In this work we present a study on the effect of the aggregation on the optical properties of star-shaped molecules. We analyzed the modification of the absorption and fluorescent properties of a 1,3,5-tristyrylbenzene core due to the formation of diverse aggregates. The nature of the aggregates in solution was investigated by different spectroscopic techniques such as electronic absorption, steady-state fluorescence, fluorescence anisotropy, time-resolved fluorescence, small-angle X-ray scattering, and dynamic lightscattering spectroscopy. In order to simulate the molecular arrangement of the aggregates, the structure and electronic properties of different clusters formed by stacking of starshaped molecules were studied by means of density functional theory calculations. The theoretical insight was performed in the gas phase as well as in solution through the polarizable continuum model, and both linear response and state-specific polarization schemes were applied. In the solid state, high quantum yields of up to 0.51 were measured for a 1,3,5-tristyrylbenzene derivative. Finally, the morphological properties of different solid samples were analyzed by differential scanning calorimetry, as well as scanning and transmission electron microscopies.

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The effects of alkyl/alkoxy pendant chains on the electronic structure and properties of a typical poly(aryl-ethynylene) (PAE) system formed by phenylethynylene units and 1,3,4-thiadiazole rings are analyzed. Optical band gaps and electronic properties (electron affinity, LUMO energy and intramolecular reorganization energy) were computed for different oligomers and then estimated for a limit polymer. The electronic properties become more influenced by the nature of the lateral chain rather than by its length. The most significant changes in the electronic properties are obtained with alkoxy side chains. The smallest band gaps were calculated for polymers containing alkoxy pendant chains. A less favored electron injection is expected for polymers with higher energy values predicted for the LUMO levels and decrease of the electron affinity. Intramolecular reorganization energies computed for all studied polymers were rather low (≤0.07 eV), which allows them to be considered as candidates for n-type semiconductors.

2,5-Bis(phenylethynyl)-1,3,4-thiadiazole (PhEtTh) and 2,5-diphenyl-1,3,4-thiadiazole (PhTh) are expected to be building blocks for polymer materials that could be employed to conduct electricity due to their narrow highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gaps. In this work, a theoretical, comparative study about the effect of the ethynyl group on the planarity and electrical conductivity of this kind of systems has been carried out. Thus, several ab initio (Hartree-Fock, Moller-Plesset) and DFT (B3LYP, B3PW91, M05, M05-2X) methods and basis sets (6-31G(*), 6-31G+G(**), 6-311G(**), cc-pVDZ, cc-pVTZ) have been tested. As a result, PhEtTh showed better properties for its use as electric conducting material relative to PhTh due to its smaller HOMO-LUMO gap, as well as its enhanced trend to retain the planarity provided the reduction in steric hindrances that the ethynyl group (-C[triple bond]C-) permits. Solvent effects were also modeled for ethanol and chloroform under the conductor-like polarizable continuum model approximation. Finally, electronic transitions in gas and solution phases were predicted by using TDDFT approximation in order to compare the theoretical lambda(max) with the experimental values reported in literature for both compounds.

This work is focused on unraveling the mechanisms responsible for the aggregation‐induced enhanced emission and solid‐state luminescence enhancement effects observed in star‐shaped molecules based on 1,3,5‐tris(styryl)benzene and tri(styryl)‐s‐triazine cores. To achieve this, the photophysical properties of this set of molecules were analyzed in three states: free molecules, molecular aggregates in solution, and the solid state. Different spectroscopy and microscopy experiments and DFT calculations were conducted to scrutinize the causative mechanisms of the luminescence enhancement phenomenon observed in some experimental conditions. Enhanced luminescence emission was interpreted in the context of short‐ and long‐range excitonic coupling mechanisms and the restriction of intramolecular vibrations. Additionally, we found that the formation of π‐stacking aggregates could block E/Z photoisomerization through torsional motions between phenylene rings in the excited state, and hence, enhancing the luminescence of the system.