Vibrational Spectra and Structures of Long-Chain Streptocyanine Dyes: Effects of Electron−Vibration Interactions and Vibrational Polarizabilities

Vibrational spectra and electron–vibration interactions of the naphthalene radical anion — Experimental and theoretical study

Vibrational analysis is carried out for the radical anions of naphthalene-h8 and -d8. Their infrared (IR) spectra are observed in tetrahydrofuran by using a cell designed for IR measurements of unstable species. The vibrational force field and the IR intensities are calculated by the ab initio molecular orbital and density functional methods at various theoretical levels. As found in the cases of the radical cations of many polycyclic aromatic hydrocarbons (PAHs), a few strong IR bands with intensities of the order of 102 km mol–1 are observed in the 1700–900 cm–1 region. These observed spectral features are well reproduced by the calculations at the CASSCF (complete active space self-consistent field) and B3LYP (Becke's three-parameter hybrid method using the Lee–Yang–Parr correlation functional) levels. The calculation at the B3LYP level gives a better fit between the observed and calculated absolute IR intensities. Normal modes and the origin of the strong IR intensities characteristic of the radical anions are discussed in terms of molecular symmetry coordinates and the dipole derivatives based on these coordinates. It is found that the IR intensities of the b2u modes below 2000 cm–1 are dominated by the contribution from one molecular symmetry coordinate, in which the C—C bonds in one ring stretch while those in the other ring shrink. The mechanism that gives rise to the strong IR intensity for this vibration is discussed by examining the changes in the electronic structure induced by this vibration.Key words: vibrational spectra, electron–vibration interaction, naphthalene, radical anion.

Vibrational polarization and opsin shift of retinal schiff bases: theoretical study

The changes in the electronic excitation energy arising from molecular structural displacement induced by external electric field (so-called vibrational polarization) are examined theoretically for the protonated and neutral 11-cis retinal Schiff bases. It is shown that the magnitude of the field-induced structural displacement is significantly large for the protonated species, so that the change in the electronic excitation energy arising from this structural displacement is of the same order of magnitude as that arising from the direct effect of electric field on the electronic wave function. These two effects contribute additively to the field-induced spectral shift. The intensity-carrying mode (ICM) theory is employed to extract a single vibrational mode (called primary infrared ICM) that is most important for the field-induced structural displacement. A simple one-dimensional model is constructed, and the extent to which we can interpret the field-induced spectral shift by such a model is examined. In the case of the neutral species, only a small change in the electronic excitation energy is induced by external electric field, mainly because the vibrational polarizability of this species is small. The meaning of these results in the spectral tuning of visual pigments is discussed.

Intensity-carrying modes important for vibrational polarizabilities and hyperpolarizabilities of molecules: derivation from the algebraic properties of formulas and applications

The intensity-carrying mode (ICM) theory is developed for analyzing the vibrational motions that mainly contribute to vibrational polarizabilities and hyperpolarizabilities, which are important for describing intermolecular electrostatic interactions and nonlinear optical properties of molecules. The ICMs are derived from dipole derivatives, polarizability derivatives, and first hyperpolarizability derivatives by using algebraic properties of intensity formulas. The way to obtain explicit forms of ICMs, including the optimization method of the basis of the ICM vector space, is discussed in detail. One- and two-dimensional models are constructed on the basis of the ICMs. The theory is applied to three molecules (a push-pull type polyene, a streptocyanine dye cation, and a symmetric neutral polyene) taken as typical examples. It is shown that the ICM theory provides a reasonable picture on the vibrational polarization properties of these molecules. On the basis of this result, the validity of the valence-bond charge transfer (VB-CT) model, which is a one-dimensional model and is widely used to describe the electronic and vibrational properties of dye molecules, is also discussed.