Photoisomerization of simple merocyanines: a theoretical and experimental comparison with polyenes and symmetric cyanines

The structure and electronic spectra of merocyanines are intermediate between those of acyclic polyenes and cationic symmetric cyanines of comparable sizes, shifting from one to the other as a result of a solvent polarity change. In this paper we address, both theoretically and experimentally, the question of how the photochemical behaviour of simple merocyanines compares with that of polyenes and symmetric cyanines. After a brief theoretical re-appraisal of the absorption maxima dependence on the chain length in the three classes of chromogens, we analyse the calculated potential-energy curves for two photoisomerization coordinates of a polyene, a symmetric cyanine and a merocyanine of comparable sizes. The results concerning both the energy curves and the nature of the relevant electronic states, particularly the TICT (twisted intramolecular charge transfer) character of the S₁ state at the perpendicular geometry, reveal the existence of a near relationship between merocyanines and symmetric ionic polymethines, traceable back to their being odd-alternant systems. The presence of a low-lying ¹(nπ*) state at the trans geometry and the associated solvent-modulated vibronic coupling with the ¹(π_Hπ_L*) state dominate the experimentally accessible photochemical behaviour of two simple merocyanines and make it peculiar with respect to those of both polyenes and symmetric ionic cyanines.