Fluorescence spectra first excited singlet state

The energy of the first excited singlet state can be determined from the wavelength at which the first vibrational band in the absorption spectrum coincides with the vibrational band in the fluorescence spectrum ... 

Fluorescence lifetimes of the first excited singlet states of HSO and DSO. Analytical expression used to compute vibrational energy dependence of non-radiative rates Dye-laser excitation spectrum of A A (004) — X A (000) band of HSO... 

Figure 12 (line A) depicts the emission spectrum of hydroxy pyrene trisulfonate dissolved in diluted buffer (pH 5.0). At this pH, the ground state is fully protonated (pK0 = 7.7), but not so the first excited singlet state (pK = 0.5). The excited molecules dissociate and 95% of the emission is at the wavelength of the excited anion (515 nm). The dissociation can be prevented if the compound is dissolved in acid solution, pH < pK., such as 2MHC1 (line B). Under such conditions, we observe the emission of the neutral form with maximum at 445 nm. Upon ligation to apomyoglobin, the fluorescence of hydroxypyrene trisulfonate consists of two... 

Another method, also applicable to the first excited singlet state, depends upon the variation of the fluorescence spectrum with pH, and is best illustrated by an example. The acridinium ion, AcrH", has pK — 5.45 in its normal state. It exhibits a green fluorescence in acid solution, attributable to the cation, and this remains unchanged with increasing pH up to about pH = 10, although the acridine has been converted almost entirely into the free base. This is explicable if the pK of the excited state is much greater than 5, so that in the pH range 7-10 the process is... 

Of course, the same energetic requirements as in the direct production of excited singlet states ( S-route ) or the production of the latter via triplet states ( T-route ) are valid here as in ECL (see p. 130). As an example, the chemiluminescent reaction of Wurster s Blue radical cation (12) with chrysene radical anion (13) must occur via the T-route . The emission spectrum matches chrysene fluorescence, but the (12)/(13) redox reaction has an enthalpy corresponding to 2.66eV only, whereas the energy of the chrysene first excited singlet state is 3.43 eV. 

Delayed fluorescence from a very-short-lived upper excited singlet state populated by hetero-TTA has been observed for the first time using the system A = anthracene and X = xanthone (Nickel and Roden, 1982). An energy-level diagram for this system is shown in Figure 5.32, and the corrected spectrum of the delayed fluorescence of anthracene and xanthone in trichlorotrifluoroe-thane is depicted in Figure 5.33. The band at 36,000-40,000 cm has been assigned to the delayed fluorescence of anthracene produced by Tf +... 

Once the molecule reaches the first excited singlet, internal conversion to the ground state is a relatively slow process. Thus, decay of the first excited state by emission of a photon can effectively compete with other decay processes. This emission process is fluorescence. Generally, fluorescence emission occurs very rapidly after excitation (10 to 10 s). Consequently, it is not possible for the eye to perceive fluorescence emission after removal of the excitation source. Because fluorescence occurs from the lowest excited state, the fluorescence spectrum, that is, the wavelengths of emitted radiation, is independent of the wavelength of... 

The molecular probe pyrene is commonly employed to elucidate solute-solvent interactions in normal liquids (18,35). Because of the high molecular symmetry, the transition between the ground and the lowest excited singlet state is only weakly allowed, subject to strong solvation effects (36-39). As a result, in the fluorescence spectrum of pyrene the relative intensities of the first (/i) and third I2) vibronic bands vary with changes in solvent polarity and polarizability. The ratio h/h serves as a convenient solvation scale, often referred to as the Py solvent polarity scale. Py values for an extensive list of common liquid solvents have been tabulated (15,16). 

No difference in the absorption spectrum is observed. The lifetimes of the two fluorescence emissions are both of the order of 10-8 sec. Another effect has been observed with hydrocarbons both as vapor and in solution, namely, a "delayed fluorescence emission with a life of a few milliseconds (56,63). This was at first interpreted as process 9, and the term excimer applied to the hypothetical excited dimer. However, C. A. Parker and C. G. Hatchard have shown that the intensity of the delayed fluorescence, which has the same spectrum as that from the excited singlet molecule, depends upon the square of the intensity of the exciting light (49). The mean life of the delayed fluorescence of anthracene solutions is (about) one-half that of the triplet state, and the effect is not observed in rigid media. These facts show that the delayed emission must be caused by an interaction between two triplet-state molecules ... 

The first electronic transition in butadiene has been the subject of many experimental and theoretical studies.The absorption, which has a maximum at 2100 A., is strong and represents a tt tt transition from a ground singlet to an upper singlet state. Analysis of the spectrum, which shows very little structure, has not been carried out. Since no fluorescent radiation has ever been detected on excitation of any of the simple dienes even at low temperature, a definite assignment of the 0 — 0 band has not been made. The 0 — 0 band had been placed at 2300 A. (124 kcal./mole), at which point the absorption is only /so as intense as at its maximum. The oscillator strength is 0.53, which leads to a radiative lifetime of 10 sec. Since emission of radiation has not... 

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