Exciplex method

Droplet temperature is of interest in practical spray processes since it influences the associated heat and mass transfer, chemical reactions, and phase changes such as evaporation or solidification. Various forms of Rayleigh, Raman and fluorescence spectroscopies have been developed for measurements of droplet temperature and species concentration in sprays.16471 Rainbow refractometry (thermometry), polarization ratioing thermometry, and exciplex method are some examples of the droplet temperature measurement techniques. 

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 basic principle of this method of recognition is a cation-induced conformational change bringing closer together (or moving away) two moieties able to interact and induce photophysical effects excimer or exciplex formation (or disappearance), electronic energy transfer and quenching. 

One may consider the relaxation process to proceed in a similar manner to other reactions in electronic excited states (proton transfer, formation of exciplexes), and it may be described as a reaction between two discrete species initial and relaxed.1-7 90 1 In this case two processes proceeding simultaneously should be considered fluorescence emission with the rate constant kF= l/xF, and transition into the relaxed state with the rate constant kR=l/xR (Figure 2.5). The spectrum of the unrelaxed form can be recorded from solid solutions using steady-state methods, but it may be also observed in the presence of the relaxed form if time-resolved spectra are recorded at very short times. The spectrum of the relaxed form can be recorded using steady-state methods in liquid media (where the relaxation is complete) or using time-resolved methods at very long observation times, even as the relaxation proceeds. 

Exciplexes are defined as molecular complexes which are stable under electronic excitation30. The picosecond (or femtosecond) laser photolysis methods are suitable for investigating the very rapid photo-induced processes related to CT complexes. 

A review of photo-cycloadditions of dienones and quinones has been published.41 The first example of a Lewis acid-catalysed 2 + 2-cycloaddition of styrene with naphthoquinone has been reported.42 FMO methods have been used to investigate the effect of substituents on the regiochemistry of the 2 + 2-photo-cycloaddition of a, fi-unsaturated carbonyl compounds with substituted alkenes.43 Evidence has been presented for the presence of a triplet exciplex intermediate in the photo-cycloaddition of 4,4-dimethylcyclohexenone to 1,1-diphenylethylene.44 The intramolecular 2-1-2-photo-cycloaddition of 2-acyloxy-3-hexenoylcyclohexenones (26) is highly diastereo-selective yielding the tricyclic adduct (27) (Scheme 10).45... 

Intermolecular and intramolecular photocycloaddition and photoaddition to aromatic rings in the electron-donor and electron-acceptor systems were discussed in this chapter. The highly stereoselective and regioselective photocycloaddition is a synthetically useful method for the construction of polycyclic carbon-skeleton compounds, including natural products. New aspects for the stereoselective intermolecular and intramolecular photocycloaddition reactions via exciplexes in less... 

Since ions are stable u = 0), the exciplex theory deals with a pure A regime where the asymptotic behavior obeys the widely recognized power law deduced by Berg [120] and then confirmed by a number of different theoretical methods [135,168,172,173] ... 

Research on carbon-centered radical cations in solution accelerated dramatically with the development of time-resolved optical absorption and emission techniques. The research group of Th. Forster in Germany pioneered photochemical methods of production of radical cations and anions, as well as exciplexes." While the Forster group focused on structure and lifetimes, the later work of D. R. Arnold in Canada, and of H. D. Roth in the United States," reported the reactivity of photochemically generated radical cations from a mechanistic perspective. These studies of radical ion chemistry evolved into the field we now know as electron donor-acceptor interactions, arich area of science in which carbon-centered radical cations are stiU actively smdied. 

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... 

Itoh s group and also Tramer, Piuzzi and their co-workers used this method extensively for the study of jet-cooled exciplexes. Examples of hole-burning spectra are shown in Figures 2 and 3. 

The photodynamics of linked systems are thus a sensitive function of their structure, and the excess energy dependence of the electron transfer depends on the details of the intramolecular dynamics. IVR and electron transfer are two important processes that may compete with each other or, conversely, operate in harmony. One way to check the role of IVR without significantly changing the electronic structure of the molecule is by isotopic substitution. The main change is in the vibrational energy-level density. Itoh and co-workers [31] applied this method to the 9-An-w-DMA system mentioned above. It was found that the Si vibrational energy thresholds for formation of the exciplex were considerably smaller in the deuterated molecules than those for the respective protonated molecules. This result is consistent with the assumption that the transition from the locally excited state (the one is initially excited) to the exciplex state, is aided by an increase in the energy level... 

