Stereoelectronic properties of photosynthetic and related systems: Ab initio configuration interaction calculations on the ground and lower excited singlet and triplet states of magnesium porphine and porphine

Abstract

Ab initio configuration interaction wavefunctions and energies are reported for the ground state and many low-lying singlet and triplet states of magnesium porphine and porphine, and are employed in an analysis of the electronic absorption spectra of these systems. For magnesium porphine, the visible spectrum is found to consist of a single low-intensity transition to a doubly degenerate 1(π,π ∗) state, whose composition may be described qualitatively in terms of a four-orbital model. The Soret band is composed of two doubly degenerate 1(π,π ∗) states of more complex configurational composition. The lower of these accounts for the major portion of the Soret band intensity, while the higher lying state carries much less oscillator strength and lies near the high energy side of the band. Several additional forbidden 1(π,π ∗) states are found to lie within the Soret region, while a fourth 1(π,π ∗) doubly degenerate state is predicted to account for the N band. In porphine the visible spectrum consists of two low intensity, perpendicularly polarized transitions to 1(π,π ∗) states similar in composition to the lowest singlet state of magnesium porphine. In contrast to magnesium porphine, however, the Soret band of porphine is shown to consist of five allowed electronic transitions. Four of these are to 1(π,π ∗) states of complex composition, which carry substantial oscillator strength, while the fifth Soret state is a weakly allowed 1(n,π ∗) state. Additional forbidden 1(π,π ∗) and 1(n,π ∗) states are located in the Soret region, and two higher lying allowed 1(π,π ∗) states which may correspond to the N band are also found. The low-lying triplet states of magnesium porphine include two doubly degenerate 3(π,π ∗) states, whose composition is described by a single configuration, which lie below the lowest lying excited singlet state, S 1. However, the composition of the lowest lying 3(π,π ∗) states of porphine, three of which lie below S 1, is found to be a balanced two-configuration description similar to that of S 1 and S 2. For both molecules many additional triplet states are found to lie in the region, relative to the ground state S 0, of the 1(π,π ∗) Soret states. Weak but detectable visible absorption is predicted for several excited singlet-singlet ( S 1 → S n ) transitions in both molecules and also for triplet-triplet ( T 1 → T n ) transitions in porphine, while triplet-triplet visible absorption in magnesium porphine is significantly less intense and likely unobservable.