DFT Studies of the Resonance Raman Spectra of Ground and Excited Triplet State Free Base meso-Tetraphenylporphyrin (H2TPP)

Abstract

DFT calculations at the B3LYP/6-31G(d) level were carried out for the ground and excited triplet states of free base meso-tetraphenylporphyrin, H2TPP, and its d2, d8, d10, d20, d22, and 13C4 isotopomers. The agreement between experimental and calculated (scaled with a single factor of 0.973) band positions for the ground state was acceptable (rms = 9.9 cm-1). In addition, although the shifts on isotopic substitution were frequently of the same order as this rms error in the absolute positions, it was found that the cancellation of errors in the calculations meant that the direction and magnitude of even small isotope shifts < 10 cm-1 were also calculated correctly. In the D2 symmetry of the calculations the lowest lying triplet state corresponded to a one-electron transition from the b1 HOMO to a b3 LUMO. It was found that the calculated changes in cm-1 of the vibrational modes on excitation to this triplet (ΔS-T) reproduced the ΔS-T values of the seven bands found in the experimental spectra. Three of these bands (ν2, ν12, and ν15) moved to lower cm-1 on excitation, two were essentially unchanged (φ4 and ν1), and two moved to higher cm-1 ( ν4 and ν6). Since the ΔS-T values are typically small (≤10's of cm-1), the correct prediction of the pattern of small shifts associated with population of an excited state with a particular electronic configuration is impressive. As before, this improved accuracy presumably arises because errors in the calculations of absolute positions cancel when values for the same modes in different electronic states are subtracted to give shifts on excitation. This level of accuracy is necessary if ΔS-T values are to be used to assign orbital parentage. Surprisingly, for several of the modes the calculated ΔS-T shifts differed dramatically between isotopomers. For example, for ν2: ΔS-T(calc), d0 = −5 cm-1, d8 = −30 cm-1; ΔS-T(obs), d0 = −15 cm-1, d8 = −26 cm-1. These differences were found to reflect not only the changes in force constants due to promotion to the triplet state (which are the same irrespective of which isotopomer is under consideration) but also changes in the mode composition on excitation which alters the isotope sensitivity. In the case of H2TPP, the mode compositions change because excitation from b1 → b3 orbitals accentuates the difference in bonding between the pairs of protonated and unprotonated pyrrole rings. In effect, excitation increases the rectangular distortion that is already present in the ground state of free base porphyrins and which distinguishes them from more regular D4h (square) metalloporphyrins. The further distortion causes modes that in the more regular systems involve all four of the pyrrole rings to become increasingly localized on just the protonated or unprotonated pyrroles. More generally, the success of the DFT calculations at this level of theory in predicting frequency shifts on excitation for an extensive series of isotopomers clearly validates the approach. Interpretation of the resonance Raman spectra of the excited states of other tetrapyrroles using DFT calculations is therefore viable, even for systems such as β substituted porphyrins and metalloporphyrins, where much less extensive isotope data are available to aid band assignments.