A reinterpretation of the unusual barochromism of azulene

Abstract

The UV–vis spectra of azulene in methylcyclohexane and acetonitrile solution at room temperature have been measured as a function of applied pressure in the 0.1–600 MPa range. The solvatochromic shifts are normal, but the barochromic shifts are unusual (hypsochromic shift in the S1–S0 absorption bands vs. normal bathochromic shifts in its S2–S0 and S3–S0 absorption bands). An analytical relationship between the barochromic shift and the differences in the polarizabilities and dipole moments of the two radiatively coupled electronic states has been derived. The unusual barochromism of azulene has been reinterpreted in terms of differences in the polarizabilities of azulene in its ground and excited states and the dominance of dispersive interactions in barochromism.Key words: barochromism, solvatochromism, azulene, polarizability, dipole moment.