Resonance Raman spectra of S₂TPP, SSeTPP, Se₂TPP, and H₂TPP: Extended tetraphenylporphine vibrational assignments and bonding effects

Resonance Raman (RR) spectra of dichalcogen (S or Se) substituted and free-base tetraphenylporphine (TPP), obtained with B-band excitation, show features corresponding to most of the expected in-plane porphyrin skeletal vibrations, and a pair of bands due to phenyl vibrations (1598, 640 cm^-1). H₂TPP (free base) also shows three depolarized bands (1383, 441, 167 cm^-1) and one anomalously polarized band (1135 cm^-1), and two extra bands, at 1327 and 674 cm^-1, which disappear in D₂TPP and are assigned to in- and out-of-plane NH bending modes. The porphyrin skeletal modes can all be correlated with previously recorded modes of (FeTPP)₂O and its d₈ and J₂₀ isotopic forms, and the symmetry lowering (D_4h → D_2h) in the X₂TPP molecules allows assignment of depolarized D_4h parent modes to B_1g or B_2g symmetry classes. The previous assignments of the TPP skeletal modes are extended with the aid of the present data. The RR relative intensities of SSeTPP are very different for excitation at 4579 or 5145 Å, consistent with x polarization (the x axis passes through S and Se) of the higher energy Q transition and y polarization of the lower energy B transition. A general lowering of all skeletal frequencies between S₂TPP and Se₂TPP, greater than expected from the mass change alone, supports the inference from previous electrochemical studies and from MO calculations that Se is more effective than S in draining π electrons from the porphyrin ring. Two specifically X-sensitive bands are assigned to C_aX stretching and symmetric X-ring deformation, occurring at frequencies close to analogous modes in thiophene and selenophene. Both pairs of bands are observed for SSeTPP, at slightly shifted frequencies, indicative of some inter-ring interaction. The interaction may be linked to direct X⋯X bonding or may be transmitted via the porphyrin conjugation. One of the low-frequency porphyrin deformations is attributed to a radial motion of the X-rings, showing a systematic decrease from S₂TPP (330 cm^-1) to SSeTPP (295 cm^-1) to Se₂TPP (280 cm^-1). The decrease is less than expected from the increased mass of the selenophene ring and is consistent with stronger SeSe than SS bonding. Differential mixing with other porphyrin deformation coordinates could also explain the trend, however. SSeTPP shows two weak bands, at 1106 and 1019 cm^-1, which are coincident with IR bands and are attributable to activation of E_u-derived modes. Their weakness, and the absence of the many other E_u-derived modes, implies that there is no large displacement of the porphyrin pseudosymmetry center in the excited state of SSeTPP.