Electronic Spectral Shifts Research Articles

Blue- and Red-Shifting Hydrogen Bonding: A Gas Phase FTIR and Ab Initio Study of RR'CO···DCCl3 and RR'S···DCCl3 Complexes.

Blue-shifting H-bonded (C-D···O) complexes between CDCl3 and CH3HCO, (CH3)2CO, and C2H5(CH3)CO, and red-shifting H-bonded (C-D···S) complexes between CDCl3 with (CH3)2S and (C2H5)2S have been identified by Fourier transform infrared spectroscopy in the gas phase at room temperature. With increasing partial pressure of the components, a new band appears in the C-D stretching region of the vibrational spectra. The intensity of this band decreases with an increase in temperature at constant pressure, which provides the basis for identification of the H-bonded bands in the spectrum. The C-D stretching frequency of CDCl3 is blue-shifted by +7.1, +4, and +3.2 cm-1 upon complexation with CH3HCO, (CH3)2CO, and C2H5(CH3)CO, respectively, and red-shifted by -14 and -19.2 cm-1 upon complexation with (CH3)2S and (C2H5)2S, respectively. By using quantum chemical calculations at the MP2/6-311++G** level, we predict the geometry, electronic structural parameters, binding energy, and spectral shift of H-bonded complexes between CDCl3 and two series of compounds named RCOR' (H2CO, CH3HCO, (CH3)2CO, and C2H5(CH3)CO) and RSR' (H2S, CH3HS, (CH3)2S, and (C2H5)2S) series. The calculated and observed spectral shifts follow the same trends. With an increase in basicity of the H-bond acceptor, the C-D bond length increases, force constant decreases, and the frequency shifts to the red from the blue. The potential energy scans of the above complexes are done, which show that electrostatic attraction between electropositive D and electron-rich O/S causes bond elongation and red shift, and the electronic and nuclear repulsions lead to bond contraction and blue shifts. The dominance of the two opposing forces at the equilibrium geometry of the complex determines the nature of the shift, which changes both in magnitude and in direction with the basicity of the hydrogen-bond acceptor.

Accurate dissociation energies of two isomers of the 1-naphthol⋅cyclopropane complex.

The 1-naphthol⋅cyclopropane intermolecular complex is formed in a supersonic jet and investigated by resonant two-photon ionization (R2PI) spectroscopy, UV holeburning, and stimulated emission pumping (SEP)-R2PI spectroscopy. Two very different structure types are inferred from the vibronic spectra and calculations. In the "edge" isomer, the OH group of 1-naphthol is directed towards a C-C bond of cyclopropane, the two ring planes are perpendicular. In the "face" isomer, the cyclopropane is adsorbed on one of the π-aromatic faces of the 1-naphthol moiety, the ring planes are nearly parallel. Accurate ground-state intermolecular dissociation energies D0 were measured with the SEP-R2PI technique. The D0(S0) of the edge isomer is bracketed as 15.35 ± 0.03 kJ/mol, while that of the face isomer is 16.96 ± 0.12 kJ/mol. The corresponding excited-state dissociation energies D0(S1) were evaluated using the respective electronic spectral shifts. Despite the D0(S0) difference of 1.6 kJ/mol, both isomers are observed in the jet in similar concentrations, so they must be separated by substantial potential energy barriers. Intermolecular binding energies, De, and dissociation energies, D0, calculated with correlated wave function methods and two dispersion-corrected density-functional methods are evaluated in the context of these results. The density functional calculations suggest that the face isomer is bound solely by dispersion interactions. Binding of the edge isomer is also dominated by dispersion, which makes up two thirds of the total binding energy.

