M. Tachiya

Transient effect in fluorescence quenching induced by electron transfer. I. Analysis by the Collins-Kimball model of diffusion-controlled reactions

Abstract Fluorescence quenching induced by electron transfer was studied by measuring the transient effect in decay curves and also the changes in stationary intensity with quencher concentration. The results were analyzed using the Collins-Kimball model of diffusion-controlled reactions, yielding the reaction radius R and the intrinsic rate constant kET of electron transfer at the reaction radius. ktET values thus obtained are much larger than the rate of diffusion. R was found to increase with increasing −ΔG of the electron transfer reaction. This is consistent with the prediction from the Marcus theory of electron transfer. The adequacy of the Collins-Kimball model in analyzing fluorescence quenching reaction is discussed.

Ultrafast fluorescence quenching by electron transfer and fluorescence from the second excited state of a charge transfer complex as studied by femtosecond up-conversion spectroscopy

Photoinduced electron transfer dynamics between fluorescer (acceptor, A) and quencher (donor, D) was investigated by measuring the fluorescence quenching using femtosecond up-conversion spectroscopy. The measurements were made in a quencher concentration range of 0.15 M–1 M and also in a neat quencher solvent. Fluorescence decay at times longer than 5 ps can be explained by combining the diffusion equation with the Marcus equation of electron transfer. At higher quencher concentrations (>0.3 M), an additional component with a time constant of ∼250 fs appears. At these concentrations, the fluorescers (9-cyanoanthracene, CA and 9,10-dicyanoanthracene) and the quenchers (N,N-dimethylaniline, DMA) were found to form “weak” CT complexes. Fluorescence from the S1 state of the CA-DMA complex was detected by steady state spectroscopy. The excitation spectrum observed at the maximum intensity of this fluorescence indicates the existence of an excited S2 state of the CT complex near the energy of D⋅A* (the locally ...

Ultrafast charge separation and exciplex formation induced by strong interaction between electron donor and acceptor at short distances

We have measured the decay of fluorescence from the electron acceptors (A) dissolved in donor (D) solvent by femtosecond up-conversion spectroscopy with a time resolution of 75 fs. The measurements have been made for several acceptors (9-cyanoanthracene, CA; 9,10-dicyanoanthracene, DCA; and 1,2,9,10-tetracyanoanthracene, TCA) in the donor solvent (N,N-dimethylaniline, DMA, and aniline, ANL). The decay times obtained are between 150 fs (CA–ANL) and 280 fs (TCA–DMA). Observation of the decay of acceptor fluorescence and the concurrent rise of fluorescence from the exciplex state indicates that the excited acceptor (A*) state directly relaxes to the exciplex state. Charge separation rates (kCS) determined from the decay of the acceptor fluorescence lie between 3.6×1012–6.7×1012/s for six combinations of donors and acceptors. Very weak energy gap (ΔE) dependence of kCS was observed in the region of ΔE of 0.36–1.47 eV. This relation between kCS and ΔE is quite different from the charge recombination rate (kCR)...

Ultrafast dynamics of the excited tetracyanoethylene–toluene electron donor–acceptor complex

Abstract The ultrafast spectroscopy of the electron donor–acceptor complex of tetracyanoethylene with toluene has been investigated in several polar and non-polar solvents. After ultrafast excitation of the charge transfer band, formation and then decay of the contact ion pair is observed. The decay time of the contact ion pair due to charge recombination is, in most cases, longer than the solvation time and is observed on a time scale from 1 ps to 1 ns, depending on the solvent. Using a conventional non-adiabatic theory of electron transfer, absolute rate constants for the non-radiative return electron transfer, varying over three orders of magnitude, can be predicted from limited information obtained from the analysis of the radiative processes in non-polar solvents only. The model fails in cases where solvent relaxation is slower or comparable to the rate of the return electron transfer.

