Triarylpyridinium-Functionalized Terpyridyl Ligand for Photosensitized Supramolecular Architectures: Intercomponent Coupling and Photoinduced Processes

The electronic (absorption spectra) and electrochemical properties of a novel series of triphenylpyridinium (H3TP+=A) electron‐acceptor‐based polyad species have been correlated with their steady‐state (emission spectra) and time‐resolved (ns and ps laser flash photolysis) photophysical behavior (at both 293 and 77 K). These d6 transition metal complexes (M=RuII, OsII) of 2,2′:6′,2″‐terpyridines (tpy) are denoted as P0 and P1, depending on whether they incorporate H3TP+‐tpy or H3TP+‐ptpy ligands (ptpy=4′‐phenyl‐substituted tpy), respectively. For the P0/Ru‐based compounds, the luminescence quantum yield and excited‐state lifetime of the “{Ru(tpy)2}2+” chromophore have been found to be considerably enhanced at 293 K (e.g., τ=0.56 ns for isolated P0/Ru in acetonitrile vs τ=55 and 27 ns for P0/Ru within P0 A/Ru and P0 A2/Ru (A=electron acceptor), respectively). In spite of the lack of conjugation between P0 and A, this behavior has been ascribed to a through‐bond mediated electronic substituent effect originating from the directly connected H3TP+ electron‐withdrawing group. For the P1‐based compounds, the possibility of photoinduced electron‐transfer (PET) processes with the formation of charge‐separated (CS) states is discussed, and the main results may be summarized as follows: 1) when involved, the electron‐donor D (D=Me2N of Me2N‐ptpy) is strongly electronically coupled to P1 but cannot facilitate a reductive quenching of *P1 to give the *[D+–P1−]‐type of CS state for thermodynamic reasons, irrespective of whether M is RuII or OsII; 2) the P1 and A components have been shown to be very weakly electronically coupled; 3) at 293 K, P1/Ru‐ and P1/Os‐based polyad systems display distinct photophysical behavior with respect to A, with only the latter exhibiting a noticeable quenching of luminescence (up to 50 % for P1 A/Os with respect to P1/Os); 4) for assemblies made up of P1/Os and A components only, comparison between their room‐temperature (RT) and low‐temperature (LT; 77 K, frozen matrix) photophysical properties, together with information gleaned from combined transient absorption experiments and spectroelectrochemical studies of P1/Os and P1 A/Os, further supported by thermodynamic considerations, allowed us to conclude that a PET process does take place within the P1 A/Os dyad leading to the *[P1+–A−] CS state. For the DP1 A/Os triad, the formation of such a CS state followed by an enhanced electron‐releasing inductive effect from D is postulated. Vari