Vibrational decoherence induced by ultrafast intramolecular charge separation of an asymmetric bianthryl derivative.

Coherent wavepacket oscillation accompanying the ultrafast photoexcited intramolecular charge separation (CS) of 9,9'-bianthryl (BA) and 10-cyano-9,9'-bianthryl (CBA) in a room temperature ionic liquid, N,N-diethyl-N-methyl-N-(methoxyethyl)ammonium tetrafluoroborate (DemeBF4), was investigated by femtosecond time-resolved transient absorption spectroscopy. The frequency of the coherent oscillation observed for CBA in nonpolar n-hexane solution (Hex) was 377 cm-1, while this oscillation was undetectable in DemeBF4. For BA in DemeBF4, coherent oscillation with a frequency of 394 cm-1 was observed, which is similar to that for CBA in Hex. CS of CBA occurs in the ultrashort time range of ≤100 fs, while that of BA occurs in a few picosecond range [E. Takeuchi et al., J. Phys. Chem. C 120, 14502-14512 (2016)]. Hence, the oscillation of CBA in Hex and that of BA in DemeBF4 are assigned to the molecular vibration in the locally excited state, while this oscillation dephases instantaneously for CBA in DemeBF4 due to the ultrafast CS and no oscillation was generated in the CS state. This result suggests that the CS reaction is not mediated by a specific intramolecular vibration in the CS state but occurs incoherently through higher levels of multiple vibrational modes.

Mechanistic studies of photoinduced intramolecular and intermolecular electron transfer processes in RuPt-centred photo-hydrogen-evolving molecular devices.

The photoinduced electron transfer properties of two photo-hydrogen-evolving molecular devices (PHEMDs) [(bpy)2Ru(II)(phen-NHCO-bpy-R)Pt(II)Cl2](2+) (i.e., condensation products of [Ru(bpy)2(5-amino-phen)](2+) and (4-carboxy-4'-R-bpy)PtCl2; bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline; RuPt-COOH for R = COOH and RuPt-CN for R = CN) were investigated. RuPt-CN demonstrates higher photocatalytic performance relative to RuPt-COOH arising from a larger driving force for the intramolecular photoinduced electron transfer (PET) associated with a stronger electron-withdrawing effect of R (ΔGPET = −0.43 eV for RuPt-CN and −0.16 eV for RuPt-COOH). This is the first study on PET events using ultrafast spectroscopy. Dramatic enhancement is achieved in the rate of PET in RuPt-CN (1.78 × 1010 s(−1)) relative to RuPt-COOH (3.1 × 109 s(−1)). For each system, the presence of three different conformers giving rise to three different PET rates is evidenced, which are also discussed with the DFT results. Formation of a charge-separated (CS) state [(bpy)2Ru(III)(phen-NHCO-bpy(−˙)-R)Pt(II)Cl2](2+) in the sub-picosecond time regime and recombination in the picosecond time regime are characterized spectrophotometrically. The CS-state formation was found to compete with reductive quenching of the triplet excited state by EDTA whose dianionic form ion-pairs with dicationic RuPt-COOH. Thus, a key intermediate [(bpy)2Ru(II)(phen-NHCO-bpy(−˙)-R)Pt(II)Cl2](+) (i.e., the one-electron-reduced species) prior to the H2 formation was found to be formed either via reduction of the CS state by EDTA or via formation of [(bpy)2Ru(II)(phen(−˙)-NHCO-bpy-R)Pt(II)Cl2](+) by reductive quenching of the triplet excited state. More importantly, it is also shown that some of the conformers in solution possess a CS lifetime sufficiently long to drive hydrogen evolution from water.

Photoinduced charge-transfer dynamics of sequentially aligned donor-acceptor systems in an ionic liquid.

Photoinduced charge separation processes of linear phenyleneethynylene derivatives (PEN) with different sequences of electron-withdrawing perfluorophenyl groups (A) and electron-donating phenyl groups (D) were investigated in an ionic liquid (IL), BmimTFSI, by picosecond time-resolved fluorescence (TRF) and transient absorption (TA) spectroscopies. Very rapid photoinduced charge separation within 10 ps in AADD was followed by the stabilization of the charge-transfer (CT) state by the solvation, while the excited states in ADAD and ADDA were ascribable to the locally excited (LE) state. Equilibrium between the LE and CT states was established for DAAD with time constants of forward and backward processes much faster than the solvation time. The relative population of the CT state increases with time owing to the dynamic stabilization of the CT state by the solvation. The elementary charge separation process, the increase in the CT population, and their relation to the solvation time were discussed.

Enhancement of vibrational coherence by femtosecond degenerate four-wave-mixing for a chromophore in 1-propanol glass.

