Collision intersystem crossing

The natural processes of intersystem crossing and internal conversion will quickly (e.g. 50 ns) carry the molecule from this excited electronic surface to the ground electronic surface without a collision,... 

The subject of delayed fluorescence was discussed in Section 5.2a. It was seen that there are two common types of delayed fluorescence, that arising from thermally activated return from the triplet state to the lowest excited singlet (E-type delayed fluorescence) and that arising from collision of two excited triplet molecules resulting in a singlet excited molecule and a ground state molecule (P-type delayed fluorescence). The P-type delayed fluorescence can be used as a convenient tool for the determination of intersystem crossing efficiencies[Pg.125]

Internal conversion refers to radiationless transition between states of the same multiplicity, whereas intersystem crossing refers to such transitions between states of different multiplicities. The difference between the electronic energies is vested as the vibrational energy of the lower state. In the liquid phase, the vibrational energy may be quickly degraded into heat by collision, and in any phase, the differential energy is shared in a polyatomic molecule among various modes of vibration. The theory of radiationless transitions developed by Robinson and Frosch (1963) stresses the Franck-Condon factor. Jortner et al. (1969) have extensively reviewed the situation from the photochemical viewpoint. 

Subsequent to the formation of a potentially chemiluminescent molecule in its lowest excited state, a series of events carries the molecule down to its ground electronic state. Thermal deactivation of the excited molecule causes the molecule to lose vibrational energy by inelastic collisions with the solvent this is known as thermal or vibrational relaxation. Certain molecules may return radia-tionlessly all the way to the ground electronic state in a process called internal conversion. Some molecules cannot return to the ground electronic state by internal conversion or vibrational relaxation. These molecules return to the ground excited state either by the direct emission of ultraviolet or visible radiation (fluorescence), or by intersystem crossing from the lowest excited singlet to the lowest triplet state. 

In solution at room temperature, non-radiative de-excitation from the triplet state Ti, is predominant over radiative de-excitation called phosphorescence. In fact, the transition Ti —> S0 is forbidden (but it can be observed because of spin-orbit coupling), and the radiative rate constant is thus very low. During such a slow process, the numerous collisions with solvent molecules favor intersystem crossing and vibrational relaxation in So-... 

It is also possible to induce intersystem crossing from an initially formed singlet state to a lower energy triplet by forcing the singlet state to suffer collisions with an inert medium (no easy task for a species as reactive as methylene ). 

In the liquid phase, singlet phenylnitrene is rapidly relaxed by collision with solvent and cannot surmount the barrier to form cyanocyclopentadiene at ambient temperature. Under these conditions PN isomerizes over a small barrier to form cyclic ketenimine K. Later, computational work of Karney and Borden would show this to be a two-step process involving benzazirine BZ, the species trapped by ethanethiol (Scheme 2). In the liquid phase, PN prefers rearrangement to intersystem crossing (ISC) to the lower-energy triplet state at ambient temperature. Intersystem crossing is not an activated process and its rate is not expected to vary with temperature. The rate of... 

A time-dependent process, such as radiative absorption, internal conversion, intersystem crossing, unimolecular isomerization, or collision, may be treated in terms of a zero-order Hamiltonian H0 and a perturbation T. An unperturbed eigenstate of H0 evolves in time, since it is not an eigenstate to the perturbed Hamiltonian... 

The intersystem crossing from CH2( /4,) to (3B,) is induced by inert gases (143). A theory dealing with the collision-induced singlet to triplet transition of methylene is developed by Chu and Dahler (211). 

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