SHAPES OF ABSORPTION-BANDS IN SOLUTIONS - ORGANIC CHROMOPHORES

The shapes of absorption bands for polyenes, cyanine dyes, and some ar omatic hydrocarbons are simulated using a combination of empirical for ce field for the ground state with quantum chemical calculations of ex citation energies as a function of nuclear coordinates. The results ar e in good agreement with experimental spectra of solutions under norma l conditions. Three factors of the band broadening are outline: (1) hi gh-frequency progression from stretch vibrations (1600-1300 cm-1); (2) low-frequency progression from bend vibrations (about 400 cm-1); and (3) inhomogeneous and rotational effects. The bandwidth of the absorpt ion band arises from the first factor. The action of the second factor simultaneously with the third may transform the absorption spectrum i nto a wide band with poorly pronounced or without vibrational structur e.