Structure and Intermolecular Vibrations of Perylene·trans-1,2-Dichloroethene, a Weak Charge-Transfer Complex

The vibronic spectra of strong charge-transfer complexes are often congested or diffuse and therefore difficult to analyze. We present the spectra of the π-stacked complex perylene trans-1,2-dichloroethene, which is in the limit of weak charge transfer, the electronic excitation remaining largely confined to the perylene moiety. The complex is formed in a supersonic jet, and its S0 ↔ S1 spectra are investigated by two-color resonant two-photon ionization (2C-R2PI) and fluorescence spectroscopies. Under optimized conditions, vibrationally cold (T(vib) ≈ 9 K) and well resolved spectra are obtained. These are dominated by vibrational progressions in the “hindered-rotation” Rc intermolecular vibration with very low frequencies of 11 (S0) and 13 cm(–1) (S1). The intermolecular Tz stretch and the Ra and Rb bend vibrations are also observed. The normally symmetry-forbidden intramolecular 1a(u) “twisting” vibration of perylene also appears, showing that the π- stacking interaction deforms the perylene moiety, lowering its local symmetry from D2h to D2. We calculate the structure and vibrations of this complex using six different density functional theory (DFT) methods (CAM-B3LYP, BH&HLYP, B97-D3, ωB97X-D, M06, and M06-2X) and compare the results to those calculated by correlated wave function methods (SCS-MP2 and SCS-CC2). The structures and vibrational frequencies predicted with the CAM-B3LYP and BH&HLYP methods disagree with the other calculations and with experiment. The other four DFT and the ab initio methods all predict a π-stacked “centered” structure with nearly coplanar perylene and dichloroethene moieties and intermolecular binding energies of D(e) = −20.8 to −26.1 kJ/mol. The 000 band of the S0 → S1 transition is red-shifted by δν = −301 cm(–1) relative to that of perylene, implying that the D(e) increases by 3.6 kJ/mol or 15% upon electronic excitation. The intermolecular vibrational frequencies are assigned to the calculated Rc, Tz, Ra, and Rb vibrations by comparing to the observed/calculated frequencies and S0 ↔ S1 Franck–Condon factors. Of the three TD-DFT methods tested, the hybrid-meta-GGA functional M06-2X shows the best agreement with the experimental electronic transition energies, spectral shifts, and vibronic spectra, closely followed by the ωB97X-D functional, while the M06 functional gives inferior results.

Response to “Comment on ‘Solvatochromic shifts of polar and non-polar molecules in ambient and supercritical water: A sequential quantum mechanics/molecular mechanics study including solute-solvent electron exchange-correlation”’ [J. Chem. Phys. 138, 217101 (2013)

In this response to Schwabe's recent comment [J. Chem. Phys. 138, 217101 (2013)10.1063/1.4807839], we discuss the validity of Schwabe's interpretation of why a large quantum mechanics (QM) region is needed to converge the quantum mechanics/molecular mechanics (QM/MM) results for aqueous benzene, which he ascribed to our insufficient electrostatic potential or neglect of polarization effect. It is shown that improving the electrostatic potential with ground-state polarizable effective fragment potential and fragment molecular orbital methods instead of simple point charge embedding still deviates much from the experimental determinations for aqueous benzene, and solvent polarization in response to the solute excitation for such a system is also very small. We then resuggest enlarging the QM region size or incorporating new exchange repulsion potentials in QM/MM calculations to account for exchange interaction between a solute and its nearby solvents for the highly accurate electronic spectral shift calculations of non-polar solutes dissolved in water.

Time-dependent density functional theory study on the electronic excited-state hydrogen bonding dynamics of BHC-nicotinamide in MeOH solution

The density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods were performed to investigate the electronic excited-state hydrogen bonding dynamics of the hydrogenbonded complex formed by BHC-nicotinamide (BHCN) and methanol (MeOH). The ground-state geometry optimizations, electronic transition energies, corresponding oscillation strengths of the low-lying electronically excited states, and the optimized S1 excited-state geometry for the isolated BHCN and MeOH monomers, the hydrogen-bonded BHCN–MeOH dimers, and BHCN–2MeOH trimer complexes have been calculated by using the DFT and TDDFT methods, respectively. We have demonstrated that the intermolecular hydrogen bond C10=O14···H40−O39−Me is weaker than C16=O17···H46−O45−Me in the hydrogen-bonded dimers and trimer no matter whether in the ground state or the excited state. In addition, our results are consistent with the relationship between the electronic spectral shifts and excited-state hydrogen bonding dynamics: hydrogen bond strengthening can induce the relative electronic spectra redshift, whereas a blueshift will be induced. In addition, we focused our attention on the frontier molecular orbital and the results could reasonably explain the hydrogen bond strengthening or weakening mechanism.

