Triarylpyridinium-functionalized terpyridyl ligand for photosensitized supramolecular architectures: intercomponent...

The electronic (absorption spectra) and electrochemical properties of a novel series of triphenylpyridinium (H(3)TP(+)=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 d(6) transition metal complexes (M=Ru(II), Os(II)) of 2,2':6',2"-terpyridines (tpy) are denoted as P0 and P1, depending on whether they incorporate H(3)TP(+)-tpy or H(3)TP(+)-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., tau=0.56 ns for isolated P0/Ru in acetonitrile vs tau=55 and 27 ns for P0/Ru within P0 A/Ru and P0 A(2)/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 H(3)TP(+) 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=Me(2)N of Me(2)N-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 Ru(II) or Os(II); 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.