Organic molecules internal vibrations

Experimental studies of liquid crystals have been used for many years to probe the dynamics of these complex molecules [12]. These experiments are usually divided into high and low-frequency spectral regions [80]. This distinction is very important in the study of liquid crystalline phases because, in principle, it can discriminate between inter- and intramolecular dynamics. For many organic materials vibrations above about 150 cm are traditionally assigned to internal vibrations and those below this value to so-called lattice modes . However, the distinction is not absolute and coupling between inter- and intramolecular modes is possible. 

The photodissociation of aromatic molecules does not always take place at the weakest bond. It has been reported that in a chlorobenzene, substituted with an aliphatic chain which holds a far-away Br atom, dissociation occurs at the aromatic C-Cl bond rather than at the much weaker aliphatic C-Br bond (Figure 4.30). This is not easily understood on the basis of a simple picture of the crossing to a dissociative state, and it is probable that the reaction takes place in the tt-tt Si excited state which is localized on the aromatic system. There are indeed cases in which the dissociation is so fast (< 10-12 s) that it competes efficiently with internal conversion. 1-Chloromethyl-Np provides a clear example of this behaviour, its fluorescence quantum yield being much smaller when excitation populates S2 than when it reaches Figure 4.31 shows a comparison of the fluorescence excitation spectrum and the absorption spectrum of this compound. This is one of the few well-documented examples of an upper excited state reaction of an organic molecule which has a normal pattern of energy levels (e.g. unlike azulene or thioketones). This unusual behaviour is related of course to the extremely fast dissociation, within a single vibration very probably. We must now... 

Both the Raman and the infrared spectrum yield a partial description of the internal vibrational motion of the molecule in terms of the normal vibrations of the constituent atoms. Neither type of spectrum alone gives a complete description of the pattern of molecular vibration, and, by analysis of the difference between the Raman and the infrared spectrum, additional information about the molecular structure can sometimes be inferred. Physical chemists have made extremely effective use of such comparisons in the elucidation of the finer structural details of small symmetrical molecules, such as methane and benzene. But the mathematical techniques of vibrational analysis are. not yet sufficiently developed to permit the extension of these differential studies to the Raman and infrared spectra of the more complex molecules that constitute the main body of both organic and inorganic chemistry. 

FIGURE 1. A schematic photochemical mechanism, showing some of the possible elementary transformations. For the purpose of illustration, it is assumed that the states A and A2 have the same multiplicity, and correspond to the ground and lowest excited singlet states of most organic molecules. The state A] would then represent the lowest triplet state. Thus 21 and 11 are radiative transitions, fluorescence and phosphorescence, respectively, and 23 and 13 (intersystem crossing) and 22 (internal conversion) are nonradiative. All of 8, C, D, and F are chemical species distinct from A. Only vibrationally equilibrated electronic states are included in this mechanism (see discussion in Section III.A.l). 

Fig. 1. Diagramme of radiative (a - absorption, fl - fluorescence, ph - phosphorescence), radiationless (ic - internal conversion, isc - intersystem crossing) transitions and processes of vibrational relaxation (vr) in organic molecules...

In the case of the vibrations, one distinguishes two types the intramolecular vibrations (molecular vibrations), which are also referred to as internal modes and whose frequencies differ from those of the free molecules only slightly or not at all and the external vibrations (lattice vibrations), which are also termed external modes, where the organic molecules as a whole oscillate around their equilibrium positions. Usually, only the external vibrations are called phonons. 

Fig. 1 A simplified Jablonski diagram for a conjugated organic molecule such as anthracene, illustrating the rate constants for key processes as defined in the text. IC represents internal conversion (an isoenergetic process) followed by vibrational relaxation similarly ISC represents intersystem crossing followed by vibrational relaxation...

An important class of photoinduced chemistry of organic molecules is cis-trans isomerization [48]. A common feature of these cis ans isomerization reactions is the ultrafast nature of the reaction dynamics taking place in a few picoseconds or less. Often, optical excitation leads to the formation of the isomerization product in its electronic ground state. Therefore, a large amount of internal vibrational energy is present immediately after isomerization. As a result, the vibrational fingerprint transitions initially appear often redshifted because of off-diagonal anharmonic... 

Fig. 6.4 Modified Jablonski diagram for an organic molecule showing ground and excited states and intramolecular photophysical processes from excited states. Radiative ntK esses—fluorescence (hvf) and phosphorescence (hvp) are shown in straight lines, radiationless processes— internal conversion (1C), inter system crossing (ISC), and vibrational cascade (vc) are shown in wavy lines. Adapted with permission fiom (Smith MB, March J 2006 March s Advanced Organic Chemistry Reactions, Mechanisms and Stiucmres, 6th Ed., John Wiley, New York). Copyright (2007) John Wiley Sons...

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