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1 Polish Journal of Chemistry

1 2008

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Enhancement of Photoacidity by Formation of an Intramolecular Charge Transfer State with Twisted Conformation

Szczepanik B., Obara R., Rothe A., Weigel W., Rettig W., Stachera M., Rotkiewicz K.

Polish Journal of Chemistry

2008

Vol. 82, nr 4

807-829

The synthesis of a pair of biphenyls with a cyano acceptor and a hydroxy (methoxy) donor, which differ in the extent of steric hindrance to planarity, induced by ortho-methyl substituents with respect to the phenyl-phenyl bond is described. It is shown that dual fluorescence can be observed, ascribed to a mesomerically stabilized less polar intramolecular charge transfer (MICT) state of near planar geometry, and a more polar one, with strongly decoupled intramolecular CT resembling a TICT state. It possesses an enhanced dipole moment with respect to the MICT state, but considerably deviates from the perpendicular geometry as judged from the transition moments. This concept is used in conjunction with the excited-state proton transfer (ESPT) mechanism. The resulting photoacidity of the compound forming the enhanced ICT is an order of magnitude larger than for the MICT-compound. The mechanistic concept allows the construction of further compounds with still increased photoacidity.