011) 32. [26] J. Zyss, D. Chemla, Non Linear Optical Properties of Organic Molecules and Crystals, Academic Press, New York, 1987. [27] P.N. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects in Molecules and Polymers, Wiley, New York, 1991. [28] J.-L. Bredas, C. Adant, P.C.E. Tackx, A. Persoons, B.M. Pierce, Chem. Rev. 94 (1994) 243. [29] G. Ponterini, D. Vanossi, A.Z. Krasnaya, A.S. Tatikolov, F. Momicchioli, Phys. Chem. Chem. Phys. 13 (2011) 9507. [30] G. Ponterini, D. Vanossi, F. Momicchioli, Phys. Chem. Chem. Phys. 14 (2012) 4171. [31] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian, Inc., Wallingford, CT, 2010. [32] D.A. Kleinman, Phys. Rev. 1962 (1977) 126. [33] M. Guillaume, V. Liégeois, B. Champagne, F. Zutterman, Chem. Phys. Lett. 446 (2007) 165.
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Electronic spectra and (hyper)polarizabilities of ketocyanine dye complexes with metal ions Mousumi Das a,⇑, Sanjib Kr Sardar a, Sanjib Bagchi b,⇑ a b
Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741 252, Nadia, West Bengal, India Department of Chemistry and Bio Chemistry, Presidency University, 86/1 College Street, Kolkata 700 073, West Bengal, India
h i g h l i g h t s " We carried out TDDFT calculation on ketocyanine and its metal ion complexes. " Theoretical calculations are in good agreement with experimental observation. " Calculation of first static hyperpolarizabilities are reported for metal-ion complexes. " Hyperpolarizability suggests promising NLO application for above complexes.
a r t i c l e
i n f o
Article history: Received 27 July 2012 Received in revised form 17 October 2012 Accepted 17 October 2012 Available online 26 October 2012 Keywords: Ketocyanine Absorption spectra Time-dependent DFT Nonlinear optical responses Hyperpolarizability
a b s t r a c t The interaction of a ketocyanine dye, an example of donor–acceptor–donor (D–A–D) chromophore, and its parent merocyanine (D–A) dye with metal ions have been investigated using density functional theory. Time dependent density functional theory (TDDFT) is employed to target the lowest singlet excited dipole allowed states of optimized structures of dye–metal ion complexes in acetonitrile solution. The calculated excitation energies are in good agreement with absorption spectra obtained in experiment. We have also calculated the linear and static first hyperpolarizabilities of those dye–ion complexes and found these complexes to be promising in the field of nonlinear optoelectronics. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction N-substituted derivatives of 2,5-bis [propylene] cyclopentanone, commonly known as ketocyanine dyes, are characterized by solvent sensitive absorption and fluorescence properties [1,2]. Due to the presence of electron donor (amino) and electron acceptor (carbonyl) groups in the molecule, the electronic transition is associated with intramolecular charge transfer (ICT) and as such the optical response of the molecule is very much sensitive to the microenvironment. Electronic spectroscopic and photophysical properties of these molecules have been extensively studied in various media in recent years [3–14]. These compounds are also used as laser dyes and have industrial application in polymer imaging system [15–17]. Recent experimental observations suggest a significant change in absorption spectra due to the formation of complex involving the dye and metal ions [18–22]. It has also been observed that interactions of these dyes with cations in acetonitrile solution cause ⇑ Corresponding authors. E-mail address: [email protected] (M. Das). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.10.042
a substantial shift of absorption and fluorescence maxima indicating the formation of dye-cation complexes in the ground and excited state. In recent times such sensitivity of spectral features have also been investigated theoretically based on density functional and time-dependent density functional theory calculations for interactions of ketocyanine dyes with Li+, Mg2+ ions taking into account of implicit and explicit solvent effect [23–25]. The objective of our present work is to study the lowest singlet dipole allowed excited state of complexes involving metal ions and a symmetric ketocyanine dye having D–A–D system of chromophores using time-dependent density functional theory and to compare the corresponding excitation energies with electronic absorption spectra observed in experiment. Work has also been done using the D–A counterpart, the parent merocyanine dye, in place of the ketocyanine dye. The symmetric ketocyanine (KD1) and its parent merocyanine dye (DN1) are shown schematically in Fig. 1. In recent times, a considerable amount of research attention has been focused on the design of molecule-based materials with large nonlinear optical (NLO) responses [26–28]. Very recently, linear and first hyperpolarizabilities of two non-centrosymmetric yet symmetric ketocyanines
237
M. Das et al. / Journal of Molecular Structure 1033 (2013) 236–242
Fig. 1. Schematic diagrams of the studied D–A–D chromophore, ketocyanine (KD1) (A) and its parent D–A chromophore, merocyanine (DN1) (B).
