Properties of Molecules in Excited States

Klessinger, M., Pdtter, T. (1991), Properties of Molecules in Excited States in Theoretical Models of Chemical Bonding Vol. 3 Maksid, Z.B., Ed. Springer Berlin. 

T vo-Hioton Absorption Spectroscopy 1.4 Properties of Molecules in Excited States... 

OL behavior is assessed simply by monitoring the transmission of a (usually solution) sample as a function of the incoming laser fluence measured in joules per square centimeter (rather than intensity in watts per square centimeter).22,23 Limiting thresholds Fth, defined as the incident fluence at which the actual transmittance falls to 50% of the corresponding linear transmittance, are then commonly quoted. Since excited-state absorption processes generally determine the OL properties of molecules, the excited-state structure and dynamics are often studied in detail. The laser pulse width is an important consideration in the study of OL effects. Compounds (1-5)58-62 are representative non-metal-containing compounds with especially large NLO and/or OL... 

Stockmayer potential is considered as a superposition of a Lennard-Jones (6-12) potential and the interaction of two point dipoles. Many of the properties of gases and liquids have been calculated in terms of these two potential functions. It should be borne in mind, however, that Lennard-Jones and Stockmayer potentials are idealizations of the true energy of interaction and that they are reasonably accurate for a number of simple molecules. The interaction of long molecules, molecules in excited states, free radicals, and ions cannot be described by these two potential functions (Ref 8a, pp 23 35)... 

These discussions provide an explanation for the fact that fluorescence emission is normally observed from the zero vibrational level of the first excited state of a molecule (Kasha s rule). The photochemical behaviour of polyatomic molecules is almost always decided by the chemical properties of their first excited state. Azulenes and substituted azulenes are some important exceptions to this rule observed so far. The fluorescence from azulene originates from S2 state and is the mirror image of S2 S0 transition in absorption. It appears that in this molecule, S1 - S0 absorption energy is lost in a time less than the fluorescence lifetime, whereas certain restrictions are imposed for S2 -> S0 nonradiative transitions. In azulene, the energy gap AE, between S2 and St is large compared with that between S2 and S0. The small value of AE facilitates radiationless conversion from 5, but that from S2 cannot compete with fluorescence emission. Recently, more sensitive measurement techniques such as picosecond flash fluorimetry have led to the observation of S - - S0 fluorescence also. The emission is extremely weak. Higher energy states of some other molecules have been observed to emit very weak fluorescence. The effect is controlled by the relative rate constants of the photophysical processes. 

The rotational and the translational freedom appear after desorption of adsorbed molecules and each energy is kept without any disturbance before detection in the present experimental condition, since there is no collision and the lifetime of the excited states for a desorbed molecule is long. The experimental data can be analyzed by a simple model using the impulse scheme, con fi ned to the momentum transferred from the substrate to an adsorbate atom, in which the form of the excited-state PES and the transition process need not be assumed [68, 69]. The energy released from the excited state is converted to the momentum and this energy is transferred impulsively. The desorption also occurs impulsively. This simple model sheds hght on the property of the intermediate excited state, and the intermediate excited state plays an important role in the DIET process. 

The impulse model is applied to the interpretation of experimental results of the rotational and translational energy distributions and is effective for obtaining the properties of the intermediate excited state [28, 68, 69], where the impulse model has widely been used in the desorption process [63-65]. The one-dimensional MGR model shown in Fig. 1 is assumed for discussion, but this assumption does not lose the essence of the phenomena. The adsorbate-substrate system is excited electronically by laser irradiation via the Franck-Condon process. The energy Ek shown in Fig. 1 is the excess energy surpassing the dissociation barrier after breaking the metal-adsorbate bond and delivered to the translational, rotational and vibrational energies of the desorbed free molecule. 

The peculiarity of the pentalene molecule resides in its central bond which connects nonalternant atoms (a perturbed [8]annulene), its completely conjugated and alternating w bond periphery, and its apparent antiaromatic nature. The transannular bond conveys planarity to the structure without contributing to its stabilization. Consequently, pentalene is expected to be highly reactive since, in a sense, it has ground state properties customarily found in excited states. The many early attempts to synthesize pentalene have been reviewed224-226 and will not be considered per se here. Suffice it to say that the lability of minimally substituted pentalenes ultimately required the implementation of rather specific reaction conditions for their successful synthesis (vide infra). 

A large amount of spectroscopic data are now available for benzene, providing much information on photophysics and photochemistry properties of the lowest excited states of this molecule. Nevertheless the nature of the various radiative and nonradiative transitions involved in the deexcitation of the first excited states is far from being completely understood. 

The photobehavior of molecules having excited states strongly perturbed by spin-orbit coupling may show large variations at low temperatures due to changes in the Boltzman populations of the various component levels in the excited states of such species. The temperature dependence of the luminescent properties of several heavy metal complexes have been interpreted in this manner (Section III-C). 

Studies on the acid-base properties of molecules in their excited states are closely related to intermolecular proton transfer. This topic has been reviewed by Weller (4) and Parker (10), and only some basic phenomena necessary for the understanding of the following sections will be discussed here. 

The problems for quantum chemists in the mid-forties were how to improve the methods of describing the electronic structure of molecules, valence theory, properties of the low excited states of small molecules, particularly aromatic hydrocarbons, and the theory of reactions. It seemed that the physics needed was by then all to hand. Quantum mechanics had been applied by Heitler, London, Slater and Pauling, and by Hund, Mulliken and Hiickei and others to the electronic structure of molecules, and there was a good basis in statistical mechanics. Although quantum electrodynamics had not yet been developed in a form convenient for treating the interaction of radiation with slow moving electrons in molecules, there were semi-classical methods that were adequate in many cases. 

Less commonly used measurement techniques include the pH dependence of partition coefficients [74], fluorescence spectra [75], nuclear magnetic resonance chemical shifts or coupling constants, HPLC or CE retention volumes [76,77], and the dependence of reaction rates for ionizable substrates on pH (also called kinetic methods). Kinetic methods were amongst the earliest methods to be used for pKg determination. In some cases, they may be the only feasible method, for example, extremely weak acids (pKa > 12) without suitable absorption spectra. The difficulty with kinetic methods is that they may not actually measure the pKg value for the substrate, but that of the reaction transition state. If the electronic configuration of the transition state is similar to that of the reactant (early transition state), then the kinetic may be quite close to the equilibrium value. However, if the transition state more nearly approximates the reaction products (late transition state), then the kinetic value may bear little resemblance to that for the reactant. This explanation might account for the lack of agreement between the first apparent kinetic pK = 4.0) and equilibrium (pK = 8.6) pKg values for hydrochlorothiazide at 60 °C [78]. Similar restrictions may be placed on the use of pKa values from the pH dependence of fluorescence spectra, as these reflect the properties of the first excited state of the molecule rather than its ground state [75]. 

A striking example of the potential sensitivity of ODMR is the report by two groups of magnetic resonance detection in a single molecule of pentacene using FDMR. It should be pointed out, however, that this accomplishment was possible because of the rather special properties of the pentacene excited states comparable sensitivity should not be expected with biopolymers. Nonetheless, PDMR of Trp in proteins, for example, is carried out successfully at the picomole level without difficulty. 

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