Role of the πσ* State in Molecular Photophysics

摘要

Photosynthesis, which depends on light-driven energy andnelectron transfer in assemblies of porphyrins, chloro-nphylls, and carotenoids, is just one example of the manyncomplex natural systems of photobiology. A fuller under-nstanding of the spectroscopy and photophysics of simple aro-nmatic molecules is central to elucidating photochemicalnprocesses in the more sophisticated assemblies of photobi-nology. Moreover, developing a better grasp of the photo-nphysics of simple aromatic molecules will also enhance ournability to create and improve practical applications in pho-ntochemical energy conversion, molecular nanophotonics, andnmolecular electronics. In this Account, we present a concertednexperimental and theoretical study of aromatic ethynes, aro-nmatic nitriles, and fluorinated benzenes, illustrating thenimportant roles that the low-lying πσ* state plays in the elec-ntronic relaxation of these aromatic compounds.nDiphenylacetylene, 4-dialkylaminobenzonitriles, 4-dialky-nlaminobenzethynes, and fluorinated benzenes exhibit fluo-nrescence that strongly quenches as the excitation energy isnincreased for gas-phase systems and at elevated temperatures in solution. Much of this interesting photophysical behaviorncan be attributed to the presence of a dark intermediate state that crosses the fluorescent ππ* state. Our quantum chem-nistry calculations, as well as time-resolved laser spectroscopies, indicate that this dark intermediate state is the πσ* statenthat arises from the promotion of an electron from the π orbital of the phenyl ring to the σ* orbital localized in the CtXngroup (where X is CH and N) or on the CsX group (where X is a halogen). These crossings not only lead to the strongnexcitation energy and temperature dependence of fluorescence but also induce highly interesting πσ*-mediated intramo-nlecular charge transfer in 4-dialkylaminobenzonitriles.