Monitoring pH and solvent proticity with donor–acceptor-substituted biphenyls: a new approach towards highly sensitive and powerful fluorescent probes by tuning the molecular structure

Abstract

The ability and structural requirements of 4-dimethylamino-4′-cyano-substituted biphenyls showing photoinduced intramolecular charge transfer (CT) to serve as hydrogen bond- or pH-sensitive fluorescent probes is investigated. The donor–acceptor (D–A) biphenyls I and II being planar in the CT excited state are most suitable as pH-sensitive fluorescent probes. The two compounds show analytically valuable features such as well-separated absorption and emission bands and signal changes spanning four orders of magnitude and can be employed in ratiometric, “self-calibrating”, and highly sensitive pH fluorosensing in the range 0<pH<4. Because of an enhanced charge separation in the excited state, the highly twisted D–A biphenyl III shows an increased proton sensitivity and can therefore be used to probe solvent proticity ia hydrogen bond formation. The molecular and electronic characteristics of the two types of probes are discussed for the different neutral and acidic alcoholic and/or aqueous solvent mixtures investigated.