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 S-1<--S-0 absorption bands of organic compounds in ethanol glass at 6 K. The application of He gas pressure (P) l eads to the shift of holes with a frequency coefficient dv/dP. The linear d ependence of dv/dP on hole position (v) within the inhomogeneous band is of considerable interest, since it can provide the S-1<--S-0 energies of nons olvated chromophores and the (local) compressibility of the surrounding mat rix. However, the extrapolated frequencies v(0(P)) at which pressure shift should vanish deviate from the actual 0-0 energies of free solutes in a vac uum. The slope (a) of the plot of dv/dP vs v varies between 1.5 x 10(-5) an d 6.2 x 10(-5) bar(-1) for different dyes, and therefore cannot be directly associated with the isothermal compressibility of the matrix. To account f or these facts, a model was developed based on the assumption that the solv ent shift is a superposition of repulsive, dispersive, electrostatic, and o ther 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 ba ndwidth and the slope a are also sensitive to the type of correlation betwe en different solvent shift mechanisms. In rhodamine dyes a dipole moment ch ange between the ground and the excited state is responsible for a strong b roadening and a shallow slope a. The conformational flexibility of a dicarb ocyanine dye HIDCI has similar consequences. The mechanisms leading to a ma trix 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 hypsochro mic solvent shift owing to repulsive forces is probably small, although the ir contribution to the slope a and the inhomogeneous bandwidth may be relat ively large.