NIR Dual Luminescence from an Extended Porphyrin. Spectroscopy, Photophysics and Theory

Spectroscopic and photophysical properties of an extended Zn porphyrin with fused bis(tetraazaanthracene) arms including a 2,9-diphenyl-1,10-phenanthroline incorporated in a polyether macrocycle are investigated in solvents of different polarity pointing to the presence of two emitting singlet excited states. The absorption and emission features are identified and ascribed, on the basis of solvent polarity dependence, to a pi-pi* and to a charge transfer (CT) state, respectively. Whereas the intraligand pi-pi* transition is assigned to the intense absorption observed at 442-455 nm, the CT states contribute to the bands at 521-525 nm and 472-481 nm. The theoretical analysis of the absorption spectrum confirms the presence of two strong bands centered at 536 and 437 nm corresponding to CT and pi-pi* states, respectively. Weak CT transitions are calculated at 657 and 486 nm. Two emission maxima are observed in toluene at 724 nm from a (1)pi-pi* state and at 800 nm from a (CT)-C-1 state, respectively. (CT)-C-1 bands shift bathochromically by increasing the solvent polarity whereas the energy of the (1)pi-pi band is less affected. Likewise, the emission yield and lifetime energy (CT)-C-1 band are strongly affected by solvent polarity. This is rationalized by a (1)pi-pi* -> (CT)-C-1 internal conversion driven by solvent polarity, this process being competitive with the (1)pi-pi* to ground state deactivation channel. Time resolved absorption spectra indicate the presence of two triplet states, a short-lived one (nanoseconds range) and a longer lived one (hundreds of microsecond range) ascribed to a (3)pi-pi* and a (CT)-C-3, respectively. For them, a conversion mechanism similar to that of the singlet excited states is suggested.