Excimer formation

In the first mechanism the excimer is represented as a common intermediate for the formation of dimer and the deactivation of the excited anthracene. In the second, excimer formation is totally nonproductive with regard to dimer formation. Again as in paragraph (b) one can think of the excimer in the second mechanism as having a structure that, if dimerization proceeded, would yield the unobserved head-to-head product ... 

By examining any correlation between excimer formation (as evidenced by characteristic excimer fluorescence) and dimerization quantum yield, one could perhaps determine whether dimerization is dependent upon prior excimer formation. Excimer fluorescence from anthracene solutions at room temperature is negligible although it has been observed in the solid state at low temperature.<75) Unfortunately, the data for substituted anthracenes allow no firm conclusions to be drawn. Some derivatives dimerize but do not exhibit excimer fluorescence. Others both dimerize and show excimer fluorescence. Still others show excimer fluorescence but do not dimerize and finally, some neither dimerize nor show excimer fluorescence. Hopefully, further work will determine what role excimer formation plays in this photodimerization. 

A2 is the dimer, HA is ethyl iodide, and Q is ferrocene. This mechanism is essentially the same as that proposed for the dimerization of acenaphthylene. The steps involving possible singlet excimer formation and singlet dimerization have been excluded since they are at best relatively unimportant processes in heavy-atom solvents. Using the steady-state approximation, we can... 

Phillips and Schug (24) have suggested that the 390 nm emission, observed when PET is excited with high energy electrons, might be from a triplet state or an excimer. Since the triplet states of both PET and DMT are lower in energy (MSO nm), it is unlikely that the emission is from a triplet state. In addition, excimer formation and emission should not effect the absorption-excitation processes therefore, it is unlikely that the 390 nm emission is from an excimer. 

Langenegger SM, Haner R (2004) Excimer formation by interstrand stacked pyrenes. Chem Commun 2792-2793... 

Bimolecular reactions with paramagnetic species, heavy atoms, some molecules, compounds, or quantum dots refer to the first group (1). The second group (2) includes electron transfer reactions, exciplex and excimer formations, and proton transfer. To the last group (3), we ascribe the reactions, in which quenching of fluorescence occurs due to radiative and nonradiative transfer of excitation energy from the fluorescent donor to another particle - energy acceptor. 

The second group of intermolecular reactions (2) includes [1, 2, 9, 10, 13, 14] electron transfer, exciplex and excimer formations, and proton transfer processes (Table 1). Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. PET is involved in many photochemical reactions and plays... 

Replacing the nitrile group by a benzothiazole produces an important subclass of fluorescent compounds represented by thioflavin T (25, Fig. 10). It is not clear if this compound undergoes deactivation via intramolecular rotation that would meet the criterion for a molecular rotor. The steady-state absorption and emission properties of thioflavin T has been attributed to micelle formation [53, 54], dimer and excimer formation [55, 56], and deactivation through intramolecular rotation [57]. 

Retna Raj C, Ramaraj R (1997) Cyclodextrin induced intermolecular excimer formation of thioflavin T. Chem Phys Lett 273(3 1) 285-290... 

Aq. micelles affect excimer formation with change in geometry of binding... 

Photoinduced excited states of the naphthalene derivatives included in the amphiphilic p-CD LB films were found to be stablized by measurements of the fluorescence lifetimes and the excimer formation of the naphthalene derivatives adsorbed by the CD monolayer occured mainly between the adjacent layers [29]. 

Many studies on side-chain modifications in PF were initially based on the idea of excimer formation, resulting in the green emission during LED operation or in solid-state PL on annealing PF films. This resulted in several proposed strategies for the design of fluorene side-chain homopolymers, where bulky substituents at position 9 of the fluorene moiety should sterically prevent (hinder) interchain interaction and thus improve the stability of blue emission. 

Polybenzobisthiazoles 608 and polybenzobisoxazole 609 have been used as efficient electron transport materials in PLEDs [71] (Chart 2.143). Although these polymers show poor fluorescence quantum yields in thin films likely due to excimer formation [700], double-layer devices ITO/PEDOT/polymer/ETL/Al with PPV or MEH-PPV as emissive polymers and... 

J.-I. Lee, G. Klaerner, and R.D. Miller, Structure-property relationship for excimer formation in poly(alkylfluorene) derivatives, Synth. Met., 101 126, 1999. 

Concentration quenching. In this case, quenching occurs because of the formation of an association between excited state and ground state, which if homo-molecular is called the excimer formation. The subsequent processes can be radiationless or the complex can emit at much longer wavelength and effectively not be detected. 

Fluorescence Rise and Decay Curves. Both monomer and excimer fluorescence decay curves of the unirradiated film are nonexponential and the excimer fluorescence shows a slow rise component. This behavior is quite similar to the result reported for the PMMA film doped with pyrene. (23) A delay in the excimer formation process was interpreted as the time taken for the two molecules in the ground state dimer to form the excimer geometry. Dynamic data of the ablated area observed at 375 no (monomer fluorescence) and 500 nm (exciner fluorescence) are shown in Figure 5. When the laser fluence increased, the monomer fluorescence decay became slower. The slow rise of the excimer fluorescence disappeared and the decay became faster. 

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