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Four tris(salicylideneanilines) (TSANs) with gradually increased steric interactions between the keto–enamine moiety and neighbouring phenyl substituent are presented. The steric interactions are induced by placing two alkyl groups at the ortho position in the N-aryl substituent. The impact of the steric effect over the radiative channels of deactivation of the excited state was evaluated through spectroscopic measurements and theoretical calculations using ab initio techniques. Our results show that the emission occurring after excited state intramolecular proton transfer (ESIPT) is favoured by placing the bulky groups in the ortho position of the N-phenyl ring of the TSAN. However, our TSANs seem to offer the opportunity to obtain a pronounced emission band at higher energy, significantly increasing the coverage of the visible spectrum, resulting in the enhancement of the dual emissive properties of tris(salicylideneanilines). Thus, TSANs may be promising molecules capable of white-like emission for use in organic electronic devices such as white OLEDs. [ABSTRACT FROM AUTHOR] more...

Photophysics of hexaazatrinaphthylene (HATN) in solution and in the solid state is determined by the nπ* character of its lowest excited singlet state and by the ππ* character of the first triplet state. The relaxation of an electronically excited state is dominated by nonradiative relaxation channels and only very weak fluorescence is observed for HATN monomers in solution. In liquid solution, the fluorescence quantum yield is 3–7 × 10−4 and the triplet formation yield ranges from 0.21 to 0.41. In methanol, the photodegradation of the molecule is observed. This molecule with a large delocalised π-electron system easily aggregates forming dimers in solution and in some crystalline forms. The band shifts observed in crystals as well as excitonic splitting in dimers provide evidence for a strong interaction in π–π stacked aggregates. The mixing of electronic states in aggregates changes their character and affects photophysical properties. The obtained experimental data are in good agreement with theoretical calculations performed using the ADC(2) method. [ABSTRACT FROM AUTHOR] more...

A structurally unique cyclic tris-coumarin possessing three identical coumarin units bridged by amide linkers as well as two linear analogs has been synthesized. There is a remarkable agreement between crystallographic data, 1H NMR and results of calculations for the cyclic tris-coumarin, showing in all cases a non-symmetric arrangement of identical coumarin moieties. Weak polarization of the coumarin subunits, resulting from the presence of only CONH– groups as electron-donors, results in a hypsochromic shift of both absorption and emission in this dye. We have proven that in non-cyclic, head-to-tail linked tris-coumarins, the photophysics is controlled not only by the substituents but also by the conformation of the molecule, which in turn depends on the nature of the linker's interactions. These can be controlled by the presence/absence of an amide-type hydrogen atom responsible for the formation of intramolecular hydrogen bonds. The presence of a hydrogen bond favors a stretched trans conformation of the dye, while in its absence, folding of the molecule occurs leading to a more compact conformation. Although, the increased number of covalently linked coumarin units does not drastically change the preferred conformation, the fluorescence quantum yields of tris-coumarins are significantly lower than for analogous bis-coumarins composed of the same units. [ABSTRACT FROM AUTHOR] more...

The fluorescence intensity of bis-coumarins linked via CONH and COO functionalities are shown to exhibit a strong dependance on solvent polarity. The presence of the intramolecular hydrogen bond between the C=O oxygen atom of coumarin and amide-NH moieties reduces the number of thermodynamically populated conformations in both ground and electronically excited states and an anti-arrangement of coumarin units is favored. Additionally, the rigidity of the linker raises the barrier to the conical intersection with the ground state, and in non-polar solvents strong fluorescence is observed. Although changing the CONH linking position from 3–7′ to 3–6′, does not remarkably affect the photophysics, replacement with a flexible ester linker allows the molecule a greater degree of conformational freedom due to the absence of the intramolecular hydrogen bonding interaction. The photophysical effect of this is the appearance of two fluorescence bands, the relative intensity and spectral positions of which are sensitive to the environment. Theoretical explorations of the excited-state potential energy surfaces performed with the aid of the ADC(2) ab initio electronic structure theory method revealed an exceptional wealth of concomitant photophysical processes. In particular, two channels for radiationless deactivation of the excited state were found; the first is related to the inter-ring twist of the coumarin units, and the second is associated with the excited-state intramolecular proton-transfer (ESIPT) from the CONH linker to the coumarin core. [ABSTRACT FROM AUTHOR] more...