Ground State Recovery Dynamics Research Articles

Establishing the origin of Marcus-inverted-region behaviour in the excited-state dynamics of cobalt(III) polypyridyl complexes.

Growing interest in the use of first-row transition metal complexes in a number of applied contexts-including but not limited to photoredox catalysis and solar energy conversion-underscores the need for a detailed understanding of their photophysical properties. A recent focus on ligand-field photocatalysis using cobalt(III) polypyridyls in particular has unlocked unprecedented excited-state reactivities. Photophysical studies on Co(III) chromophores in general are relatively uncommon, and so here we carry out a systematic study of a series of Co(III) polypyridyl complexes in order to delineate their excited-state dynamics. Compounds with varying ligand-field strengths were prepared and studied using variable-temperature ultrafast transient absorption spectroscopy. Analysis of the data establishes that the ground-state recovery dynamics are operating in the Marcus inverted region, in stark contrast to what is typically observed in other first-row metal complexes. The analysis has further revealed the underlying reasons driving this excited-state behaviour, thereby enabling potential advancements in the targeted use of the Marcus inverted region for a variety of photolytic applications.

The Excited-State Lifetime of Poly(NDI2OD-T2) Is Intrinsically Short.

Conjugated polymers composed of alternating electron donor and acceptor segments have come to dominate the materials being considered for organic photoelectrodes and solar cells, in large part because of their favorable near-infrared absorption. The prototypical electron-transporting push-pull polymer poly(NDI2OD-T2) (N2200) is one such material. While reasonably efficient organic solar cells can be fabricated with N2200 as the acceptor, it generally fails to contribute as much photocurrent from its absorption bands as the donor with which it is paired. Moreover, transient absorption studies have shown N2200 to have a consistently short excited-state lifetime (∼100 ps) that is dominated by a ground-state recovery. In this paper, we investigate whether these characteristics are intrinsic to the backbone structure of this polymer or if these are extrinsic effects from ubiquitous solution-phase and thin-film aggregates. We compare the solution-phase photophysics of N2200 with those of a pair of model compounds composed of alternating bithiophene (T2) donor and naphthalene diimide (NDI) acceptor units, NDI-T2-NDI and T2-NDI-T2, in a dilute solution. We find that the model compounds have even faster ground-state recovery dynamics (τ = 45, 27 ps) than the polymer (τ = 133 ps), despite remaining molecularly isolated in solution. In these molecules, as in the case of the N2200 polymer, the lowest excited state has a T2 to NDI charge-transfer (CT) character. Electronic-structure calculations indicate that the short lifetime of this state is due to fast nonradiative decay to the ground state (GS) promoted by strong CT-GS electronic coupling and strong electron-vibrational coupling with high-frequency (quantum) normal modes.

DNA-Templated Aggregates of Strongly Coupled Cyanine Dyes: Nonradiative Decay Governs Exciton Lifetimes.

Molecular excitons are used in a variety of applications including light harvesting, optoelectronics, and nanoscale computing. Controlled aggregation via covalent attachment of dyes to DNA templates is a promising aggregate assembly technique that enables the design of extended dye networks. However, there are few studies of exciton dynamics in DNA-templated dye aggregates. We report time-resolved excited-state dynamics measurements of two cyanine-based dye aggregates, a J-like dimer and an H-like tetramer, formed through DNA-templating of covalently attached dyes. Time-resolved fluorescence and transient absorption indicate that nonradiative decay, in the form of internal conversion, dominates the aggregate ground state recovery dynamics, with singlet exciton lifetimes on the order of tens of picoseconds for the aggregates versus nanoseconds for the monomer. These results highlight the importance of circumventing nonradiative decay pathways in the future design of DNA-templated dye aggregates.

Redox-Based Photostabilizing Agents in Fluorescence Imaging: The Hidden Role of Intersystem Crossing in Geminate Radical Ion Pairs.

