Carbazole-Based Cyano-Stilbene Highly Fluorescent Microcrystals

Abstract

Aggregation-induced enhanced emission (AIEE) is reported for 1-cyano-trans-1,2-bis-(4-carbazolyl)phenylethylene (CN-CPE). The weak luminescence of dilute CN-CPE solutions is enhanced upon aggregate formation into 2-3 μm sized crystals. In contrast to general observations, crystal formation of CN-CPE causes a blue-shift in emission and enhances the intensity. X-ray crystallographic analysis revealed that key factors causing high luminescence efficiency in the crystal are a lack of strong cofacial π-π alignment and the existence of the strong supramolecular interactions due to the intermolecular H-bonding. These factors seem to be responsible for the AIEE phenomenon as molecules of CN-CPE are held in a rigid twisted conformation, thereby increasing the fluorescence intensity in the solid or aggregated states. Accordingly, conformational twisting in the crystal packing process may be responsible for the unusual emission blue-shift in the aggregate.

Determining the Critical Particle Size to Induce Enhanced Emission in Aggregates of a Highly Twisted Triarylamine

“…Both the magnitude and trends with polarity for t 1 are consistent with the previous 9DAAA studies [17] in varying polarity organic solvents. [13,21] Also, as the water fraction increases from pure ACN, the overall weighted mean lifetime at first decreases-consistent with excited state quenching in more polar media [1,18,22] -but then increases in concert with increase in the slow decay channel lifetime above the 70 % water fraction, where AIEE onset is observed. As water content increases above 40 %, the slow decay channel process appears and becomes more prominent relative to the fast channel process (Table 1); this behavior is qualitatively similar to that of AIEE previously reported for other systems.…”

“…[1,4,8,9] Thus, photophysical behavior of organic chromophores in aggregate states is an increasingly important topic of activity. [12][13][14] AIEE has been reported in donor-acceptor dyes with strong electron-donating and -withdrawing groups, with increased aggregated-state emission attributed to restricted intramolecular rotation, and reduced co-facial intermolecular p overlap. [11,12] This phenomenon has been termed aggregation-induced enhanced emission (AIEE), attributed to the suppression of non-radiative pathways for relaxation of the excited state through aggregation-assisted charge transfer, intramolecular rotations, p-p stacking or a combination of these.…”

“…[10] Some organic dyes display strongly increased PL upon aggregation, in contrast to the commonly observed PL quenching. [13,15,16] AIEE occurring just at onset of aggregation has been associated with hypsochromic shift of the emission maximum at a slightly increased intensity, followed by a large rise in emission intensity with a stronger hypsochromic shift. [12][13][14] AIEE has been reported in donor-acceptor dyes with strong electron-donating and -withdrawing groups, with increased aggregated-state emission attributed to restricted intramolecular rotation, and reduced co-facial intermolecular p overlap.…”

Nanoscaled rhodamine dyes with 4.5-fold longer fluorescence lifetimes and 150 nm red shifted fluorescence spectra: Preparation, size tuning and optical properties

“…The studies on fluorescent pure organic nanoparticles have been focusing on perylene [12,13], carbazoles [14,15], and cyanine dyes [16,17]. Nanoscaled rhodamine dyes have not been explored in depth on the preparation and properties except for a recent report in which UV-induced formation of rhodamine B nanoparticles was found [18].…”

Crystal Multi‐Conformational Control Through Deformable Carbon‐Sulfur Bond for Singlet‐Triplet Emissive Tuning

“…[12][13][14][15][16] Thep ast years witnessed substantial progress in developing crystal-state emissions,t he study and application of which have been principally focused on luminescent color tuning, brightness adjustment and device fabrication. [17][18][19][20][21][22] Nevertheless,r egulation of emissive path-ways (e.g.,r egarding singlet-triplet excited state features or the transfer in between) on single materials by crystal engineering has received little attention, probably because such ae xcited state regulation often lacks sensitivity in ac rystalline system. [14] In contrast, it is known that emissive pathways can basically respond to diverse structural and selfassembly factors in amorphous phases.…”