Monique M. Martin

Hydrogen bond effects on radiationless electronic transitions in xanthene dyes

Abstract Intermolecular hydrogen bonding effects on internal conversion (S1 ⇝ S0) and intersystem crossing (S1 ⇝ T1) in 6-hydroxy-9-phenyl-fluoron and fluorescein were determined from fluorescence and triplet yield measurements in alkaline solutions of CF3CH2OH, H2O, CH3OH, CH3CH2OH, HCONH2, HCON(CH3)2 and CH3SOCH3. Hydrogen bonding between the dye and the solvent leads to a blue-shift of the absorption and fluorescence spectra and to an enhancement of the fluorescence yield: the triplet yield remains small in all solvents. It was found that the internal conversion rate constant decrease exponentially when the 0-0 band energy of the S0-S1 transition increases.

Study of triplet quantum yields using a tunable dye laser

Abstract A method of determining triplet quantum yields using a dye laser is described. Triplet yields were measured for eosin, fluorescein, acridine orange, proflavine, and pentacene in fluid solutions. The results reveal the occurrence of an efficient internal conversion process for the three latter compounds.

Excited-state relaxation dynamics of a PYP chromophore model in solution: influence of the thioester group

Abstract Cis–trans photoisomerization of a photoactive yellow protein chromophore model, the deprotonated trans S -phenyl thio- p -hydroxycinnamate, is studied in aqueous solution by subpicosecond transient absorption and gain spectroscopy. The excited-state deactivation is found to involve the formation, in 1.7 ps, of an intermediate state which decays in 2.8 ps. A persistent bleaching signal is observed at longer times indicating that the excited state not only relaxes to the ground state but also partly forms a stable photoproduct, possibly the cis isomer. This behavior is analogous to that of the native photoactive yellow protein.

Ultrafast intramolecular charge transfer in the merocyanine dye DCM

Abstract Time-resolved transient absorption and gain spectra with subpicosecond laser excitation are reported for weakly polar and highly polar solutions of the merocyanine dye DCM (4-dicyanomethylene-2-methyl-6-( p -dimethylamino-styryl)-4-pyran) in the 380–800 nm spectral range. A temporary isosbestic point is observed in the transient gain spectra during 3 to 18 ps after excitation depending on the solvent. The results are interpreted in terms of the rapid formation of an emissive intramolecular charge-transfer state from the emissive initially excited singlet state. A further evolution of the transient spectra includes a blue-shift of the gain band during a few tens of picoseconds in solvents having ultrashort solvation time whereas a red-shift is found for solvent of slow response. A relaxation mechanism including vibrational cooling and solvation of the charge-transfer state is discussed to explain the observed resolved differential spectra. Other processes such as isomerization or single bond twisting are also discussed.

Spectro-temporal characterization of the photoactivation mechanism of two new oxidized cryptochrome/photolyase photoreceptors.

The photoactivation dynamics of two new flavoproteins (OtCPF1 and OtCPF2) of the cryptochrome photolyase family (CPF), belonging to the green alga Ostreococcus tauri , was studied by broadband UV-vis femtosecond absorption spectroscopy. Upon excitation of the protein chromophoric cofactor, flavin adenine dinucleotide in its oxidized form (FAD(ox)), we observed in both cases the ultrafast photoreduction of FAD(ox): in 390 fs for OtCPF1 and 590 fs for OtCPF2. Although such ultrafast electron transfer has already been reported for other flavoproteins and CPF members, the present result is the first demonstration with full spectral characterization of the mechanism. Analysis of the photoproduct spectra allowed identifying tryptophan as the primary electron donor. This residue is found to be oxidized to its protonated radical cation form (WH(*+)), while FAD(ox) is reduced to FAD(*-). Subsequent kinetics were observed in the picosecond and subnanosecond regime, mostly described by a biexponential partial decay of the photoproduct transient signal (9 and 81 ps for OtCPF1, and 13 and 340 ps for OtCPF2), with reduced spectral changes, while a long-lived photoproduct remains in the nanosecond time scale. We interpret these observations within the model proposed by the groups of Brettel and Vos, which describes the photoreduction of FADH(*) within E. coli CPD photolyase (EcCPD) as a sequential electron transfer along a chain of three tryptophan residues, although in that case the rate limiting step was the primary photoreduction in 30 ps. In the present study, excitation of FAD(ox) permitted to reveal the following steps and spectroscopically assign them to the hole-hopping process along the tryptophan chain, accompanied by partial charge recombination at each step. In addition, structural analysis performed by homology modeling allowed us to propose a tentative structure of the relative orientations of FAD and the conserved tryptophan triad. The results of preliminary transient anisotropy measurements performed on OtCPF2 finally showed good compatibility with the oxidation of the distal tryptophan residue (WH(351)) in 340 ps, hence, with the overall Brettel-Vos mechanism.

Primary events in the photoactive yellow protein chromophore in solution

Abstract The photoinduced cis–trans isomerization of the photoactive yellow protein chromophore, the deprotonated trans-p -coumaric acid, is studied in solution by subpicosecond transient absorption and gain spectroscopy. A nonemissive photoproduct absorbing in the UV region, attributed to the cis isomer, is found to appear in 10 ps at the rate of the excited-state decay, demonstrating that there is no detectable intermediate in the relaxation pathway of the photoactive yellow protein chromophore in solution.