Here we report transient absorption studies on the ground-state recovery dynamics of the single-molecule fluorophore Cy3B in the presence of four different photostabilizing agents, namely β-mercaptoethanol (β-ME), Trolox (TX), n-propyl gallate (n-PG), and ascorbic acid (AA). These are triplet-state quenchers that operate via photoinduced electron transfer (PeT). While quantitative geminate recombination was recorded following PeT for β-ME (∼100%), for Trolox, n-propyl gallate, and ascorbic acid the extent of geminate recombination was >48%, >27%, and >13%, respectively. The results are rationalized in terms of the rates of intersystem crossing (ISC) in the newly formed geminate radical ion pairs (GRIPs). Rapid spin relaxation in the radicals formed accounts for quantitative geminate recombination with β-ME and efficient geminate recombination with TX. Our results illustrate how the interplay of PeT quenching efficiency and geminate recombination dynamics may lead to improved photostabilization strategies, critical for single-molecule fluorescence and super-resolution imaging.

Ultrafast electronic deactivation dynamics of the rare natural nucleobase hypoxanthine

The electronic deactivation of the 6-oxopurine derivative hypoxanthine, its nucleoside inosine and the nucleotide inosine monophosphate have been studied by femtosecond time-resolved spectroscopy after ππ∗ photoexcitation at λ=260nm. The development of a highly sensitive parallel broadband (near-UV/VIS) and single-color (deep-UV) transient absorption setup enabled us to monitor the excited-state decay and the ground-state recovery dynamics in one and the same experiment. The measurements revealed similar relaxation behavior, with time constants of τ1≲0.1ps, τ2≈0.21±0.08ps and τ3≈1.8±0.4ps, for all three investigated molecules. The observed dynamics are assumed to take place through a conical intersection involving an out-of-plane puckering mode of the six-membered ring similar to guanine.

Ultrafast transient mid IR to visible spectroscopy of fully reduced flavins

The light sensing apparatus of many organisms includes a flavoprotein. In any spectroscopic analysis of the photocycle of flavoproteins a detailed knowledge of the spectroscopy and excited state dynamics of potential intermediates is required. Here we correlate transient vibrational and electronic spectra of the two fully reduced forms of flavin adenine dinucleotide (FAD): FADH(-) and FADH(2). Ground and excited state frequencies of the characteristic carbonyl modes are observed and assigned with the aid of DFT calculations. Excited state decay and ground state recovery dynamics of the two states are reported. Excited state decay occurs on the picosecond timescale, in agreement with the low fluorescence yield, and is markedly non single exponential in FADH(-). Further, an unusual 'inverse' isotope effect is observed in the decay time of FADH(-), suggesting the involvement in the radiationless relaxation coordinate of an NH or hydrogen bond mode that strengthens in the excited electronic state. Ground state recovery also occurs on the picosecond time scale, consistent with radiationless decay by internal conversion, but is slower than the excited state decay.

Ultrafast Excited State Dynamics of the Perylene Radical Cation Generated upon Bimolecular Photoinduced Electron Transfer Reaction

The ultrafast ground state recovery (GSR) dynamics of the radical cation of perylene, Pe(*+), generated upon bimolecular photoinduced electron transfer in acetonitrile, has been investigated using pump-pump-probe spectroscopy. With 1,4-dicyanobenzene as electron acceptor, the free ion yield is substantial and the GSR dynamics of Pe(*+) was found to depend on the time delay between the first and second pump pulses, Deltat(12), i.e., on the "age" of the ion. At short Deltat(12), the GSR dynamics is biphasic, and at Deltat(12) larger than about 500 ps, it becomes exponential with a time constant around 3 ps. With trans-1,2-dicyanoethylene as acceptor, the free ion yield is essentially zero and the GSR dynamics of Pe(*+) remains biphasic independently of Deltat(12). The change of dynamics observed with 1,4-dicyanobenzene is ascribed to the transition from paired to free solvated ion, because in the pair, the excited ion has an additional decay channel to the ground state, i.e., charge recombination followed by charge separation. The rate constants deduced from the analysis of these GSR dynamics are all fully consistent with this hypothesis.

