UNUSUAL PICOSECOND (1)(PI,PI-ASTERISK) DEACTIVATION OF RUFFLED NONPLANAR PORPHYRINS

Time-resolved and steady-state optical data are presented for metal-fr ee meso-tetraalkylporphyrins, H2T(alkyl)P. Introduction of bulky terti ary-butyl or adamantyl substituents at the meso positions induces larg e ruffling distortions of the macrocycle skeleton that give rise to hi ghly novel photophysical properties of the porphyrins. These include u ltrashort (1)(pi, pi) lifetimes of 10-50 ps at 296 K that, intriguing ly, lengthen to 10-15 ns at 78 K, the typical timescale found at both temperatures for planar derivatives such as tetra(n-pentyl) and tetrap henyl porphyrins. The ability of the nonplanar ruffled porphyrins to u ndergo additional structural deformations in the excited singlet state likely underlies their dramatically enhanced rates of internal conver sion.