The enhancement effect of vibrational coherence by femtosecond degenerate four-wave-mixing (DFWM) technique was investigated with a dye, NK-2990, doped in low temperature 1-propanol glass. The strongest enhancement was observed with the delay between the first and the second pulses, t(13), set at about quarter of the oscillation period, which does not coincide with the theoretical prediction of half the period. One of the enhanced vibrational modes at ca. 90 cm(-1) became significantly broader below the glass transition temperature, indicating its sensitivity towards microscopic viscosity of the environment.

Ultrafast solvation dynamics in room temperature ionic liquids observed by three-pulse photon echo peak shift measurements.

Three-pulse photon echo peak shift (3PEPS) measurement was applied to the investigation of the primary part (<100 ps) of the solvation dynamics in a series of imidazolium ionic liquids (IL) with an organic dye, oxazine 4 (Ox4), utilized as a probe. The ultrafast solvent response in the range of ≤300 fs exhibited dependence on the square root of the anion mass, indicating its relation with the inertial motion of anion. The inertial response of ILs with chloride anion was the fastest among other ILs with heavier and larger anions. Because Ox4 is a cationic dye, it holds a stronger interaction with the anion of IL, thus the ultrafast part of the solvation is strongly affected by the inertial motion of anions. The second solvation component in the range of ≤3.5 ps had better correlation with the reduced mass and the size of both ions included, indicating the beginning of a more global solvation process.

Photoinduced electron transfer in tris(2,2'-bipyridine)ruthenium(II)-viologen dyads with peptide backbones leading to long-lived charge separation and hydrogen evolution.

Three 5,5'-disubstituted-2,2'-bipyridine ligands tethered to l-Asp-based peptide backbones having pendant viologen-modified branches, i.e., 5-ethoxycarbonyl-5'-(N-G(1)-carbamoyl)-2,2'-bipyridine ((4+)), 5,5'-bis(N-G(1)-carbamoyl)-2,2'-bipyridine ((8+)), and 5,5'-bis(N-G(2)-carbamoyl)-2,2'-bipyridine ((12+)), were prepared, where G(1) = Asp(NHG(3))-NHG(3), G(2) = Asp(NHG(3))-Asp(NHG(3))-NHG(3), and G(3) = -(CH(2))(2)-(+)NC(5)H(4)-C(5)H(4)N(+)-CH(3), i.e., 2-(1'-methyl-4,4'-bipyridinediium-1-yl)ethyl. These were reacted with cis-Ru(bpy)(2)Cl(2) to give three new dyads [Ru(bpy)(2)()](6+) ((6+)), [Ru(bpy)(2)()](10+) ((10+)), and [Ru(bpy)(2)()](14+) ((14+)), respectively, where bpy = 2,2'-bipyridine. All these dyads undergo extremely efficient intramolecular quenching leading to the formation of charge separated (CS) states (Ru(III)-MV(+) ), and display a triple exponential decay due to the presence of three classes of conformers with each exhibiting the individual rate of electron transfer. The lifetimes (contributions) were determined as 12.5 ps (94.2%), 791 ps (4.5%), and 18.3 ns (1.2%) for , 82.2 ps (79.9%), 1.12 ns (12.4%), and 4.60 ns (7.7%) for , and 43.6 ps (71.6%), 593 ps (20.2%), and 3.75 ns (8.1%) for . The forward electron transfer rate constants (k(ET)) for the major components were calculated as k(ET) = 8.3 x 10(10) s(-1) for , k(ET) = 1.2 x 10(10) s(-1) for , and k(ET) = 2.3 x 10(10) s(-1) for . Further, the lifetimes and quantum yields of charge separated states were determined as tau(CS) = 16 +/- 3 ns and Phi(CS) = 0.81 for , tau(CS) = 20 +/- 3 ns and Phi(CS) = 0.92 for , and tau(CS) = 20 +/- 3 ns and Phi(CS) = 0.64 for . The backward electron transfer rate constants (k(BET)) were also determined as 6.3 x 10(7), 5.0 x 10(7), and 5.0 x 10(7) s(-1) for , , and , respectively. From the analysis of electrical conductivity, the major ion-pair adducts in aqueous media were characterized as (PF(6))(5+) (52%) for , (PF(6))(2)(8+) (29%) and (PF(6))(3)(7+) (32%) for , and (PF(6))(3)(11+) (27%) and (PF(6))(4)(10+) (29%) for , at a total complex concentration of 0.04 mM. The present family is found to be the first example of a Ru(bpy)(3)(2+)-MV(2+) system in which three orders of magnitude of difference is achieved between the forward and backward electron transfer rate constants. These dyads were finally combined with a Pt(ii)-based H(2)-evolving catalyst, i.e., cis-diamminedichloroplatinum(ii), to ascertain the applicability of the system towards the visible light-induced water splitting processes.