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

• Published in: The Journal of chemical physics 2009-11, Vol.131 (17), p.174305-174305
• Main Authors: Beames, Joseph M, Nix, Michael G D, Hudson, Andrew J
• Format: Article
• Language: English
• Subjects: ABSORPTION; ASYMPTOTIC SOLUTIONS; AZOLES; CALCULATION METHODS; CARBOXYLIC ACIDS; CHEMISTRY; COMPARATIVE EVALUATIONS; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; DEACTIVATION; DENSITY FUNCTIONAL METHOD; DIRECT ENERGY CONVERTERS; ELECTRONIC STRUCTURE; Electrons; ENERGY LEVELS; EVALUATION; EXCITED STATES; HETEROCYCLIC ACIDS; HETEROCYCLIC COMPOUNDS; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; IONIZATION; MATERIALS SCIENCE; MATHEMATICAL SOLUTIONS; Models, Molecular; Molecular Conformation; MOLECULAR ORBITAL METHOD; ORGANIC ACIDS; ORGANIC COMPOUNDS; ORGANIC NITROGEN COMPOUNDS; Photochemical Processes; PHOTOCHEMISTRY; PHOTOELECTRIC CELLS; PHOTOIONIZATION; Photons; PHOTOVOLTAIC CELLS; PORPHYRINS; Porphyrins - chemistry; PYRROLES; Pyrroles - chemistry; Quantum Theory; RESONANCE; RYDBERG STATES; SIMULATION; SOLAR CELLS; SOLAR EQUIPMENT; SORPTION; SPECTRA; Spectrum Analysis; Surface Properties; TIME DEPENDENCE; Time Factors; ULTRAVIOLET SPECTRA; VARIATIONAL METHODS; VISIBLE SPECTRA
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.3257681

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 (1)B(2) (pipi( *)) and (1)A(1) (pipi( *)) states in pyrrole due to ultrafast deactivation via conical intersections with the dissociative (1)A(2) (pisigma( *)) and (1)B(1) (pisigma( *)) 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 3d