Piotr Fita

Ground-and Excited-State Tautomerization Rates in Porphycenes

The rates of double hydrogen transfer in the ground and excited electronic states have been measured for porphycene and its derivatives by using a new method based on pump-probe polarization spectroscopy. Changing the strength of two intramolecular hydrogen bonds by altering the NHN distance leads to differences in the tautomerization rate exceeding three orders of magnitude. The reaction is considerably slower in the lowest electronically excited state. A correlation was found between the tautomerization rates and (1)H chemical shifts of the internal protons.

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.

Ultrafast Excited-State Dynamics of Eosin B: A Potential Probe of the Hydrogen-Bonding Properties of the Environment

The photophysics of two dyes from the xanthene family, eosin B (EB), and eosin Y (EY) has been investigated in various solvents by femtosecond transient absorption spectroscopy, first, to clarify the huge disparity of the EB fluorescence lifetimes reported in literature, and, second, to understand the mechanism responsible for the ultrafast excited-state deactivation of EB in water. The excited-state lifetime of EB was found to be much shorter in water and in other protic solvents, due to the occurrence of hydrogen-bond assisted nonradiative deactivation. This mechanism is associated with the hydrogen bonds between the solvent molecules and the nitro groups of EB, which become stronger upon optical excitation due to the charge-transfer character of the excited-state. This process is not operative with EY, where the nitro groups are replaced by bromine atoms. Therefore, the excited-state lifetime of EB in solution is directly related to the strength of the solvent as a hydrogen-bond donor, offering the possibility to build a corresponding scale based on the fluorescence quantum yield or lifetime of EB. This scale of hydrogen-bonding strength could be especially useful for studies of liquid interfaces by time-resolved surface second harmonic generation.

Femtosecond transient fluorescence spectrometer based on parametric amplification

We report an experimental proof-of-principle of a method for recording femtosecond, time-resolved fluorescence spectra in the visible range. The method is based on a noncollinear parametric amplification in a beta barium borate crystal and provides time resolution of the order of 100fs. We demonstrate that with this method, transient fluorescence spectra as wide as 6000cm−1 can be recorded in a single time-delay scan. Fluorescence decay dynamics and transient spectra of Coumarin 6dye dissolved in aniline were measured to test the usefulness of the method.

Ground and Excited State Double Hydrogen Transfer in Symmetric and Asymmetric Potentials: Comparison of 2,7,12,17-Tetra-n-propylporphycene with 9-Acetoxy-2,7,12,17-tetra-n-propylporphycene

Analysis of time-resolved anisotropy of transient absorption enabled determination of room temperature ground and excited state rate constants for intramolecular double hydrogen transfer in two similar porphycenes, one of them with symmetric and the other, with asymmetric character of a double minimum potential for hydrogen motion. The perturbation preserves a quasi-symmetric minimum in S(0), but the rate decreases approximately two times. In S(1), the perturbed potential becomes strongly asymmetric, and the downhill hydrogen transfer occurs with a rate higher than that observed for a symmetrical compound.

Unusually Slow Intermolecular Proton-Deuteron Exchange in Porphycene

Abstract The rates of the intermolecular exchange of two internal protons by deuterons have been determined for alcohol solutions of porphycene and its 2,7,12,17-tetra-t-butyl derivative using femtosecond pump-probe polarization spectroscopy. The process is very slow, requiring approximately twenty hours to substitute both protons by deuterons in more than 90% of the molecules dissolved in bulk EtOD or BuOD at 293 K. Equal rates were found for the exchange of the first and the second proton.

Hydrogen-Bond-Assisted Excited-State Deactivation at Liquid/Water Interfaces

The excited-state dynamics of eosin B (EB) at dodecane/water and decanol/water interfaces has been investigated with polarization-dependent and time-resolved surface second harmonic generation. The results of the polarization-dependent measurements vary substantially with (1) the EB concentration, (2) the age of the sample, and (3) the nature of the organic phase. All of these effects are ascribed to the formation of EB aggregates at the interface. Aggregation also manifests itself in the time-resolved measurements as a substantial shortening of the excited-state lifetime of EB. However, independently of the dye concentration used, the excited-state lifetime of EB at both dodecane/water and decanol/water interfaces is much longer than in bulk water, where the excited-state population undergoes hydrogen-bond-assisted non-radiative deactivation in a few picoseconds. These results indicate that hydrogen bonding between EB and water molecules at liquid/water interfaces is either much less efficient than in bulk water or does not enhance non-radiative deactivation. This strong increase of the excited-state lifetime of EB at liquid/water interfaces opens promising avenues of applying this molecule as a fluorescent interfacial probe.

Molecular Dynamics Simulations of Liquid Phase Interfaces: Understanding the Structure of the Glycerol/Water−Dodecane System

Modern spectroscopic techniques such as time-resolved second-harmonic-generation spectroscopy allow molecules to be examined selectively directly at phase interfaces. Two-phase systems formed by glycerol/water and alkane layers have previously been studied by time-resolved second-harmonic-generation spectroscopic measurements. In this molecular dynamics study, a triphenylmethane dye was inserted at the glycerol/water-alkane interface and was used as a probe for local properties such as viscosity. We now show how extensive simulations over a wide range of concentrations can be used to obtain a detailed view of the molecular structure at the glycerol/water-alkane interface. Glycerol is accumulated in a double layer adjacent to the alkane interface, which results in increased viscosity of the glycerol/water phase in the direct vicinity of the interface. We also show that conformational ensembles created by classical molecular-dynamics simulations can serve as input for QM/MM calculations, yielding further information such as transition dipoles, which can be compared with spectroscopic measurements.

Tautomerism in porphycenes: analysis of rate-affecting factors.

Double hydrogen transfer occurring in both ground and the lowest electronically excited singlet states was studied for a series of 19 differently substituted porphycenes. The rates of tautomerization have been determined using femtosecond pump-probe spectroscopy with polarized light. The values vary by over 3 orders of magnitude, suggesting the importance of tunneling. Good correlation exists between the values of the rates and the parameters characterizing the strength of two intramolecular hydrogen bonds: proton NMR shift, distance between the hydrogen-bonded nitrogen atoms, and the NH stretching frequency. While hydrogen-bond strength is the main factor determining the rate of double hydrogen transfer, other factors, such as static and dynamic symmetry breaking and the population of low-frequency vibrations also have to be taken into account.

Evidence for Dominant Role of Tunneling in Condensed Phases and at High Temperatures: Double Hydrogen Transfer in Porphycenes.

Investigation of the double hydrogen transfer in porphycene, its 2,7,12,17-tetra-tert-butyl derivative, and their N-deuterated isotopologues revealed the dominant role of tunneling, even at room temperature in condensed phase. Ultrafast optical spectroscopy with polarized light employed in a wide range of temperatures allowed the identification and evaluation of contributions of two tunneling modes: vibrational ground-state tunneling, occurring from the zero vibrational level, and vibrationally activated, via a large amplitude, low-frequency mode. Good correspondence was found between the rates of incoherent tunneling occurring in condensed phase and the values estimated on the basis of tunneling splittings observed in molecules isolated in supersonic jets or helium nanodroplets. The results provide solid experimental insight into widely proposed quantum facets of ubiquitous hydrogen-transfer phenomena.