Transient Fluorescence Spectroscopy Research Articles

Highly Efficient Triplet−Triplet Intramolecular Energy Transfer and Enhanced Intersystem Crossing in Rigidly Linked Copper(II) Porphyrin−Free Base Porphyrin Hybrid Dimers

Intramolecular energy transfer as well as excited-state relaxation of gable-type copper(II) porphyrin-free base porphyrin dimers, in which the two halves are linked via a benzene or a naphthalene, was studied by transient absorption and fluorescence spectroscopy. Photoexcitation at 532 nm of the dimer gives rise to transient absorption spectra identical to that of triplet−triplet (T−T) absorption in the free base monomer; however, the absorption intensities in the dimers are more than four times larger than that of the monomer, indicating efficient intramolecular energy transfer from the copper porphyrin to the free base counterpart. Presence of pyridine in the dimer solutions decreases the initial optical densities, while the free base monomer shows no change. The decrease is ascribed to competition between energy transfer via triplet manifolds and quenching by pyridine in the excited trip−quartet and/or trip−doublet states of the copper porphyrin moiety. The analysis of the generated yield of the T1 fre...

Specific Mutation Near the Primary Donor in Photosystem I from Chlamydomonas reinhardtii Alters the Trapping Time and Spectroscopic Properties of P700

Time-resolved absorption and fluorescence spectroscopy were used to investigate the energy and electron transfer processes in the detergent-isolated photosystem I core particles from the site-directed mutant of Chlamydomonas reinhardtii with the histidine-656 of PsaB replaced by asparagine [HN(B656) mutation]. The specific mutation near the primary donor molecule results in a 40 mV increase in the P700/P700+ midpoint potential [Webber, A. N., Su Hui, Bingham, S. E., Kass, H., Krabben, L., Kuhn, M., Jordan, R., Schlodder, E., & Lubitz, W. (1996) Biochemistry 35, 12857-12863]. There is no indication that the HN(B656) mutation affects the spectral distribution of the antenna pigments. However, the lifetime of the trapping process measured independently by transient absorption and fluorescence spectroscopy in the mutant PSI core antenna is increased by a factor of approximately 2 (approximately 65 ps compared to approximately 30 ps in the wild-type PSI). This implies that the trapping process in the PSI antenna is limited by the process where the primary donor molecule directly participates. The HN(B656) mutation results in the appearance of a new bleaching band at 670 nm in the spectrum which is due to formation of P700+ upon photooxidation. The difference spectrum of the photoreduction of the possible primary acceptor, A0 in the mutant PSI is very similar to wild type, indicating that it is unaffected by the HN(B656) mutation. Possible mechanisms for slowing of the trapping process and the appearance of a new band in the P700 - P700+ difference spectrum of the HN(B656) PSI are discussed.

Photophysical and Photochemical Properties of the Fullerenes

Abstract This article primarily reviews the recent work of this laboratory on the photophysical and photochemical properties of the fullerenes, C60 and C70, in solution. Properties of the excited singlet and triplet states of them have been characterised by static and transient fluorescence spectroscopy. Charge and/or proton transfer dynamics in ground and excited singlet state of the fullerenes with the amines have been investigated in detail. Strong evidence has been obtained for weak interaction between the fullerenes and the aromatic solvents, like benzene, toluene etc. A novel method has been described to solubilise C60 in aqueous solution via formation of inclusion complex with γ-cylodextrin.

Transient absorption and fluorescence spectroscopy in fused silica induced by pulsed KrF excimer laser irradiation

We have carried out in situ transient absorption and fluorescence spectroscopy measurements in two ‘‘wet’’(OH content ∼0.1%) fused silica samples (Suprasil II from Heraeus Amersil and P-30 from Shin-Etsu Quartz Product) during KrF laser irradiation. Both samples exhibit an absorption peak at 210 nm corresponding to the E′ center. For Suprasil II, there is also a 265 nm absorption peak, and both peaks increase with the number of irradiated pulses showing little relaxation after the laser was turned off. The region irradiated with three million pulses at 400 mJ/cm2 fluence ten months ago has a residual absorption of about 10%/cm at 210 nm. On the other hand, the P-30 shows a rapid increase in the 210 nm absorption in both the unirradiated and previously irradiated regions during the initial irradiation and levels off after a few thousand pulses. There is no residual absorption at the spot irradiated for 63 million pulses ten months ago. However, the initial rate of increase in the previously irradiated spot is twice as high as compared to the unirradiated spot. This suggests the density of the precursor state for the E′ center is higher in the previously irradiated region. The fluorescence intensity at 650 nm increases with the induced absorption for Suprasil II, but is almost independent of the number of irradiation pulses in P-30. The quasilinear repetition-rate dependence suggests the fluorescence is transient in nature and relaxes partially between successive laser pulses.

Oxidation of carbon monoxide with nitrous oxide over CeO2

With the help of the transient response method and fluorescence spectroscopy, it has been revealed that the title reaction proceeds by the catalyst redox transformation Ce(III)⇆Ce(IV) over CeO2 in great contrast to previous observations over MgO.

Time-resolved spectroscopic measurements on microscopic solvation dynamics

This paper reinvestigates the use of transient fluorescence spectroscopy of polar aromatics in solution as a method to determine microscopic solvation dynamics. It is shown that the compounds previously employed as polar fluorescent probes tend to fall into three photophysical classes depending upon: (i) whether the photon induced change in μ occurs simultaneously with photon absorption (ii) whether solvent motion subsequent to photon absorption is required to induce the change in μ; or (iii) whether two excited-state isomers with different μ’s are present simultaneously. The consequence of the different classes on microscopic solvation dynamic measurements is discussed with a molecular example for each class: (i) 4-aminophthalimide, (ii) 4-(9-anthryl)-N, N-dimethylaniline, and (iii) bianthryl, respectively. In addition, we introduce a new transient fluorescence procedure for the determination of solvation dynamics that has advantages over the traditional transient Stokes-shift method. Finally, for the first time, extensive measurements on the solvation dynamics of a polar aprotic solvent have been made. The observed dynamics of the solvent (glycerol triacetate) are highly nonexponential; this has important implications for chemical reactions in similar solvents. Interestingly, the experimentally observed microscopic dynamics are in qualitative agreement with predictions of the dielectric continuum model.

Hydrogen/deuterium isotope effects on the excited-state proton transfer kinetics of 3-hydroxyflavone

The static and transient fluorescence spectroscopy of 3-hydroxyflavone, 3HF, has been investigated as a function of deuteration of the 3-hydroxy group and the solvent. Small H/D isotope effects are observed for at least two of the simple rate constants for the excited-state dynamics. The isotope effects are not significantly temperature dependent. The results suggest that the proton transfer process is not a simple barrier crossing reaction.