A Theoretical Study on Electronically Excited States of the Hydrogen-Bonded Clusters for Fluorenone and Fluorenone Derivatives in Methanol Solvent

The time-dependent density functional theory (TDDFT) method has been carried out to study the hydrogen-bonding of fluorenone (FN) and FN derivatives (FODs) in hydrogen-donating methanol solvent. The ground-state geometry structure optimizations, electronic excitation energies and corresponding oscillation strengths of the low-lying electronically excited states for the isolated FN, FODs and methanol monomers and their corresponding complexes have been calculated using DFT and TDDFT methods respectively. Comparing FODs with FN, we have obtained the strength change of the hydrogen bonds and the electronic spectral shift in different excited states. At the same time, the nature of the FODs in the electronic excited states and the influence of the different substituent group have been summed up.

Effect of Substituents on Electronic Spectral Shifts and Phase Stability of Liquid Crystalline Biphenylcyclohexane Molecules—A Theoretical Approach

The structures of liquid crystalline disubstituted biphenylcyclohexanes (BCHs) of the general formula R-C6H10-C6H4-C6H4-X with R: C3H7; X: H (BCH30), and R: C5H11; X: CN (BCH5CN) have been optimized using the density function B3LYP with 6–31+G (d) basis set using crystallographic geometry as input. Using the optimized geometry, electronic structures of the molecules have been evaluated using the density functional theory (DFT) calculations. Mulliken atomic charges for each molecule have been compared with Loewdin atomic charges to understand the molecular charge distribution and phase stability. The electronic absorption spectra and circular dichroism (CD) spectra of BCH molecules have been simulated by employing the DFT method. The excited states have been calculated via configuration interaction single level (CIS) with semiempirical Hamiltonian ZINDO. Two types of calculations have been performed for model systems containing single and double molecules of BCHs. Further, the spectra have been reported for all single molecules, and the ultraviolet (UV) stability of the molecules has been discussed. The stacked and in-plane dimer complexes have also been reported to enable the comparison between single and double molecules.

Excited-state hydrogen-bonding dynamics of trans-acetanilide in an aqueous environment: a theoretical study

In this work, the time-dependent density functional theory (TDDFT) method was carried out to study the hydrogen-bonding dynamics in both singlet and triplet excited states of trans-acetanilide (AA)–(H2O) complexes formed by trans-acetanilide in a water (H2O) solvent. The ground-state geometric structure optimisations were calculated by the density functional theory method, but the electronic excitation energies and corresponding oscillation strengths of the low-lying electronically excited states for isolated AA, H2O monomers and hydrogen-bonded AA(CO)–(H2O)1, AA(NH)–(H2O)1 dimers as well as the dihydrogen-bonded AA–(H2O)2 trimer were calculated by the TDDFT method. In our system, the intermolecular hydrogen bonds C = O…H–O and N–H…O–H can also be formed. From the TDDFT results, we depicted the changes of different hydrogen-bonded complexes in various electronic excited states. According to Zhao's rule on the relationship between the electronic spectral shift and excited-state hydrogen-bonding dynamics, hydrogen bond strengthening can bring the relative electronic spectra redshift, whereas hydrogen bond weakening can make the corresponding electronic spectra shift to blue. In addition, we also discussed the frontier molecular orbitals and the electron density transition.