have been calculated by Ponterini et al. on the basis of essential three state model showing substantial vectorial components of first hyperpolarizability which are quite comparable with their parent merocyanines and thus proved to possess significant nonlinear optical properties [29]. In this context their very recent linear and first hyperpolarizability calculation based on three-state-model and TDDFT-SOS analysis of a penta-heptamethine ketocyanine is quite important [30]. As large hyperpolarizabilites are the key features of strong NLO responses, we are interested in studying the static linear and first hyperpolarizabilities of the above dyes (KD1, DN1) and dye–metal ion complexes in view of their promising applicability in the field of nonlinear photonics.
2. Theoretical and computational approach 2.1. Optimized structures in the ground state The molecular structures of pure ketocyanine (KD1) and its parent merocyanine (DN1) dye as well as corresponding dyecation complexes are fully optimized using density functional theory (DFT) implemented in Gaussian 09 software [31]. Becke’s three-parameter exchange functional combined with LYP correlation functional (B3LYP) is employed in all calculation. The basis set 6-311G++(d,p) are used to study the free merocyanine and its metal ion complexes where as relatively economical 6-311G(d,p) was used to study the free ketocyanine dye and its metal ion complexes. In case of merocyanine (DN1) dye, we have also performed ground state geometry optimization using relatively economical basis set 6-311G(d,p) for comparison with the results obtained from 6-311G++(d,p) basis set. The metal ions considered in our calculations are Li+, Ca2+ and Mg2+. At all optimized structures of merocyanine dye and their complexes obtained from DFT/6-311G++(d,p) (DFT/6-311G(d,p)) level of theory, excitation spectra were calculated in 6-311G++(d,p) (6-311G(d,p)) basis with B3LYP functional using time-dependent density functional theory (TDDFT) and comparatively economical 6-311G(d,p) basis set is used for ketocyanine dye (KD1) and their complexes with same B3LYP functional in TDDFT calculation. The effect of solvent on excitation spectra of the above TDDFT calculations had been incorporated considering modeling the solvent implicitly. The above implicit modeling termed as Polarizable Continuum Model (PCM) implemented in Gaussian 09 and default values for acetonitrile solution were taken as the parameters of the solvent. In calculation the default values of dielectric constants
of acetonitrile solvent are taken as 36.64 at static constant frequency and 1.806 at infinite frequency. 2.2. Molecular polarizabilities The molecular polarizability is defined by its response to applied electric field e. The perturbed system Hamiltonian is given ! ! by H0 ¼ M ~ e where M stands for the induced dipole moment and this will modify the system energy. Under the influence of static electric field, the modified energy can be expanded as a Taylor series.
