Temperature- and pressure-induced shifts of Shpol’skii spectra. I: Pressure shifts

Abstract

Abstract Pressure-induced shifts of absorption, fluorescence and phosphorescence lines of organic chromophores embedded in crystalline n-alkanes were measured at 6 K under He gas pressure up to 200 bar. The shifts are reversible (elastic) and linear with the change of pressure P. The P shift coefficients of transition frequency dν/dP are compared with vacuum-to-matrix shifts of optical energies ν−ν00. In alternant polyarenes the dν/dP lie between −0.7 and −0.16 GHz/bar for relatively weak 1 L b or α-type transitions. The ν−ν00 vary considerably less, from −535 to −313 cm−1. In strong 1 L a or p-type transitions both the pressure and solvent shifts are much larger, ranging from −1.3 to −0.75 GHz/bar and from −2000 to −1200 cm−1, respectively. The phosphorescence lines shift very little with pressure (from −0.3 to 0.2 GHz/bar). Both dν/dP and ν−ν00 are small for the S1–S0 lines in simple tetrapyrrolic pigments porphine, chlorin and cis-iso-bacteriochlorin, lying between −0.16 and 0.08 GHz/bar and −280 and 10 cm−1, respectively. Octaethylporphine in n-octane exhibits, besides of the broad and complicated multiplet structure, a huge dispersion in dν/dP values between −0.52 and 0.36 GHz/bar. The relationships between the P-induced shifts and absolute solvent shifts are analysed in terms of Lennard-Jones 6-12 potential that takes into account both the bathochromic shifts of dispersive origin and the repulsive hypsochromism.