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.