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The room-temperature fluorescence properties of DNA nucleoside and nucleotide aqueous solutions are studied by steady-state and time-resolved spectroscopy. The steady-state fluorescence spectra, although peaking in the near-UV region, are very broad, extending over the whole visible domain. Quantum yields are found to be mostly higher and the fluorescence decays faster than those reported in the literature. The fluorescence spectra of the 2‘-deoxynucleosides are identical to those of the 2‘-deoxynucleotides, with the exception of 2‘-deoxyadenosine, for which a difference in the spectral width is observed. The steady-state absorption and fluorescence spectra do not show any concentration dependence in the range 5 × 10^-6 to 2 × 10^-3 M. All fluorescence decays are complex and cannot be described by monoexponential functions. From the zero-time fluorescence anisotropies recorded at 330 nm, it is deduced that after excitation at 267 nm the largest modification in the electronic structure is exhibited by 2‘-deoxyguanosine. In the case of purines, the fluorescence decays and quantum yields are the same for 2‘-deoxynucleosides and 2‘-deoxynucleotides. In contrast, for pyrimidines, the fluorescence quantum yields of nucleotides are higher and the fluorescence decays slower as compared to those of the corresponding nucleosides showing that the phosphate moiety affects the excited-state relaxation.

Triplet excitation transport occurring in the columnar liquid crystalline phase of a triphenylene derivative at room temperature is studied by transient absorption spectroscopy with nanosecond resolution. The properties of the triplet excitons are evidenced by doping the mesophase with different concentrations of 2,4,6-trinitrofluoren-9-one (TNF) which is inserted in the stacks of the triphenylene cores (T) and acts as energy trap. It is shown that triplet migration and trapping leads to the formation of the ion-pair ³(T⁺, TNF^-), whose recombination rate constant is 5.5 × 10⁵ s^-1. The comparison of the experimentally determined time dependence of the ion-pair concentration with numerically simulated curves on the basis of an one-dimensional random walk model allows the determination of the hopping time (2 ± 1 ps). The latter value is close to that found, in a previous study, for the singlet excitation transport (1.2 ± 0.5 ps) in the same mesophase. This is in agreement with the finding that interactions due to intermolecular orbital overlap, responsible for energy transport in the triplet state, are also the main driving force for singlet excitation transport. The migration length of the triplet exciton is limited by structural defects to a few hundreds of triphenylene cores.

The time evolution of the fluorescence and the photoinduced absorption spectra of four triarylpyrylium cations, differing in the number of dodecyl chains attached to the chromophore, was studied by means of femtosecond fluorescence upconversion and picosecond pump−probe absorption techniques. The dependence on solvent viscosity was also examined. The results are rationalized in terms of excited and ground-state relaxation dynamics and may be differentiated into intramolecular conformational changes and intermolecular solute−solvent contributions. The dynamic fluorescence shift is related to solvent relaxation, whereas the fluorescence intensity decay is attributed to molecular twisting acting in an intricate manner. First, the geometrical relaxation reduces rapidly the fluorescence oscillator strength, but it also greatly enhances the nonradiative S0 ← S1 internal conversion rate. Molecular back-twisting combined with vibrational relaxation in the ground state is shown to be the last and the slowest process of the reaction.

The spectroscopic properties of discotic hexa-alkylthiotriphenylenes are studied in solution and thin films and compared to those of hexa-alkyloxytriphenylenes. The solution properties are analyzed in the light of CS-INDO-CIPSI quantum chemistry calculations. The absorption maximum is assigned to the degenerate S0 → S4 transition. The fluorescence of the neat phases is attributed to weakly bound excimers. The phase transition leading from ordered to disordered columnar stacks induces an increase in the oscillator strength of the S0 → S1 transition and favors excimer formation. The influence of structural disorder on the properties of the delocalized states is rationalized by using various approximations within the frame of the exciton theory; three models for the calculation of the exciton coupling (point dipole, extended dipole, atomic transition charge distribution) are tested, short and long range interactions are considered, and the introduction of a dielectric constant is discussed. The best agreement between experimental and calculated absorption maxima is obtained using the atomic charge transition model. Off-diagonal disorder is correlated to structural disorder by changing the orientation and the position of the molecules within the aggregate. The case of degenerate molecular states is compared to that of nondegenerate ones. Orientational disorder has a dramatic effect on the energy and the localization of the upper eigenstate when molecular states are nondegenerate. Conversely, the properties of degenerate eigenstates are quite insensitive to orientational disorder. The magnitude of the off-diagonal disorder induced by positional disorder largely depends on the model used in the calculation of the exciton coupling. The results of the numerical calculations are in agreement with the small change observed in the neat phases absorption maxima upon a quasi one-dimensional melting of columnar stacks.

The photophysical properties of a homologous series of 2, 4-6-triaryl pyrylium tetrafluoroborates substituted by six alkoxy chains (CnH2n+1O, n = 2, 3, 4, 5, 8, 12) are reported. in dilute solution, the electronic absorption and fluorescence spectra do not depend on the length of the alkoxy chains while both fluorescence lifetimes and fluorescence quantum yields increase when the chain length increases. Monomer and excimer fluorescence is observed with the pure compounds; the steady state and time resolved emission spectra and the decay kinetics which do not depend on the chain length suggest that the columnar structure exist with very short lateral chains. Excimer lifetimes determined in the liquid-crystalline phases follow an exponential law as a function of the reciprocal temperature: τ ∝ exp (EM/RT). The activation energy for excimer migration is 0.046 eV, 0.061 eV and 0.087 eV respectively for n = 5, 8 and 12.

The Annelides represent a new class of surfactants able to selectively complex a predetermined metallic ion. They are shown to form a novel type of lyotropic phase. This phase is thoroughly characterized by polarizing microscopy and X-rays determinations. It is found to be lamellar with a transition at 32°C to an isotropic phase. The main geometrical parameters are determined, in particular, the surface available for each polar head is calculated.

The present paper deals with the geometry and the electronic transitions of ion-pair dimers formed by 2,6-diphenyl-4-(4‘-(dialkylamino)phenyl)pyrylium tetrafluoroborates. The geometry is determined theoretically by minimizing the potential interaction energy calculated at a molecular level. The most stable configuration corresponds to a noncentrosymmetric oyster-like arrangement where the anions are sandwiched between the pyrylium cores forming an angle of ca. 30°. Such an arrangement is corroborated by¹H-NMR and nonlinear optical measurements. The properties of the electronic transitions of the dimers are determined in the framework of the exciton theory, taking into account the calculated geometry, and compared to those obtained experimentally. It is shown that fluorescence originates from an excited state localized on one of the chromophores. Localization of the excitation is accompanied by a moving of the counterions by ca. 1.5 Å, due to the variation of the atomic charge distribution. The calculated decrease (3350 cm^-1) in the transition energy upon relaxation is practically the same as the experimentally observed Stokes shift (3000 ± 400 cm^-1).