! 1 XX @ 2 E Eð~ eÞ ¼ Eð0Þ þ ei þ ei ej @ ei 0 2 i j @ ei @ ej i 0 ! 1 XXX @3E þ ei ej ek þ . . . 6 i j k @ ei @ ej @ ek X @E
ð1Þ
0
The energy derivatives taken at zero electric field provide the static response properties of the system with respect to the applied electric field. The measure of interaction is attributed to first, second and higher order properties of the system based on the degree of differentiation of the energy. The single energy derivatives are the dipole moments (li) and higher order terms introduce static linear polarizabilities (aij), the first order hyperpolarizability (bijk) respectively and are defined as,
li
@E ¼ ; @ ei 0
@2E aij ¼ @ ei @ ej
! ; 0
bijk
@3E ¼ @ ei @ ej @ ek
! ð2Þ 0
However the above expressions for linear and first hyperpolarizabilities are not used explicitly for calculation whereas the individual tensor components of static linear polarizability and first hyperpolarzabilities have been calculated within the framework of density functional theory (DFT) using static finite field approach implemented in Gaussian 09 at zero frequency respectively. The step size in the finite electric field is used as 0.001 atomic unit in polarizability calculation. Along with its vector components, we report the average linear polarizability (aavg) for DN1 and KD1. The average polarizability is defined as,
1 3
aavg ¼ ðaXX þ aYY þ aZZ Þ
ð3Þ
In general, the experimental accessible vector component of first hyperpolarizability tensor is along the direction of the ground state dipole moment. The molecular long axis is taken along X-axis
238
M. Das et al. / Journal of Molecular Structure 1033 (2013) 236–242
therefore X-component of linear (a) and first hyperpolarizabilities (b) are contributing most for merocyanine (DN1) molecule as its ground state dipole moment coincides along molecular X axis. According to choice of Cartesian coordinate axes for symmetrical ketocyanine (KD1) (c2v molecular symmetry), the dipole moment is aligned along the Y axis, so Y-component of first hyperpolarizability will be most important towards NLO responses. The first hyperpolarizability is a third rank tensor with 27 components that can be reduced to 10 components due to Kleinman symmetry [32]. Computational method mostly gives important hyperpolarizability components that would be interesting from the experimental point of view. 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,
1 hbi ¼ b2X þ b2Y þ b2Z 2 ;
bi ¼
1X ðb þ bjij þ bjji Þ 3 j¼1;3 ijj
ð4Þ
To interpret electric field induced second harmonic generation (EFISH) measured in experiment, the hyperpolarizability average required to compute is bl, known as the strength of hyperpolarizability in the direction of molecular ground state dipole moment and defined as,
bl ¼
X
li bi =jlj
ð5Þ
i¼1;3
We will address those two average values of static first hyperpolarizabilities for isolated and metal ion complexes of merocyanine and ketocyanine molecules. 3. Results and discussion 3.1. Optimized geometry in the ground state and optical absorption The optimized geometries of the above dyes (DN1, KD1) and their corresponding metal-ion complexes are shown in Figs. 2 and 3 respectively. The dyes and corresponding dye–metal ion complexes are planar as C–N–C–C dihedral angles are almost zero. KD1 has a symmetry plane passing through the carbonyl group with a dipole moment 5.25 D along the carbonyl axis. The effective dipole moment of the parent DN1 molecule is 6.74 D. As KD1 and DN1 have both oxygen and nitrogen centers, the metal ion can bind on either of these two centers. Earlier work has shown that metal ion binding on the oxygen of KD1 was to be lower in energy than
that binding on the nitrogen by nearly 20 kcal mol1 [22]. The cation-oxygen distances were obtained in vacuum and also in solvent for both DN1 and KD1 dye–metal ion complexes. The geometry optimizations have been done employing polarization continuum model implemented in Gaussian to incorporate the effect of solvent to obtain the cation-oxygen distance in solvent. In case of DN1 dye–metal complexes, the Li+–O, Ca2+–O and Mg2+–O distances were obtained to be 1.67 (1.85), 1.97 (2.22), and 1.92 (1.91) Å respectively in vacuum (solvent). Similarly for KD1 dye– metal complexes, Li+–O, Ca2+–O and Mg2+–O distances were obtained as 1.65 (1.81), 1.96 (2.17), and 1.85 (1.87) Å respectively in vacuum (solvent). For both DN1 and KD1 dye–metal complexes the Li+–O, Ca2+–O distances are expected to increase in presence of acetonitrile solvent as the polarization of the solvent stabilizes the free cations better than complex. So the interaction strength between the metal cation and the molecule become weaker hence cation-dye distance increases. On the other hand Mg2+–O distance shows almost no change in solvent medium for both dye–metal complexes. However for DN1–Mg2+ complex in vacuum Mg2+ cation inclines more towards the carbonyl bond and the angle associated with carbonyl bond (
Fig. 2. Optimized structures in the ground state of the merocyanine dye (DN1) (A), Li+–DN1 complex (B), Ca2+–DN1 complex (C), Mg2+–DN1 complex (D).
