M. Glasbeek

Femtosecond fluorescence upconversion studies of excited-state proton transfer dynamics in 2-(2'-hydroxyphenyl)benzoxazole (HBO) in liquid solution and DNA

Abstract A femtosecond fluorescence upconversion study is reported for HBO in solution, as well as for HBO incorporated in DNA. The typical time for the excited-state intramolecular proton-transfer reaction of the syn -enol tautomer in solution and in DNA has been determined to be 150 fs. In addition, the lifetimes of the keto, the anti -enol and the ‘solvated enol’ tautomer forms were determined in protic solvents, aprotic solvents and DNA. Picosecond rise and decay components in the fluorescence transients with characteristic times between 3 and 25 ps are also observed and attributed to the effects of vibrational cooling.

Optically detected spin coherence of the diamond N-V centre in its triplet ground state

For the N-V centre in type Ib diamond the optical detection of spin coherence in the 3A state is reported. The 3A-state lifetime is studied as a function of the light intensity used for the optical excitation of the N-V centre by means of spin-locking experiments. The shortening of the lifetime at higher excitation intensities provides evidence for the previously proposed idea that the 3A state of the N-V centre is the ground state.

Femtosecond fluorescence upconversion studies of barrierless bond twisting of auramine in solution

Femtosecond fluorescence upconversion studies have been performed for auramine (a diphenylmethane dye), dissolved in ethanol, as a function of temperature. It is found that the (sub)picosecond decay components in the fluorescence slow down as the temperature is lowered from 293 K to 173 K. From the observation of a residual fluorescence, with a viscosity-dependent lifetime of about 30 ps (or longer at higher viscosity), and transient absorption results it is concluded that the two-state sink function model [B. Bagchi, G. R. Fleming, and D. W. Oxtoby, J. Chem. Phys. 78, 7375 (1983)] does not apply in the case of auramine. Comparison of the auramine fluorescence kinetics in ethanol and decanol shows that diffusional twisting and not solvation is the main cause for the (sub)picosecond excited state relaxation. To explain the experimental results, adiabatic coupling between a locally excited emissive state (F) and a nonemissive excited state (D) is considered. Torsional diffusion motions of the phenyl groups ...

Ultrafast single and double proton transfer in photo-excited (2,2'-bipyridyl)-3,3'-diol

Abstract We report on a subpicosecond fluorescence study of [2,2′-bipyridyl]-3,3′-diol using the fluorescent up-conversion technique with a time resolution of ≈300 fs. A new short-living fluorescence band, peaking at 568 nm, is observed, which is attributed to the emissive keto-enol tautomer. This tautomer is discussed to be formed in its excited singlet state in a stepwise proton transfer mechanism; eventually the keto-enol tautomer decays into the double-protonated di-keto product, which alternatively could also have been formed in a direct double proton transfer reaction.

Subpicosecond fluorescence upconversion measurements of primary events in yellow proteins

Abstract The primary step in the photocycle of photoactive yellow protein (PYP) has been investigated by means of the subpicosecond fluorescence upconversion technique. The nature of the chromophore is found to affect the fluorescence decay dynamics: upon replacement of the coumaric acid chromophore by 7-hydroxy-coumarin-3-carboxylic acid, the fluorescence decay becomes much longer (with typical decay times larger than 60 ps), whereas the 700 fs and 3 ps components have disappeared. It is discussed that the obtained results are strong support for the idea that the initial step in the photocycle involves trans–cis isomerization inside the coumaric acid chromophore in native PYP.

Femtosecond fluorescence upconversion study of a boron dipyrromethene dye in solution

Abstract A femtosecond fluorescence upconversion study is reported for the boron-dipyrromethene (BDP) dye, 2,6-diethyl-1,3,5,7-tetramethyl-8-phenyl-4-difluorobora-3 a ,4 a -diaza-( s )-indacene ( BDP-1 ), dissolved in cyclohexane, chloroform and acetonitrile. After pulsed photoexcitation into the higher excited states, S 2 or S 3 (at energies of ∼ 26 300 cm −1 and ∼ 30 000 cm −1 above the S 0 state, respectively), the fluorescence from the S 1 state shows an ultrafast solvent-dependent rise, with characteristic times between 100 and 230 fs. The rise is attributed to S n →S 1 ( n =2,3) internal conversion followed by vibrational relaxation. In addition, the BDP-1 fluorescence shows picosecond transient behavior that is attributed to vibrational cooling in the excited S 1 state.

Femtosecond laser selective intramolecular double proton transfer in [2,2'-bipyridyl]-3,3'-diol

Abstract We report an excitation energy dependence of the dynamics of the excited-state intramolecular double-proton transfer of [2,2′-bipyridyl]-3,3′-diol in liquid solution. By means of femtosecond fluorescence upconversion experiments it is shown that an increase of the vibrational energy in the excited electronic S 1 state results in an increase of the ratio of the reaction yields of the monoketo and diketo tautomeric products. The dominance of the concerted double-proton transfer process at the lower excitation energies is evidence for an energy barrier in the dienol–monoketo reaction pathway.

Femtosecond studies of charge separation in photo-excited DCM in liquid solution

Abstract The solvatochromism of DCM in the fluorescent state has been studied using the femtosecond fluorescent up-conversion technique with a time-resolution of ≈ 150 fs. It is shown that intramolecular charge separation is not complete within 300 fs after the excitation pulse. A significant portion of the charge separation trajectory ( ≈ 30%) is controlled by the solvation process on a picosecond time scale. It is inferred that in photo-excited DCM the locally excited and charge transfer states mix in the strong adiabatic coupling limit in the inverted region.

Unusual, Solvent Viscosity-Controlled Tautomerism and Photophysics: Meso-Alkylated Porphycenes

Stationary and time-resolved studies of 9,10,19,20-tetramethylporphycene and 9,10,19,20-tetra-n-propylporphycene in condensed phases reveal the coexistence of trans and cis tautomeric forms. Two cis configurations, cis-1 and cis-2, play a crucial role in understanding the excited-state deactivation and tautomer conversion dynamics. The trans-trans tautomerization, involving intramolecular transfer of two hydrogen atoms, is extremely rapid (k ≥ 10(13) s(-1)), both in the ground and lowest electronically excited states. The cis-1-trans conversion rate, even though the process is thermodynamically more favorable, is much slower and solvent-dependent. This is explained by the coupling of alkyl group rotation with the hydrogen motion. Excited-state deactivation is controlled by solvent viscosity: the S(1) depopulation rate decreases by more than 2 orders of magnitude when the chromophore is transferred from a low-viscosity solution to a polymer film. Such behavior confirms a model for excited state deactivation in porphycene, which postulates that a conical intersection exists along the single hydrogen transfer path leading from the trans to a high energy cis-2 tautomeric form. For this process, the tautomerization coordinate includes not only hydrogen translocation but also large-amplitude twisting of the two protonated pyrrole moieties attached to the opposite sides of the ethylene bridge.

Femto- and picosecond fluorescence studies of solvation and non-radiative deactivation of ionic styryl dyes in liquid solution

Abstract A comparative study of the solvation and non-radiative relaxation dynamics of bridged and unbridged ionic styryl dye compounds is reported. Femto- and picosecond fluorescence transient measurements reveal solvation dynamics on a picosecond timescale of the solutes in ethanol, benzonitrile and decanol. Bridging is found to strongly affect the lifetime of the emissive state. It is shown that the presence of unbridged styryl-group single bonds allows for an effective non-radiative decay process. This decay process is suppressed by double chemical bridging.