Stokes shift solvent

The discriminatory emission properties between two-coordinate d ° gold(I) complexes and their respective three-coordinate counterparts have been demonstrated in the literature [6, 10-13]. As discussed in the later sections, Che and coworkers have rationalized that the extraordinarily large Stokes shift of the visible emission of [Au2(diphosphine)2] from the [5da 6pa] transition is due to the exciplex formation ofthe excited state with solvent or counterions [6]. Fackler [14—16] reported the photophysical properties of monomeric [AUL3] complexes, which show visible luminescence with large Stokes shifts (typically lOOOOcm ), suggesting significant excited-state distortion. Gray et al. [10] examined the spectroscopic properties of... 

Evidence for specific solvent-solute interactions can be seen in the Lippert or others solvatochromic plots. One notices that the Stokes shift is generally larger in H-bonding solvents (water, alcohols) than in solvents with less probability to form... 

H-bonds. Such behavior of the Stokes shifts in protic solvents is typical for specific solvent-solute interactions, and has been seen for many solutes phthalimides, FLs, oxazines, and others [1, 2, 4, 50, 51, 72-74]. 

In methanol/DMSO solvent mixtures the fluorescence spectrum of TIN (A.max = 400 nm) displays a normal Stokes shift indicating that this emission arises from a non proton-transferred, excited state of TIN. The fluorescence excitation spectrum for this emission coincides with the absorption spectrum of the resolved non-planar species suggesting that this conformer is the ground-state precursor responsible for the observed emission. As the amount of DMSO in the mixture increases the fluorescence maximum undergoes a bathochromic shift from 415 nm in pure methanol to 440 nm in pure DMSO. 

This important parameter can provide information on the excited states. For instance, when the dipole moment of a fluorescent molecule is higher in the excited state than in the ground state, the Stokes shift increases with solvent polarity. The consequences of this in the estimation of polarity using fluorescent polarity probes is discussed in Chapter 7. 

When Stokes shifts are plotted as a function of the orientation polarizability A f (Lippert s plot, see Section 7.2.2), solvents are distributed in a rather complex manner. A linear relationship is found only in the case of aprotic solvents of relatively low polarity. The very large Stokes shifts observed in protic solvents (methanol, ethanol, water) are related to their ability to form hydrogen bonds. 

If the molecule carries its own base, excited-state proton transfer can occur intramolecularly (ESIPT = excited-state intramolecular proton transfer(37)) and becomes more or less independent of the surrounding solvent. The ESIPT reaction is extremely fast (subpicosecond kinetics(38)) and occurs also in rigid glasses and at very low temperatures.06 39) Very often, only the ESIPTproduct P fluoresces, and this is the source of extremely large Stokes shifts which are fairly independent of medium... 

Tyrosine fluorescence emission in proteins and polypeptides usually has a maximum between 303 and 305 nm, the same as that for tyrosine in solution. Compared to the Stokes shift for tryptophan fluorescence, that for tyrosine appears to be relatively insensitive to the local environment, although neighboring residues do have a strong effect on the emission intensity. While it is possible for a tyrosine residue in a protein to have a higher quantum yield than that of model compounds in water, for example, if the phenol side chain is shielded from solvent and the local environment contains no proton acceptors, many intra- and intermolecular interactions result in a reduction of the quantum yield. As discussed below, this is evident from metal- and ionbinding data, from pH titration data, and from comparisons of the spectral characteristics of tyrosine in native and denatured proteins. 

Little is known about the fluorescence of the chla spectral forms. It was recently suggested, on the basis of gaussian curve analysis combined with band calculations, that each of the spectral forms of PSII antenna has a separate emission, with Stokes shifts between 2nm and 3nm [133]. These values are much smaller than those for chla in non-polar solvents (6-8 nm). This is due to the narrow band widths of the spectral forms, as the shift is determined by the absorption band width for thermally relaxed excited states [157]. The fluorescence rate constants are expected to be rather similar for the different forms as their gaussian band widths are similar [71], It is thought that the fluorescence yields are also probably rather similar as the emission of the sj tral forms is closely approximated by a Boltzmann distribution at room temperature for both LHCII and total PSII antenna [71, 133]. 

The fluorescence spectrum of the nonsteroidal anti-inflammatory agent piroxicam 21 has been determined in a variety of solvents (Scheme 7) <1999PCP4213>. The key observations are that the molecule exists with a strong H-bond between the phenolic OH and the adjacent amide. A very high Stokes shift in the excited state was observed and attributed to the proton-transfer event (tautomerization) between the phenolic and amide oxygens (cf. 21 —>63). In the case of protic solvents, such as water, the open conformation 64 was observed. 

The central question in liquid-phase chemistry is How do solvents affect the rate, mechanism and outcome of chemical reactions Understanding solvation dynamics (SD), i.e., the rate of solvent reorganization in response to a perturbation in solute-solvent interachons, is an essential step in answering this central question. SD is most often measured by monitoring the time-evolution in the Stokes shift in the fluorescence of a probe molecule. In this experiment, the solute-solvent interactions are perturbed by solute electronic excitation, Sq Si, which occurs essenhaUy instantaneously on the time scale relevant to nuclear motions. Large solvatochromic shifts are found whenever the Sq Si electroiuc... 

