The dynamics of solvation of an excited chromophore in pure water and in a restricted space with a limited number of water molecules have been studied. The time-dependent Stokes shift of Coumarin 480 (C480) and Coumarin 460 (C460) were measured using femtosecond fluorescence upconversion and time-correlated singlephoton-counting techniques. The system with a limited number of water molecules was an inclusion complex of Coumarin dyes with y-cyclodextrin (yCD). The results of molecular dynamics simulations are compared with the observed solvent response in pure water and in the yCD cavity. The observed relaxation times range from < fs to 1.2 ns. Solvation of C480 in pure water is observed to occur with time constants of <50 and 310 fs. In sharp contrast with the solvation response in pure water, in the case of the C48O/yCD inclusion complex, additional long solvation time constants of 13, 109 and 1200 ps are observed. The stoichiometry, structure and dynamics of the CoumarinlyCD complexes are also discussed.
Fluorescence decay and polarization of dansyl-labeled poly(methacrylic acid) (PMA) and poly-(acrylic acid) (PAA) have been studied as a function of pH. The decay measurements have shown that, at low pH, PMA chains form highly compact hydrophobic clusters, which are joined by short extended polymer chains. During the transition from the compact to the expanded form, the size of the clusters decreases up to a limit, beyond which the clusters disintegrate completely to an expanded polymer chain. The effect of PMA molar mass and ionic strength on this process was investigated.
The solvent dependence of absorption and fluorescence spectra, fluorescence lifetimes (τ Fl ), and quantum yields (q Fl ) of various 3-substituted benzanthrone derivatives have been investigated. A consistent correlation between fluorescence quantum yield and emitting state energy has been found that holds for all 6 derivatives in 11 solvents. The experimental data together with the results of semiempirical quantum chemical calculations indicate that the main quenching channel of the fluorescent S 1 (π,π*) excited state is intersystem crossing to an upper (n,π*) triplet state, T N . The rate constant and efficiency of intersystem crossing between these two states are strongly influenced by the substituent and by the solvent polarity, as both modulate the singlet state energy and the S 1 -T N energy gap. The rate constant of direct S 1 f T 1 intersystem crossing is small in most systems but appears to increase with a decrease in the energy of the S 1 state.
The wavelength dependent fluorescence decay properties of bovine prothrombin fragment 1 have been investigated employing a picosecond time-correlated single photon counting technique. All observations are discussed with using the crystal structure (Soriano-Garcia et al., Biochemistry 31:2554-2566. Fluorescence lifetimes distribution and conventional multiexponential analysis, as well as acrylamide quenching studies lead to the identification of six distinguishable tryptophan excited-states. Accessibility to the quencher and the known structure are used to associate a fluorescence decay of the tryptophan present in the Gla domain (Trp42) with two red shifted components (2.3 and 4.9 ns). The two kringle domain tryptophans (Trp90 and Trp126) exhibit four decay times (0.06,0.24,0.68, and 2.3 ns), which are blue shifted. The calciuminduced fluorescence quenching is a result of static quenching: the five decay times remain unchanged, whereas the fluorescence intensity of Trp42 is decreased. The static quenching process is a consequence of a ground state interaction between the Cys18-Cys23 disulfide bridge and Trp42. The monomolecular equilibrium constant for this disulfide-n-electron interaction is found as 4.8.
The intramolecular electronic excitation energy flow was investigated in a specially designed bichromophoric molecule, 2-(3-benzanthronylamino)-4-(1-pyrenylamino)-6-chloro-1,3,5-triazine (1) and was compared with the behaviour of two relevant component model compounds that closely mimic the photophysical properties of acceptor and donor sub-units in the bichromophore. Electronic absorption and fluorescence spectroscopy was applied (including fluorescence anisotropy and decay kinetics measurements with nanosecond to femtosecond time resolution) in order to resolve the energy relaxation process on a real time. An unambiguous piece of evidence is reported for an ultrafast process which leads to practically instantaneous population of the emitting state of the acceptor sub-unit after selective ≈200-fs-excitation of the donor sub-unit. This first direct observation of extremely fast energy transfer in a stiff bichromophore is significant for further development of relevant theory. Two conceptually different approaches to explaining such fast energy flow are discussed.
Ultrafast time resolved fluorescence anisotropy decay measurements were performed to gain insight into the energy gap dependence of donor-acceptor inter-chromophoric coupling within one supra-molecule. Three new compounds, each consisting of two semi-rigidly linked and strongly coupled chromophores, were designed and synthesized for this study. Their general structure is donor-spacer-acceptor, where "donor" is phenylamino, pyrenylamino, or benzanthronylamino moiety, and acceptor is aminobenzanthrone. While being similar structurally, the compounds differ significantly in the excitation energy difference of the two chromophores in a supra-molecule. Experimental data show an ultrafast initial fluorescence emission anisotropy decrease (within less then 1 ps) when the excited state energies of the interacting chromophores are close to each other or equal. No such fast fluorescence anisotropy dynamics is observed for a compound with a large energy gap.
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