Solvatochromic shift mechanisms of centrosymmetric chromophores in polar liquids

Origins of absorption shifts in polar liquids are considered for centrosymmetric chromophores devoid of dipole moments. The refractive index (n) dependent bathochromism shows that the excited state is always more polarizable than ground state. With increasing solvent dielectric constant (ɛ), mainly red shifts occur, but there are exceptions. Hypsochromism of n–π* transition in s-tetrazine is ascribed to a decrease of quadrupole moment in the excited state. In this case the upper level is pushed up in the reaction field of the ground-state quadrupole. Red shifts of intense bands in polyenes (11Bu or S2) and polyarenes (1Bb or β), reaching ∼−250 cm−1 are assigned to local polarization in the reaction fields of chromophore bond dipoles. The hyperbolic-like ɛ-dependence of solvent shift, approximated to an empirical function (ɛ − n2)/(ɛ − n2 + c), is much steeper (with “stretching” factor c = 3 ± 0.5) in this case, as compared to that for the linear effect (for s-tetrazine, c = 9). Polarization mechanism also causes less band broadening in polar environments. Contrary to a common view, the spectral shift is probing local reaction fields of chromophores, rather than random fields in the cavities accommodating the dye. Negligible polarization in solvent “cavity” fields is in accordance with earlier hole-burning study of matrix induced dipole moments in tetrapyrrolic pigments that depend surprisingly little on the polarity of polymeric hosts (R.B. Altmann, I. Renge, L. Kador, D. Haarer, J. Chem. Phys. 97 (1992) 5316 [30]).