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

Abstract

Nanosecond time-resolved transient absorption (TA) and time-resolved resonance Raman (TR 3 ) spectroscopies have been used to study the first triplet excited state, T 1 , of Zn(II) octaethylchlorin (ZnOEC). Additionally, TR 3 spectra are reported for Zn(II) etiochlorin I (ZnEtI) and the meso-d 4 and 15 N isotopomers of ZnOEC. Formation of the T 1 state in ZnOEC results in a red shift of the Soret absorption band from 400 to 421 nm. Features in the resonance Raman spectra of the ground and T 1 states are assigned on the basis of the isotope shift patterns and of normal coordinate analyses of MOECs. Bands at 1464 and 1597 cm -1 in the T 1 spectrum are assigned to C R C m stretching modes, ν 3 and ν 10 respectively, downshifted by 21 and 19 cm -1 from their ground state frequencies. The only observed C β C β stretching mode seen in the T 1 spectrum is at 1542 cm -1 and is assigned to ν 2 , downshifted from 1571 cm -1 in the ground state. The C R N and C R C β stretching modes located in the 1350-1400 cm -1 region of the ground state downshift in the triplet excited state to an unresolved set of peaks in the 1325-1380 cm -1 region. The frequency downshifts indicate that the observed peaks arise from vibrational modes which are localized on bonds that are weakened in the excited state. Peaks corresponding to substituent vibrational modes are not shifted in the excited state spectrum, implying a lack of hyperconjugation in the T 1 state.

Resonance Raman Spectroscopic Characterization of the π-Anion and π-Cation Radicals of Zinc(II) Octaethylchlorin

“…Different solvents were utilized because the ZnOEC anion was found to be stable in THF, while the cation was stable in CH 2 Cl 2 . Assignments are based upon previous RR studies and normal mode analyses of β-substituted metallochlorins ,,, and metalloporphyrins, with peak labeling based on the parent metalloporphyrin modes, as worked out for NiOEC . The high-frequency stretching modes are at slightly lower frequencies in THF than in CH 2 Cl 2 , because THF coordinates the Zn(II) and expands the chlorin core …”

“…7 Triplet State. RR spectra and assignments for the T 1 state of ZnOEC are reported elsewhere, 15 and it is of interest to compare the excited-state shifts with the radical anion and cation shifts. Since the T 1 electronic excitation involves transfer of an electron from the HOMO to the LUMO, the accompanying structural effects might be expected to be additive.…”

“… a Asterisks indicate not assignable. b From ref . c Frequency shift determined from ground-state value measured in a different solvent (THF) than was the cation (CH 2 Cl 2 ). …”

“…In neither study was isotopic labeling used to assign the Raman features. We have recently investigated the anion of Zn(II) tetraphenylchlorin, ZnTPC, and several of its isotopomers and have also used time-resolved RR spectroscopy in conjunction with isotope labeling to characterize the first triplet, T 1 , excited states of ZnTPC and ZnOEC …”

“…We have recently investigated the anion of Zn(II) tetraphenylchlorin, ZnTPC, and several of its isotopomers 13 and have also used time-resolved RR spectroscopy in conjunction with isotope labeling to characterize the first triplet, T 1 , excited states of ZnTPC 14 and ZnOEC. 15 In the present study, we use RR spectroscopy to characterize the π-anion and π-cation species of ZnOEC (Figure 1). Naturally occurring metallochlorins are β-substituted, and it is important to determine how this substitution pattern affects the properties of the radicals.…”