Probing the intrachain and interchain effects on the fluorescence behavior of pentiptycene-derived oligo(p-phenyleneethynylene)s.

The synthesis, crystal structure, and fluorescence behavior of acetylene-bridged pentiptycene dimer (2), trimer (3), and tetramer (4) are reported. For comparison, a phenylene-pentiptycene-phenylene three-ring system (5) is also investigated. As a result of the unique intrachain pentiptycene-pentiptycene interactions in 3 and 4, their twisted conformers are populated in polar solvents and at low temperatures, and the phenomenon of nonequilibration of excited rotational conformers is observed. Twisting of the pi-conjugated backbones leads to blue-shifted absorption and fluorescence spectra and increased fluorescence quantum yields and lifetimes. The fluorescence spectra of 2-4 undergo small red shifts but large intensity variations in the 0-1 vs 0-0 bands on going from solutions to thin solid films, which can be accounted for by the reabsorption effect. However, the reduction in fluorescence quantum yields for 2-4 in films vs solutions is mainly attributed to efficient interchain exciton migration to nonfluorescent energy traps. In contrast, the behavior of nonequilibration of excited rotamers is not observed for 5 in solutions. Compound 5 forms J-type aggregates through terminal phenylene pi-stackings in the solid state, resulting in a new absorption band at 377 nm and large red shifts of the structured fluorescence spectra.

Spectroscopic correlations between supermolecules and molecules. Anatomy of the ion-modulated electronic properties of the nitrogen donor in monoazacrown-derived intrinsic fluoroionophores.

The synthesis, absorption and emission spectra, fluorescence quantum yields, and fluorescence lifetimes of three compound series of trans-4,4'-disubstituted aminostilbenes (1-3) are reported. The chromo-/fluoroionophoric behavior of the monoaza-15-crown-5- (A15C5) and monoaza-18-crown-6 (A18C6)-derived species (1A(5)()-3A(5)() and 1A(6)()-3A(6)()) in acetonitrile and dichloromethane are also investigated. Great similarities in electronic spectroscopic properties (chemical shifts, wavelength, intensity, and lifetime) between the metal ion-complexed supermolecules and the corresponding chloro-substituted molecules have been observed: namely, 1A(5)()/Ca(2+)-3A(5)()/Ca(2+) approximately 1A(6)()/Ba(2+)-3A(6)()/Ba(2+) approximately 1C-3C in acetonitrile and 1A(5)()/Na(+)-2A(5)()/Na(+) approximately 1A(6)()/K(+)-2A(6)()/K(+) approximately 1C-2C in dichloromethane. Such spectroscopic correlations allow us to define the metal ion-modulated electronic character of the azacrown nitrogen atom in the ground and excited states and, in turn, to gain insights into the observed fluoroionophoric behavior of these probes in terms of the size and direction of fluorescence shifts and intensity variations.

Origin of the N-methyl and N-phenyl substituent effects on the fluorescence vibronic structures of trans-4-aminostilbene and its derivatives in hexane.

The origin of the N-substituent effect on the room temperature fluorescence vibronic structures of trans-4-aminostilbene (1), its disubstituted derivatives (2 and 3), and the fused-ring analogs (4) in hexane has been investigated. The fluorescence spectra of aminostilbenes 1-4 in hexane are structured and the intensity ratio of the 0,1 vs. the 0,0 band decreases upon N-methyl or N-phenyl substitution, indicating that the N-substituents reduce the displacement in the equilibrium configuration (deltaQe) between the ground and the excited states. Our data are consistent with a planar fluorescing state for 1-4 in hexane. The N-substituents affect the planarity of the ground state structures as well as the conjugation length allowed for exciton resonance, both of which are in accord with the observed progression in fluorescence vibronic structures. According to our analysis, the conformation effect that is associated with the geometry of the amino group appears to play a major role. The corresponding studies on the solvent effect suggest that the fluorescing state of 1-4 in more polar solvents is also a planar charge-transfer state.

Fluorescence enhancement of trans-4-aminostilbene by N-phenyl substitutions: the "amino conjugation effect".

The synthesis, structure, and photochemical behavior of the trans isomers of 4-(N-phenylamino)stilbene (1c), 4-(N-methyl-N-phenylamino)stilbene (1d), 4-(N,N-diphenylamino)stilbene (1e), and 4-(N-(2,6-dimethylphenyl)amino)stilbene (1f) are reported and compared to that of 4-aminostilbene (1a) and 4-N,N-dimethylaminostilbene (1b). Results for the corresponding 3-styrylpyridine (2) and 2-styrylnaphthalene analogues (3) are also included. The introduction of N-phenyl substituents to 4-aminostilbenes leads to a more planar ground-state geometry about the nitrogen atom, a red shift of the absorption and fluorescence spectra, and a less distorted structure with a larger charge-transfer character for the fluorescent excited state. Consequently, the N-phenyl derivatives 1c-e have low photoisomerization quantum yields and high fluorescence quantum yields at room temperature, in contrast to the behavior of 1a, 1b, and most unconstrained monosubstituted trans-stilbenes. The isomerization of 1c and 1d is a singlet-state process, whereas it is a triplet-state process for 1e, presumably due to a relatively higher singlet-state torsional barrier. The excited-state behavior of 1f resembles 1a and 1b instead of 1c-e as a consequence of the less planar amine geometry and weaker orbital interactions between the N-phenyl and the aminostilbene groups. Such an N-phenyl substituent effect is also found for 2 and 3 and thus appears to be general for stilbenoid systems. The nature of this effect can be described as an "amino conjugation effect".