The nature of transitions in electronic absorption spectra of radical cations of dipyrroles with phenylene bridging groups

The electronic absorption spectra of radical cations of dipyrroles with a phenylene bridge were studied by laser flash photolysis and quantum chemical methods. Intense absorption bands of the radical cations in the visible region (lambda(max) a parts per thousand 500 nm, epsilon(max) > 2 center dot 10(4) L mol(-1) cm(-1)) are caused by excitation of electrons from single occupied MOs to the LUMO. In the near IR region, calculations predict additional, relatively intense (fa parts per thousand 0.27-0.29) electronic transitions associated with excitation of electrons from low-lying MOs to the single occupied MO.