Solvent and Intramolecular Effects on the Absorption Spectrum of Betaine-30

Abstract

The effect of solvent electrostatics and solute torsional modes on the absorption spectrum of betaine-30 in acetonitrile is examined. Combined quantum/classical molecular dynamics ground state simulations are used to calculate the electronic absorption spectrum in acetonitrile. The model for betaine-30 includes the electronic degrees of freedom of the π system of the molecule and their interactions with the electric field of the solvent, treating the electronic wave function at the level of Pariser−Parr−Pople semiempirical electronic structure theory. The absorption intensity, width, and maximum of the S0 to S1 band are well reproduced by the model. In solution, the S0 molecular dipole moment is found to be strongly enhanced due to solvent-induced electronic reorganization. The width of the absorption band in acetonitrile is found to be a function of solvent orientational fluctuations and is not correlated with conformational changes caused by torsional motion in the molecule. This fact, combined with the good agreement between the classical reorganization energies inferred from the simulated and experimental spectra indicates that, at least in acetonitrile, the classical component of the reorganization energy is fully determined by solvent orientational polarization. The spectral band maximum of the lowest energy transition is found to be blue shifted over 7000 cm-1, compared to a calculation in which the coupling of the betaine-30 electronic structure to the solvent molecules is eliminated, in agreement with the shift found experimentally for betaine-30 in acetonitrile compared to alkanes. However, in contrast to the result found in acetonitrile, the transition energy in the absence of solvent interactions is found to be strongly correlated with the central phenolate−pyridinium dihedral ring angle. This contrasting behavior implies that in nonpolar solvents, the classical reorganization energy does have a contribution from that torsional mode. Correspondingly, this difference in behavior with solvent indicates that the assumption of a solvent independent intramolecular contribution to the reorganization energy is questionable.

Motional narrowing of broadband absorption of solvatochromic indicator Betaine 30

“…There is little doubt that, as a result of disorder in protic glasses, this specific solvent shift is subject to individual variation, and contributes to the width of IDF. Besides universal and specific solvation, in flexible chromophore structures a geometry modulation can generate additional inhomogeneous broadening [26][27][28]. Unfortunately, there seems to be little experimental evidence for S 1 -S 0 frequency variation as a result of conformational change.…”

Excited state molecular dynamics simulations of nonlinear push–pull chromophores

“…In addition, an amino wag angle and single bond and double bond twist angles were defined for JM as (180 À D (2, 1, 8, 3))/2, D (12,13,14,15), and D (13, 14, 15, 17), respectively. As in PNA, the amino wag angle is strongly correlated to the hybridization of the amino nitrogen; a decrease in this angle signifies the evolution from sp 3 to sp 2 hybridization, sp 2 hybridization being associated with the zwitterionic basis state.…”

“…Gao and Alhambra [9] obtained similar results for the some of the same molecules considered in [6] using a hybrid quantum mechanical and molecular mechanics methodology. Recent work on the betaine-30 dye molecule [11,12] has focused on electronic effects related to the torsional coordinate between its p conjugated rings, and the importance of this contribution as a function of media polarity; the general conclusion was that the potential along the torsional coordinate has a much larger curvature in the isolated molecule than in solution. Farztdinov et al [13] used semiempirical methods to explore the excited state potential energy surface of p-nitroaniline as a function of solvent.…”

Multistate empirical valence bond study of temperature and confinement effects on proton transfer in water inside hydrophobic nanochannels

“…In this study, we employed a MS-EVB approach to model the quantum nature of proton, observed through its delocalization, together with MD techniques which allowed us to monitor the trajectory of all particles in time. Although this combined methodology has been widely used to study chemical reactivity in solution, [11,14,17,19,26,[43][44][45][46][47][48][49][50][51] we will describe the most important details of the methodology below.…”