Synthesis, Transient Absorption, and Transient Resonance Raman Spectroscopy of Novel Electron Donor−Acceptor Complexes: [5,15-Bis[(4‘-nitrophenyl)ethynyl]-10,20- diphenylporphinato]copper(II) and [5-[[4‘-(Dimethylamino)phenyl]ethynyl]-15-[(4‘‘-nitrophenyl)ethynyl]-10,20-diphenylporphinato]copper(II)

Abstract

We report the synthesis, transient absorption, FT Raman, resonance Raman, time-resolved resonance Raman, and transient resonance Raman spectra of pseudo-D 2h symmetric [5,15-bis[(4'-nitrophenyl)ethynyl]-10,20-diphenylporphinato]copper(II) (I) and electronically asymmetric [5-[4'-(dimethylamino)phenyl]ethynyl]-15-[(4''-nitrophenyl)ethynyl]-10,20-diphenylporphinato]copper(II) (II), which bears both electron-releasing and electron-withdrawing groups conjugated directly to the porphyrin periphery. The spectroscopic results suggest extensive electronic communication between the 5- and 15-arylethynyl groups and the porphyrin core. Relative to the parent compound, (tetraphenylporphinato)copper(II) (CuTPP), the arylethynyl substituents increase the lifetime of the excited trip-multiplet states. CuTPP, as well as compounds I and II, however, shows similar solvent-dependent dynamics: the trip-multiplet lifetimes are longer in a noncoordinating solvent such as benzene than in a coordinating solvent such as THF. This behavior is consistent with the existence of a quenching state whose effect is more pronounced upon coordination of solvent. The time-resolved resonance Raman spectrum of compound II shows features commonly associated with the relatively long-lived triplet excited states of copper(II) porphyrins. The transient resonance Raman spectrum of a short-lived excited state present in both compounds I and II is characterized by marked shifts in the nitro and porphyrin stretching frequencies relative to that observed for the ground states of both (4-nitrophenyl)ethyne and (tetraphenylporphinato)copper(II). We interpret these results for the compounds I and II as arising from (i) a short-lived excited state present at early time that possesses enhanced porphyrin-to-nitro charge-transfer character with respect to the ground state and (ii) a longer-lived excited state deriving from this initially probed charge-transfer state that is largely porphyrin localized.