Excimer, binding energies state

The excimer binding energy deduced by Gregory and Helman (136) from transient luminescence decay data at low temperature (0.26 eV) is at variance with the conclusions of other authors (125,134) (>0.36 eV) using steady state luminescence data. 

Figure 8.1 Schematic potential energy diagram of an excimer-forming pair of moiecuies. The iower curve shows both molecules in the ground state. The upper curve shows the excimer formation on the approach between an excited molecule and a molecule in the ground state. AEfm is the excitation energy of the monomer, AEfd is the excitation energy of the excimer, and 6 is the excimer-binding energy.

The theoretical approaches taken to calculate the binding energy of the excimer have been reviewed 68-70). Most authors have assumed a sandwich structure for the excimer in which the ring planes are parallel and the molecular axes are aligned. By matching the calculated and experimental values of the excimer fluorescence peak, the interplanar distance of the excimer can be computed. All such calculations yield values of the interplanar distance which are 0.2-0.5 A less than the ground-state van der Waals ring separation. For the naphthalene excimer, an interplanar distance of 3.3 0.3 A has been computed. 

Excimer may relax (i) by emission of characteristic structureless band shifted to about 6000 cm-1 to the red of the normal fluorescence, (ii) dissociate nonradiatively into original molecules, (iii) form a photodimer. Those systems which give rise to photodimers may not decay by excimer emission. The binding energy for excimer formation is provided by interaction between charge transfer (CT) state A+A- A A+ and charge resonance state AA A A. 

As both the excitation resonance and charge transfer should be weaker in the triplet state than in the singlet, the binding energy in triplet excimers is appreciably smaller. As the entropy loss is supposed to be smaller in Intramolecular exclmer formation, triplet excimers are more likely to be observed in bichromophoric systems than in monofunctional ones (9). A further difficulty that arises is that the study of phosphorescence in liquid solutions is very difficult. Molecules with a ground state... 

The intensity dependence of UV laser flash excitation of diphenylaraine in methanol shows the occurrence of two quantum photoionization involving the triplet state.Also the binding energies of the triplet excimers observed in poly(N-vinylcarbazole) films between 15 and 55 K have been estimated.Triplet states, biradicals, radical ions and heavy atom effects can all be involved in the photodimerization of aceanthrylene. It is established that the triplet state reaction mechanisms give rise to four stereoisomeric dimers. The basicity of the Ti state of phenazine is found to have a pKa of 1.9, which is appreciably different from an earlier value, by flash photolysis. The discordant results obtained in earlier work are satisfactorily explained. 

The classic example for the tt-tt electron interaction between polycyclic arenes is the pyrene excimer (11). Upon UV excitation of a 10 5 M pyrene solution, the structured fluorescence of monomeric pyrene molecules is mainly observed. The increase of the concentration to 10 3 M diminishes the monomeric fluorescence, and a new broad and completely structureless excimer band appears, which is red-shifted by 5000-6000 cm-1. This phenomenon can be explained through potential curves of the electronic ground state and the excited singlet state (I, 12). The spectroscopic shift between the fluorescence of the excimer and the monomer depends on the depth of the potential well in the excited state that is, the red shift is proportional to the binding energy of the excimer. 

Since excimer formation apparently involves substantial geometric movement, and since the binding energy of the excimer is large in such materials as pyrene, it might be assumed that once an excimer is formed in the crystal the excitation energy will be immobilized. In other words, it seems possible that the excimer state provides a deep trap. 

The ground-state manifold consists of two states, of which the S state has the lowest energy, and is referred to as the X state. This X state is generally nearly flat or weakly bound with the exception of an XeF excimer having a strong bound state with a 1065-cm binding energy. The other manifold is the H state, which is always repulsive as shown in Fig. 1. This H state is referred to as the A state. 

An ab initio 8CF-CI study predicts a binding energy of 3.7 kcal/mol for [BNe] " (X Ab initio calculations also suggest excimer-type bound-free broad bands in the vacuum ultraviolet region. The emission band from the v = 0 of the lowest singlet excited state 11 of [BNe]+ has a peak at 72000 cm" (139 nm), whereas the spectrum from the v =1 state exhibits two peaks at 73500 (136 nm) and 70200 cm (142.5 nm). The calculated lifetime of the v = 0 state is 1 ns [5]. 

Ab initio SCF-CI calculations (multireference configuration interaction approach) predict binding energies for [BAr]" (8.1 kcal/mol in its X state) and show that excimer-type bound-free broad banc s are expected in the vacuum ultraviolet region. In relation to v = 0 of the lowest singlet excited state n, the emission band has a peak at 49000 cm" (204 nm) the calculated lifetime is 6 ns [5]. 

The various examples of photoresponsive supramolecular systems that have been described in this chapter illustrate how these systems can be characterized by steady-state and time-resolved spectroscopic techniques based on either absorption or emission of light. Pertinent use of steady-state methods can provide important information in a simple vay stoichiometry and stability constant(s) of host-guest complexes, evidence for the existence of photoinduced processes such as electron transfer, energy transfer, excimer formation, etc. Investigation of the dynamics of these processes and characterization of reaction intermediates requires in most cases time-resolved techniques. Time-resolved fluorometry and transient absorption spectroscopy are frequently complementary, as illustrated by the study of photoinduced electron transfer processes. Time-resolved fluorometry is restricted to phenomena whose duration is of the same order of magnitude as the lifetime of the excited state of the fluorophores, whereas transient absorption spectroscopy allows one to monitor longer processes such as diffusion-controlled binding. 

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