Pyrene exciplex

TABLE 1. Activation parameters for pyrene- exciplex formation in CM ---------------------------------------------------------------... 

Pyrene exciplex methods allow key values to be determined over the range of 10 s" to 10 s i. Accurate measur ent of weak pyrene excimer emission is somewhat easier key values down to ca. 3 x 10 s can be determined. Triplet methods are needed to determine cyclization rates in the millisecond domain. 

The best evidence for a charge-transfer exciplex (hetero excimer) has been provided by Thomaz and Stevens.<148,149) They note a reduction in fluorescence yield of pyrene with increasing heavy-atom concentration and proposed the following set of reactions to explain their results ... 

Weller24 has estimated enthalpies of exciplex formation from the energy separation vg, — i>5 ax of the molecular 0"-0 and exciplex fluorescence maximum using the appropriate form of Eq. (27) with ER assumed to have the value found for pyrene despite the doubtful validity of this approximation the values listed for AHa in Table VI are sufficiently low to permit exciplex dissociation during its radiative lifetime and the total emission spectrum of these systems may be expected to vary with temperature in the manner described above for one-component systems. This has recently been confirmed by Knibbe, Rehm, and Weller30 who obtain the enthalpies and entropies of photoassociation of the donor-acceptor pairs listed in Table XI. From a detailed analysis of the fluorescence decay curves for the perylene-diethyl-aniline system in benzene, Ware and Richter34 find that... 

An example of exciplex formation in the solid state may be afforded by perylene doped crystals of pyrene which emit a green structureless fluorescence in addition to the blue and orange-red excimer bands of pyrene and perylene, respectively. Hochstrasser112 has shown that the energy of the emitting species is consistent with that of a charge transfer complex of pyrene and perylene molecules in a bimolecular unit of the pyrene lattice. 

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...

Quantum yields for isomerization of t-1 to c-1 via intersystem crossing are similar to those obtained in triplet sensitization, indicating that t is formed with near unit efficiency. Similar results were reported for the interaction of t with inorganic anion electron donors (127b). Rate constants for intersystem crossing of the t-1-ethyldiisopropylamine and triethylamine exciplexes obtained from the measured exciplex lifetimes are 5 x 107 s"1 (116), similar to the values for the exciplexes of pyrene (128) and naphthalene (121) with triethylamine. Larger intersystem crossing rate constants are observed for the exciplexes of t-1 with the bicyclic amines l-azabicyclo[2.2.2]-... 

Steady-state fluorescence spectroscopy has also been used to study solvation processes in supercritical fluids. For example, Okada et al. (29) and Kajimoto and co-workers (30) studied intramolecular excited-state complexation (exciplex) and charge-transfer formation, respectively, in supercritical CHF3. In the latter studies, the observed spectral shift was more than expected based on the McRae theory (56,57), this was attributed to cluster formation. In other studies, Brennecke and Eckert (5,31,44,45) examined the fluorescence of pyrene in supercritical CO2, C2HSteady-state emission spectra were used to show density augmentation near the critical point. Additional studies investigated the formation of the pyrene excimer (i.e., the reaction of excited- and ground-state pyrene monomers to form the excited-state dimer). These authors concluded that the observance of the pyrene excimer in the supercritical fluid medium was a consequence of increased solute-solute interactions. 

In some cases excited state chromophores form supramolecular complexes either with ground state chromophores on the same molecule or one nearby resulting in the formation of an excimer (excited state dimer) or, if the two chromophores are different to one another an exciplex. Formally an excimer as defined as a dimer which is associated in an electronic excited state and which is dissociative in its ground state.1 Formation of the pyrene excimer is illustrated in Figure 11.3. 

Figure 11.3 The excited state of a chromophore such as pyrene can form a complex with a ground state molecule. If the result is an excited state dimer the complex is known as an excimer, while if the excited complex is formed between two different molecule it is termed and exciplex. Excimers and exciplexes emit at lower energy than the corresponding monomers.

