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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

J. Chem. Phys. 131, 174305 (2009); doi:10.1063/1.3257681

Published 3 November 2009

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Joseph M. Beames,1 Michael G. D. Nix,2 and Andrew J. Hudson3
1School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, United Kingdom
2Department of Chemistry, University of York, Heslington, York YO10 5DD, United Kingdom
3Department 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−1 (390 nm) and 48 500  cm−1 (206 nm). We did not observe a (1+1) REMPI signal through the optically bright 1B2 (pipi*) and 1A1 (pipi*) states in pyrrole due to ultrafast deactivation via conical intersections with the dissociative 1A2 (pisigma*) and 1B1 (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<--pi 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 1B2 (pipi*) state, and it does not show the 3d<--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 1pisigma* states (dissociative) and optically bright 1pipi* states well, predicting the barrierless deactivation of the 1B2 (pipi*) state of pyrrole and the bound minimum of the 1B2 (pipi*) 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. ©2009 American Institute of Physics
History: Received 27 June 2009; accepted 9 October 2009; published 3 November 2009
Permalink: http://link.aip.org/link/?JCPSA6/131/174305/1
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KEYWORDS and PACS

Keywords
PACS
  • 82.50.Hp
    Chemical processes caused by visible and UV light
  • 71.15.Mb
    Density functional theory, local density approximation, gradient and other corrections (condensed matter electronic structure)
  • 78.40.Me
    Visible and ultraviolet spectra of organic compounds and polymers
  • YEAR: 2009

PUBLICATION DATA

ISSN:
0021-9606 (print)   1089-7690 (online)
Publisher:
AIP is a member of CrossRef AIP

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