Nature of the optical transition in polymethine dyes and J-aggregates

Using a new theory of photoinduced electron transfer beyond the Landau–Zenner ideology or, more precisely, electrodynamics of extended multiphonon transitions [V. V. Egorov, Chem. Phys. 269, 251 (2001)], we give an explanation for the well-known experimental data [L. G. S. Brooker et al., J. Am. Chem. Soc. 62, 1116 (1940)] for the absorption line shape in the vinylogous series of an ideal polymethine dye represented by a thiacarbocyanine. Then, using this explanation together with our earlier explanation [V. V. Egorov, Chem. Phys. Lett. 336, 284 (2001)] for the nature of the well-known intense narrow J-band due to an aggregation of polymethine dyes, we predict very intense narrow absorption lines for short optical transitions. Interpretation of these results is given on a basis of the Heisenberg uncertainty relation. A process of creation of the pure (quantum-mechanical) electron-transfer state is considered for the two complementary cases: the electron-transfer state is determined by interaction of the electron with its environment through spontaneous pumping of this state by an ordered or disordered environmental motion. The latter case corresponds to the Landau–Zenner-type picture of adiabatic and nonadiabatic electron transfers. The former case is used to account for the nature of the intense narrow bands.