M. Das et al. / Journal of Molecular Structure 1033 (2013) 236–242
239
Fig. 3. Optimized structures in the ground state of the ketocyanine dye (KD1) (A), Li+–KD1 complex (B), Ca2+–KD1 complex (C), Mg2+–KD1 complex (D).
Table 1 The ground state optimized C–O bond lengths in carbonyl group along with carbon– carbon bond lengths involved in conjugation for pure merocyanine (DN1) and its metal ion complexes. All distances are in Å. Metal ion
C5–O
C1–C2
C2–C3
C3–C4
C4–C5
Pure Li+ Ca2+ Mg2+
1.22 1.27 1.33 1.31
1.48 1.42 1.39 1.39
1.35 1.37 1.40 1.41
1.43 1.40 1.38 1.37
1.36 1.38 1.40 1.41
Table 2 The ground state optimized C–O bond lengths in carbonyl group along with carbon– carbon bond lengths involved in conjugation for pure ketocyanine (KD1) and its metal ion complexes. All bond lengths are in Å. Metal ion
C5–O
C1–C2
C2–C3
C3–C4
C4–C5
Pure Li+ Ca2+ Mg2+
1.22 1.28 1.33 1.30
1.36 1.37 1.38 1.39
1.43 1.41 1.39 1.39
1.35 1.37 1.38 1.39
1.48 1.43 1.41 1.42
enolate forms 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 ‘enol’ form increases due to complex formation with metal ions. Position of the band maximum depends on the contribution of the enol form, a larger contribution of enol structure causes a shift of the band towards higher wave length. In an aprotic solvent like acetonitrile the actual structure is closer to the ‘keto’ form; on the other hand, the actual structure is closer to other extreme form namely the enol form in protic solvents like alcohols. A red1 shift of the HOMO to LUMO transition of the dye (equivalently, shortening of the HOMO–LUMO gap) is thus expected on complex formation. The calculated values of wave length corresponding to maximum absorption have been shown in Tables 3 and 4 for DN1 and KD1 respectively. The agreement with experimental results is good. Fig. 5 shows the lowest energy band of both merocyanine (DN1) and ketocyanice (KD1) dyes and their metal complexes in acetonitrile solution observed in experiment. The absorption spectra of the dye–metal ion complexes are characterized by a shoulder at higher wave length. The spectrum can be resolved into two Gaussian components. The lowest singlet transitions are tabulated in Table 3 and in Table 4 for merocyanine 1 For interpretation of color in Figs. 5 and 6, the reader is referred to the web version of this article.