Furthermore, it seems reasonable to ascribe the regiochemistry observed to that of an unencumbered radical anion or solvent separated species in the radical anion reductions and the triethylamine photochemical runs. Various studies on lithium biphenylide have shown that it exists both as a contact ion pair and a solvent separated ion pair in THF, although at or below 20 °C mostly as a solvent separated ion pair [47,48], In the case of the exciplex between the radical anion of 1 and Et3N+, various studies of pyrene and aliphatic or aromatic amine exciplexes [49, 50] and related excited singlet states of EDA complexes... 

As with most discoveries, these ideas then passed through a lull before takeoff. Then the Chicago-New York axis of Yang and Turro in the USA investigated the first example (6) in the host-guest mould [10]. Their focus was on the demonstration of exciplex emission of (6) in aqueous solution due to protection by an encapsulating /J-cyclodextrin. Ever since Weller s experiments, exciplex emissions had been observed to fall off rapidly in intensity as solvent polarity was increased [4], A formally related example due to Tazuke from 1982 must, however, be pointed out, where a hydrophobic polymer microdomain in mixed and neat aqueous solution allows exciplex emission from pyrene and dimethyl-aniline pendants [11], The importance of (6) in the present context stems from the correlation of pH-dependent emissions from the naphthalene moiety and from the exciplex. Co-occurrence of externally switchable photophysics and supramolecular phenomena would later become common in the research literature [12]. 

One of the problems facing spin chemists performing these measurements is that the observed field effects can be rather small. Thus the method of detection should be as sensitive as possible. In some systems, it is possible to use the inherent fluorescence of one of the species involved in the reaction as a probe of RP activity. The most common of these approaches is the situation with the formation of RlPs that can often lead to spin-selective exciplex formation via the singlet RIP. Systems involving conjugated aromatic molecules, for example, anthracene and pyrene as electron donors/acceptors, amines as electron donors, and substituted benzenes (e.g., dicyanobenzenes) as electron acceptors, have been commonly employed and are now extremely well... 

When primary and secondary co-arylalkylamines are excited, exciplex fluorescence is not observed, but fluorescence quenching does take place (Shizuka et al., 1979). Presumably chemical reaction competes effectively with exciplex formation. However, by means of picosecond laser flash photolysis, the time-resolved absorption spectra of the species produced on reaction of pyrene with diphenylamine have been obtained (Okada et al., 1980b). It was shown that the reaction leads to neutral radicals via an exciplex. Although the formation of neutral radicals in such systems had been previously identified (Okada et al, 1976a) the role of exciplexes had been purely speculative. 

Triplet production in the quenching of aromatic hydrocarbon fluorescence by amines has usually been monitored by flash photolysis. A particularly good example is the study of the quenching of the excited singlet state of pyrene by l,4-diazabicyclo[2.2.2]octane in cyclohexane solutions (Delouis et al., 1979). Fluorescent exciplex formation is not observed with this system but triplet formation does occur. By triplet-triplet absorption spectroscopy... 

Weller s group also measured the MFEs on the exciplex fluoreseence intensity (1(B)) observed in acetonitril at room temperature for the same polymethylene-linked compounds with A=pyrene and D=dimethylaniline [3b]. The exciplex fluorescence (ho ex) is emitted from the following processes ... 

Quenching Excimers and Exciplexes.—By measurements of decay times and fluorescence anisotropy of pyrene and the excimer in cellulose acetate films it has been found that the medium consists of spaces where small pyrene molecules have considerable freedom, Dissado and Walmsley have developed a complete theory of excimer formation and exciton-induced lattice distortion in crystals. Reference is made to data on 9-cyanoanthracene. The spectroscopy of chemically linked dimers of l,3-(l,l -dinaphthyl)propane in a... 

A similar situation is encountered in other systems. For instance, in the case of the pair pyrene (acceptor)-dimethylaniline (donor) connected by a propyl [( 112)3] chain, one of the conformers is found to exhibit a charge-transfer band at the origin [19]. Another conformer, possibly a stretched one, shows only locally excited fluorescence at the origin, and exciplex type as the vibronic energy is increased. It seems that for strong acceptors, such as pyrene, an n = 3 chain is sufficiently long to allow... 

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