On the Role of Hydrogen Bonds in Photoinduced Electron-Transfer Dynamics between 9-Fluorenone and Amine Solvents (original) (raw)
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Chemical Physics Letters, 2017
We have prepared a molecular triad containing a triphenylamine as the electron donor D subunit, an anthraquinone derivative as the acceptor A group, and a dinuclear Ru(II) species (P-P) based on terpyridine-like ligands as light-harvesting subunit, so that a linearly arranged D-(P-P)-A molecular triad is obtained. In acetonitrile solution containing 1% (in volume) of methanol, photoinduced oxidative electron transfer occurs in 380 ps, with formation of the D-(P-P) +-A À charge-separated (CS) state. However, although formation of the fully-developed D +-(P-P)-A À state is favored by about 0.23 eV, such species is not formed since charge recombination within D-(P-P) +-A À appears to be faster.
The Journal of Physical Chemistry C, 2017
Controlling the ultrafast dynamical process of photoinduced charge-transfer at donor-acceptor interfaces remains a major challenge for physical chemistry and solar cell communities. The process is complicated by the involvement of other complex dynamical processes, including hydrogen bond formation, energy transfer and solvation dynamics occurring on similar time scales. In this study, we explore the remarkable impact of hydrogen-bond formation on the interfacial charge transfer between a negatively charged electron donating anionic porphyrin and a positively charged electron accepting -conjugated polymer, as a model system in solvents with different polarities and capabilities for hydrogen bonding using femtosecond transient absorption spectroscopy. Unlike the conventional understanding of the key role of hydrogen bonding in promoting the charge transfer process, our steady-state and timeresolved results reveal that the intervening hydrogen-bonding environment and consequently the probable longer spacing between the donor and acceptor molecules significantly hinders the charge-transfer process between them. These results show that site-specific hydrogen bonding and geometric considerations between donor and acceptor can be exploited to control both the chargetransfer dynamics and its efficiency not only at donor-acceptor interfaces, but also in complex biological systems.
Journal of the American Chemical Society, 1987
A series of molecules 1 was synthesized containing a 1,4-dimethoxynaphthaIene donor (D) and a 1,l-dicyanoethylene acceptor (A) interconnected by five different, rigid, nonconjugated bridges. The length of the bridges varies with increments of two u-bonds from four in l(4) to 12 u-bonds in 1(12), to provide donor-acceptor center-to-center separations (R,) ranging from 7.0-14.9 A. In solvents of medium and high polarity, excitation of the donor D is followed by rapid intramolecular electron transfer. The rate constant (keJ shows only small dependence upon the solvent polarity (a factor of 2-3 between benzene and acetonitrile, for example) but decreases with increasing separation ranging from > 10" s-l for a four-bond separation to ==4 X lo8 s? for a 12-bond separation. In saturated hydrocarbon solvents photoinduced electron transfer is not observed for 10-and 12-bond separations, while it is not significantly decreased for the shorter homologues. Therefore the absence of electron transfer at IO-and 12-bond separations in saturated hydrocarbon solvents is attributed to a thermodynamic rather than to a kinetic effect. In solvents where electron transfer is thermodynamically feasible, its rate is considerably greater than that found from various other experimental studies where either different bridges were used or intermolecular electron transfer was studied. Through-bond interaction involving U / T interaction between the bridge and the donor-acceptor pair is proposed to explain the very high electron transfer rates observed in 1; this is qualitatively correlated with independent information about this coupling derived from both theory and experiment (photoelectron spectroscopy). The observation of intramolecular charge-transfer absorption and emission for 1(4), 1(6), and l (8) confirms the operation of such through-bond interaction. Rigid systems like 1 can therefore not only provide more insight into the thermodynamics of electron transfer and its solvent dependence but especially also into the role of the nature of the coupling of donor and acceptor. The latter is of crucial importance for a better understanding of the factors governing the rates of electron transfer between redox centers in, e.g., biologically important redox proteins involved in photosynthesis and in the respiratory cycle. It is shown that the near-independence of the photoassisted charge separation dynamics on solvent polarity and the overall free-energy change (even for a calculated variation of-0.9 eV) is consistent with predictions of electron-transfer theory on the assumption that the solvent is a continuous dielectric. It is also shown how the parameters entering into the theoretical expressions (in particular, the intramolecular reorganization energy) may be correlated with those obtained from radiative transitions (e.g., charge recombination fluorescence). The dependence of the effective electron interaction element (J), which couples donor and acceptor, on the bridge length is discussed. Single electron transfer constitutes one of the most fundamental chemical reactions and plays a crucial role in many synthetic as well as biological processes, occurring under either thermal2 or under photochemical3 conditions. Various lines of experimental evidence have led to the conclusion that both thermal2 and photoind~ced'*~-~ electron transfer may occur between species separated by a distance significantly exceeding the sum of their van der Waals radii. In fact many biological electron transport processes-including the primary steps of photosynthesisg-involve such long-range electron-transfer events. Biological and modified biological systems containing a variety of redox centers at widely different distances, between which electron transfer occurs on a time scale extending from the picosecond regime to many seconds, have been studied.'