J. Phys. Chem. 1993,97, 11599-1 1602
11599
Time-Resolved Resonance Raman Spectroscopy of Excited Singlet and Triplet States of Free-Base meseTetraphenylporphyrin Steven E. J. Bell,’qt Ala H. R. AI-Obaidi,* Martin Hegarty,t Ronald E. Hester,* and John J. McGarveyt School of Chemistry, The Queens University of Belfast, Belfast BT9 JAG, U.K., and Department of Chemistry, University of York,York YO1 5DD, U.K. Received: June 29, 1993’
Two-color time-resolved resonance Raman spectroscopy has been used to probe the lowest excited singlet (Sl) and triplet (TI)states of free-base meso-tetraphenylporphyrinand meso-tetrakis(4-sulphonatopheny1)prphyrin in solution at room temperature. The spectra were recorded using 532-nm excitation pulses and time-delayed probe pulses ( A T = 0-30 ns, 447 and 460 nm) near,,,A of the S1 and TI states. Significant shifts in frequency of the porphyrin core vibrations were observed upon excitation to either the SIor T1 state. Several of the strongest polarized bands in the spectra of both excited states, including u1, 4,u4, Ug, and 44, are assigned, and the information they give on the differences in electron distribution in the ground, SI, and T1 states is discussed.
Introduction The electronically-excitedstates of simple synthetic porphyrins such as meso-tetraphenylporphyrin (TPP) have attracted considerable interest because of their use as sensitizers in solarenergy conversion systems,1 as components of model systems for the study of excited-state electron-transfer reactions,2 and, more recently, as dyes for photodynamictherapy.3 Although resonance Raman spectroscopy has been extensively applied to porphyrin and porphyrin-related materials in their ground electronic states, surprisingly little work has been carried out on their electronically excited r-a* states. Two groups have reported the results of time-resolved resonance Raman (TR9 studies of triplet-state Zn(TPP),4J and recently, resonance Raman spectra of the lowest lying singlet and triplet excited states of free-base octaethylporphyrin (H2OEP) have been reportedS6 We have chosen to investigate the free-base forms of TPP (H2TPP) and a watersoluble derivative meso-tetrakis(4-sulphonatopheny1)porphyrin (HzTPPS) since these bridge the gap between the metalated TPP complexes and free-base OEP systems which have previously been studied.
Experimental Section TR3 spectra were obtained using two pulsed Nd:YAG lasers (Spectra Physics DCR2 and GCR3) with a pump wavelength of 532 nm and probe wavelength of 447 nm (second Stokes Raman shifted from 355 nm in CH4) or 460 nm (Spectra Physics PDL3 dye laser). Pump/probe delay was controlled using a digital pulse generator (Stanford Research Instruments DG 535) which was used to trigger both lasers. The timing jitter was measured with a fast-response silicon photodiode (Radio Spares BPX65) mounted within the Raman sample compartment, whose output was displayed on a Tektronix 7904 oscilloscope; the jitter between the peak positions of the oscilloscope traces corresponding to the laser pulses was found to be
