Visualization of rigorous sum rules for Franck–Condon factors: spectroscopic applications to Xe2

A fully quantum mechanical approach to the calculation and normalization of the Franck–Condon factors for diatomic species is described. The treatment is based on the fundamental demand of completeness of the energy eigenfunctions, which results in the rigorous sum rule for the Franck–Condon overlap integrals. The importance of this general rule has been discussed and thoroughly illustrated in the case of diatomic xenon molecules. Exactly solvable reference potentials for this system have been constructed and a complete basis of the actual energy eigenstates (including both bound and scattering states) has been created. Several direct spectroscopic applications to xenon excimers are presented, and their good agreement with relevant experimental data demonstrated. In particular, a kinetic model is proposed to explain the observed oscillatory structures in the fluorescence spectra of Xe₂^* [Chem. Phys. Lett. 117 (1985) 301] related to their classical left turning points. The same model gives a uniform explanation to the well-known first and second emission continua of rare gases.     

Chemiluminescence in O2 oxidation of organosodium compounds of polycyclic aromatic hydrocarbons and benzophenone

Chemiluminescence (CL) in oxidation of organosodium compounds by O₂ in THF was studied. Emitters of CL are excited complexes of polycyclic aromatic hydrocarbons, excimers¹(R·R)^*. The mechanism of their formation was proposed. The Na⁺, R^.−+O₂ CL system is a unique source for the selective generation of excimers of aromatic hydrocarbons. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 284–288, February, 1997.     

Excimer and Exciplex Formation in Gold(I) Complexes Preconditioned by Aurophilic Interactions

Excimers and exciplexes are defined as assemblies of atoms or molecules A / A ′ where interatomic/intermolecular bonding appears only in excited states such as [ A ₂]* (for excimers) and [ AA ′]* (for exciplexes). Their formation has become widely known because of their role in gas‐phase laser technologies, but their significance in general chemistry terms has been given little attention. Recent investigations in gold chemistry have opened up a new field of excimer and exciplex chemistry that relies largely on the preorganization of gold(I) compounds (electronic configuration Au^I(5d¹⁰)) through aurophilic contacts. In the corresponding excimers, a new type of Au???Au bonding arises, with bond energies and lengths approaching those of ground‐state Au?Au bonds between metal atoms in the Au⁰(5d¹⁰6s¹) and Au^II(5d⁹) configurations. Excimer formation gives rise to a broad range of photophysical effects, for which some of the relaxation dynamics have recently been clarified. Excimers have also been shown to play an important role in photoredox binuclear gold catalysis.     

Controllable Electronic Structures and Photoinduced Processes of Bay‐Linked Perylenediimide Dimers and a Ferrocene‐Linked Triad

A series of perylene‐3,4,9,10‐bis(dicarboximide) (PDI) dimers linked through the bay regions was systematically synthesized to examine the electronic structures and photophysical properties in dependence on the distance and orientation between the two PDI units. The spectroscopic and electrochemical measurements suggested that the coupling value of a directly linked PDI dimer (PDI)₂ is much larger than those of para‐ and meta‐phenylene‐bridged PDI dimers p‐(PDI)₂ and m‐(PDI)₂. The width of Davydov splitting was quantitatively evaluated to compare the coupling values between the two PDI units in these dimers by absorption spectroscopy in frozen 2‐methyl‐THF. Excimer formation of PDI dimers induced the strong fluorescence quenching and large red‐shifts. Femtosecond transient absorption revealed a broad absorption derived from an excimer in the range from about 600 nm to the near‐IR region. The rate constants of formation and decay of the excimer are strongly dependent on the coupling values. Time‐resolved measurements on ferrocene‐linked p‐(PDI)₂ revealed a competition between the photoinduced processes of electron transfer and excimer formation in PhCN, which is in sharp contrast with the sole electron‐transfer process in toluene.     

Aggregation‐Controlled Excimer Emission from Anthracene‐Containing Polyamidoamine Dendrimers

Lower generations of polyamidoamine (PAMAM) dendrimers were peripherally modified with anthracene moieties, and excimer emission from anthracene chromophores was investigated in an acetonitrile–water mixture at acidic and basic pH values. Results from fluorescence spectroscopic experiments suggest that 1) the propensity of anthracene‐modified PAMAM dendrimers to aggregate in acetonitrile is substantial in the presence of 15–20 vol % of water, and 2) aggregate formation in anthracene‐modified PAMAM dendrimers leads to unique morphologies in the ground state, where the anthracene units are pre‐arranged to form stable excimers upon photoexcitation. Three types of anthracene excimers are generated in the system, with face‐to‐face, angular, and T‐shaped geometry. The formation of different types of anthracene excimers was confirmed by steady‐state and time‐resolved fluorescence spectroscopic experiments. Experimental results further suggest that it is feasible to alter the type of excimer formed by anthracene units attached to the PAMAM dendrimers through altering the propensity for ground‐state aggregation. Most excitingly, increased π conjugation in the molecular framework of anthracene‐substituted PAMAM dendrimers leads to intense and exclusive excimer emission from anthracene at room temperature.     

Regularities of triplet-triplet annihilation of aromatic hydrocarbons in aqueous micellar solutions of sodium dodecyl sulfate

Investigation of deactivation processes in triplet states of a series of aromatic hydrocarbon molecules (antracene, 1,2-benzantracene, and 3,4-benzpyrene) made it possible to reveal the presence of triplet-triplet annihilation of the molecules in aqueous micellar solutions of sodium dodecyl sulfate. It is shown that the effect of microheterogeneous solutions on the process of triplet-triplet annihilation manifests itself in an increase in the probability of excimerization upon dissociation of triplet pairs of 1,2-benzantracene and 3,4-benpyrene molecules compared to one-component solutions. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 5, pp. 626–628, September–October, 1997.     

Fluorescence Study of Intermolecular Interactions in Diluted Aqueous Solutions of Some Cationic Amphiphilic Polysaccharides

The intermolecular hydrophobic association in diluted aqueous solutions of some cationic amphiphilic polysaccharides was investigated using fluorescence techniques. Dextran and dextran carrying N-alkyl-N,N-dimethyl-N(2-hydroxypropylene) ammonium chloride groups as side chains were single labeled with pyrene or naphthalene. The intensity of the pyrene excimer peak and the ratio I₃/I₁ determined from the fluorescence emission spectra of pyrene-labeled amphiphilic polymer increased with increasing polymer concentration and were higher than in the solution containing pyrene-labeled dextran. Emission spectra of diluted solution (0.008–0.2 g/dl) containing mixtures of pyrene and naphthalene single-labeled amphiphilic polymers proved the occurrence of a nonradiative energy transfer between labels at very a low polymer concentration (<10^?2 g/dl). The energy transfer was not observed in mixtures of single-labeled unmodified dextran. All these results suggest that the intermolecular hydrophobic association of alkyl substituents takes place at a very low concentration of amphiphilic polymers.