Excited-State Dynamics of Organic Radical Ions in Liquids and in Low-Temperature Matrices

The excited-state dynamics of the radical cations of perylene (PE•+), tetracene (TE•+), and thianthrene (TH•+), as well as the radical anions of anthraquinone (AQ•-) and tetracenequinone (TQ•-), formed by γ irradiation in low-temperature matrices (PE•+, TH•+, AQ•-, and TQ•-) or by oxidation in sulfuric acid (PE•+, TE•+, and TH•+) have been investigated using ultrafast pump−probe spectroscopy. The longest ground-state recovery time measured was 100 ps. The excited-state lifetime of PE•+ is substantially longer in low-temperature matrices than in H2SO4, where the effects of perdeuteration and of temperature on the ground-state recovery dynamics indicate that internal conversion is not the major decay channel of PE•+*. The data suggest that both PE•+* and TE•+* decay mainly through an intermolecular quenching process, most probably a reversible charge transfer reaction. Contrarily to AQ•-*, TQ•-* exhibits an emission in the visible which, according to theoretical calculations, occurs from an upper excited state.

Solvent dependence of the ultrafast ground state recovery dynamics of phenol blue

Abstract We have studied femtosecond ground state recovery dynamics of phenol blue (PB) by pump-probe spectroscopy with a time resolution of 33 fs. Multiexponential decay with time constants extending from 300 fs to 10 ps were observed. Some low frequency intramolecular quantum beats were also observed. Solvent dependence of these decays and beats are discussed.

Solvent Dependent Ultrafast Ground State Recovery Dynamics of Triphenylmethane Dyes

We have studied ground state recovery dynamics of triphenylmethane dyes, brilliant green and malachite green, by a single wavelength pump-probe spectroscopy at 635 nm with a time-resolution of 33 fs. The recovery of the ground state dependeded on solvent viscosity and also on solute molecular size. We observed a plateau or a rise component in the signal which indicates an intermediate state. The rise time also showed solvent viscosity dependence even in the ultrafast time domain, although no solute dependence was observed. It is suggested that the lack of solute dependece indicates that only small conformational change is involved in the initial process.

Solvent dependence of ultrafast ground state recovery of the triphenylmethane dyes, brilliant green and malachite green

We have studied femtosecond ground state recovery dynamics of the triphenylmethane dyes brilliant green (BG) and malachite green (MG) by pump–probe spectroscopy at the center wavelength of 635 nm with a time resolution of 33 fs. The ultrafast recovery of the ground state bleach was highly nonexponential and depended on the solvent viscosity, although all time constants were shorter than the solvation times obtained from other measurements. We observed a plateau or a rise component in the signal, which indicates an intermediate state. The rise time showed a viscosity dependence, even in the ultrafast time domain. It should be noted that the decay times were always longer for BG than MG, while the rise time did not show a solute dependence. The torsional motion of the amino-substituted phenyl group may be involved in the ultrafast process to the intermediate state, but lack of a solute dependence indicates that only a small conformational change is involved.

Picosecond dynamics of torsional motion in malachite green adsorbed on silica. A time-resolved surface second harmonic generation study

The ground-state recovery dynamics of malachite green adsorbed on silica have been studied by picosecond time-resolved surface second harmonic generation. Reaction dynamics were investigated as a function of coverage, laser intensity and wave-length. A fast component was observed to be independent of all three parameters. This suggests that intramolecular motion may be facile in adsorbates, and that energy disposal is efficient for low-frequency modes. In addition an intensity and coverage dependent disruption of the surface layer was detected. This is tentatively assigned to ultrafast multiphoton chemistry of the aggregates formed at high coverage.