Charge-Transfer Transitions Govern the Reactivity and Photophysics of Vicinally Diphosphanyl-Substituted Diborapentacenes

Abstract

2,3-Difluoro-5,14-dihydro-5,14-diborapentacene (DBP) was endowed with two vicinal Ph2P groups by an SNAr reaction at both CF sites using Ph2PSiMe3. Computations reveal the ambipolar product P to undergo P-to-B charge transfer under ambient light irradiation. Consequently, P is prone to photooxidation by air, yielding the Ph2P(O) species PO. With S8 or [Me3O][BF4], P furnishes the Ph2P(S) or Ph2P(Me)+ derivatives PS or [PMe][BF4]2. Along the series P, PO, PS, and [PMe][BF4]2, the redox potentials shift anodically from E1/2=−1.89 V to −1.02 V (CH2Cl2). Thus, derivatization of the Ph2P group allows late-stage modulation of the LUMO-energy level of the DBP. Derivatization also influences the emission properties of the compounds, as PO shows green (521 nm) and [PMe][BF4]2 red (622 nm) fluorescence in C6H6, while P and PS are dark. With CuBr and AgBr, P forms dimeric [M(μ-Br)]2 complexes [PCu]2 and [PAg]2, which show pronounced metal-to-ligand charge transfer (MLCT), making P a promising ligand for photocatalysts.