A theoretical forecast of the hydrogen bond changes in the electronic excited state for BN and its derivatives

Abstract The relationship between electronic spectral shifts and hydrogen-bonding dynamics in electronically excited states of the hydrogen-bonded complex is put forward. Hydrogen bond strengthening will induce a redshift of the corresponding electronic spectra, while hydrogen bond weakening will cause a blueshift. Time-dependent density function theory (TDDFT) was used to study the excitation energies in both singlet and triplet electronically excited states of Benzonitrile (BN), 4-aminobenzonitrile (ABN), and 4-dimethylaminobenzonitrile (DMABN) in methanol solvents. Only the intermolecular hydrogen bond C≡N...H-O was involved in our system. A fairly accurate forecast of the hydrogen bond changes in lowlying electronically excited states were presented in light of a very thorough consideration of their related electronic spectra. The deduction we used to depict the trend of the hydrogen bond changes in excited states could help others understand hydrogen-bonding dynamics more effectively.

Experimental and quantum chemical studies of the electronic absorption spectra of pyrimidine derivatives

The electronic absorption spectra of different pyrimidine derivatives have been measured experimentally and calculated theoretically by the PPP and CNDO/S methods. These pyrimidine derivatives are: 4,6-dichloro-pyrimidine (I), 4,6-dichloro, 5-amino-pyrimidine (II), 2,4,6-trichloro-pyrimidine (III), 4,6-dihxdroxy-pyrimidine (IV), 4,6-dihxdroxy-5-nitro-pyrimidine (V), 2,4-diamino-pyrimidine (VI), 2,4-diamino-6-hydroxy-pyrimidine (VII), 2,4-dihydroxy-5-carboxy-pyrimidine (VIII), 2,4-dimethyl-6-hydroxy-pyrimidine (IX), 5-nitro-uracil (X), and orotic acid (XI). The observed electronic spectral shifts are quantitatively analyzed in relation to different solute–solvent interaction mechanisms. The effects of solvent polarity and hydrogen bonding on the spectra are discussed in the light of theoretical predictions. This comparative analysis provides a reasonable picture of the solvent effects on the absorption spectral properties of pyrimidine nucleobases.

Hydrogen Bonding in the Electronic Excited State

Because of its fundamental importance in many branches of science, hydrogen bonding is a subject of intense contemporary research interest. The physical and chemical properties of hydrogen bonds in the ground state have been widely studied both experimentally and theoretically by chemists, physicists, and biologists. However, hydrogen bonding in the electronic excited state, which plays an important role in many photophysical processes and photochemical reactions, has scarcely been investigated. Upon electronic excitation of hydrogen-bonded systems by light, the hydrogen donor and acceptor molecules must reorganize in the electronic excited state because of the significant charge distribution difference between the different electronic states. The electronic excited-state hydrogen-bonding dynamics, which are predominantly determined by the vibrational motions of the hydrogen donor and acceptor groups, generally occur on ultrafast time scales of hundreds of femtoseconds. As a result, state-of-the-art femtosecond time-resolved vibrational spectroscopy is used to directly monitor the ultrafast dynamical behavior of hydrogen bonds in the electronic excited state. It is important to note that the excited-state hydrogen-bonding dynamics are coupled to the electronic excitation. Fortunately, the combination of femtosecond time-resolved spectroscopy and accurate quantum chemistry calculations of excited states resolves this issue in laser experiments. Through a comparison of the hydrogen-bonded complex to the separated hydrogen donor or acceptor in ground and electronic excited states, the excited-state hydrogen-bonding structure and dynamics have been obtained. Moreover, we have also demonstrated the importance of hydrogen bonding in many photophysical processes and photochemical reactions. In this Account, we review our recent advances in electronic excited-state hydrogen-bonding dynamics and the significant role of electronic excited-state hydrogen bonding on internal conversion (IC), electronic spectral shifts (ESS), photoinduced electron transfer (PET), fluorescence quenching (FQ), intramolecular charge transfer (ICT), and metal-to-ligand charge transfer (MLCT). The combination of various spectroscopic experiments with theoretical calculations has led to tremendous progress in excited-state hydrogen-bonding research. We first demonstrated that the intermolecular hydrogen bond in the electronic excited state is greatly strengthened for coumarin chromophores and weakened for thiocarbonyl chromophores. We have also clarified that the intermolecular hydrogen-bond strengthening and weakening correspond to red-shifts and blue-shifts, respectively, in the electronic spectra. Moreover, radiationless deactivations (via IC, PET, ICT, MLCT, and so on) can be dramatically influenced through the regulation of electronic states by hydrogen-bonding interactions. Consequently, the fluorescence of chromophores in hydrogen-bonded surroundings is quenched or enhanced by hydrogen bonds. Our research expands our understanding of the nature of hydrogen bonding by delineating the interaction between hydrogen bonds and photons, thereby providing a basis for excited-state hydrogen bonding studies in photophysics, photochemistry, and photobiology.