The goal of theory and computer simulation is to predict S i) and relate it to solvent and solute properties. In order to accomplish this, it is necessary to determine how the presence of the solvent affects the So —> Si electronic transition energy. The usual assmnption is that the chromophore undergoes a Franck-Condon transition, i.e., that the transition occurs essentially instantaneously on the time scale of nuclear motions. The time-evolution of the fluorescence Stokes shift is then due the solvent effects on the vertical energy gap between the So and Si solute states. In most models for SD, the time-evolution of the solute electronic stracture in response to the changes in solvent environment is not taken into accoimt and one focuses on the portion AE of the energy gap due to nuclear coordinates. 

Fig. 1 represents schematically the usual physical interpretation of polar SD The solute undergoes vertical electroitic excitation and the dynamic fluorescence Stokes shift arises Ifom the reorganization of the solvent molecules. In the case... 

Ultraviolet (UV) spectroscopy does not tend to be the method of choice for structure determination, but a list of UV absorptions was given in the review by Knowles <1996CHEC-II(7)489>. Fluorescence properties and triplet yields of [l,2,3]triazolo[4,5-r/ pyridazines in various solvents have been reported <2002JPH83>. These heterocyclic systems were found to be photochemically very stable. In a recent paper, Wierzchowski et al. studied the fluorescence emission properties of 8-azaxanthine ([l,2,3]triazolo[4,5-r/ pyrimidine-5,7-dione) and its A -alkyl derivatives at various pH s <2006JPH276>. For the 8-azaxanthines, an important characteristic of emission spectra in aqueous solutions was the unusually large Stokes shift. Since 8-azaxanthine is a substrate for purine nucleoside phosphorylase II from Escherichia coli, the reaction is now monitored fluorimetrically. The fluorescence properties of [l,2,3]triazolo[4,5-r/ -pyrimidine ribonucleosides were earlier described by Seela et al. <2005HCA751>. 

Indole in cyclohexane and ethanol is excited at 270 nm populating a mixture of the La and Lb states. The La and Lb states differ in their spectral structure (Fig. la)) and Stokes shifts [3]. The unstructured spectrum of the La state shows a large Stokes shift due to the large change of the dipole moment upon electronic excitation. The dipole moment of the Lb state is similar to the ground state value. In nonpolar solvents like cyclohexane, the Lb state is energetically below the La state and its emission spectrum exhibits vibronic structure. In the polar solvent ethanol state reversal occurs after the electronic excitation and the La state becomes responsible for the more red shifted fluorescence [4],... 

Apart from pure benzene and pure polar solvents, either acetonitrile or methanol, we have considered xp = 0.2 and xp = 0.7 molar fractions of the polar solvent. Systems ranging from 256 (pure benzene) to 512 (pure polar solvents) molecules were used. From well equilibrated (1 ns) simulations with the coumarin in the ground state So, one to two hundred equally distant configurations were selected. In these configurations the coumarin state was switched to the Si state and the solvent was let to relax in a series of 10 ps long NVE simulations. The solvent response was monitored using the normalized time-dependent stokes-shift function ... 

The variations of the simulated steady state solvatochromism, as a function of the polar solvent molarity, were found to be in good agreement with the experimental work of Krolicki et al. [4], both for absorption and fluorescence. The difference of Stokes-shifts between benzene and acetonitrile is 981 cm-1, compared to 1230 cm-1 obtained experimentally. These numbers are 870 cm-1 and 1910 cm-1, respectively, for methanol. 

In this work we presented the results of Molecular Dynamics simulations performed to study the solvatochromism and the dynamic stokes-shift of coumarin 153 in mixtures of solvents. We showed the ability of MD to reproduce available data of the time-dependent Stokes-shifts. Moreover, MD allowed us to interpret these dynamics in benzene-acetonitrile mixtures in terms of motions of benzene around the coumarin or rotation of acetonitrile. The role of benzene in the solvation process of Cl53 seems to be more important than usually assumed. 

The temporal evolution of the simulated peak position of the fluorescence spectrum is shown in Fig. 3a The corresponding experimental peak-shift (figure 2 in [5]) consists of an oscillatory and an exponential part. Our model reproduces the weakly damped oscillatory part of the peak shift, but does not describe the large Stokes shift of 1400 cm-1. The reason is that our single-mode model does not take into account other system modes and, what is more important, solvent modes, which contribute to the overall shift of the SE spectrum. The model may be improved by including an additional overdamped solvent mode. 

The steady-state spectra obtained for different alcohols are depicted in fig. 1. While the absorption spectra red shift with increasing solvent polarisability (from methanol to octanol), the fluorescence shows a red-shift when going from octanol to methanol. The total Stokes shifts are very large 7.900 100 cm 1 for PSBR/MeOH and 6.870 100 cm 1 for octanol. Another striking observation is the 30 % smaller width of the fluorescence spectrum of methanol (AE = 3.420 cm 1) compared with other alcohols. While the widths of the fluorescence spectra are solvent-dependent, the absorption spectra have a FWHM of -5.100 cm"1, irrespective of the solvent. As we will substantiate in the following, this behavior indicates that the potential energy surface around the fluorescent point is different than near the Franck-Condon zone probed by absorption, as suggested by quantum chemistry calculations [7]. 

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