Pyrimidyl-substituted anthracene fluorophores: Syntheses, absorption spectra, and photophysical properties

Twelve new polycyclic aromatic chromophores whose structures recall 9,10-substituted anthracene have been prepared and their absorption spectra, and luminescence properties (both in ethanol and dichloromethane solution at room temperature and in MeOH/EtOH (4:1 v/v) rigid matrix at 77 K) have been analyzed as well as pump-probe transient absorption spectroscopy and computational studies have been performed. The compounds have variously decorated pyrimidyl groups as substituents of the anthracene framework. The compounds are conveniently grouped in two series: 1-4 and 11, in which the pyrimidine nitrogen atoms are on the external side with respect to the anthracene framework (Ext-type compounds), and 5-10, where the pyrimidine nitrogen atoms are pointing towards the anthracene platform (Int-type compounds). Compound 12 contains both a pyrimidine substituent with "inner" nitrogen atoms ("Int") and a pyrimidine with "outer" nitrogen atoms ("Ext"). All the new species are quite efficient luminophores (spectral range of emission maxima: 400-450 nm at 77 K; 415-520 nm at room temperature; emission lifetimes: 0.25-8.7 ns range; emission quantum yields ranging between 0.08 to 0.99, with a single exception), with their photophysical properties depending on the connection scheme of the pyrimidyl groups to the anthryl platform (i.e., Int-type vs Ext-type compounds). Luminescence originating from locally excited pi-pi* anthracene-based singlet states, delocalized states largely involving the pyrimidyl moieties, and charge transfer states has been identified. For the int-type series, the initially-prepared excited state can deactivate via interconversion to a saddle-shaped conformation, opening the way to fast nonradiative decays.