Anthracene fluorescence quantum yields

Interesting examples are found in substituted anthracenes. The lifetimes and quantum yields of fluorescence of substituted anthracenes show different dependencies on temperature. The position of the substituent is more important than its nature. For 9- and 9,10-substituted anthracenes, fluorescence quantum yields increase steeply with decrease of temperature, while side-substituted derivatives have low yield and small temperature dependence. The variation is of the form... 

TABLE 5 Deactivation of Singlet and Triplet Excited Linked Anthracenes. (Fluorescence Quantum Yields < >F in Cyclohexane. Reaction Quantum Yields < >R in Benzene)... 

We have now seen how fluorescence quantum yields and lifetimes are experimentally determined, along with some of the strengths and weaknesses of the methods used. For anthracene these constants have been determined to be... 

In most cases, the linear absorption is measured with standard spectrometers, and the fluorescence properties are obtained with commercially available spectrofluo-rometers using reference samples with well-known <1>F for calibration of the fluorescence quantum yield. In the ultraviolet and visible range, there are many well-known fluorescence quantum yield standards. Anthracene in ethanol (Cresyl Violet in methanol (commonly used reference samples for wavelengths of 350-650 nm. For wavelengths longer than 650 nm, there is a lack of fluorescence references. Recently, a photochemically stable, D-ji-D polymethine molecule has been proposed as a fluorescence standard near 800 nm [57]. This molecule, PD 2631 (chemical structure shown in Fig. 5) in ethanol, has linear absorption and fluorescence spectra of the reference PD 2631 in ethanol to... 

As in the former cases, k2 was calculated from the integrated extinction coefficients,149 k3 + kt was derived from fluorescence quantum yields,149 while k3 and k4 were separately estimated from the maximum quantum yields of photooxygenations at high oxygen concentrations.150 Flash spectroscopy techniques were used in order to determine k5 and k7, while kB was obtained from the Stern-Volmer quenching constant of oxygen.149 The ratio ke/kg was determined from the variation of AOz with the concentration of the anthracene.71 When photodimerization occurred, k13l(kia + k13) was calculated from the maximum yield of... 

The acridizinium (benzo[6]quinolizinium) ion, being isoelectronic with anthracene, is fluorescent (55JA4812). The fluorescent quantum yield for acridizinium perchlorate in methanol was reported to be 0.52 (80MI21000). The rate of quenching of this fluorescence by alkyl halides was found to be related to the ionization potential of the halide (78MI21001). Quenching by anions was also measured (79JPR420). 

PAHs also generally have well-structured emission spectra (see Figs. 10.6-10.10) and relatively large fluorescence quantum yields. For example, in degassed n-heptane at room temperature, the fluorescence quantum yields are as follows fluoranthene, 0.35 benz[ ]anthracene, 0.23 chrysene, 0.18 BaP, 0.60 BeP, 0.11 and benzo[g/zi]perylene, 0.29 (Heinrich and Giisten,1980). Cyclopenta[crf]pyrene, however, does not fluoresce. 

As an example of the effect of level shifts in the crystalline state, as just described, consider the observed rates of radiationless transitions in anthracene.45 The first excited 1BSu of the isolated anthracene molecule is located about 600 cm-1 above the second triplet state. Hence, 8 < vv and the intersystem crossing process is quite rapid at room temperature. The fluorescence quantum yield is about 0.3 for this molecule in the gas phase and in solution. In the crystal the first excited singlet state is red shifted (from the gas level) by about 1880 cm- while the second triplet state is hardly affected, so that in this case the energy gap between those two states increases in the crystal. Then the coupling term, v, is smaller in the crystalline state than in solution, thereby leading to a decrease in the rate of the intersystem crossing. The result is that the fluorescence yield in the crystal is close to unity.40... 

The effects of limited molecular flexibility and increasing deviation from coplanarity of the anthracene and ethylene ir-systems on the radiative properties have been assessed in a series of symmetrically 2,2-substituted l-(9-anthryl)ethylenes 87 [63]. As for structurally rigid 9-anthrylethylenes 88 and 89, for which rotation about the anthryl-ethylene single bond is not possible, and in which the ethylene double bond has been forced to be coplanar with the anthracene -system, their fluorescence quantum yields in cyclohexane are exceptionally high, i.e., 0.94 and 0.96, respectively, and the Stokes shifts are less than 200 cm 1 (see Figure 15). For nonplanar 9-vinylanthracene 87a and its dimethyl derivative 87b, whose ethylene double bond may be twisted out of the plane of the anthracene by about 60°, the quantum yield is 0.63, and the Stokes shifts are around 1000cm-1 (see Table 16). 

