Environment sensitive phenothiazine dyes strongly fluorescence in protic solvents

1,10-Fused ring phenothiazines were found to be a new kind of fluorescent dyes that strongly fluorescence in protic solvents than in aprotic solvents, with higher fluorescence quantum yields (Φ) and longer emission wavelength than the previously reported dyes that with similar solvent sensitivity profile. For example, compound 5 shows a Φ-value of 0.075 in diethyl ether and 0.517 in methanol. Interestingly, a transition from CC double bond (5) to CC single bond (4) in the fused ring switch the photophysical properties completely, i.e. the analogue compound 4 demonstrates the opposite sensitivity, for which Φ = 0.099 in diethyl ether but the fluorescence is quenched completely in methanol. Further derivatization of 5 is convenient, demonstrated by the preparation of compound 6, for which the unique solvent sensitivity is reserved and higher Φ in protic solvents was observed (Φ = 0.739 in methanol). The solvatochromism of the compounds were analysed with Lippert–Mataga correlation, ETN(30) values and multilinear regressions with Catalán and Kamlet–Taft solvents scales. The aprotic and protic solvents appeared as two isolated domains in the Lippert–Mataga plots, whereas fitting with ETN(30) scales gives a linear regression comprising both aprotic and protic solvents. Kamlet–Taft scales are more appropriate than the Catalán solvent scales for describing the solvatofluorochromism of the compounds. An intermolecular hydrogen bonding mechanism is proposed for the observed switching of the fluorescence with protic solvents.