Unusual picosecond 1(π, π∗) deactivation of ruffled nonplanar porphyrins

Time-resolved and steady-state optical data are presented for metal-free meso-tetraalkylporphyrins, H2T(alkyl)P. Introduction of bulky tertiary-butyl or adamantyl substituents at the meso positions induces large ruffling distortions of the macrocycle skeleton that give rise to highly novel photophysical properties of the porphyrins. These include ultrashort 1(π, π∗) lifetimes of 10–50 ps at 296 K that, intriguingly, lenghten to 10–15 ns at 78 K, the typical timescale found at both temperatures for planar derivatives such as tetra(n-pentyl) and tetraphenyl porphyrins. The ability of the nonplanar ruffled porphyrins to undergo additional structural deformations in the excited singlet state likely underlies their dramatically enhanced rates of internal conversion.