DYNAMICS OF THE FIRST EXCITED STATE OF THE DECATUNGSTATE ANION STUDIED BY SUBPICOSECOND LASER SPECTROSCOPY

Abstract The primary events in decatungstate photolysis are investigated by subpicosecond spectroscopy. Steady-state and picosecond fluorescence experiments are also reported. The absorption peak at 776 nm of the transient state called X, involved in photocatalytic reactions with organic substrates on the nanosecond time scale, rises in a few ps after subtle changes of an initial band, extending from the visible to the near-infrared (NIR). The decay in the NIR edge occurs in 25 ps in acetonitrile and 15 ps in water. X is interpreted as a delocalized ligand-to-metal charge transfer (LMCT) excited state, originating from a localized excited state absorbing in the NIR. Re-excitation of X could contribute to the 580 nm fluorescence observed under picosecond excitation.

Excited state relaxation paths in 9,9′-bianthryl and 9-carbazolyl-anthracene: a sub-ps transient absorption study

Abstract Transient absorption spectra with sub-picosecond resolution are reported for 9,9′-bianthryl (BA) and 9-carbazolyl-anthracene (C9A) in ethanol, n -butanol and diethylether, from 320 to 800 nm. Ground-state bleaching and excited-state absorption (ESA) are seen in the UV region. In alcohols, the ESA decay (≈20 ps in ethanol and ≈70 ps in butanol) matches the rise time of the well-known charge-transfer (CT) band in the visible, with a time constant comparable to the average solvation time. An additional change is observed for BA in the fluorescence region (400–480 nm), characterized by the disappearance of a structured gain contribution in less than 1 ps. This is also observed in diethylether, although the subsequent evolution is different. This ultrafast process, which is attributed to the unhindered torsional relaxation of BA out of the Franck–Condon geometry, is less clear for C9A. However, for both compounds in alcohols, the CT state formation is a second step and is more than one order of magnitude slower. In diethylether, unhindered and diffusive conformational changes are invoked. The data support a two-coordinate (internal twisting and solvent motion) relaxation model for BA, the relative importance of these coordinates being strongly solvent dependent. A similar model is proposed for C9A.

Primary photoprocesses involved in the sensory protein for the photophobic response of Blepharisma japonicum.

We present new femtosecond transient-absorption and picosecond fluorescence experiments performed on OBIP, the oxyblepharismin-binding protein believed to trigger the photophobic response of the ciliate Blepharisma japonicum. The formerly identified heterogeneity of the sample is confirmed and rationalized in terms of two independent populations, called rOBIP and nrOBIP. The rOBIP population undergoes a fast photocycle restoring the initial ground state in less than 500 ps. Intermolecular electron transfer followed by electron recombination is identified as the excited-state decay route. The experimental results support the coexistence of the oxyblepharismin (OxyBP) radical cation signature with a stimulated-emission signal at all times of the evolution of the transient-absorption spectra. This observation is interpreted by an equilibrium being reached between the locally excited state and a charge-transfer state on the ground of a theory developed by Mataga and co-workers to explain the fluorescence quenching of aromatic hydrogen-bonded donor-acceptor pairs in nonpolar solvents. OxyBP is supposed to bind to an as yet unknown electron acceptor by a hydrogen-bond (HB) and the coordinate along which forward and backward electron transfer proceed is assumed to be the shift of the HB proton. The observed kinetic isotope effect supports this interpretation. Protein relaxation is finally proposed to accompany the whole process and give rise to the highly multiexponential observed dynamics. As previously reported, the fast photocycle of rOBIP can be interpreted as an efficient sunscreen mechanism that protects Blepharisma japonicum from continuous irradiation. The nrOBIP population, the transient-absorption of which strongly reminds that of free OxyBP in solution, might be proposed to actually trigger the photophobic response of the organism through excited-state deprotonation of the chromophore occurring in the nanosecond regime. Additional femtosecond transient-absorption spectra of OxyBP and peri-deprotonated OxyBP are also reported and used as a comparison basis to interpret the results on OBIP.

Ultrafast light-induced response of photoactive yellow protein chromophore analogues

The fluorescence decays of several analogues of the photoactive yellow protein (PYP) chromophore in aqueous solution have been measured by femtosecond fluorescence up-conversion and the corresponding time-resolved fluorescence spectra have been reconstructed. The native chromophore of PYP is a thioester derivative of p-coumaric acid in its trans deprotonated form. Fluorescence kinetics are reported for a thioester phenyl analogue and for two analogues where the thioester group has been changed to amide and carboxylate groups. The kinetics are compared to those we previously reported for the analogues bearing ketone and ester groups. The fluorescence decays of the full series are found to lie in the 1-10 ps range depending on the electron-acceptor character of the substituent, in good agreement with the excited-state relaxation kinetics extracted from transient absorption measurements. Steady-state photolysis is also examined and found to depend strongly on the nature of the substituent. While it has been shown that the ultrafast light-induced response of the chromophore in PYP is controlled by the properties of the protein nanospace, the present results demonstrate that, in solution, the relaxation dynamics and pathway of the chromophore is controlled by its electron donor-acceptor structure: structures of stronger electron donor-acceptor character lead to faster decays and less photoisomerisation.