Molecules, excited states fluorescent emission

The efficient emission of molecules in this class is due to the exceptional stability of the 1st excited singlet state, as shown by the detailed analysis carried out in the case of 44 (see also Sect. VII). The fluorescence spectra in this series show the normal mirror relationship with the absorption spectra indicating emission from the 1 st excited state. The emission vibrational maxima " X and the vibrational spacings v are given in Table 15. 

Emissions from both the and the previously unreported lli states of the IF molecule have been observed in the gas-phase reaction of L with F2 at low pressure a four-centre complex has been proposed as the reaction intermediate. A combined theoretical-experimental programme has been conducted to establish techniques for the study of excited-state transitions in Ij and IC1. Experimental techniques based on two-step excitation using two synchronized, tunable lasers have been developed, and successfully applied to excited-state fluorescence measurements on ICl. lodine(i) chloride adsorbed on silica gives the same Raman spectrum as that obtained from adsorbed l2. ... 

Once the molecule reaches the first excited singlet, internal conversion to the ground state is a relatively slow process. Thus, decay of the first excited state by emission of a photon can effectively compete with other decay processes. This emission process is fluorescence. Generally, fluorescence emission occurs very rapidly after excitation (10 to 10 s). Consequently, it is not possible for the eye to perceive fluorescence emission after removal of the excitation source. Because fluorescence occurs from the lowest excited state, the fluorescence spectrum, that is, the wavelengths of emitted radiation, is independent of the wavelength of... 

Very little is known about the nature of rotational energy transfer in a collision between an electronically excited molecule and a ground-state atom or molecule. In the few reported studies the experimental method is fundamentally the same as that described at the beginning of Section III.A. An initial rotational distribution is established by narrow-band excitation. The fluorescence emission contour is recorded twice, under collision-free and thermal equilibrium conditions, and then again under conditions such that there is one collision during the lifetime of the excited state. The differences in the rotational contours of the three emission spectra are then used to infer the pathway of rotational energy transfer, and the rate of that transfer. Some examples of the emission spectra recorded under these conditions are shown in Fig. 22. Because of the small spacings between the rotational levels of polyatomic molecules most excitation sources prepare nonthermal superpositions of rotational states rather than pure rotational states, and this complicates interpretation of the observations. 

There are several experimental approaches to obtain lifetime data. The primary objective of these approaches is to obtain data representing the time dependence of the decay of the luminescence. For a single component sample in which the excited state decays by first-order processes the radiative fluorescence lifetime tr represents the time the fluorophore remains on average in the excited state, before emission takes place. If there are N molecules in the excited state, the decrease dN of molecules that revert... 

Several phenomena can render the measured anisotropy to values lower than the maximum achievable theoretical values. The most common cause is diffusion of a macromolecule to which the fluorophore is attached. Such rotational diffusion occurs during the lifetime of the excited state and displaces the emission dipole of the fluorophore. Measurement of this parameter provides information regarding the relative angular displacement of the fluorophore between the times of absorption and emission. In fluid solution, most fluorophores rotate extensively in 50-100ps. Hence, the molecules can rotate many times during the typical 1-10 ns excited-state fluorescence lifetime, and the orientation of the polarized emission easily becomes randomized or depolarized. For... 

Typical singlet lifetimes are measured in nanoseconds while triplet lifetimes of organic molecules in rigid solutions are usually measured in milliseconds or even seconds. In liquid media where drfifiision is rapid the triplet states are usually quenched, often by tire nearly iibiqitoiis molecular oxygen. Because of that, phosphorescence is seldom observed in liquid solutions. In the spectroscopy of molecules the tenn fluorescence is now usually used to refer to emission from an excited singlet state and phosphorescence to emission from a triplet state, regardless of the actual lifetimes. 

Fluorescence from the ExcitedSj State. In Figure 1, after absorption (A) and vibrational deactivation (VD) occur, the lowest or nearly lowest level of the singlet excited state is reached. If the molecule is fluorescent with a high quantum efficiency, fluorescent emission of a quantum of... 

Fluorescent small molecules are used as dopants in either electron- or hole-transporting binders. These emitters are selected for their high photoluminescent quantum efficiency and for the color of their emission. Typical examples include perylene and its derivatives 44], quinacridones [45, penlaphenylcyclopenlcne [46], dicyanomethylene pyrans [47, 48], and rubrene [3(3, 49]. The emissive dopant is chosen to have a lower excited state energy than the host, such that if an exciton forms on a host molecule it will spontaneously transfer to the dopant. Relatively small concentrations of dopant are used, typically in the order of 1%, in order to avoid concentration quenching of their luminescence. 

The fluorescence spectrum of dibenz[7>,/]oxepin shows that this molecule adopts a planar structure in the excited state whereas the ground state has bent geometry as expected.19 The emission spectrum is similar to that of anthracene. 

Though theories have been proposed (32-35) to explain this phenomenon, the mechanism of fluorescence is still not yet fully understood. Jankow and Willis (36) proposed a mechanism which involves a direct excitation of the molecule or an impurity to an excited state, followed by internal conversion and then reversion back to the original state with emission of light. This mechanism can be explained as follows A molecule in the lowest vibrational level of the ground state A is transferred to a certain vibrational level in the excited state D. The molecule tends to cascade into the lowest vibrational level of state D by collisions with other excited molecules. It passes from state D to state C and then to state B by radiationless transi-... 

---