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A Model of Inhomogeneous Broadening and Pressure Induced Hole Shifts in the Optical Spectra of Organic Chromophores in Glasses

Zero phonon holes were burned in the S1rS0 absorption bands of organic compounds in ethanol glass at 6 K. The application of He gas pressure (P) leads to the shift of holes with a frequency coefficient d O/dP. The linear dependence of dO/dP on hole position (O) within the inhomogeneous band is of considerable interest, since it can provide the S1rS0 energies of nonsolvated chromophores and the (local) compressibility of the surrounding matrix. However, the extrapolated frequencies O0(P) at which pressure shift should vanish deviate from the actual 0-0 energies of free solutes in a vacuum. The slope (a) of the plot of dO/dP vs O varies between 1.5 10 -5 and 6.2 10 -5 bar -1 for different dyes, and therefore cannot be directly associated with the isothermal compressibility of the matrix. To account for these facts, a model was developed based on the assumption that the solvent shift is a superposition of repulsive, dispersive, electrostatic, and other possible interactions. Each interaction has a specific intermolecular distance ( r) dependence (e.g. r -6 for dispersive and r -12 for repulsive interactions) and is assigned a Gaussian frequency distribution function. It follows from the model that the observed slope a depends on, in addition to the volume compressibility of the matrix, the widths of the constituent Gaussians and the respective power coefficients of r. Both the measured bandwidth and the slope a are also sensitive to the type of correlation between different solvent shift mechanisms. In rhodamine dyes a dipole moment change between the ground and the excited state is responsible for a strong broadening and a shallow slope a. The conformational flexibility of a dicarbocyanine dye HIDCI has similar consequences. The mechanisms leading to a matrix polarity dependent hypsochromism in open chain cyanine dyes, free base tetrapyrroles, and s-tetrazine correspond to a distance dependence of r -6 and are deemed to be of multipolar nature. The magnitude of the hypsochromic solvent shift owing to repulsive forces is probably small, although their contribution to the slope a and the inhomogeneous bandwidth may be relatively large.