Picosecond spectroscopy of electronically excited singlet states in biphenylene

A photophys. properties of biphenylene-h8 and biphenylene-d8 in soln. at room temp. was investigated with picosecond absorption and fluorescence spectroscopy. The weak fluorescence with complex vibrational structure originates entirely from the S1 state. It decays monoexponentially with a lifetime of 240 +- 20 ps. Upon photoexcitation to S2 no emission attributable to the S2 state was obsd. Lifetime and quantum yield of fluorescence (FF = 2.3 * 10-4) are the same for biphenylene-h8 and for the completely deuterated compd., indicating the C-H vibrations are not involved in the major radiationless decay process of the S1 level, i.e. internal conversion (jIC > 0.99). This conclusion is supported by INDO/S CI calcns. where strong changes of the CC bond orders (particularly in the central C4 ring) with S0 -> S1 excitation are found. The time-resolved excited-state absorption, measured at several wavelengths, decays biexponentially with time consts. of 8 +- 3 and 250 +- 40 ps. Various interpretations of the 8 ps lifetime are discussed; the conclusion results that the 8 ps transient has to be assigned to the lifetime of the S2 state even though the emission expected for an allowed transition with such a long lifetime is not obsd. Strong S1 -> Sn and S2 -> Sn excited-state absorption is found as predicted by INDO/S calcns