Comparison of the resonance-enhanced multiphoton ionization spectra of pyrrole and 2,5-dimethylpyrrole: Building toward an understanding of the electronic structure and photochemistry of porphyrins

The photophysical properties of porphyrins have relevance for their use as light-activated drugs in cancer treatment and sensitizers in solid-state solar cells. However, the appearance of their UV-visible spectra is usually explained inadequately by qualitative molecular-orbital theories. We intend to gain a better insight into the intense absorption bands, and excited-state dynamics, that make porphyrins appropriate for both of these applications by gradually building toward an understanding of the macrocyclic structure, starting with studies of smaller pyrrolic subunits. We have recorded the (1+1) and (2+1) resonance-enhanced multiphoton ionization (REMPI) spectra of pyrrole and 2,5-dimethylpyrrole between 25 600 cm^-1 (390 nm) and 48 500 cm ^-1 (206 nm). We did not observe a (1+1) REMPI signal through the optically bright ¹B ₂ (π π ^* ) and A1 (π π ^* ) states in pyrrole due to ultrafast deactivation via conical intersections with the dissociative A1 ₂ (π σ) and ¹B ₁ (π σ) states. However, we did observe (2+1) REMPI through Rydberg states with a dominant feature at 27 432 cm^-1 (two-photon energy, 54 864 cm^-1) assigned to a 3dπ transition. In contrast, 2,5-dimethylpyrrole has a broad and structured (1+1) REMPI spectrum between 36 000 and 42 500 cm^-1 as a result of vibronic transitions to the ¹B ₂ (π π ) state, and it does not show the 3dπ Rydberg transition via (2+1) REMPI. We have complemented the experimental studies by a theoretical treatment of the excited states of both molecules using time-dependent density functional theory (TD-DFT) and accounted for the contrasting features in the spectra. TD-DFT modeled the photochemical activity of both the optically dark π 1 σ states (dissociative) and optically bright π 1 π ^* states well, predicting the barrierless deactivation of the ¹B ₂ (π π ^* ) state of pyrrole and the bound minimum of the ¹B ₂ (π π ^* ) state in 2,5-dimethylpyrrole. However, the quantitative agreement between vibronic transition energies and the excited-state frequencies calculated by TD-DFT was hampered by inaccurate modeling of Rydberg orbital mixing with the valence states, caused by the lack of an asymptotic correction to the exchange-correlation functionals used.