and ketocyanine dye and their metal ion complexes respectively. The corresponding experimental data are shown for comparison. The lowest singlet excitation (S0–S1) with non-negligible intensity in free merocyanine dye (DN1) molecule corresponds to the HOMO to LUMO transition. The calculated excitation energy (440.2 nm) is well comparable with the absorption maximum (445 nm) obtained in experiment for DN1. Similarly for free ketocyanine dye, the calculated lowest singlet excitation energy (503.7 nm) is in very good agreement with absorption maxima obtained in experiment (503 nm). PCM description failed to give good results when applied to an alcohol, e.g., methanol. Thus for the dyes under TDDFT calculation using PCM, the values of wavelength corresponding to maximum absorption came as 489.4 and 439.4 nm for KD1 and DN1 respectively. Values differ widely from the experimental values of 525 and 463 nm respectively for the dyes. This is intelligible in view of the fact that methanol interacts with the dye through specific hydrogen bonding interaction (between carbonyl group and the –OH group of the solvent). This is not taken into account into a continuum model. To take H-bonding into account we optimized the dye–methanol hydrogen bonded structure and that under TDDFT calculation using PCM taking methanol as solvent (static and optical dielectric constant as 32.63 and 1.758 respectively) gave 502.6 and 444.6 nm as the wavelength of maximum absorption for KD1 and DN1 respectively. However the calculated excitation energies due to transition from HOMO to LUMO for Li+–DN1 (457.2 nm), Ca2+–DN1 (463.8 nm) and Mg2+–DN1 (489.1 nm) complexes are different from corresponding experimental observations (494, 492, 500 nm). The excitation energies calculated at TDDFT/6-311G(d,p) level of theory have also been reported in Table 3 for comparison. The HOMO– LUMO energy gap is 3.31 eV for free DN1 molecule and decreases to 2.82 eV (Li+–DN1), 1.33 eV (Ca2+–DN1), 1.30 eV (Mg2+–DN1) on complex formation as expected. On complex formation both HOMO and LUMO energies of free dye molecule decrease and dye complex formation results in the red shift of observed singlet transition in agreement with experimental data. The HOMO to LUMO singlet excitation energies (546.2 nm, 578.9 nm, 576.5 nm) in the complexes with Li+, Ca2+, Mg2+ ions respectively are also in good agreement with experimental data (540 nm, 570 nm, 580 nm) for ketocyanine (KD1). The diagrams of frontier orbitals contributing to the observed singlet transitions in calculation for free ketocyanine dye and its metal ion complexes are shown in Fig. 6. Like parent merocyanine, the HOMO–LUMO energy gap (3.03 eV) for free ketocyanine decreases to 2.32 eV, 0.45 eV, 0.47 eV for its Li+, Ca2+, Mg2+ dye complexes. Although we have correlated the singlet vertical transition energy with the position of the absorption maxima for
240
M. Das et al. / Journal of Molecular Structure 1033 (2013) 236–242
Fig. 4. ‘Keto’ and the zwitterionic ‘enol’ form of KD1 and DN1 dyes.
Table 3 Energies E (nm) of lowest singlet transitions, oscillator strengths f of the merocyanine dye molecule (DN1) and corresponding dye–metal ion complexes in acetonitrile solution. The calculated energy values at maximum absorption are compared with experimental observations. H and L stands for HOMO and LUMO orbital. The superscripts a and b indicate calculated energy values for 6311G++(d,p) and 6311G(d,p) basis sets respectively for comparison. Excitation DN1 Li+–DN1 Ca2+–DN1 Mg2+–DN1
H?L H?L H?L H?L
E (calculated) a
440.2 , 457.2a, 463.8a, 489.1a,
b
427.0 450.9b 456.8b 484.9b
E (experiment)
f
445 494 492 500
1.27 1.51 1.68 1.50
Table 4 Energies E (nm) of lowest singlet transitions, oscillator strengths f of the ketocyanine dye (KD1) molecule and corresponding dye–metal ion complexes in acetonitrile solution. The calculated energy values at maximum absorption are compared with experimental observations. H and L stand for HOMO and LUMO orbital respectively.
KD1 Li+–KD1 Ca2+–KD1 Mg2+–KD1
Excitation
E (calculated)
E (experiment)
f
H?L H?L H?L H?L
503.7 546.2 578.9 576.5
503 540 570 580
2.32 2.57 2.71 2.71
both dyes and their corresponding metal ion complexes, we would like to emphasize that in principle they can be compared approximately as the position of absorption maxima depends predominately on the vibronic structure of the absorption band. The exact shape of the absorption spectra can be simulated and compared with experimental data by employing Franck–Condon vibrational analysis performed in earlier studies [25,33].
