Effects of Through-Bond and Through-Space Conjugations on the Photoluminescence of Small Aromatic and Aliphatic Aldimines

Abstract

Through-bond conjugation (TBC) and/or through-space conjugation (TSC) determine the photophysical properties of organic luminescent compounds. No systematic studies have been carried out to understand the transition from aromatic TBC to non-aromatic TSC on the photoluminescence of organic luminescent compounds. In this work, a series of small aromatic and aliphatic aldimines were synthesized. For the aromatic imines, surprisingly, N,1-diphenylmethanimine with the highest TBC is non-emissive, while N-benzyl-1-phenylmethanimine and N-cyclohexyl-1-phenylmethanimine emit bright fluorescence in aggregate states. The aliphatic imines are all emissive, and their maximum emission wavelength decreases while the quantum yield increases with a decrease in steric hindrance. The imines show concentration-dependent and excitation-dependent emissions. Theoretical calculations show that the TBC extents in the aromatic imines are not strong enough to induce photoluminescence in a single molecule state, while the intermolecular TSC becomes dominant for the fluorescence emissions of both aromatic and aliphatic imines in aggregate states, and the configurations and spatial conformations of the molecules in aggregate states play a key role in the formation of effective TSC. This study provides an understanding of how chemical and spatial structures affect the formation of TBC and TSC and their functions on the photoluminescence of organic luminescent materials.

Space and Bond Synergistic Conjugation Controlling Multiple‐Aniline NIR‐II Absorption for Photoacoustic Imaging Guided Photothermal Therapy

“…[ 27 ] The combination of intramolecular BC and SC made the excited molecules form excimer/dissociator, blocking the path of radiation transition and eventually leading to fluorescence quenching. [ 28 ] According to the structure optimization model, the nearest intermolecular distance between adjacent benzene rings of TPD molecule and TB molecule was 3.877 and 4.165 Å, respectively (Figure 3e; Figure S10, Supporting Information), which provided spaces for the intermolecular vibration under light excitation. Besides, the presence of alkane chains in TPD molecules greatly improved the intramolecular movement, which further opened the channel for non‐radiative transition.…”

New betulin imine derivatives with antioxidant and selective antitumor activity

“…60 By comparison, the studied imines displayed a new absorption band with an absorption maximum at intermediate values, between 231–243 nm, characteristic of the π → π* transition of the – CN groups. 61 The band was hypsochromically shifted once the length of the aliphatic chain increased, from 243 nm ( I3 ) to 231 nm ( I9 ), attributable to the decrease of the conjugation effect due to the increased repulsive effect of the longer alkyl chain. 61 All the samples showed slight emission of bluish light, both in solid and in liquid states, when exposed to an UV lamp, and showed an intense emission band with maximum around 440 nm (in the blue domain) when excited with a wavelength of 365 nm (Fig.…”

Pyrazine as a More Efficient Luminophore than Benzene for Producing Red-Shifted and Enhanced Photoluminescence

“…Theoretical calculations show that in the crystal state, the electrons of the intermolecular phenyl groups of s -TPE overlap, leading to obvious intermolecular TSC and a lowered HOMO–LUMO energy gap. In addition, both TBC and TSC stiffen the molecular conformation, and thus nonradiative decay is suppressed while fluorescence emission is promoted . Enhanced intramolecular TBC and intermolecular TSC are both feasible strategies to construct PL compounds with red-shifted and enhanced emissions.…”

“…In addition, both TBC and TSC stiffen the molecular conformation, and thus nonradiative decay is suppressed while fluorescence emission is promoted. 26 Enhanced intramolecular TBC and intermolecular TSC are both feasible strategies to construct PL compounds with red-shifted and enhanced emissions. The TSC mechanism is important for the structural design of heterocyclic fluorescent compounds.…”