Quantum-Chemical Calculations of the Excited Electronic States of the Prodan Molecule and Its Complexes in Water

Abstract

A quantum-chemical calculation of the prodan molecule and its complexes in water for the geometry of the ground and fluorescent states is carried out. To describe the fluorescent state, changes in the electronic state (population) on bonds and atoms during transition of the molecule into an excited state are taken into account. A model of interaction of the prodan molecule with a polar proton-donor solvent (water) is suggested. It is shown that interaction with the ionic forms (H 3 O + ) provides an explanation for the sensitivity of prodan to the solvent (displacement of the fluorescence bands). The nature of the electronic excited states of the prodan molecule and its complexes has been investigated. The constants of the rates of radiative and nonradiative processes and the fluorescence quantum yields have been calculated.Introduction. Fluorescing organic molecules are widely used in biochemical investigations as probes for studying the physical-chemical properties of solvents, surfaces of various physical nature, large biological molecules, membranes, cells, etc.[1]. In this connection, a molecule of prodan (6-propionyl-2-dimethyl-aminonaphthalene) and its derivatives, for example, a molecule of laurdan (6-dodecanoyl-2-dimethyl-aminonaphthalene), are interesting objects for investigations. In [2][3][4], it was shown that the solvatochromism of laurdan is associated with the considerable change in the dipole moment in excited states. The fluorescence of laurdan in glycerin at a temperature from -15 to +50 o C consists of two bands; at 75 o C it has one band [3]. In the prodan molecule, a fluorochromic effect was observed at room temperature: ν max = 24,940 cm -1 (cyclohexane) and 18,830 cm -1 (water) [5,6].The prodan molecule is a derivative of naphthalene ( Fig. 1) that contains amino-and carbonyl groups. By elongating the CH 2 chain we obtain different derivatives of prodan (prodan at n = 1 and laurdan at n = 10).In the present work, we carried out quantum-chemical investigations of the prodan molecule and its complexes in water. We studied the influence of intermolecular interactions between the prodan molecule and a polar protondonor solvent on the position of electronic states and radiative and nonradiative processes.The sensitivity of the prodan molecule to a solvent is monitored by measuring the displacement of the absorption and fluorescence bands. The properties of this molecule were investigated experimentally in [6,7] in a series of the solvents, the polarity of which differs greatly. On transition from cyclohexane to water, an appreciable displacement of the fluorescence band (6100 cm -1 ) is observed; the maximum of the absorption band in this case is shifted to 1770 cm -1 . The authors note a low quantum yield of fluorescence for the given molecule but do not cite its value. It is pointed out that the quantum yield in a nonpolar solvent is considerably higher than in water [5].Investigation Techniques. The quantum-chemical calculation of the prodan molecule was performed with the aid of a package ...

Spectral Heterogeneity of PRODAN Fluorescence in Isotropic Solvents Revealed by Multivariate Photokinetic Analysis

“…29 It is also noteworthy that recent theoretical work supports a significant role for specific PRODAN solvent interactions. Artyukhov and coworkers 30 describe semiempirical calculations that reproduce PRODAN spectral properties in water within 5% when the hydronium ion was used as a complexing ligand. Recent calculations at a much higher level of theory 23 included two hydrogen-bonded waters in the description of PRODAN in water but generated less accurate emission energies.…”