3.2. Nonlinear polarizabilities At each optimized molecular geometry, static linear and first hyperpolarizability tensor components have been calculated using finite field approach within the framework of density functional theory employing B3LYP exchange–correlation functional using the same basis sets which have been used in the study of excitation spectra. Numerical derivatives have been used to obtain the NLO responses. Tables 5 and 6 summarizes the vector components of linear polarizabilities of merocyanine (DN1) and its metal-ion complexes and same for ketocyanine molecule and its metal ion complexes respectively. The linear polarizability is mostly contributed and dominated by aXX components for DN1 and KD1. We also observed a slight increase in the average value of a from isolated through singly to doubly charged metal ion complexes. Such observation holds good for both DN1 and KD1. The average value of linear polarizability increases about 12% from isolated DN1 to singly charged metal ion DN1 complex whereas for KD1 such increment
Fig. 5. Normalised absorption spectrum of the symmetrical dye (KD1) (A) and its parent unsymmetrical dye (DN1) (B) for the different metal ions in acetonitrile solvent.
is nearly 25%. The maximum increment is nearly 40% from singly to doubly charged metal ion complexes for DN1 and KD1 both. Tables 7 and 8 lists the tensor components of static first hyperpolarizability, average static first hyperpolarizabilty (hbi), vector hyperpolarizability along the dipole moment (bl) for DN1 and KD1 respectively. In case of DN1 and its metal ion complexes, the static first hyperpolarizability is mostly dominated by its tensor component along its molecular axis X. Therefore the vector component along the direction of ground state dipole moment (bl) is smaller than to average static first hyperpolarizabilty (hbi).
241
M. Das et al. / Journal of Molecular Structure 1033 (2013) 236–242
HOMO
LUMO
(A)
HOMO
LUMO
(B)
HOMO
LUMO
(C)
HOMO
LUMO
(D) Fig. 6. Plots of frontier orbitals for free KD1 dye (A), Li+–KD1 complex (B), Ca2+–KD1 complex (C), Mg2+–KD1 complex (D).
Table 5 The linear polarizabilitiy (1024 cm3) of parent merocyanine (DN1) and its metal ion complexes.
DN1 Li+–DN1 Ca2+–DN1 Mg2+–DN1
aXX
aYY
aZZ
h ai
85.34 101.64 113.83 152.05
36.40 37.41 41.52 44.10
19.37 18.88 18.75 21.07
47.04 52.64 58.04 72.39
Table 6 The linear polarizabilitiy (1024 cm3) of ketocyanine (KD1) and its metal ion complexes.
KD1 Li+–KD1 Ca2+–KD1 Mg2+–KD1
aXX
aYY
aZZ
h ai
151.31 205.81 257.85 293.39
46.12 47.62 121.70 93.21
21.97 22.16 24.11 25.90
73.14 91.86 134.56 137.50
The merocyanine D–A chromophore (DN1) and also its metal ion complexes show substantial first hyperpolarizability and thus can be considered to be good candidates for nonlinear optics
Table 7 The first hyperpolarizabilitiy (1030 esu1 cm5) of parent merocyanine (DN1) and its metal ion complexes.
DN1 Li+–DN1 Ca2+–DN1 Mg2+–DN1
bx
by
bz
hbi
bl
62.47 86.11 80.73 620.14
19.67 30.94 37.97 144.18
0.01 0.048 0.09 76.49
65.49 91.50 89.22 641.26
57.67 65.15 78.57 631.36
Table 8 The first hyperpolarizabilitiy (1030 esu1 cm5) of ketocyanine (KD1) and its metal ion complexes.