Contribution of the ultrafast, short-distance intermolecular electron transfer to the fluorescence quenching rate in solution

In a previous paper [S. Iwai et al., J. Chem. Phys. 112, 7111 (2000)] we have found ultrafast electron transfer (ET) which occurs between donor and acceptor molecules at short distances. We incorporate this ultrafast ET in the calculation of the second-order ET rate in order to explain the discrepancies between experimental [D. Rehm and A. Weller, Isr. J. Chem. 8, 259 (1970)] and theoretical [M. Tachiya and S. Murata, J. Phys. Chem. 96, 8441 (1992)] ET rates. The effect of the short-distance ET is significant in the Marcus normal region where the Marcus-type ET is not very fast. Compared to the case where the original Marcus equation is used as the first-order ET rate, the calculated second-order rate is found to increase by several orders of magnitude in the normal region. Thus the discrepancy between theory and experiment is potentially resolved and the importance of the short-distance ET in fluorescence quenching demonstrated.

Ultrafast relaxation processes of excitons to the relaxed excited state in one-dimensional dimethylglyoxime platinum complexes driven by intramolecular vibration

Optical response due to the photo excited carrier or exciton dynamics in a one-dimensional (1D) dimethylglyoxime Pt complex [Pt(dmg)2] has been investigated by femtosecond pump–probe spectroscopy. Measurements were made at several excitation energies between the exciton state and the free electron hole pair state. Induced absorption was observed in the low-energy side of the exciton band at any excitation energy. The spectral shape of the induced absorption changed with time between 0.1 and 0.3 ps after instantaneous rise up. These results can be explained by the formation and thermalization processes of the relaxed excited state on the adiabatic potential surface. The unthermalized relaxed excited state is formed in a very short time (<50 fs). From the Raman spectrum, the frequency of the intermolecular stretching mode along the chain was estimated to be 80 cm−1 (T∼400u200afs). These facts suggest that, in contrast to other 1D systems such as polydiacetylenes (PDA) and halogen bridged mixed valence metal com...

Synthesis and binding properties of calix[4]arene dual porphyrin conjugate: tweezers for DABCO

A dual porphyrin system built on a calix[4]arene spacer was prepared and the corresponding Zn(II) complex 4 showed a marked affinity for 1,4-diazobicyclo[2.2.2]octane (DABCO) as compared with other amines.

Ultrafast relaxation processes of excitons in one-dimensional platinum complexes

Optical response due to the photo-excited carrier or exciton dynamics in the excited state of a one-dimensional metal complex (Pt(dmg)2) has been investigated by femtosecond pump-probe spectroscopy. Measurements were made under various excitation photon energies within the absorption band which originates from the delocalized electronic states along the 1-D chain. Around the lowest-energy absorption peak, absorption bleaching was found to be larger and its decay faster compared to those in higher-energy region. Furthermore, an induced absorption which is considered to be due to a bi-exciton or relaxed excited state was observed.

Two-Dimensional Free Energy Surfaces for Electron Transfer Reactions in Solution

Change in intermolecular distance between electron donor (D) and acceptor (A) can induce intermolecular electron transfer (ET) even in nonpolar solvent, where solvent orientational polarization is absent. This was shown by making simple calculations of the energies of the initial and final states of ET. In the case of polar solvent, the free energies are functions of both D-A distance and solvent orientational polarization. On the basis of 2-dimensional free energy surfaces, the relation of Marcus ET and exciplex formation is discussed. The transient effect in fluorescence quenching was measured for several D-A pairs in a nonpolar solvent. The results were analyzed by assuming a distance dependence of the ET rate that is consistent with the above model.

Ultrafast adiabatic elctron transfer cooperated with exciplex formation induced by strong interaction between donor and acceptor at short distances

A relation between free energy change vs. electron transfer (ET) rate between donor and acceptor in contact was investigated by time-resolved fluorescence study. NonMarcus type ET was found to occur on the adiabatic barrierless potential surface which can be realized by the strong interaction between donor and acceptor molecules.