From a systematic study of bichromophoric compounds 97-99, the importance of substituents and solvent polarity in intramolecular deactivation processes of photoexcited anthracenes by nonconjugatively tethered, and spatially separated, aromatic ketones in their electronic ground state is apparent. For 97a-d, in which the electron acceptor properties of the aromatic ketone moiety have been varied by appropriate p-substitution of the phenyl ring (R is methoxy, H, phenyl, and acetyl, respectively), the longest-wavelength absorption maximum band lies at 388 nm, i.e., any ground state effects of substitution are not detectable by UV spectroscopy. Also, the fluorescence spectra of 97a-d in cyclohexane are all related to the absorption spectra by mirror symmetry. However, the fluorescence quantum yields for 97a-d in cyclohexane dramatically are substituent dependent (see Table 19), ranging from 0.20 for the methoxy derivative to 0.00059 for the acetyl compound [33,109],... 

In polar solvents such as chloroform, dichloromethane, acetone, and acetonitrile, the fluorescence quantum yields of 97a-d decrease by varying degrees (see Table 19). Moreover, in the case of the phenyl and acetyl derivatives 97c and 97d, the rather drastic decrease of the structured fluorescence from the locally excited anthracene is associated with the appearance of a structureless, red-shifted emission which is attributable to an intramolecular exciplex. For 97d, in which the electron acceptor properties of the aromatic carbonyl moiety are enhanced by p-acetyl substitution, exciplex emission is dominant even in toluene solution (see Figure 22). 

Photoexcitation of lepidopterene in solution also gives rise to a structured emission of low intensity around 400 nm. This emission is attributable to the deactivation of the locally excited state of the E rotamer A, formed mainly by inadvertent direct excitation of the ground state cycloreversion product 114 [131]. The absorption and emission spectra of 114 are typical of the anthracene chromophore (see Figure 33). Selective excitation of 114, experimentally possible because of the suitable ground state [L]/[A] equilibrium ratio, gives rise to locally excited A, which in cyclohexane solution at room temperature has a fluorescence quantum yield of 0.84 [131]. The adiabatic conversion of A into E is difficult to detect because it proceeds at 298 K... 

Trichromophoric systems having the structure R2N-(CH2)n-NH-(CH2)m-Ar generally form either normal exciplexes involving only one amine function or triple exciplexes incorporating both amines in a linear arrangement (A-D-D) [116]. Nonlinear triple exciplexes with an unusually high fluorescence quantum yield have recently been observed between anthracene and two anchored amines of an anthraceno-cryptand [117]. lust as exdplexes, triplexes are important intermediates in a variety of photoreactions [lc, 118], too. 

Another recent study makes use of the participation of the T2 state in the S - TISC process in anthracene [31]. 1,3-Octadiene was used to intercept some of the T2 states before they relaxed to Tx and the decrease in 7, yield was used to estimate the T2 lifetime. Further, this study compensated for the effects of static and time-dependent quenching that comes into play at the relatively large quencher concentrations that are required when quenching sub-nanosecond-lifetime transients. The lifetimes obtained (given in Table 5) were significantly less than previously estimated from other quenching studies and are in line with the lifetimes implied from the T-T fluorescence quantum yields discussed above. 

The lifetimes of molecular fluorescence emissions are determined by the competition between radiative and nonradiative processes. If the radiative channel is dominant, as in the anthracene molecule, the fluorescence quantum yield is about unity-and the lifetime lies in the nanosecond range. In molecular assemblies, however, due to the cooperative emission of interacting molecules, much shorter lifetimes—in the picosecond or even in the femtosecond range—can theoretically be expected an upper limit has been calculated for 2D excitons [see (3.15) and Fig. 3.7] and for /V-multilayer systems with 100 > N > 2.78 The nonradiative molecular process is local, so unless fluorescence is in resonance by fission (Section II.C.2), its contribution to the lifetime of the molecular-assembly emission remains constant it is usually overwhelmed by the radiative process.118121 The phenomenon of collective spontaneous emission is often related to Dicke s model of superradiance,144 with the difference that only a very small density of excitation is involved. Direct measurement of such short radiative lifetimes of collective emissions, in the picosecond range, have recently been reported for two very different 2D systems ... 

Another type of temperature dependence that may be thought of as competition between temperature-dependent and temperature-independent processes has been observed for substituted anthracenes. It is due to the fact that the T2 state of anthracene is energetically very close to the lowest excited singlet state S,. When Tj is just above S, the intersystem crossing S T2 will be associated with a barrier E. The rate constant of this transition can be expressed in the form of a normal Arrhenius equation k = intersystem crossing becomes temperature dependent. This situation is found in 9- and 9,10-substituted anthracenes. (Cf. Example 5.3.) The fluorescence quantum yield d>p increases strongly with decreasing temperature. In other substituted anthracenes as well as in anthracene itself T2... 

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