KD1 Li+–KD1 Ca2+–KD1 Mg2+–KD1
bx
by
bz
hbi
bl
0.04 0.08 0.01 0.13
47.46 87.18 332.33 167.56
0.05 0.02 0.17 0.06
47.46 87.18 332.33 167.56
47.29 87.03 332.23 167.07
applications. Table 7 exhibits Mg2+–DN1 complex would be most promising among them having comparatively larger values of static first hyperpolarizability. On the other hand for symmetric (C2v)
242
M. Das et al. / Journal of Molecular Structure 1033 (2013) 236–242
ketocyanine (KD1) with D–A–D chromophore and its metal ion complexes the ground state dipole moment is aligned along the carbonyl group or Y axis. Therefore the average static first hyperpolarizability (hbi) is nearly same as bl. The most significant observation is that the studied ketocyanine and its metal ion complexes possess significant first hyperpolarizability and the order of magnitude of hbi and bl are well comparable with those calculated for their parent merocyanine (DN1) with D–A chromophore. So KD1 dye and its metal ion complexes studied also possess significant nonlinear optical properties. Such results is unexpected in view of the fact that symmetric ketocyanines are quasi linear and would have an affinity with centrosymmetric D–A–D chromophore that have vanishing first hyperpolarizabilities. Earlier work on other symmetric ketocyanines has shown similar observation as the calculated first hyperpolarizabilties are quite significant in agreement with our calculations [29]. 4. Conclusions In our paper we have studied linear and nonlinear optical properties of symmetric ketocyanine, an example of D–A–D chromophore and its parent merocyanine (D–A) dyes. Besides isolated molecule we were also interested in studying the linear and nonlinear optical responses of metal ion complexes of those dyes. The electronic absorption spectra have been investigated for DN1 and KD1 dyes and their complexes within the framework of time-dependent density functional theory considering the effect from acetonitrile solvent using Polarizable Continuum Model. The calculated absorption maxima are well compared with experimental observations. We have also calculated the static linear and nonlinear optical responses of above dyes and their metal ion complexes using finite field approach based on density functional theory. The calculated static first hyperpolarizailities are quite significant for DN1 and its metal complexes and are considered to be promising for nonlinear optical applications. Interestingly the D–A–D symmetric ketocyanine dye and their metal ion complexes feature same order of magnitude of first hyperpolarizabities as that of their parent merocyanines in spite of the fact that quasi linear ketocyanines tend to have affinity with centrosymmetric D–A–D chromophores which have vanishing first hyperpolarizabilities. Our calculation on first hyperpolarizabilities strongly suggests that both isolated and metal ion complexes of merocyanine and ketocyanine studied above would find significant application in the field of nonlinear optoelectronics. It is worthwhile to mention that the practical application of NLO usually based on solid state devices. Although the dyes KD1 and DN1 can be isolated in the crystalline form, their complexes with the metal ions could not be isolated in crystalline form yet. In view of the enhanced NLO activity of the metal ion complexes of these dyes, isolation of these complexes would be an interesting future study. Acknowledgements MD acknowledges financial support from Department of Science and Technology, Government of India through a SERC
