Photophysical properties of [N]phenylenes

In the present study, photophysical properties of [N] phenylenes were studied by means of stationary and time-resolved absorption and fluorescence spectroscopy ( in THF at room temperature). For biphenylene ( 1) and linear [3] phenylene (2a), internal conversion (IC) with quantum yields Phi(IC) > 0.99 is by far the dominant mechanism of S-1 state deactivation. Angular [3] phenylene (3a), the zig-zag [4]- and [5] phenylenes (3b), (3c), and the triangular [ 4] phenylene ( 4) show fluorescence emission with fluorescence quantum yields and lifetimes between Phi(F) = 0.07 for (3a) and 0.21 for (3c) and tau(F) = 20 ns for (3a) and 81 ns for (4). Also, compounds (3) and (4) exhibit triplet formation upon photoexcitation with quantum yields as high as Phi(ISC) = 0.45 for (3c). The strong differences in the fluorescence properties and in the triplet formation efficiencies between (1) and ( 2a) on one hand and ( 3) and ( 4) on the other are related to the remarkable variation of the internal conversion ( IC) rate constants k(IC). A tentative classification of (1) and (2a) as "fast IC compounds", with k(IC) > 10(9) s(-1), and of (3) and (4) as "slow IC compounds", with k(IC) approximate to 10(7) s(-1), is suggested. This classification cannot simply be related to Huckel's rule-type concepts of aromaticity, because the group of fast IC compounds consists of antiaromatic (1) and aromatic (2a), and the group of slow IC compounds consists of antiaromatic (3b), (4) and aromatic (3a), (3c). The IC in the [N] phenylenes is discussed within the framework of the so-called energy gap law established for non-radiative processes in benzenoid hydrocarbons.