Mechanisms of solvent shifts, pressure shifts, and inhomogeneous broadening of the optical spectra of dyes in liquids and low-temperature glasses

Optical absorption spectra were measured in liquid solutions at ambient tem perature for comparatively nonpolar chromophores: polymethine dyes, polycyc lic hydrocarbons, and tetrapyrrolic compounds. The analysis of solvent shif ts of band maxima as a function of polarity, polarizability, and hydrogen b onding properties of the medium allows one to distinguish several solvent s hift mechanisms. Solvent polarizability dependent red shifts are assigned t o dispersive interaction. Hypsochromism in the spectra of open chain cyanin e dyes and s-tetrazine in polar media may be understood in terms of a multi polar reaction field. Blue shifts of the visible bands of anionic dyes, res orufin, and resazurin occur in alcohols due to the hydrogen bonding with th e solvent. Both the polar solvation and the H-bonding with water in the cen ter of tetrapyrrolic macrocycle is responsible for the blue shifts of the S -1-S-0 band in porphyrins. inhomogeneous bandwidths were measured in ethano l glass at 6 K. The reason of inhomogeneous broadening is the spread of mic roscopic solvent shifts in the disordered matrix that can have the same mec hanisms as the macroscopic shifts of band maxima. Alternatively, other broa dening mechanisms such as the linear Stark effect in the solvent cavity fie ld do not shift the spectral band as a whole. Further, spectral holes were burned in the inhomogeneous S-1 <-- S-0 absorption bands in glassy ethanol and the pressure shift coefficients of the holes d nu/dP were determined us ing gaseous He as pressure transmitter. d nu/dP shows a linear dependence o n hole burning wavenumber that can be extrapolated to the frequency nu(0(P) ) where pressure shift disappears. The nu(0(P)) values deviate significantl y from the actual 0-0 origins of nonsolvated chromophores. The slope of the dependence of d nu/dP on hole frequency generally differs from the value o f 2 beta(T) (beta(T) is the isothermal compressibility of the matrix), pred icted for the dispersive solvent shift. The slopes steeper than 2 beta(T) w ere assigned to short-range repulsive forces. The long-range electrostatic interactions must lead to the slope values less than beta(T).