Clustering‐Triggered Emission of Nonconjugated Polyacrylonitrile

Abstract

Intrinsic emission from nonconjugated polymers has attracted considerable attention owing to its fundamental importance and intensive applications in diverse fields. The emission mechanism, however, is still in debate. Herein, nonconjugated polyacrylonitrile (PAN) molecules are found to be virtually nonluminescent in dilute solutions, while being highly emissive when concentrated or aggregated as nanosuspensions, solid powders, and films, exhibiting distinct aggregation-induced emission (AIE) characteristics. Moreover, triplet emissions of delayed fluorescence and room temperature phosphorescence are detected from the solid powders. Such unique emission of nonconjugated PAN is ascribed to the formation of cyano clusters, which act as the exact chromophores. In these clusters, through space electronic interactions, namely overlap of π and lone pair (n) electrons among cyano groups extend the conjugation and meanwhile rigidify the molecular conformations, thus offering remarkable emission upon irradiation. The AIE phenomenon can also be well rationalized by the formation of cyano clusters together with conformation rigidification. And the triplet emissions shall be originated from the n-π* transition owing to the presence of lone pairs. It is believed that such clustering-triggered emission mechanism is instructive for further development of unorthodox luminogens.

Emission and Emissive Mechanism of Nonaromatic Oxygen Clusters

“…Recently, we proposed the clustering‐triggered emission (CTE) mechanism to rationalize the unique photophysical behaviors of nonconventional luminophores . Namely, the clustering of nonconventional chromophores bearing π and lone‐paired ( n ) electrons and subsequent electron cloud overlap together with simultaneous conformation rigidification can rationalize the emission .…”

“…Despite the absence of conjugation in PEG, overlaps of electronic clouds provided by oxygen atoms, according to the CTE mechanism, may afford bright photoluminescence (PL) through proper contact of oxygen in the aggregated states. Exactly, under 312 nm UV irradiation, PEG powders could exhilaratingly emit blue light with a quantum efficiency ( Φ ) of 8.1% ( Figure a).…”

Nonconventional Luminescence Enhanced by Silicone‐Induced Aggregation

“…[5] Another effective strategy is developing nonconventional luminogens only containing nonconventional chromophores such as an aliphatic tertiary amine, [6] amide, [7] nitrile, [8] C = O, [9] C = S, [10] C = C, [11] epoxy, [12] or heteroatoms. [13] Generally,n onconventional luminogens are not composed of traditional aromatic units or constructed by bulky conjugation.…”

“…The fluorescence of BCCN and BSCN can be explained by the CTE mechanism.I nb oth cases, when cyano groups clustered in close proximityt oe ach other,t hey afford throughspace electronic communications, including electron overlap between lone pairs and p electrons, dipole-dipolei nteractions, and n-p interactions, which form an extended electronic conjugation [8] that facilitatese xcitation and subsequentr adiative deactivation. The existence of Si-O should make the molecules more flexible, and internal rotationo ccur more easily,r educing the fluorescence emission in an on-radiative-decay pathway.H owever,l uminescence with remarkable enhancementw as observed in BSCN (Table S1 and Figure 1), which was much brighter than the fluorescence of BCCN under the same conditions.…”

“…Therefore, designing nonconventional luminescentm aterials with improved performance becomes an impending task and has attracted ag reat deal of interest. [8,10, 14] Since the unexpected luminescence from archetypal poly(amidoamine)s (PAMAM) was reported, more and more nonconventional fluorescent polymers have been designed and synthesized. The nonconventional organic luminogens always exist as dendrimers, [15] hyperbranched polymers, [16] or even linear polymers, [17] such as polyurea dendrimers, [9] poly(aminoester)s (PAE), [8] polyacrylonitrile (PAN), [8] poly(acrylic acid), [14b] hyperbranched polyether epoxy, [12] and even some supramolecular assemblies [18] .D espite their structural diversity and interesting fluorescence emission, the emission mechanism is still under debate.…”