Excitation wavelength-reliant light-induced energy and electron processes in pyrene and naphthalene functionalized dual-dye integrated polyaromatic azaborondipyrromethenes

Abstract

Polyaromatic-based dual-dye integrated azaborondipyrromethene (azaBODIPY), 1 containing pyrene and naphthalene at the 1,7- and 3,5 positions, respectively, was synthesized and the photo-induced energy (PEnT) and electron transfer (PET) events were explored. The contribution of the individual constituents i.e., pyrene, naphthalene, and azaBODIPY moieties, in the light-initiated events is investigated by preparing the azaBODIPY centric control compounds containing pyrene, 2 or naphthalene, 3 and tolyl moieties, TABPY. The structural integrity of the synthesized compounds is elucidated by NMR and HRMS techniques. Spectral overlap of the emission of pristine naphthalene, Naph and absorption of TABPY suggested that the naphthalene moieties in 1 and 3 would behave as energy donors and azaBODIPY as an energy acceptor. On the other hand, electrochemical studies on 1 and 2 suggested that, upon photoexcitation, pyrene moieties flanked on the azaBODIPY core can act as electron donors and an azaBODIPY moiety as an acceptor, which was further supported by in silico studies. Steady-state fluorescence studies revealed that, upon selective excitation of 3 at 275 nm, the fluorescence corresponding to the naphthalene moiety was observed to be quenched, followed by the simultaneous appearance of the azaBODIPY emission at 694 nm indicating the occurrence of PEnT from ¹Naph* → azaBODIPY. In the case of 2, the photoexcitation of either pyrene or an azaBODIPY moiety at 335 and 660 nm, respectively, displayed diminished emission of pyrene and azaBODIPY indicating the occurrence of PET from ¹Pyr* → azaBODIPY or Pyr → ¹azaBODIPY*. Interestingly, excitation of the naphthalene moiety in 1 resulted in the quenching of its emission with a concurrent quenching of the azaBODIPY fluorescence indicating the occurrence of PEnT from singlet excited naphthalene to azaBODIPY i.e., (Pyr)₂-azaBODIPY-¹(Naph)₂* → (Pyr)₂-¹azaBODIPY*-(Naph)₂ followed by PET from the ground state of the pyrene moiety to ¹azaBODIPY* leading to the formation of the charge separated state, (Pyr)₂˙⁺-(azaBODIPY)˙⁻-(Naph)₂. Parallel time resolved fluorescence studies revealed that the PEnT and PET are occurring in the range of ∼10⁹–10¹⁰ s⁻¹ in these donor–acceptor (D–A) systems.