Picosecond to microsecond photodynamics of a nonplanar nickel porphyrin: Solvent dielectric and temperature effects

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Distortion of the Ni(porphyrin) ring from planarity does not appreciably affect the decay of the intraligand 1ππ* excited state to the dd state and the vibrational relaxation of the latter. However, the dd excited state is kinetically stabilized at least 100 times in nonpolar solvents at room temperature and 104 times at 80 K, as compared with analogous planar porphyrins. This is caused by a trapping of the excited molecule in a polar conformation that is different from the stable conformation of the ground state. Excited state lifetime of the nonplanar Ni porphyrins is strongly solvent dependent, varying from a few picoseconds in highly polar solvents to about 50 ns in nonpolar solvents and to a few microseconds in solid solutions at 80 K. This remarkable solvent dependence of the excited state dynamics originates in the coupling of the excited state decay with a conformational change. Because of the inherent polarity of the excited state conformation, the rate of the latter increases with the polarity of the solvent.

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