A two‐colour Raman polarization experiment and structure of a chlorophyll dimer in solution

Abstract

It is well known that chlorophyll a (Chl a) forms aggregates if dissolved in non-polar solvents such as hexane. A chemical equilibrium of these aggregates and the monomer exists, but for solutions which are 10-5-10-4 M the concentration of the dimer is higher than that of the other species of this large molecule. The chemical bonding of the molecules occurs via a linkage of the C=O keto group of ring V of one Chl a (donor) to the Mg atom of another, the acceptor molecule (see Fig. 1). The bonding of the molecules follows a model which assumes weak coupling of the n-electrons of the macrocycles. If compared with that of the monomer,' the energy level diagram of the dimer consists of pairs of levels. Of these pairs, that of the donor is lowest in energy and the position of the acceptor level for the dimer in hexane lies close to the level of the monomer. For example, transitions from the ground state to the first excited singlet state of the donor give rise to an absorption band at 676 nm and the corresponding fluorescence is Stokes shifted by 9 nm. The absorption band of the acceptor is at 662 nm while S , t So for the monomer is at 659 nm. The fluorescence from the first i 0-c Me0 Figure 1 . The dirner of chlorophyll a without the phytyl group on ring IV. The y-axis has the longer path of conjugation. excited singlet of the monomer to the ground state is Stokes shifted by 5 nm.Similarly to the Q, absorption band of Chl a, the Soret band is intense. For the monomer in hexane, this band peaks at 426 nm and the intensity of the band is caused by S, +-So transitions. We have measured the depolarization ratio for a Raman band with a shift of 1555 cm-' of monomeric Chl a and obtained a value of p1 = 0.15. The Raman band is caused by scattering from C-C stretching motions and the intensity of this band is resonance enhanced. Although the symmetry of Chl a is low, we assume that the non-resonance tensor for the C-C stretches contains diagonal elements only. From the value of the depolarization ratio at resonance, we deduce that the value of two of these elements is larger than that of the third, i.e. resonance occurs with an accidently degenerate pair of electronic excited states which lie hidden in the Soret band of the monomeric species. This conclusion is supported by an analysis of the energy level diagram as discussed by Weiss.2The Soret band of the dimer of Chl a is split into components which give rise to a weak absorption at 448 nm and a stronger band at 432 nm. The resonance Raman profile for the intensity of a band of the dimer at 1550 cm-', and also that for other bands, reveals maxima at 435.2 and 448 nm.' On analysis of the resonance Raman excitation profile which includes the intensity pattern of the Raman band at 1705 cm-' (assigned to C-0 motions of the keto group on ring V of the acceptor) and 1665 cm-' (C-0Mg link), we find that the maxima represent the positions of electronic origins for S, t So of the acceptor and donor molecules. The depolarization ratio of the C=C stretch at 1550 cm-' at rigorous re...