Solvent dependence fluorescence

A number of drawbacks in the application of the 0PA/2-ME reagent system include the instability of the fluorescent isoindole derivative (5-7) the use of the noisome reagent 2-mercaptoethanol the low and solvent-dependent fluorescence efficiencies (8,9) of the isoindole and—perhaps the most limiting—the effective restriction of the OPA assay to primary aliphatic amines and to amino acids. 

Kalyanasundaram K, Thomas JK (1997) Solvent-dependent fluorescence of pyrene-3-car-boxaldehyde and its applications in the estimation of polarity at micelle-water interfaces. J Phys Chem 81 2176-2180... 

Kalyanasundaran K. and Thomas J. K. (1977a) Solvent-Dependent Fluorescence of Pyrene-3-Carboxaldehyde and its Applications in the Estimation of Polarity at Micelle-Water Interfaces,/. Phys. Chem. 81, 2176-2180. 

R = p-phenylsulfonyl) have been found to enhance the proposed adiabatic isomerization [90,93]. The hypothetical existence of two distinct conformers of cis-9-styrylanthracenes has also been invoked in order to explain the solvent-dependent fluorescence properties of cis-9-styrylanthracenes [92]. 

The existence of two interconvertible ICT and TICT excited-state molecules can lead to a dual, variable solvent-dependent fluorescence. This dual fluorescence was first discovered by Lippert et al. [342] using 4-(dimethylamino)benzonitrile as the fluorescent compound, and then correctly interpreted by Grabowski et al. [343]. They identified the... 

TICT excited state as the origin of the long-wavelength solvent-dependent fluorescence of this nitrile see also references [484, 485], The large number of organic molecules with two fluorescent states and the underlying concepts of the TICT excited state have been extensively reviewed [116, 344],... 

Solvatochromic fluorescent probe molecules have also been used to establish solvent polarity scales. The solvent-dependent fluorescence maximum of 4-amino-V-methylphthalimide was used by Zelinskii et al. to establish a universal scale for the effect of solvents on the electronic spectra of organic compounds [80, 213], More recently, a comprehensive Py scale of solvent polarity including 95 solvents has been proposed by Winnik et al. [222]. This is based on the relative band intensities of the vibronic bands I and III of the % - n emission spectrum of monomeric pyrene cf. Section 6.2.4. A significant enhancement is observed in the 0 0 vibronic band intensity h relative to the 0 2 vibronic band intensity /m with increasing solvent polarity. The ratio of emission intensities for bands I and III serves as an empirical measure of solvent polarity Py = /i/Zm [222]. However, there seems to be some difficulty in determining precise Py values, as shown by the varying Py values from different laboratories the reasons for these deviations have been investigated [223]. 

Polar solvents and in particular hydrogen bonds are able to stabilize polar molecules. This is also true for the excited states of a molecule that may have quite different polarities, and solvent variation may possibly change the energy order of the excited states as well as transition moments. (Cf. Section 2.7.2.) As an example we recall quinoline, whose solvent-dependent fluorescence quantum yield has been discussed in Section 5.3.2. 

Figure 3.36. Solvent-dependent fluorescence of 43b3 in various solvents. (From Ref. [234] with permission of the American Chemical Society.)...

Absorption and emission spectra of six 2-substituted imidazo[4,5-/]quinolines (R = H, Me, CH2Ph, Ph, 2-Py, R = H CH2Ph, R = Ph) were studied in various solvents. These studies revealed a solvent-independent, substituent-dependent character of the title compounds. They also exhibited bathochromic shifts in acidic and basic solutions. The phenyl group in the 2-position is in complete conjugation with the imidazoquinoline moiety. The fluorescence spectra of the compounds exhibited a solvent dependency, and, on changing to polar solvents, bathochromic shifts occur. Anomalous bathochromic shifts in water, acidic solution, and a new emission band in methanol are attributed to the protonated imidazoquinoline in the excited state. Basic solutions quench fluorescence (87IJC187). 

Molecular rotors are useful as reporters of their microenvironment, because their fluorescence emission allows to probe TICT formation and solvent interaction. Measurements are possible through steady-state spectroscopy and time-resolved spectroscopy. Three primary effects were identified in Sect. 2, namely, the solvent-dependent reorientation rate, the solvent-dependent quantum yield (which directly links to the reorientation rate), and the solvatochromic shift. Most commonly, molecular rotors exhibit a change in quantum yield as a consequence of nonradia-tive relaxation. Therefore, the fluorophore s quantum yield needs to be determined as accurately as possible. In steady-state spectroscopy, emission intensity can be calibrated with quantum yield standards. Alternatively, relative changes in emission intensity can be used, because the ratio of two intensities is identical to the ratio of the corresponding quantum yields if the fluid optical properties remain constant. For molecular rotors with nonradiative relaxation, the calibrated measurement of the quantum yield allows to approximately compute the rotational relaxation rate kor from the measured quantum yield [Pg.284]

This approximation requires that cos. This behavior in fact follows from a Debye dielectric continuum model of the solvent when it is coupled to the solute nuclear motion [21,22] and then xs would be proportional to the longitudinal dielectric relaxation time of the solvent indeed, in the context of time dependent fluorescence (TDF), the Debye model leads to such an exponential dependence of the analogue... 

However, the fluorescence characteristics of these compounds are strongly solvent-dependent. In protic solvents such as alcohols, hydrogen bonds can be formed between the nitrogen atoms and the solvent molecules. This results in an inversion of the lowest-lying rt-n and n-n states. As the lowest-lying transition becomes of n —> n character in these solvents, the fluorescence quantum yield is much higher than in hydrocarbon solvents. 

Tab. 7.4. Solvent dependence of the ratio / //m of the fluorescence intensities of the first and third vibronic bands in the fluorescence spectrum of pyrene.

The TICT model was put forward to account for the observation that the dual fluorescence of DMABN with its normal band (B band) at around 350 nm and its anomalous one (A band, around 450 nm in medium polar solvents) depends on the... 

G. van der Zwan and J. T. Hynes, Time-dependent fluorescence solvent shifts, dielectric friction and nonequilibrium solvation in polar solvents, J. Phys. Chem. 89, 418M188 (1985). 

Viscosity-dependent fluorescence (VDF) typically occurs in molecules which, following absorption of excitation light, undergo nonradiative decay by intramolecular twisting or torsional motions (12-19). In ordinary low-viscosity solvents, VDF compounds exhibit... 

The UV spectrum has been measured in hexane and cyclohexane other solvents also seem to have been used. In the 305-343 nm region, the UV spectrum strongly resembles that of benzo[c]thiophene.2 The lack of solvent dependence together with a mirror relationship to its fluorescence spectrum (in isopentane-methylcyclohexane 3 1 at 77°K) is indicative of the 7t,7r -character of the lowest singlet transition. ... 

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