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

Journal Article · Sat Nov 07 00:00:00 EST 2009 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.3257681· OSTI ID:21559801

Beames, Joseph M ^[1]; Nix, Michael G. D. ^[2]; Hudson, Andrew J ^[3]

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1. School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS (United Kingdom)
2. Department of Chemistry, University of York, Heslington, York YO10 5DD (United Kingdom)
3. Department of Chemistry, University of Leicester, Leicester LE1 7RH (United Kingdom)

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{sup -1} (390 nm) and 48 500 cm{sup -1} (206 nm). We did not observe a (1+1) REMPI signal through the optically bright {sup 1}B{sub 2} ({pi}{pi}*) and {sup 1}A{sub 1} ({pi}{pi}*) states in pyrrole due to ultrafast deactivation via conical intersections with the dissociative {sup 1}A{sub 2} ({pi}{sigma}*) and {sup 1}B{sub 1} ({pi}{sigma}{sup *}) states. However, we did observe (2+1) REMPI through Rydberg states with a dominant feature at 27 432 cm{sup -1} (two-photon energy, 54 864 cm{sup -1}) assigned to a 3d(leftarrow){pi} transition. In contrast, 2,5-dimethylpyrrole has a broad and structured (1+1) REMPI spectrum between 36 000 and 42 500 cm{sup -1} as a result of vibronic transitions to the {sup 1}B{sub 2} ({pi}{pi}*) state, and it does not show the 3d(leftarrow){pi} 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 {sup 1}{pi}{sigma}* states (dissociative) and optically bright {sup 1}{pi}{pi}* states well, predicting the barrierless deactivation of the {sup 1}B{sub 2} ({pi}{pi}*) state of pyrrole and the bound minimum of the {sup 1}B{sub 2} ({pi}{pi}*) 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.

• OSTI ID: 21559801
• Journal Information: Journal of Chemical Physics, Vol. 131, Issue 17; Other Information: DOI: 10.1063/1.3257681; (c) 2009 American Institute of Physics; ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
36 MATERIALS SCIENCE
ABSORPTION
ASYMPTOTIC SOLUTIONS
COMPARATIVE EVALUATIONS
DEACTIVATION
DENSITY FUNCTIONAL METHOD
ELECTRONIC STRUCTURE
MOLECULAR ORBITAL METHOD
PHOTOCHEMISTRY
PHOTOIONIZATION
PORPHYRINS
PYRROLES
RESONANCE
RYDBERG STATES
SIMULATION
SOLAR CELLS
TIME DEPENDENCE
ULTRAVIOLET SPECTRA
VISIBLE SPECTRA
AZOLES
CALCULATION METHODS
CARBOXYLIC ACIDS
CHEMISTRY
DIRECT ENERGY CONVERTERS
ENERGY LEVELS
EQUIPMENT
EVALUATION
EXCITED STATES
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
IONIZATION
MATHEMATICAL SOLUTIONS
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
PHOTOELECTRIC CELLS
PHOTOVOLTAIC CELLS
SOLAR EQUIPMENT
SORPTION
SPECTRA
VARIATIONAL METHODS