A DFT/TDDFT study of hydrogen bonding interactions between resorufin anion and water molecules in the excited state

In order to explore the hydrogen bonding interactions between resorufin anion (denoted as Res − in this paper) and water molecules, we have calculated the geometric structures as well as the total energies of the various hydrogen-bonded Res −–water complexes composed based on the atomic charges obtained from the NBO analysis. We found that there are four sites (O 1–3, N 1) within the Res − molecule through which intermolecular hydrogen bonds can be formed with water molecule. Furthermore, by comparing the hydrogen bond lengths and the relative energies of the four singly hydrogen-bonded complexes Res −–H 2O, we found that hydrogen bonds formed at the two carbonyl oxygen atoms (O 2 and O 3) are stronger than those formed at heteroatom O 1 and N 1 which can be reasonably predicted by the highest negative charge localized on the two former atoms. Moreover, as the number of the water molecules hydrogen-bonded to the Res − increases, the hydrogen-bonded complex Res −–water becomes more stable although the strength of each hydrogen bond decreases. In addition, the electronic spectra of the various hydrogen-bonded Res −–water complexes as well as the Res − monomer are also calculated. We found that, compared with that of the Res − monomer, the excitation spectra of the hydrogen-bonded Res −–water complexes are mostly blue-shifted in the S 2 and S 3 states while they are all red-shifted in the higher excited states, especially in states S 10, S 11 and S 12. According to the relationship between electronic spectral shifts and electronic excited-state hydrogen-bonding dynamics first clarified by Zhao and coworkers [51], we demonstrate that, for all the hydrogen-bonded Res −–water complexes, most of the hydrogen bonds are weakened in the S 2 and S 3 states while they are all strengthened in excited states S 10, S 11 and S 12.

Excited-state hydrogen bonding and deprotonation of esculetin in solution: A DFT/TDDFT study

The combined density functional theory (DFT) and time-dependent density functional theory (TDDFT) method was used to study the electronic spectral properties of different deprotonated forms of esculetin. By comparing the experimental absorption and fluorescence bands with the calculated electronic spectra, it is evidently demonstrated that the minor absorption and fluorescence bands observed at slightly longer wavelengths than the principal bands in experiments are predominantly from the de-H3 form of the esculetin monomer. Furthermore, we clarified the relationship between electronic spectral shifts and electronic excited-state intramolecular hydrogen bonding changes: the strengthening of intramolecular hydrogen bond can induce an electronic spectral blueshift while the intramolecular hydrogen bond weakening can result in an electronic spectral redshift.

