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STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS — V. AB INITIO CONFIGURATION INTERACTION CALCULATIONS ON THE GROUND AND LOWER EXCITED SINGLET AND TRIPLET STATES OF ETHYL CHLOROPHYLLIDE a AND ETHYL PHEOPHORBIDE a
Author(s) -
Petke J. D.,
Maggiora Gerald M.,
Shipman Lester,
CHRISTOFFERSEN Ralph E.
Publication year - 1979
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1979.tb07138.x
Subject(s) - chemistry , excited state , singlet state , molecular orbital , ab initio , dipole , configuration interaction , atomic orbital , molecular electronic transition , ground state , molecule , molecular physics , atomic physics , electron , physics , organic chemistry , quantum mechanics
— Ab initio configuration interaction wavefunctions and energies are reported for the ground state and many low‐lying excited singlet and triplet states of ethyl pheophorbide a (Et‐Pheo a ) and ethyl chlorophyllide a (Et‐Chl a), and are employed in an analysis of the electronic absorption spectra of these systems. In both molecules the visible spectrum is found to consist of transitions to the two lowest‐lying 1 (π, π*) states, S 1 and S 2 . The configurational compositions of S 1 and S 2 in both molecules are similar, and are described qualitatively in terms of a four‐orbital model. The S 1 ← S 0 transition in each case is predicted to be intense, and is largely in‐plane y‐polarized, while the S 2 ← S 0 transition is predicted to be extremely weak and in‐plane polarized. The orientation of the S 2 ← S 0 transition dipole is not conclusively established in the present calculations. The Soret band in both molecules is composed of transitions to no less than ten states ( S 3‐ S 12 in Et‐Chl a and S 3 ‐S 7 S 9 ‐S 12 . and S 14 in Et‐Pheo a ), which exhibit primarily ( π, π* ) character. The configurational compositions of these states are generally a complex mixture of excitations from both occupied macrocyclic π molecular orbitals and occupied orbitals with electron density in the cyclopen‐tanone ring and the carbomethoxy chain. No clear correspondences are evident between respective Soret states of the two systems. Transitions to these states are generally intense and display a variety of in‐plane polarizations. Two additional Soret states of Et‐Pheo a , S 8 and S 13 , exhibit primarily (n. π* ) character. S 8 is characterized by excitations from u and non‐bonding regions of the carbomethoxy chain, while S 13 is described by n →π * excitations involving the nitrogen atom of ring II. No corresponding (n, π * ) states were found for Et‐Chl a . In both molecules the lowest two triplet states, T 1 and T 2 , are found to lie lower in energy than S 1 . while T, and S 1 are approximately degenerate. The configurational compositions of T 1 ‐T 4 of both molecules are nearly identical, and may be described by a four‐orbital model. However, the compositions of T 1 ‐T 4 differ sharply from those of S 1 and S 2 . A number of higher‐lying 3 (π, π*) states of both molecules ( T 5 ‐ T 13 in Et‐Chi a and T 8 ‐T 9 , T 11 ‐T 13 in Et‐Pheo a ) are found to have energies similar to the singlet Soret states, relative to S 0 . They are characterized by a complex mixture of configurations which do not include significant contributions involving the four‐orbital model. In addition, two 3 (n, π * ) states of Et‐Pheo a, T 10 and T 14 , are found, which are somewhat analogous to S 8 and S 13 . Additional data presented include the charge distributions and molecular dipole moments of the S 0 . S 1 , and T 1 states of both molecules, as well as energies and oscillator strengths of computed S n ←S 1 and T n ← 1 transitions.