Fast-Track Grant SR/FT/CS-003/2010 and SKS and SB acknowledge financial support from UGC, India. References [1] (a) M.A. Kessler, O.S. Wolfbeis, Spectrochim. Acta Part A 47 (1991) 187; (b) P.K. Das, R. Pramanik, D. Banerjee, S. Bagchi, Spectrochim. Acta 56A (2000) 2763; (c) M. Shannigrahi, R. Pramanik, S. Bagchi, Spectrochim. Acta 59A (2003) 2921. [2] C. Reichardt, Chem. Rev. 94 (1994) 2319. [3] D. Banerjee, A.K. Laha, S. Bagchi, J. Photochem. Photobiol. A 85 (1995) 153. [4] D. Banerjee, S. Mondal, S. Ghosh, S. Bagchi, J. Photochem. Photobiol. A 90 (1995) 171. [5] D. Banerjee, S. Bagchi, J. Photochem. Photobiol. A 101 (1996) 57. [6] M. Shannigrahi, S. Bagchi, J. Photochem. Photobiol. A 168 (2004) 133. [7] M. Shannigrahi, S. Bagchi, J. Phys. Chem. B 108 (2004) 17703. [8] N. Marcotte, S. Fery-Forgues, J. Photochem. Photobiol. A 130 (2000) 13. [9] A.O. Doroshenko, V.G. Pivovarenko, J. Photochem. Photobiol. A 156 (2003) 5. [10] A.O. Doroshenko, M.D. Bilokin, V.G. Pivovarenko, J. Photochem. Photobiol. A 163 (2004) 95. [11] V.G. Pivovarenko, A.V. Klueva, A.O. Doroshenko, A.P. Demchenko, Chem. Phys. Lett. 325 (2000) 389. [12] A.O. Doroshenko, A.V. Grigorovich, E.A. Posokhov, V.G. Pivovarenko, A.P. Demchenko, J. Mol. Eng. 8 (1999) 199. [13] A.O. Doroshenko, L.B. Sychevskaya, A.V. Grygorovych, V.G. Pivovarenko, J. Fluoresc. 12 (2002) 451. [14] K. Rurack, M.L. Dekhtyar, J.L. Bricks, U. Resch-Genger, W. Rettig, J. Phys. Chem. A 103 (1999) 9626. [15] M.V. Barnabas, A. Liu, A.D. Trifanac, V.V. Krougaug, C.T. Chang, J. Phys. Chem. 96 (1992) 212. [16] W.J. Chambers, D.F. Eaton, J. Imaging Sci. 13 (1986) 230. [17] M.D. Baun, C.P. Henry, Ger. Offen. 2,133,315, January 13, 1972 (U.S. Patent Appl. 53,686, July 09, 1970). [18] N. Ray, J. Basu, M. Shannigrahi, S. Bagchi, Chem. Phys. Lett. 404 (2005) 63. [19] J. Basu, M. Shannigrahi, S. Bagchi, J. Phys. Chem. A 101 (2006) 9051. [20] J. Basu, M. Shannigrahi, S. Bagchi, Chem. Phys. Lett. 431 (2006) 278. [21] J. Basu, M. Shannigrahi, S. Bagchi, J. Phys. Chem. A 111 (2007) 7066. [22] S.K. Sardar, K. Srikanth, S. Bagchi, J. Phys. Chem. A 114 (2010) 10388. [23] A. Eilmes, J. Mol. Struct. Theochem. 915 (2009) 141–148. [24] A. Eilmes, Theor. Chem. Acc. 127 (2010) 743. [25] A. Eilmes, Comp. Theor. Chem. 972 (2011) 32. [26] J. Zyss, D. Chemla, Non Linear Optical Properties of Organic Molecules and Crystals, Academic Press, New York, 1987. [27] P.N. Prasad, D.J. Williams, Introduction to Nonlinear Optical Effects in Molecules and Polymers, Wiley, New York, 1991. [28] J.-L. Bredas, C. Adant, P.C.E. Tackx, A. Persoons, B.M. Pierce, Chem. Rev. 94 (1994) 243. [29] G. Ponterini, D. Vanossi, A.Z. Krasnaya, A.S. Tatikolov, F. Momicchioli, Phys. Chem. Chem. Phys. 13 (2011) 9507. [30] G. Ponterini, D. Vanossi, F. Momicchioli, Phys. Chem. Chem. Phys. 14 (2012) 4171. [31] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian, Inc., Wallingford, CT, 2010. [32] D.A. Kleinman, Phys. Rev. 1962 (1977) 126. [33] M. Guillaume, V. Liégeois, B. Champagne, F. Zutterman, Chem. Phys. Lett. 446 (2007) 165.