Electron transfer was followed by the exciplex fluorescence method LE and exciplex-type emissions were recorded as a function of internal energy, as in the previously cited papers (Section 4.2). Here, however, the initial excitation was either of the electron donor or the acceptor. The electron-donating power was varied by simple substitution, with minimal interference with the nature of the spacer. Typical donors were para-substituted anilines or substituted naphthalenes. Typical acceptors were cyano-substituted naphthalenes (when anilines served as the donors) and dicyanoethylene or dicarbomethoxyethylene when naphthalenes served as donors. Spacers were of varying flexibility semi-flexible (a piperidine ring which may have a... 

Electrochemical excitation, photochemistry without light, exhibits many phenomena that are unique to ECL as compared to photochemistry. The efficient production of emission from excimers or exciplexes as compared to excited monomers, efficient generation of excited triplet states, and intense delayed fluorescence caused by triplet-triplet annihilation are the most typical examples. On the other hand, the method offers a chance to populate the excited states that are inaccessible by the processes following photoexcitation. 

The [4 + 2]-cycloaddition of 02( Ag) to rubrene has been shown to be a simple method for the determination of oxygen concentrations in organic solvents.Irradiation of benzene solutions of tropone (86) in the presence of 9,10-dicyanoanthracene leads to the formation of four products, (87), (88), (89) and (90), and in acetonitrile-dichloromethane there is also an [8 + 4]tc adduct (91) produced. It has been suggested that this latter compound arises by coupling of the radical cation of (86) with the radical anion of the dicyanoanthracene (Scheme 2). Solvent-dependent quenching of the lowest excited state of 9,10-dibromoanthracene by 2,5-dimethylhexa-2,4-diene has been studied and appears to proceed by an exciplex. Flash photolysis investigations have shown that a neutral radical species is an intermediate in the formation of the [4 + 2] adduct which is a dibenzobicyclo[2.2.2]octadiene-type compound. Irradiation of 9,10-dicyanophenanthrene (DCA) in the presence of buta-1,3-diene gives a mixture of the product of [3 + 2]-photo-... 

Offermans et al. used several spectroscopic methods to study the formation and luminescence decay of exciplexes as a function of photoexcitation (of either polymer), and subsequent charge transfer at the heterojunction [229]. Exciplex formation was followed by a relaxation to the lower lying triplet state Ti of the MDMO-PPV. The energetic scheme is depicted in Fig. 48. 

In these solutions, K-(A -X)/(Ai-X2), F-K/(Ai-A2), A-(Ai X)/(X-Y), X-ki+k2+k3, Y-k4+k5, and Aj and A2 are given by Eqn. 16. For several well known (30) limiting cases, A3 and A2 are equivalent to and r2, the lifetimes of the pyrene singlet state and excited state complexes, respectively (see Eqns. 9-11). Activation parameters for pyrene excimer formation were calculated by two Independent methods. Since kx+k2 is known to be virtually temperature independent and k4 and ky are negligible(31), the ratios of fluorescent intensity maxima from the pyrene excimer and monomer maxima (Ig/Itl) the inverse of temperature yield the activation energy for pyrene excimer formation, E3. A similar experiment for the pyrene-CA system was not possible since its exciplex is not emissive. Activation parameters for the excimer and exciplex were also obtained from temperature and phase dependent pyrene fluorescent lifetime data. In the 1iquid-crystalline and isotropic phases of M, all pyrene decays were single exponential and the excimer decays could be expressed as the difference between two exponentials. 

Following a similar approach to that of pyrene exclmer formation, activation energies for pyrene- 4. exciplex formation can be obtained from expression of Eqn. 9 in an Arrhenius form and differentiation by 1/T. ki+k2 are obtained from data taken in eyelohexane (32), and A3 and E3 from the lifetime taken at f CA1 - 0. E3 can also be obtained from the slope of the phase dependent dynamic Stern-Volmer plots. As seen in Table 1 the data from each method are in good agreement. The small differences in activation parameters measured in the cholesteric and isotropic phases probably reflect changes in viscosity that accompany phase transitions. 

Z, -Isomerizations of 1,2-disubstituted alkenes are well documented and can occur either by direct irradiation via the singlet state or via the triplet by sensitization. Examples of electron transfer activation have also been reported and mainly involve radical cations of alkenes. For the reasons discussed above, in general, the sensitization is the method of choice. There are numerous examples collected in the textbooks and the reviews. The ZE-isomerization of cyclooctenes presented by Y. Inoue belong to rare examples of a singlet sensitization. The advantage of such a process is that the reaction probably involve exciplex intermediates. By choosing a chiral sensitizer an asymmetric Z,E-isomerization is possible with an enantiomeric excess up to 53% (at -85 °C). 

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