Ultrafast intramolecular energy relaxation dynamics of benzoporphyrins: influence of fused benzo rings on singlet excited states.

We have investigated the role of fused benzo rings on the electronic structures and intramolecular energy relaxation dynamics in a series of benzoporphyrins (Bp1, syn-Bp2, anti-Bp2, Bp3, and Bp4) by using time-resolved fluorescence measurements and theoretical calculations. Interestingly, even though anti- and syn-Bp2 have the same number of fused benzo rings, in the respective absorption spectra, anti-Bp2 shows an obvious splitting of B(x) (Q(x)) and B(y) (Q(y)) states, whereas syn-Bp2 exhibits degenerate B and Q bands. These features provide two dynamical models for the effect of the position of substituted benzo rings on the intramolecular energy relaxation dynamics. syn-Bp2 gives rise to similar intramolecular dynamics from the B state to the Q state in the case of ZnTPP having D(4h) molecular symmetry. On the other hand, anti-Bp2 shows split B and Q bands in the order B(y) > B(x) > Q(x) > Q(y), which leads a superimposition of the Q(x) (0,0) and Q(y) (1,0) bands. This overlap generates a strong coupling between these two states, which results in a direct internal conversion from B(x) (0,0) to Q(y) (0,0). This observation suggests that the anti-type fused position of benzo rings leads to a new mechanism in internal conversion from the B to the Q state. On the basis of this work, further insight was obtained into the effect of fused benzo rings on the photophysical properties of benzoporphyrins, providing a detailed understanding of the structure-property relationship in a series of benzoporphyrins.