Vibronic Spectra of Jet-Cooled 2-Aminopurine·H2O Clusters Studied by UV Resonant Two-Photon Ionization Spectroscopy and Quantum Chemical Calculations

For understanding the major- and minor-groove hydration patterns of DNAs and RNAs, it is important to understand the local solvation of individual nucleobases at the molecular level. We have investigated the 2-aminopurine·H(2)O monohydrate by two-color resonant two-photon ionization and UV/UV hole-burning spectroscopies, which reveal two isomers, denoted A and B. The electronic spectral shift δν of the S(1) ← S(0) transition relative to bare 9H-2-aminopurine (9H-2AP) is small for isomer A (-70 cm(-1)), while that of isomer B is much larger (δν = -889 cm(-1)). B3LYP geometry optimizations with the TZVP basis set predict four cluster isomers, of which three are doubly H-bonded, with H(2)O acting as an acceptor to a N-H or -NH2 group and as a donor to either of the pyrimidine N sites. The "sugar-edge" isomer A is calculated to be the most stable form with binding energy D(e) = 56.4 kJ/mol. Isomers B and C are H-bonded between the -NH2 group and pyrimidine moieties and are 2.5 and 6.9 kJ/mol less stable, respectively. Time-dependent (TD) B3LYP/TZVP calculations predict the adiabatic energies of the lowest (1)ππ* states of A and B in excellent agreement with the observed 0(0)(0) bands; also, the relative intensities of the A and B origin bands agree well with the calculated S(0) state relative energies. This allows unequivocal identification of the isomers. The R2PI spectra of 9H-2AP and of isomer A exhibit intense low-frequency out-of-plane overtone and combination bands, which is interpreted as a coupling of the optically excited (1)ππ* state to the lower-lying (1)nπ* dark state. In contrast, these overtone and combination bands are much weaker for isomer B, implying that the (1)ππ* state of B is planar and decoupled from the (1)nπ* state. These observations agree with the calculations, which predict the (1)nπ* above the (1)ππ* state for isomer B but below the (1)ππ* for both 9H-2AP and isomer A.

Coordination equilibria in the complex formation of guanylurea with CuII: Formation and stability of binary CuII-guanylurea and ternary CuII-guanylurea-glycinate complexes

Combined pH-metric and spectrophotometric investigations on the complex formation equilibria of CuII with guanylurea (H2 1NC(=O) 2NH.C(=3NH) 4NH2), hereafter, GuH, in the absence and in the presence of glycine (GlyH), in aqueous solution indicates variety of binary and mixed-ligand complexes: Cu(Gu)+, Cu(Gu)(OH); Cu(Gu)2, Cu(Gu-H)(Gu)−, Cu(Gu-H) 2−2 , Cu(Gu-H)(Gu-2H)3−; Cu(Gly)+, Cu(Gly)(OH); Cu(Gly)(Gu); Cu(Gly)(Gu-H)−, Cu(Gly)(Gu-2H)2−; (Gly)Cu(Gu)Cu(Gly)+, (Gly)Cu(Gu-H)Cu(Gly) and (Gly)Cu(Gu-2H)Cu(Gly)−. At pH < 6, guanylurea anion (Gu−) acts as a [(C=O), 3N−] or [=]NH, 3N−] bidentate ligand and above pH 7 it is transformed through a coordination equilibrium into a (=1N−, =3N−) bidentate ligand, similar to biguanide dianion. Occurrence of dinuclear complex species, (Gly) Cu(Gu)Cu(Gly)+, in the complexation equilibria, indicates bridging double bidentate [(1NH2, 3N−), (C=O, 4NH2)] and/or [(1NH2, 4NH2), (C=O, 3N−)] chelation by Gu− ion in an isomeric equilibrium. Above pH 6·5, the dinuclear complex decomposes mostly to the mononuclear species, Cu(Gly)(OH) and Cu(Gu)(OH) and only partly deprotonates to (Gly)Cu(Gu-H)Cu(Gly) and (Gly)Cu(Gu-2H)Cu(Gly)−. Electronic spectral shifts, with change of pH have been correlated with the possible modes of coordination of guanylurea species.

