Intermolecular Vibronic Coupling Research Articles

Solute–solvent intermolecular vibronic coupling as manifested by the molecular near-field effect in resonance hyper-Raman scattering

Vibronic coupling within the excited electronic manifold of the solute all-trans-β-carotene through the vibrational motions of the solvent cyclohexane is shown to manifest as the "molecular near-field effect," in which the solvent hyper-Raman bands are subject to marked intensity enhancements under the presence of all-trans-β-carotene. The resonance hyper-Raman excitation profiles of the enhanced solvent bands exhibit similar peaks to those of the solute bands in the wavenumber region of 21,700-25,000 cm(-1) (10,850-12,500 cm(-1) in the hyper-Raman exciting wavenumber), where the solute all-trans-β-carotene shows a strong absorption assigned to the 1A(g) → 1B(u) transition. This fact indicates that the solvent hyper-Raman bands gain their intensities through resonances with the electronic states of the solute. The observed excitation profiles are quantitatively analyzed and are successfully accounted for by an extended vibronic theory of resonance hyper-Raman scattering that incorporates the vibronic coupling within the excited electronic manifold of all-trans-β-carotene through the vibrational motions of cyclohexane. It is shown that the major resonance arises from the B-term (vibronic) coupling between the first excited vibrational level (v = 1) of the 1B(u) state and the ground vibrational level (v = 0) of a nearby A(g) state through ungerade vibrational modes of both the solute and the solvent molecules. The inversion symmetry of the solute all-trans-β-carotene is preserved, suggesting the weak perturbative nature of the solute-solvent interaction in the molecular near-field effect. The present study introduces a new concept, "intermolecular vibronic coupling," which may provide an experimentally accessible∕theoretically tractable model for understanding weak solute-solvent interactions in liquid.

Intensity enhancement and selective detection of proximate solvent molecules by molecular near-field effect in resonance hyper-Raman scattering

A new molecular phenomenon associated with resonance hyper-Raman (HR) scattering in solution has been discovered. Resonance HR spectra of all-trans-beta-carotene and all-trans-lycopene in various solvents exhibited several extra bands that were not assignable to the solute but were unequivocally assigned to the solvents. Neat solvents did not show detectable HR signals under the same experimental conditions. Similar experiments with all-trans-retinal did not exhibit such enhancement either. All-trans-beta-carotene and all-trans-lycopene have thus been shown to induce enhanced HR scattering of solvent molecules through a novel molecular effect that is not associated with all-trans-retinal. We call this new effect the "molecular near-field effect." In order to explain this newly found effect, an extended vibronic theory of resonance HR scattering is developed where the vibronic interaction including the proximate solvent molecule (intermolecular vibronic coupling) is explicitly introduced in the solute hyperpolarizability tensor. The potential of "molecular near-field HR spectroscopy," which selectively detects molecules existing in the close vicinity of a HR probe in complex chemical or biological systems, is discussed.

Solid-state optical properties of linear polyconjugated molecules: π-stack contra herringbone

The intermolecular arrangement in the solid state and the consequences on the optical and photophysical properties are studied on different derivatives of oligophenylenevinylenes by UV/VIS absorption and angular-resolved polarized fluorescence spectroscopy. Unsubstituted distyrylbenzene (DSB) organizes in a herringbone manner, with the long axes of the molecules oriented in parallel, but the short axes almost perpendicular to each other. Fluorinated distyrylbenzene (F(12)DSB) as well as the DSB:F(12)DSB cocrystals prefer cofacial pi-stacking in the solid state. For all structures, the consequence of the parallel alignment of the transition moments is a strongly blueshifted H-type absorption spectrum and a low radiative rate constant k(F). Significant differences are observed for the emission spectra: the perpendicular arrangement of the short axes in DSB crystals leads to only very weak intermolecular vibronic coupling. Hence the emission spectrum is well structured, very similar to the one in solution. For F(12)DSB and DSB:F(12)DSB, the cofacial arrangement of the adjacent molecules enables strong intermolecular vibronic coupling of adjacent molecules. Thus, an unstructured and strongly redshifted excimerlike emission spectrum is observed. The differences in the electronic nature of the excited states are highlighted by quantum-chemical calculations, revealing the contribution of interchain excitations to the electronic transitions.

