Ultrafast Intramolecular Energy Relaxation Dynamics of Benzoporphyrins: Influence of Fused Benzo Rings on Singlet Excited States KimPyosang SungJooyoung UoyamaHiroki OkujimaTetsuo UnoHidemitsu KimDongho 2011 We have investigated the role of fused benzo rings on the electronic structures and intramolecular energy relaxation dynamics in a series of benzoporphyrins (<b>Bp1</b>, <i>syn</i>-<b>Bp2</b>, <i>anti</i>-<b>Bp2</b>, <b>Bp3</b>, and <b>Bp4</b>) by using time-resolved fluorescence measurements and theoretical calculations. Interestingly, even though <i>anti</i><b></b>- and <i>syn</i><b></b>-<b>Bp2</b> have the same number of fused benzo rings, in the respective absorption spectra, <i>anti</i><b></b>-<b>Bp2</b> shows an obvious splitting of B<sub><i>x</i></sub> (Q<sub><i>x</i></sub>) and B<sub><i>y</i></sub> (Q<sub><i>y</i></sub>) states, whereas <i>syn</i>-<b>Bp2</b> 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. <i>syn</i>-<b>Bp2</b> gives rise to similar intramolecular dynamics from the B state to the Q state in the case of <b>ZnTPP</b> having <i>D</i><sub>4<i>h</i></sub> molecular symmetry. On the other hand, <i>anti</i>-<b>Bp2</b> shows split B and Q bands in the order B<sub><i>y</i></sub> > B<sub><i>x</i></sub> > Q<sub><i>x</i></sub> > Q<sub><i>y</i></sub>, which leads a superimposition of the Q<sub><i>x</i></sub> (0,0) and Q<sub><i>y</i></sub> (1,0) bands. This overlap generates a strong coupling between these two states, which results in a direct internal conversion from B<sub><i>x</i></sub> (0,0) to Q<sub><i>y</i></sub> (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.