Relaxation Dynamics and Structural Characterization of Organic Nanobelts with Aggregation-Induced Emission

Abstract

We prepared the fluorescent nanobelts of cyano-substituted 1,4distyrylbenzene derivative (CNDSB) with the reprecipitation method. CNDSB is nonemissive in solution but with strong aggregation-induced emission (AIE) in belt-like structure. The molecular structure predicted by quantum chemical calculations indicates a twisted conformation of CNDSB monomer, while the solid-state X-ray structure of the CNDSB crystal features herringbone-type arrangement and three benzene rings arrange in nearly planar conformation. Femto-and picosecond time-resolved fluorescence spectroscopy are implemented to study the photophysics and the origin of the AIE effect in CNDSB nanobelts. The fluorescence quenching of CNDSB in THF solution arises from the efficient nonradiative isomerization channel. In this study, the fluorescence decay of CNDSB embedded in poly(methyl methacrylate) matrix is slowed down due to the restriction of intramolecular motions in the solid matrix. In nanobelt structure, the fluorescence lifetime further increases due to the intermolecular interaction between H-type aggregates. Both the steady-state and time-resolved studies suggests that there are at least two different packing structures might exist in nanobelts, and the relative composition of these two different structures gradually changes in different sizes of nanobelts.

Twist‐Elasticity‐Controlled Crystal Emission in Highly Luminescent Polymorphs of Cyano‐Substituted Distyrylbenzene (βDCS)

“…The prediction for PM3 agrees with the observed rate constant for fluorescence which was extracted from the fluorescence lifetime τ F and quantum yield Φ F by k F = Φ F /τ F = 0.07 ns −1 (Table ) which is significantly lower than in solution ( k F = 0.33 ns −1 ) as expected for H‐aggregates . Moreover, the absorption spectrum of nanoparticle suspensions and thin films, which show very similar characteristics as PM3 (vide supra), show a strong hypsochromic shift against solution, again indicating H‐aggregation. The remarkably high Φ F of 69% for the PM3 crystal is a further example for the realization of highly luminescent H‐aggregate type single crystals, as discussed earlier …”

“…It should be however mentioned that crystal spectra suffer from substantial reabsorption due to the high optical density; this effect can be very considerable (with reabsorption probabilities of ϕ = 0.5 and more), which leads to apparent (though artificial) bathochromic shifts of up to 0.1 eV. This effect is pronounced for the reported PM1 and PM2 crystals as can be seen from the steep slope of the high‐energy side of the spectra in Figure , while for PM3 we made much effort to minimize ϕ by reducing light penetration in the anyway small needled shaped crystal (≈0.35 mm length, 0.06 mm width; see the Supporting Information). Thus, the spectral positions of emission for PM2 and PM3 are rather comparable.…”

“…Emission spectra of βDCS: solution (CHCl 3 ), nanoparticle (NP) suspension, thin film, crystals of PM1 and PM2 crystals, and the present PM3 crystal. The inset shows the fluorescence image of PM3 under UV irradiation.…”

“…In the quest of in‐depth understanding of the twist elasticity concept, we here investigate cyano‐substituted distyrylbenzene (βDCS) (i.e., with CN substitution in the outer vinylene position, see Figure ), for which two different polymorphs with very different crystal structures were successfully determined in the past by SC X‐ray analysis (polymorphs PM1, PM2 in Figure ); there, both crystals were grown from the gas phase under different conditions. However, in this case, the different structure of PM2 compared to PM1 was mainly driven by the coexistence of two different rotamers— syn and anti (with respect to the mutual arrangement of the two CN groups; see Figure ) in a 1:1 stoichiometry, while in PM1 only anti ‐rotamers exist.…”

“…Single‐crystal X‐ray structures of βDCS: nearest neighbor arrangement for PM1 (CCDC 815708), PM2 (CCDC 860278), and PM3 (CCDC 1542079) along different view directions; a–c) crystallographic axes and rotamers in PM2 ( syn , anti ) are indicated.…”