Modeling of the Optical Properties of Cofacial Chromophore Pairs: Stilbenophane

Using electronic absorption and fluorescence spectroscopic techniques, as well as quantum chemical calculations, we have studied the electronic spectra of thia-bridged stilbenophane (TSP) with close cofacial contact of two trans-stilbene (t-SB) units. Compared to the t-SB monomer, the experimental consequences of the cofacial arrangement are (i) a splitting of the main absorption band with a weakly allowed emitting state, and (ii) a strongly red-shifted, unstructured emission spectrum with long fluorescence decay times. According to the theoretical investigations, the two t-SB units are strongly bent in the electronic ground state (So), because of repulsive π-π overlap. In the first excited state (S 1 ), the t-SB units become almost planar, because of attractive π*-π* overlap. As a consequence, the symmetry-forbidden S 0 ↔ S 1 transition couples strongly to interchromophore breathing modes of low frequency (v 1 = 67 cm - 1 , v 2 = 117 cm - 1 ), yielding structureless spectra with large Stokes shifts. The features of the calculated spectra are in good agreement with the experimental data. The results indicate that strong intermolecular vibronic coupling is also responsible for excimer-like emission in organic molecular crystals of cofacially arranged molecules. Furthermore, the different geometries in the S 0 and S 1 states of TSP give evidence for the mechanism of [2+2]photodimerization of t-SB in solutions.

Luminescence of Conjugated Molecules Confined in Nanochannels

The optical properties, molecular orientations and host-guest ratios of a series of p,p′-disubstituted oligophenylenevinylenes (OPVs), incorporated in the nanochannels of perhydrotriphenylene, are investigated by polarized fluorescence and UV absorption spectroscopy. The spectral positions and fine-structures of the fluorescence bands are very similar to those obtained in solution, thus indicating only weak interactions within the channels. In three-dimensional arrangements, the spectra are additionally red-shifted due to an increase of intermolecular interactions. Strongly red-shifted emission spectra due to intermolecular vibronic coupling are observed for films and single crystals of p,p′-dicyano-distyrylbenzene with π,π-stacking of the molecules.

Relationships Between the Electron-Phonon Coupling and the Elastic Properties of Polymer Matrices: A Hole-Burning Study

Abstract Spectral holes have been investigated between 7 and 50K on tetra-tert-butyl-tetra-azaporphine(t-Bu-TAP) embedded in a number of polymers. The maximum position(vph) of the pseudo wing and the temperature dependence of the quasi-homogeneous hole width(Γqh) have been measured and compared to the known values of the Young's modulus(E') of the matrix. We found that in polymers with larger E' the vph is higher and the hole width is smaller. The influence of the dynamic mechanical properties as well as the polarity and hydrogen bonding on intermolecular vibronic coupling is discussed.

Effect of intermolecular vibronic coupling on nonradiative transitions and optical absorption of dimers

The theory of intermolecular vibronic coupling of dimers is considered. It will be shown that even small intermolecular interaction has significant effect on nonradiative lifetime and optical absorption band shape of dimers. Results of calculations are compared to experimental data on optical absorption and nonradiative lifetime of Mg-methyl bacteri-opheophorbide-d dimers.

Intermolecular vibronic coupling and energy transfer in a flat molecular aggregate

The migration of excitation within a small flat molecular aggregate composed of identical molecules is described using a Davidov-like model and a mechanism of excitation transfer of Forster type. We consider in this model the changes that take place in the equilibrium position of each molecule upon excitation and construct energy surfaces that describe “paths”, that is, conditions for excitation localization and transfer that govern, in first-order, the motion of excitation within the aggregate.

Intermolecular vibronic coupling and the mechanism of formation of hexatrienes from benzene in viscous media

The photochemistry of benzene and some of its alkyl derivatives has been studied in deuterated alcohol at 77°K. Intermolecular vibronic coupling of triplet benzene occurs with C-H but not O-H vibrational modes in the solvent. The coupling is sensitive to the relative orientation of the neighbouring molecules and in favourable cases, leads to the formation of a hexatriene in a concerted process.