Exciplexes formation

Upon exposure to uv light, ground-state benzophenone is excited to the ttiplet state (a diradical) which abstracts an alpha H atom from the alcohol, resulting in the formation of two separate initiating radicals. With amine H atom donors, an electron transfer may precede the H-transfer, as in ttiplet exciplex formation between benzophenone and amine (eq. 43) ... 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

This result reveals that exciplex formation plays a principal role in the initiation of polymerization. Since the absorption band is broadened toward longer wavelengths as the result of formation of CTC between AN and aniline, a certain concentration of aniline can be chosen so that 365-nm light is absorbed only by the CTC but not by the aniline molecule. Therefore, in this case the photopolymerization may be ascribed to the CTC excitation selected. For example, a 5 x 10 mol/L aniline solution in AN could absorb light of 365 nm, while solutions in DMF or cyclohexane with the same concentration will show no absorption. Obviously, in this case the polymerization of AN is caused by CTC excitation. The rates of polymerization for different amines were found to be in the following order (Table 12) ... 

The discriminatory emission properties between two-coordinate d ° gold(I) complexes and their respective three-coordinate counterparts have been demonstrated in the literature [6, 10-13]. As discussed in the later sections, Che and coworkers have rationalized that the extraordinarily large Stokes shift of the visible emission of [Au2(diphosphine)2] from the [5da 6pa] transition is due to the exciplex formation ofthe excited state with solvent or counterions [6]. Fackler [14—16] reported the photophysical properties of monomeric [AUL3] complexes, which show visible luminescence with large Stokes shifts (typically lOOOOcm ), suggesting significant excited-state distortion. Gray et al. [10] examined the spectroscopic properties of... 

Argentophilic attraction has been found in Tl[Ag(CN)2] this compound displays photoluminescence that has been explained in terms of excited-state Ag—Ag interactions leading to exciplex formation, [Ag(CN)2-]3. 51 Several photochemical studies have been carried out with this type of compound.252-255... 

Not all sensitized photochemical reactions occur by electronic energy transfer. Schenck<77,78) has proposed that many sensitized photoreactions involve a sensitizer-substrate complex. The nature of this interaction could vary from case to case. At one extreme this interaction could involve a-bond formation and at the other extreme involve loose charge transfer or exciton interaction (exciplex formation). The Schenck mechanism for a photosensitized reaction is illustrated by the following hypothetical reaction ... 

Another point of view is that exciplex formation can in some cases involve bonding changes in the acceptor. Solomon, Steel, and Weller 104 proposed that the quenching by quadricyclene leads to an exciplex where the quadricyclene is distorted. 

Besides the excited molecule can interact physically with a second molecule, i.e. undergo bimolecular processes. These are either energy transfer (1.7) or exciplex formation (1.8) depending on the relative excitation energies of the molecule to be studied and its partner. 

Explain the deactivation of excited states by other molecules in terms of quenching processes, excimer/exciplex formation, energy transfer and electron transfer. 

Figure 6.7 Energy diagram for exciplex formation in solvents of differing polarities...

The exciplex emission is also affected by solvent polarity, where an increase in the solvent polarity results in a lowering of the energy level of the exciplex, at the same time allowing stabilisation of charged species formed by electron transfer (Figure 6.7). Thus, in polar solvents the exciplex emission is shifted to even higher wavelength and accompanied by a decrease in the intensity of the emission, due to competition between exciplex formation and electron transfer. 

Exciplex formation occurs only in the liquid phase, where a green emission occurs on irradiation, while in the vapour phase a blue emission is observed because of the monomer. Using careful calibration techniques it is possible to perform accurate concentration measurements of the fuel/air mixtures. 

The separation of binding site and fluorophore by a nonconjugating spacer opens the path to other mechanisms of communication, most prominently ET and exci-mer/exciplex formation. In the first case, the electronic nature of both fluorophore and receptor unit and the steric nature of the spacer are the important parameters for signal generation. In the second case, for most systems the electronic nature of the fluorophores and the steric nature of the receptor as well as its change upon analyte binding determine the signal. 

In acetonitrile-dichloromethane 1 1 v/v solution, their absorption spectra are dominated by naphthalene absorption bands and they exhibit three types of emission bands, assigned to naphthyl localized excited states (/Wx = 337 nm), naphthyl excimers (Amax ca. 390 nm), and naphthyl-amine exciplexes (/lmax = 480 nm) (solid lines in Fig. 3). The tetraamine cyclam core undergoes only two protonation reactions, which not only prevent exciplex formation for electronic reasons but also cause strong nuclear rearrangements in the cyclam structure which affect excimer formation between the peripheral naphthyl units of the dendrimers. 

Extensive investigation has been performed on the interaction of dendrimers 1 and 2 with metal ions [17a, e-h]. Complexation with Zn2+ engages the nitrogen lone pairs and thereby prevents exciplex formation, with a resulting intense naphthyl fluorescence (dashed lines in Fig. 3). This strong fluorescent signal is... 

This chapter describes the characteristics of the fluorescence emission of an excited molecule in solution. We do not consider here the photophysical processes involving interactions with other molecules (electron transfer, proton transfer, energy transfer, excimer or exciplex formation, etc.). These processes will be examined in Chapter 4. 

Class 3 fluorophores linked, via a spacer or not, to a receptor. The design of such sensors, which are based on molecule or ion recognition by a receptor, requires special care in order to fulfil the criteria of affinity and selectivity. These aspects are relevant to the field of supramolecular chemistry. The changes in photophysical properties of the fluorophore upon interaction with the bound analyte are due to the perturbation by the latter of photoinduced processes such as electron transfer, charge transfer, energy transfer, excimer or exciplex formation or disappearance, etc. These aspects are relevant to the field of photophysics. In the case of ion recognition, the receptor is called an ionophore, and the whole molecular sensor is... 

The effects of photophysical intermolecular processes on fluorescence emission are described in Chapter 4, which starts with an overview of the de-excitation processes leading to fluorescence quenching of excited molecules. The main excited-state processes are then presented electron transfer, excimer formation or exciplex formation, proton transfer and energy transfer. 

The pH dependence of the regioselectivity for the nucleophilic photosubstitution of 3,4-dimethoxy-l-nitrobenzene by n-butylamine gives21 2-methoxy-5-nitro-Af-butylaniline as the major product at pH = 11 (equation 19). At pH = 12, the ratio of the major product to 2-methoxy-4-nitro-7V-butylaniline increases to 12 1 the increased selectivity is caused by hydroxide ion, which can either promote exciplex formation or act as a base catalyst in deprotonation steps following the cr-complex formation22. 

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. 

F. Pages, J.-P. Desvergne, and H. Bouas-Laurent, Nonlinear triple exciplexes Thermodynamic and kinetic aspects of the intramolecular exciplex formation between anthracene and the two anchored nitrogens of an anthraceno-cryptand, /. Am, Chem. Soc. Ill, 96-102(1989). 

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