Solvent-dependent fluorescence

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). 

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... 

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]

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.

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. 

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). 

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. ... 

Benzo[c]furan (4) exhibits a long-wave absorption band of medium intensity in the region of 340 nm. Lack of solvent dependence together with mirror relationship to the fluorescence spectrum signifies a tt-ti band a rotational analysis of the vapor phase spectrum led to an assignment as 82 <- Ap 1,3-Diaryl-substituted benzo[c]furans show a strong absorption band in the region of 415 nm in sterically hindered compounds, this... 

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]. 

Fluorescence spectra of a range of flavones have been reported (80P2443). The excited state proton transfer in 3-hydroxyflavone has been found to exhibit a solvent dependence (81JA6916, 82JA4146), whilst the fluorescence of flavone has been shown to be pH dependent... 

There are a number of empirical tests for the near ideality of potential probes. Ideal probes should exhibit a solvent dependence of the fluorescence Stokes-shift (hvabs - hvn) that is in account with the Lippert-Mataga equation [9,10]. 

Alternatively, the solvent dependence of X(t = oo) can be estimated from the equilibrated fluorescence maximum hvfl of the probe in each solvent. Thus the dependence of the photophysical properties of the probe, on its fluorescence maximum hvfl, can be established. This is demonstrated in Figure 14. The usefulness of the curves in Figure 14 is clear if one considers Eq. (23) and the fact that in the subpicosecond solvation experiments, the excited state population Se can be assumed to be a constant during the solvation process. It follows that the curves in Figure 10 represent how the fluorescence intensity at different wavelengths should change as the emission maximum... 

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]. 

As for trans-l,2-dimethoxy-l,2-di-9-anthrylethylene 39f, its fluorescence quantum yield, uniquely, is strongly solvent dependent. In cyclohexane the quantum yield is as high as 0.15, but in dichloromethane the quantum yield has decreased to 0.0077. 

Figure 4.24 Example of the solvent dependence of an exciplex fluorescence the pyrene/N,N-dime-thylaniline exciplex in cyclohexane (broken line) and tetrahydrofuran (full line). Wavelength A (XlOO) nm...

Some properties of the t -dimethylhexadiene exciplex are summarized in Table 7. Its fluorescence maximum is at slightly shorter wavelength than that of the anthracene-dimethylhexadiene exciplex (435 nm) (51). While data on other unsubstituted arene-diene exciplexes are not available, t appears to be more reactive and to form more stable exciplexes with dienes than arenes of comparable electron affinity (101). The dipole moment of the - -t -dimethylhexadiene exciplex is estimated to be 7 D from the solvent dependence of its fluorescence maxima (36). This value is substantially lower than those for pure charge-transfer exciplexes (p > 15 D) and indicates that this exciplex is relatively nonpolar and might be better categorized as a hetero-excimer, than as an exciplex (83). That is, using the normal resonance description of an exciplex... 

It is commonly assumed, therefore, that solvent reorganization will dominate electron transfer kinetics and that the reorganization energy in the same medium will be constant within a series of closely related redox partners. With a value of 2.4 kcal/mole for solvent reorganization (as obtained by Rehm and Weller (7) for fluorescence quenching of a series of arenes by substituted anilines in a polar medium) the curve shown in Fig. 2 is obtained. It is clear that substantial solvent-dependent barrier to electron exchange can be encountered. 

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