Isolated molecules, radiationless transitions

As an example of the effect of level shifts in the crystalline state, as just described, consider the observed rates of radiationless transitions in anthracene.45 The first excited 1BSu of the isolated anthracene molecule is located about 600 cm-1 above the second triplet state. Hence, 8 < vv and the intersystem crossing process is quite rapid at room temperature. The fluorescence quantum yield is about 0.3 for this molecule in the gas phase and in solution. In the crystal the first excited singlet state is red shifted (from the gas level) by about 1880 cm- while the second triplet state is hardly affected, so that in this case the energy gap between those two states increases in the crystal. Then the coupling term, v, is smaller in the crystalline state than in solution, thereby leading to a decrease in the rate of the intersystem crossing. The result is that the fluorescence yield in the crystal is close to unity.40... 

The theory of rotation effects on prolate luminescent molecules in solution and its experimental verification have been developed and compared. Generalized diffusion equations for the rotational motion of an asymmetric rigid motor have been used to given an expression for steady-state fluorescence depolarization. " The radiationless transition from the first excited singlet state of Eosin has been measured by optoacoustic relaxation, and the absolute fluorescence quantum yields of organic dyes in poly(vinyl alcohol) have also been measured by the photoacoustic method. The accuracy of the method has been discussed in the latter paper. Actinometry in flash photolysis experiments has been assisted by new measurements on the extinction coefficient of triplet benzophenone. Matrix-isolation fluorescence spectrometry has been used to detect polycyclic aromatic hydrocarbons from gas chromatography. ... 

Although the radiationless processes appeared to be intramolecular in character, the presence of the solvent or rigid matrix presented an essential complication to a theoretical description of the radiationless phenomena. A complete theory of radiationless processes must begin with a description of electronic relaxation in isolated molecules. Once the isolated molecule case is properly understood, the effects of the external media can be considered and the bulk of the experimental data concerning radiationless transitions can be confronted. 

II. Basic Facets of Radiationless Transitions in Isolated Molecules.298... 

Early studies of the photophysical radiationless processes of molecular systems were carried out on molecules in condensed media, liquids, rigid matrices, and high-pressure gases. This experimental situation introduces the complication associated with the presence of the possible occurrence of a number of different competing photophysical relaxation processes in the same molecular system in a fashion that mimics the complexity of a full photochemical reaction scheme. In order to study the primary photophysical radiationless transitions, it is optimal to consider experiments in which only the elementary individual processes of interest appear. Such investigations often involve the experimental determination of radiationless transition rates in isolated collision-free molecules. " For instance, collision-free experiments enable the consideration of the important phenomena of electronic relaxation and intramolecular vibrational redistribution. Studies on isolated molecules have greatly contributed to our... 

A previous review provides a description of the theory of electronic relaxation in polyatomic molecules with particular emphasis on the vibronic state dependence of radiationless transition rates. A sequal review considers the general question of collisional effects on electronic relaxation, while the present one covers only the special phenomenon of collision-induced intersystem crossing. It departs from the other collisional effects review in presenting only a qualitative description of the theory the full theoretical details can be obtained from the previous review and the original papers.As a review of the basic concepts of radiationless transitions theory is necessary as a prelude to a discussion of collision-induced intersystem crossing, considerable overlap exists between this section and Section II of the previous collision effects review. However, since many concepts from radiationless transition theory, such as the nature and criteria for irreversible decay, the role of the preparation of the initial state, the occurrence of intramolecular vibrational relaxation, etc. pervade the other papers on laser chemistry in these volumes, it is useful to recall the primary results of the theory of electronic relaxation in isolated molecules and its relevance to the material in the present volume as well as to this review. 

At an early stage it has been recognized that relaxation of a small molecule in a periodic rare-gas lattice represents a prototype case of radiationless transition, which lends itself to rather straightforward and mathematically tractable modeling. Sun and Rice have been the first to consider the simplest situation of an isolated diatomic (Nj) in an Ar lattice. Their semiclassical model, using realistic parameters predicted an ssl0 s lifetime for the vibrationally excited Nj, which was contrary to the generally accepted ideas of fast vibrational relaxation in condensed phases. 

Studies of radiationless transitions in matrix-isolated molecules represent a nice case of constructive interaction between theory and experiment. Most early experimental studies showed very poor agreement with the theoretical predictions. Thus in NH and OH, neither the expected temperature dependence, nor the energy-gap law predictions were fulfilled. Similarly, no steep temperature dependence of the relaxation rates was found in matrix-isolated CO. The experimental studies, however, permitted to identify the reasons for the failure of the simple theories. This in turn led to development of new models, describing more adequately the experimental results. " ... 

The existence of radiationless transitions between electronic states within a large isolated molecule thus was demonstrated before 1964, but... 

Zilberg S, Haas Y. The photochemistry of 1,4-cyclohexadiene in solution and in the gas phase conical intersections and the origin of the helicopter-type motion of H-2 photo-generated in the isolated molecule. Phys Chem Chem Phys. 2002 4 34-42. Tamura H, Nanbu S, Nakamura H, Ishida T. A theoretical study of cyclohexadiene/ hexatriene photochemical interconversion multireference configuration interaction potential energy surfaces and transition probabilities for the radiationless decays. Chem Phys Lett. 2005 401 487-491. 

Radiationless transitions occur even in dilute gases indicating that such processes are possible in isolated molecules. If a molecule were excited to a stationary state of the Hamiltonian the matrix element V would, of necessity, be zero. However, excitation at a precise energy is not possible unless exceptionally monochromatic radiation is used (as could be the case with laser excitation). Hence, the energetic distinction between nearly degenerate vibronic states is lost if the line width of the exciting radiation is sufficiently large. As a result transition may be to a nonstationary state. See A. H. Zewail, T. E. Orlowski, and K. E. Jones, Proc. Nat. Acad. Sci, U.S.A. 74, 1310 (1977). 

Theory of radiationless transitions in an isolated molecule. /. Chem. Phys., 49, 610. 

Some reports on fluorescence occurring in, for instance, porous materials such as Nafion or aluminophosphates, " do not refer to azobenzene but to protonated azobenzene, which is classified as a pseudostilbene see Section 1.5). Emission from nonprotonated, isolated azobenzene-type molecules is still very rare. Aggregated systems, however, seem more prone to sho%v fluorescence emission. Shinomura and Kunitake have detected fluorescence bands with a maximum of near 600 nm in bilayer systems built from the monomers of 15. They have shown that the ability to emit is tied to the type of aggregation Head-to-tail aggregates emit relatively strongly, with quantum yields of up to < ) = 10" and lifetimes below 2 ns. Their prototype of card-packed dimers does not emit at all. This is expected because of the low transition probability at the lower band edge, which favors radiationless deactivation, probably via the Si state (see Figure 1.7). 

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