Resonance Raman characterization of the triplet state of zinc tetraphenylchlorin

Abstract

The TI excited state of zinc(I1) tetraphenylchlorin, ZnTPC, has been studied by nanosecond time-resolved transient absorption (TA) and transient resonance Raman (TR3) spectroscopy. The TA difference spectrum shows an induced absorption at 450 nm that is assigned as arising from a 3 (~, n * ) state. The most striking feature of the 3(ZnTPC) TR3 spectrum is the significant frequency downshifts and strong intensity enhancements of the pyrrole half-ring stretch, v4, and of other pyrrole ring motions which together produce a broad band in the 1150-1300-~m-~ region. In addition, the enhancement of C,C, modes (~2 and V I I ) changes with their phasing between the SO and TI states. Weaker intensity enhancements, with no frequency shifts, are seen for the phenyl substituent ring mode, 0 4 and V I , and C,-phenyl stretch. These findings suggest that there is less electronic participation from the phenyl rings in the resonant triplet state, T,, for ZnTPC than for its parent metalloporphyrin, ZnTPP.The excited states of metalloporphyrins and metallochlorins play an integral role in biological energy and electron transfer, photodynamic therapy, and photocatalysi~.'-~ The lowest-lying excited states of metalloporphyrins have been studied by resonance Raman (RR)6a-C and resonance coherent anti-Stokes Raman'"-' (RCAR) spectroscopy in order to obtain detailed structural information on the excited states. In comparison to metalloporphyrins, far less is known about the excited state dynamics of metallochlorins. Although ground state absorption, resonance Raman, and infrared spectra of metallochlorins have been studied in some detail,8-'s there have been few reports dealing with their excited state spectra.Hydroporphyrins such as ZnTPC (TPC = tetraphenylchlorin) represent a distinct class of biologically relevant compounds.16-'8 The reduction of one pyrrole ring in the chlorin macrocycle lowers the symmetry from D4h to Czv, resulting in further separation of the highest occupied molecular orbitals, HOMOS, and a loss of degeneracy in the lowest unoccupied molecular orbitals, LUMOs. The electronic and structural changes alter the photophysical, redox, and ligand-binding properties of metallochlorins relative to those of the parent metalloporphyrins.18a-b However, it is not clear how the reduction of the pyrrole ring affects the excited state dynamics of metallochlorins relative to metalloporhyrins. In order to address this question, we have investigated the character of the lowest-lying excited state of ZnTPC by transient absorption and TR3 spectroscopy.ZnTPC was purchased from Midcentury Chemical Co. (Posen, IL). Its punty was checked via thin layer chromatography. ZnTpC-d20 and ZnTPC-d8 isotopomers were synthesized according to standard literature preparation^.'^-*' The ZnTPC complexes were purified on 1000-pm silica gel plates (Analtech), using a degassed 3:l mixture of THF and hexane as eluant. The purification was done in a glovebox under an inert atmosphere (Vacuum Atmosphere Model D1-00 1-SD equipped with an He-493 Dri-Train). ...

A Resonance Raman Spectroelectrochemical Study of the Zn(II) Tetraphenylchlorin Anion

“…Extensive RR studies are beginning to unravel the structural properties of metalloporphyrin π-radicals . In contrast, with few exceptions, RR studies of metallochlorins have been limited to neutral, ground state species …”

Structural Distortion of the Vanadyltetraphenylporphine Anion Radical Probed by Resonance Raman Spectroelectrochemistry

“…Because of its high sensitivity and selective enhancement for the vibrational modes of chromophores, resonance Raman (RR) spectroscopy is a useful probe of chromophore structure and has been increasingly applied to investigate the ground state, , excited state, − and π radical species − of many tetrapyrrolic chromophores. The nature of the J−T effect in radical anions can be characterized by examining depolarization ratios, enhancement patterns, and frequency shifts in RR spectra.…”

Resonance Raman Study of the T1 Excited State of Zinc(II) Complexes of β-Substituted Chlorins

“…While RR spectroscopy has been employed extensively to characterize natural and synthetic metallochlorins in the ground state, 2 studies of their excited states have been limited. [3][4][5] RR studies of the excited states of metallochlorin model compounds have been confined to the more stable mesotetraphenylchlorins. In a nanosecond time-resolved Raman (TR 3 ) spectroscopic study of ZnTPC (TPC ) tetraphenylchlorin) the lowest triplet state, T 1 , of 3 (π,π * ) character was detected.…”

“…[3][4][5] RR studies of the excited states of metallochlorin model compounds have been confined to the more stable mesotetraphenylchlorins. In a nanosecond time-resolved Raman (TR 3 ) spectroscopic study of ZnTPC (TPC ) tetraphenylchlorin) the lowest triplet state, T 1 , of 3 (π,π * ) character was detected. 3 When the closed-shell Zn(II) is replaced by d 9 Cu(II), de Paula et al identified a (π,d) charge transfer state in the difference spectrum of Raman spectra of CuTPC obtained at high and low power pulsed laser excitation.…”

“…In a nanosecond time-resolved Raman (TR 3 ) spectroscopic study of ZnTPC (TPC ) tetraphenylchlorin) the lowest triplet state, T 1 , of 3 (π,π * ) character was detected. 3 When the closed-shell Zn(II) is replaced by d 9 Cu(II), de Paula et al identified a (π,d) charge transfer state in the difference spectrum of Raman spectra of CuTPC obtained at high and low power pulsed laser excitation. 4 Natural metallochlorins are substituted at the pyrrole β positions.…”