Multiple solvation configurations around phthalocyanine in helium droplets

Recent measurements of the emission spectrum of phthalocyanine solvated in superfluid helium nanodroplets exhibit a constant 10.3 cm(-1) splitting of each emission line relative to the absorption spectrum. This splitting has been attributed to two distinct helium environments near the surface of the phthalocyanine molecule. Rigid-body path-integral Monte Carlo provides a means of investigating the origin of the splitting on a detailed microscopic level. Path-integral Monte Carlo simulations of 4He(N)-phthalocyanine at 0.625 K with N ranging from 24 to 150 show two distinct helium configurations. One configuration is commensurate with the molecular substrate and the other is a triangular lattice. We investigate the energetics of these two configurations and use a method for calculating electronic spectral shifts for aromatic molecule-rare-gas clusters due to dispersive interactions to estimate the spectral splitting that would arise from the two helium configurations seen for N=150. The results are in reasonable agreement with the experimentally measured splitting, supporting the existence of two distinct local helium environments near the surface of the molecule in the nanodroplets.

Rotational Dynamics of Coumarin 153 in Supercritical Fluoroform

Subpicosecond fluorescence anisotropy decay curves have been measured using the fluorescence up-conversion technique to examine the rotational dynamics of coumarin 153 (C 153) in supercritical fluoroform (T = 302 and 310 K). For reduced densities (p, = ρ/ρ c ) above 0.9, the rotation times of C153 increase with density. For reduced densities lower than 0.9, the rotation times increase with decreasing density and has a maximum at a density near p, = 0.5. The comparison with the extrapolation of the data in polar aprotic solvents indicates that the local solvent density around the solute exceeds about 4 times the bulk density at a density near ρ r = 0.5, which is consistent with those obtained from the steady-state electronic spectral shifts.

Electric polarization effects on the electronic spectral shift of centrosymmetric compounds

The classic dielectric dipolar Onsager model was extended to include quadrupolar interactions between solute molecules and solvents with different polarities. A multiparametric solvatochromic expression, based on the point quadrupole moment inside a spherical cavity embedded in a dielectric continuum, is applied to centrosymmetric sulfonamide porphyrins, zinc tetraphenyl porphyrin, squaraine and 9,10-dicyanoanthracene, in order to account for the quadrupolar polarization effect of solute molecules. The reaction field polarity functions created respectively by dipole and quadrupole moments are compared and found to be linearly correlated.

MOLECULAR AGGREGATION IN A HEMICYANINE DYE: MODELING BY A COMBINED CRYSTALLOGRAPHIC AND COMPUTATIONAL APPROACH

Molecular aggregation in hemicyanine dye molecules, a problem of fundamental relevance to their linear and nonlinear optical properties, is addressed through a novel approach based on crystal structure investigation combined with semiempirical quantum chemical computations. Crystal structure of the hemicyanine salt N-n-butyl-4-[2-(4-dimethylaminophenyl) ethenyl] pyridinium bromide is investigated. The electronic absorption spectra of this compound in the solid state and in solution are modeled using semiempirical AM1/CI computations on the molecule and its dimers extracted from the crystal lattice; the molecular environment is mimicked by invoking a solvation model. This approach is shown to provide insight into the electronic absorption spectral shifts reported earlier for LB films of amphiphiles based on these chromophores and should prove useful in guiding further efforts at achieving deaggregation in these LB films.

MOLECULAR AGGREGATION IN A HEMICYANINE DYE: MODELING BY A COMBINED CRYSTALLOGRAPHIC AND COMPUTATIONAL APPROACH

Molecular aggregation in hemicyanine dye molecules, a problem of fundamental relevance to their linear and nonlinear optical properties, is addressed through a novel approach based on crystal structure investigation combined with semiempirical quantum chemical computations. Crystal structure of the hemicyanine salt N-n-butyl-4-[2-(4-dimethylaminophenyl) ethenyl] pyridinium bromide is investigated. The electronic absorption spectra of this compound in the solid state and in solution are modeled using semiempirical AM1/CI computations on the molecule and its dimers extracted from the crystal lattice; the molecular environment is mimicked by invoking a solvation model. This approach is shown to provide insight into the electronic absorption spectral shifts reported earlier for LB films of amphiphiles based on these chromophores and should prove useful in guiding further efforts at achieving deaggregation in these LB films.