The photophysical properties of an extended series of donor-bridge-acceptor systems (D-B-A) based on modular polyphenylene spacers has been investigated. In these D-B-A systems, the metal complex Ru(bpy)32+ acts as a photoexcitable donor and the quaternarized bipyridinium unit (DQ2+) is the electron acceptor. The intense emission of metal-to-ligand charge-transfer (MLCT) nature, exhibited by the parent [Ru-phn-bpy]2+, is strongly quenched in all the quaternarized systems. Evidences from both stationary and time-resolved spectroscopic techniques, including single-photon counting and ultrafast transient absorption spectroscopy, indicate donor-to-acceptor photoinduced electron transfer (PET) as the quenching mechanism for n = 1-3, whereas a different quenching pathway involving bridge-to-acceptor CT states takes place for n = 4-5. Although there is no accumulation of the D+-B-A- charge-separated state, likely due to charge-recombination faster than charge-separation, validation for the PET occurrence in compounds with n = 1-3 is attained by solvent effect investigation. With regard to the quenching mechanism for compounds with n = 4-5, new solvent-sensitive bands in the UV-visible spectra, increasing in intensity and red-shifting with increasing number of phenylene units, suggest the presence of new low-lying excited states of bridge-to-acceptor CT in nature. Support for this hypothesis has been obtained from DFT (Density Functional Theory) calculations performed on systems with n = 2-4. The calculations predict the lowest excited state to be a D+-B-A- charge-separated state for n = 2 and a D-B+-A- CT state for n = 4. The presence of these low-lying CT states also accounts for the fast charge-recombination taking place in the systems with n = 1-3. In conclusion, this experimental and computational investigation shows that oligophenylene bridges are efficient mediators of photoinduced electron transfer within inorganic dyads. Nonetheless, as the oligophenylene bridges are lengthened, CT excited states of the D-B+-A- type become sufficiently low in energy to introduce new quenching pathways.
PHOTOINDUCED ELECTRON TRANSFER THROUGH OLIGOPHENYLENE BRIDGES IN [Ru(bpy)3–(ph)n–DQ]4+ (n=1-5) COMPLEXES.
ORLANDI, Michele;RAVAGLIA, Marcella;GHIROTTI, Marco;INDELLI, Maria Teresa;CHIORBOLI, Claudio;SCANDOLA, Franco;
2008
Abstract
The photophysical properties of an extended series of donor-bridge-acceptor systems (D-B-A) based on modular polyphenylene spacers has been investigated. In these D-B-A systems, the metal complex Ru(bpy)32+ acts as a photoexcitable donor and the quaternarized bipyridinium unit (DQ2+) is the electron acceptor. The intense emission of metal-to-ligand charge-transfer (MLCT) nature, exhibited by the parent [Ru-phn-bpy]2+, is strongly quenched in all the quaternarized systems. Evidences from both stationary and time-resolved spectroscopic techniques, including single-photon counting and ultrafast transient absorption spectroscopy, indicate donor-to-acceptor photoinduced electron transfer (PET) as the quenching mechanism for n = 1-3, whereas a different quenching pathway involving bridge-to-acceptor CT states takes place for n = 4-5. Although there is no accumulation of the D+-B-A- charge-separated state, likely due to charge-recombination faster than charge-separation, validation for the PET occurrence in compounds with n = 1-3 is attained by solvent effect investigation. With regard to the quenching mechanism for compounds with n = 4-5, new solvent-sensitive bands in the UV-visible spectra, increasing in intensity and red-shifting with increasing number of phenylene units, suggest the presence of new low-lying excited states of bridge-to-acceptor CT in nature. Support for this hypothesis has been obtained from DFT (Density Functional Theory) calculations performed on systems with n = 2-4. The calculations predict the lowest excited state to be a D+-B-A- charge-separated state for n = 2 and a D-B+-A- CT state for n = 4. The presence of these low-lying CT states also accounts for the fast charge-recombination taking place in the systems with n = 1-3. In conclusion, this experimental and computational investigation shows that oligophenylene bridges are efficient mediators of photoinduced electron transfer within inorganic dyads. Nonetheless, as the oligophenylene bridges are lengthened, CT excited states of the D-B+-A- type become sufficiently low in energy to introduce new quenching pathways.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
