"Ultrafast Dynamics of Meso-tetraphenylmetalloporphyrins: The Role of Dark States"Y. Liang, M. Bradler, M. Klinger, O. Schalk, M. C. Balaban, T. S. Balaban, E. Riedle and A.-N. Unterreiner
ChemPlusChem 78, 1244 - 1251 (2013).
Studying the relaxation pathways of porphyrins and related structures upon light absorption is crucial for understanding the fundamental processes of light harvesting in biosystems and many applications. Here, we show by means of transient absorption studies following Q- and Soret-band excitation and ab initio calculations on meso-tetraphenylporphyrinato magnesium (II) (MgTPP) and meso-tetraphenylporphyrinato cadmium (II) (CdTPP) that electronic relaxation following Soret-band excitation of porphyrins with a heavy central atom is mediated by a hitherto disregarded dark state. This accounts for an increased rate for internal conversion. The dark state originates from an orbital localized at the central N-atoms and its energy continuously drops in the series Mg, Zn, Cd to below 2.75 eV for CdTPP solvated in tetrahydrofuran. Furthermore, we are able to directly trace the fast intersystem crossing in the cadmium derivative which takes place within (110 ± 20) ps.
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