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
Authors: Petke J. D. , Maggiora Gerald M. , Shipman Lester , CHRISTOFFERSEN Ralph E. 
Published: January 2008

Abstract— 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, S1 and S2. The configurational compositions of S1 and S2 in both molecules are similar, and are described qualitatively in terms of a four‐orbital model. The S1← S0 transition in each case is predicted to be intense, and is largely in‐plane y‐polarized, while the S2← S0 transition is predicted to be extremely weak and in‐plane polarized. The orientation of the S2← S0 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 (S3‐S12 in Et‐Chl a and S3‐S7S9‐S12. and S14 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, S8 and S13, exhibit primarily (n. π*) character. S8 is characterized by excitations from u and non‐bonding regions of the carbomethoxy chain, while S13 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, T1 and T2, are found to lie lower in energy than S1. while T, and S1 are approximately degenerate. The configurational compositions of T1‐T4 of both molecules are nearly identical, and may be described by a four‐orbital model. However, the compositions of T1‐T4 differ sharply from those of S1 and S2.

A number of higher‐lying 3(π, π*) states of both molecules (T5‐T13 in Et‐Chi a and T8‐T9, T11‐T13 in Et‐Pheo a) are found to have energies similar to the singlet Soret states, relative to S0. 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, T10 and T14, are found, which are somewhat analogous to S8 and S13.

Additional data presented include the charge distributions and molecular dipole moments of the S0. S1, and T1 states of both molecules, as well as energies and oscillator strengths of computed Sn←S1 and Tn←1 transitions.