ChemInform Abstract: FLUORESCENCE AND ABSORPTION SPECTRA OF STERICALLY HINDERED ARYLETHYLENES IN THE NEAT GLASSY STATE

Luminescence in Crystalline Organic Materials: From Molecules to Molecular Solids

“…[15] In any case, the frequently used AIE terminology remains highly controversial due to its ambiguous character, as the vast majority of the molecules reported do not require chromophore aggregation for the PL switch-on, but simply a restriction of large amplitude motions in the solid. This is equivalent to what was mentioned above for the early studies on solid and frozen solutions, [20,21,[58][59][60][61][62][63][64]98] and further observed in host-guest compounds, [46][47][48][49] or by intercalation in DNA. [105] For the sake of an unambiguous definition of "aggregation", [106] it was thus suggested to term the phenomenon in a physically sound, phenomenological way simply as "solid state luminescence [98,101,102] providing all combinations of bright versus dark character of (dicyano-)distyrylbenzenes (DCS) in fluid solution versus nanocrystalline/polycrystalline state.…”

“…[257] In general, for more extensive molecular structures, large amplitude motions are expected to be involved (see, e.g., the example of DPBDF [109] ), which make the nonradiative deactivation prone to steric effects. This was early realized, [15] and viscosity studies have indeed demonstrated a significant effect on excited state deactivation of various dyes, [20,58] stilbene, [59] and TPE compounds; [61][62][63] for structures, see Scheme 1. Thus, in rigid matrixes, as for [73,248] The table contains the compounds' photophysical data.…”

“…Notably, Φ F does not reach unity in PMMA, and thus non-negligible residual nonradiative decay is observed; this is associated with the free volume, [44,45,62] which to some extent allows for the structural reorganization necessary to reach the CI. [72] For this reason, significant differences are observed in PMMA for Φ F of differently substituted DCS compounds (nature, number and position of functional groups), as the steric demands of the substituents will largely determine the access to the CI in the restricted environment of PMMA.…”

“…[53][54][55][56][57] On the other side, the nonradiative pathway still poses a significant challenge. Early studies on various dyes since the 1890s, [15,58] stilbenes in the 1960s [21,44,59,60] as well as derivatives such as tetraphenylethylene (TPE; Scheme 1) [44,[61][62][63][64][65] triggered a description of nonradiative processes beyond Fermi's golden rule (FGR) (i.e., the overlap of vibrational eigenstates in the initial and final electronic states). [27] The standard model of stilbene deactivation assumed an "avoided crossing" between the ground and excited state potential energy surfaces along the double bond torsion.…”

“…[15,44,[58][59][60][61] This was substantiated for stilbenes by extensive viscosity studies (including freezing experiments), [44,59,60] supramolecular host-guest formation, [46,47,[68][69][70] and in particular intramolecular rigidification, [59,60] being textbook knowledge. [71] Thermal activation of nonradiative deactivation was further recognized as a key property and intensively studied; [59][60][61][62] finally, electronic substituent and polarity effects were explored experimentally. [59,60] In the meantime, for specific families of stilbenoid compounds, systematic library-based insights to the influence of substituents on the deactivation process were gained, [59,72] giving guidelines for targeted molecular design.…”