Understanding the Driving Mechanisms of Enhanced Luminescence Emission of Oligo(styryl)benzenes and Tri(styryl)‐s‐triazine

Abstract

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.

Shedding Light on the Origin of Solid‐State Luminescence Enhancement in Butterfly Molecules

“…In general, the wavelengths calculated for the lowest-lying electronic transitions (l calcd abs )m atch well with the experimental absorption maxima( l exptl abs ), with Dl jj ¼ l calcd abs À l exptl abs 21 nm in all cases (see Table 1). As reported for other 1,3,5-tris(styryl)benzenes, [11,20,34] some degeneracy was observed in the frontier molecular orbitals of compound 1 (see Figure 5). As ac onsequence, two electronic transitions (S 0 !S 1 and S 0 !S 2 )w ith similar energies and oscillator strengths were found for this compound.…”

“…Thisr esult is in contrast to the clear vibronic structure observed in the fluorescencee mission spectra of the parents tyrylarene compounds 4 and 6. [11] Av ibronic structure is also clearly discerned in the emission spectrum of 3,w ith two bandsa t4 00 and 421 nm. The calculated emissione nergies for the S 1 !S 0 transition are collected in Table 2.…”

“…As imilar observation, namely,t he planarisation of two diagonal styryl branches upon photoexcitation, was previously reported for parentc ompound 6. [11] In the case of compound 3,t he lowest-lying electronic transition was assigned to aH OMO!LUMO transition (contribution of 89 %) and these molecular orbitals were fully delocalised.…”

“…[1,2] Aggregation-induced emission (AIE) is also used as as ynonym of SLE, although the concept of "molecular aggregation" is more frequently associated with solution phases. [2] Considerablee ffort is currently being made to obtain SLE luminogens and to unravel the mechanisms responsible for fluorescence "on/off" switching between solution and the solid state, such as exciton coupling effects, [3][4][5][6][7] restriction of intramolecularm otions (RIMs), [8][9][10] blockingo fZ/E photoisomerisation [11] and restricted accesst oc onical intersections( RACIs). [2,12,13] Controlo ft he supramolecular structure is therefore key to obtaining SLE luminogens because short-a nd long-range intermoleculari nteractions determine the solid-state photophysical properties.…”

“…[2,11,19,20] In compound 6,s ome rocking movements occur in the phenyl rings (associated with high Huang-Rhys (HR) factors) and these are not totally restricted in the solid state, andh ence, SLE is not observed. [11] The new molecular structuresd esigned here (1)(2)(3) contain the styrylarene core (in blue in Scheme 1), but also incorporate new phenyl rotors to generate triphenylethylene moieties (in red in Scheme1). The presence of several triphenylethylene moieties in the same molecule leads to butterflylike structures that resemble some TPEd erivatives for which SLE has been reported.…”