Nonradiative pathways

Another example of the role played by a nonradiative relaxation pathway is found in the photochemistry of octatetraene. Here, the fluorescence lifetime is found to decrease dramatically with increasing temperature [175]. This can be assigned to the opening up of an efficient nonradiative pathway back to the ground state [6]. In recent years, nonradiative relaxation pathways have been frequently implicated in organic photochemistry, and a number of articles published on this subject [4-8]. 

Soper SA, Mattingly QL (1994) Steady-state and picosecond laser fluorescence studies of nonradiative pathways in tricarbocyanine dyes implications to the design of near-IR fluor-ochromes with high fluorescence efficiencies. J Am Chem Soc 116 3744—3752... 

In summary, it appears that phosphorescence at room temperature is a function of burial or rigidity of the site, but, as for all excited states, the competing nonradiative pathways are influenced by the polarizability, polarity, and mobility of the local environment. 

Figure 5.16 Configurational coordinate diagrams to explain (a) radiative and (b) nonradiative (multiphonon emission) de-excitation process. The sinusoidal arrows indicate the nonradiative pathways.

The rate of relaxation by nonradiative pathways can be increased by addition of quenchers. Quenching of fluorescence occurs by several mechanisms, many of which involve collision of the excited chro-mophore with the quenching molecule. Some substances such as iodide ion are especially effective quenchers. The fluorescence efficiency of a substance in the absence of a quencher can be expressed (Eq. 23-lb) in terms of the rate constants for fluorescence (fcf), for nonradiative decay (km), and for phosphorescence ( r )=... 

The work of van der Werf et al. (246) has clarified the nonradiative pathways that depopulate Sj. Since the results of this work have been discussed in a previous section, it will not be mentioned here, except to confirm that both the aforementioned low-energy studies involve collision-induced intersystem crossing. 

Nonradiative Processes. The nonradiative pathways which depopulate are still a matter of some discussion. It has been pointed out that the short lifetime of precludes collisional quenching of this electronic state, and the failure to observe sensitized triplet emission from biacetyl (92) indicates that the... 

The complexes often undergo radiative decay from their lowest excited state both in fluid solutions at room temperature and in glassy media at 77 K [51, 63, 64, 71]. Emission lifetimes are typically 20 ns to 2 ps at room temperature and are summarized in Table 7. The excited state can decay by two nonradiative pathways by internal conversion to the ground state and by a thermally activated process through a higher energy excited state that rapidly decays to the ground state. The exact parameters for the two pathways depend on X, L, solvent and temperature. 

Energy transfer can occur either radiatively through absorption of the emitted radiation or by a nonradiative pathway. The nonradiative energy transfer can also occur via two different mechanisms—the Coulomb or the exchange mechanism. 

Fignre 20.16 schematically illustrates these two pathways for loss of energy by radiation from an electronically excited molecnle, along with one nonradiative pathway. In the nonradiative pathway, the molecnle may cross over to the gronnd electronic state and gradually lose its energy as heat to the snrronndings as it cascades down to lower vibrational and rotational levels. 

ESPT has been identified as the main nonradiative pathway in the excited state of ethidium bromide (EB, I), a popular DNA probe [46a]. In aqueous solution, on addition of DNA, EB intercalates in the double helix of DNA [44-46]. This causes a nearly 11-fold increase in the emission intensity and lifetime of EB. The emission quantum yield and lifetime of EB are very similar in methanol and glycerol, whose viscosities differ by a factor of 2000 [46a]. Thus, the fluorescence enhancement of EB on intercalation is not due to high local viscosity. Emission intensity of EB is low in highly polar, protic solvents, such as alcohol... 

One other feature of the data in Table 3 and elsewhere [56,79, 82] relates to the pressure effects on the nonradiative pathways from the LF excited states of the Rhm complexes. For these photoactive complexes, the pressure dependence of kn generally has the same sign as that for the major photoreaction pathway but a smaller absolute value. This finding may indicate the parallel character of ks and a strong-coupling contribution [88] to ka. Such a pattern would be consistent with a reaction coordinate, which approaches the GS surface in a manner that allows partitioning between reactive deactivation and nonradiative deactivation. 

It is a well documented fact that the main pathway for the Si state of polyme-thinic dyes at room temperature is the trans-cis isomerization (see [16, 88] and references quoted therein). For monomeric cyanines and in the absence of steric hindrance, both fluorescence quantum yields and intersystem crossing quantum yields are usually very low. For TCC and 9-MeTCC entrapped within the polymer chains of microcrystalline cellulose F becomes close to unity, evidence of the decrease of the nonradiative pathways of deactivation. 

For 2,2 -cyanine the fluorescence quantum yields (0/-) determined were about 0.08 whenever dichloromethane (solvent which does not swell cellulose) was used for sample preparation, while, with ethanol, <(>f was approximately 0.30. Similar values were also obtained for l,l -diethyl-2,2 -carbocyanine as Figure 42 shows. These values are about three orders of magnitude higher than in solution, showing the importance of the rigid dry matrix in reducing the nonradiative pathways of deactivation of the (rr, Jt ) first excited singlet state of this cyanine. 

F. 1.9 Luminescent mechanisms of thermal effects on inorganic solids, a Emission from a luminescent activator on excitation, b TQ results in a nonradiative pathway associated with heat, c TI excites electrons toward the conduction band through heat d TD can lead to other emissions as a result of heat. A and A represent the ground and excited states of the activator, respectively. A and A represent the ground and excited states of the activator with different charges, respectively. VB and CB represent the valence and conduction bands of the host, respectively. Reprinted from Ref. [27], Open Access... 

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