&*O Systematic variation of donor and acceptor at a single rigidly fixed distance has recently been used15321-23 to establish the dependence of the rate of electron transfer upon the thermodynamic driving force. These studies corroborated earlier theoretical predictions of the existence of an optimal rate constant. In addition, significant progress has been made in the study of the distance dependence of electron transfer especially by measuring the time resolved evolution of the number of electron-transfer events in dilute rigid solutions containing donor and acceptor species.* The evaluation of such measurements, however, rests inter alia on the assumption of a statistical distribution of donoracceptor distances and relative
The role of hydrogen bonding in excited state intramolecular charge transfer
Physical Chemistry Chemical Physics, 2012
Intramolecular charge transfer (ICT) that occurs upon photoexcitation of molecules is a vital process in nature and it has ample applications in chemistry and biology. The ICT process of the excited molecules is affected by several environmental factors including polarity, viscosity and hydrogen bonding. The effect of polarity and viscosity on the ICT processes is well understood. But, despite the fact that hydrogen bonding significantly influences the ICT process, the specific role of hydrogen bonding in the formation and stabilization of the ICT state is not unambiguously established. Some literature reports predicted that the hydrogen bonding of the solvent with a donor promotes the formation of a twisted intramolecular charge transfer (TICT) state. Some other reports stated that it inhibits the formation of the TICT state. Alternatively, it was proposed that the hydrogen bonding of the solvent with an acceptor favors the TICT state. It is also observed that a dynamic equilibrium is established between the free and the hydrogen bonded ICT states. This perspective focuses on the specific role played by hydrogen bonding of the solvent with the donor and the acceptor, and by proton transfer in the ICT process. The utility of such influence in molecular recognition and anion sensing is discussed with a few recent literature examples in the end.
Dynamics of intermolecular electron transfer from amines to the excited states of 9-fluorenone
Journal of Photochemistry and Photobiology A: Chemistry, 2013
Dynamics of photoinduced electron transfer (PET) reactions between the singlet (S 1) and the triplet (T 1) excited states of 9-fluorenone or simply fluorenone and a few aromatic and aliphatic amines have been investigated under both diffusive and non-diffusive conditions. Formation of the fluorenone anion radical confirms the electron transfer (ET) from the amines to the excited states of fluorenone. Rate constants for both the forward ET process, k CS , and the charge recombination (CR) process, k CR , have been determined in acetonitrile and benzene solutions. Sub-picosecond time-resolved transient absorption study reveals that quenching of the S 1 state in acetonitrile is biexponential. Lifetime of one of these two components is independent of quencher concentration and its values determined for aniline (7.1 ps), dimethylaniline (8.5 ps) and diethylaniline (6.8 ps) are very similar. It represents the non-diffusive component of the PET reaction. But the lifetime of the other component decreases with increasing quencher concentration and the rate of this diffusive component of the PET reaction nearly agrees with the value determined using steady state fluorescence quenching method. To determine the intrinsic values of the rates of the PET reactions involving the S 1 state of fluorenone, the PET dynamics have been studied in these three neat donor solvents. The forward ET process is biexponential and the lifetimes of these two components are very similar in these solvents and vary in the range 0.3-0.5 ps and 6-8 ps. Nonexponential dynamics of the PET reactions conducted in neat donor-solvents have been discussed using a simple solvent reorientational model.
The Journal of Physical Chemistry, 1995
We have investigated intermolecular electron transfer (ET) from electron-donating solvents (aniline and N&dimethylaniline) to coumarins in the excited state by means of the femtosecond fluorescence up-conversion technique. The coumarins we studied have a variety of structures with different substituents in the 4and 7-positions. The ET occurs on a time scale ranging from a few nanoseconds to a couple of hundred femtoseconds depending on the structure of the coumarins and solvent. As for the 7-position, as the length of the alkyl chain on the amino group is longer, the ET is slower, and when the amino group is fixed by a double-hexagonal ring, it is slowest. When the electron-accepting ability of the substituent in the 4-position is increased, the reaction occurs faster. The origin of this substituent effect is mainly attributed to the variation of the energy gap between the reactant and product states. This is confirmed by theoretical calculations in terms of the extended Sumi-Marcus two-dimensional model. Good agreement between the experiment and calculation indicates that some of the reactions take place from the relaxed vibrational state of reactant to the excited vibrational states of high-frequency modes of product states. The simulated population decays for nonequilibrium configuration of solvents agreed well with experimental data. In the steady-state fluorescence spectra was also observed an effect of very fast fluorescence quenching due to ET; i.e., the amount of fluorescence Stokes shift depends on the rate of ET because the excited state is quenched in competition with thermal equilibration of the solvent configuration. We regard this spectral shift as the result of the "chemical timing" effect in solution.