Tuning Photophysical Properties via Positional Isomerization of the Pyridine Ring in Donor-Acceptor-Structured Aggregation-Induced Emission Luminogens Based on Phenylmethylene Pyridineacetonitrile Derivatives

Abstract

A series of aggregation-induced emission (AIE)-featured phenylmethylene pyridineacetonitrile derivatives named o-DBCNPy ((Z)-3-(4-(di-p-tolylamino)phenyl)-2-(pyridin-2-yl)acrylonitrile), m-DBCNPy ((Z)-3-(4-(di-p-tolylamino)phenyl)-2-(pyridin-3-yl)acrylonitrile), and p-DBCNPy ((Z)-3-(4-(di-p-tolylamino)phenyl)-2-(pyridin-4-yl)acrylonitrile) have been synthesized by tuning the substitution position of the pyridine ring. The linkage manner of the pyridine ring had influences on the molecular configuration and conjugation, thus leading to different photophysical properties. The absorption and fluorescence emission peak showed a bathochromic shift when the linking position of the pyridine ring changed from the meta to the ortho and para position. Meanwhile, o-DBCNPy exhibited the highest fluorescence quantum yield of 0.81 and the longest fluorescence lifetime of 7.96 ns as a neat film among all three isomers. Moreover, non-doped organic light-emitting diodes (OLEDs) were assembled in which the molecules acted as the light-emitting layer. Due to the relatively prominent emission properties, the electroluminescence (EL) performance of the o-DBCNPy-based OLED was superior to those of the devices based on the other two isomers with an external quantum efficiency (EQE) of 4.31%. The results indicate that delicate molecular modulation of AIE molecules could endow them with improved photophysical properties, making them potential candidates for organic photoelectronic devices.

Keywords: aggregation-induced emission; fluorescence emission; non-doped organic light-emitting diode; phenylmethylene pyridineacetonitrile; positional isomerization.

Scheme 1

Synthetic routes of the compounds.

Figure 1

Absorption spectra of o-DBCNPy (a), m-DBCNPy (b), and p-DBCNPy (c) in 1 × 10⁻⁵ M solutions (THF, tetrahydrofuran; DCM, dichloromethane; DMF, N, N-dimethylformamide).

Figure 2

Fluorescence emission spectra of o-DBCNPy (a), m-DBCNPy (b), and p-DBCNPy (c) in 1 × 10⁻⁵ M solutions (THF, tetrahydrofuran; DCM, dichloromethane; DMF, N, N-dimethylformamide); the plot of Stokes shifts of the three isomers in different solvents versus solvent orientation polarizability (Δf) (d).

Figure 3

Fluorescence lifetimes of o-DBCNPy (a), m-DBCNPy (b), and p-DBCNPy (c) in THF and as neat film.

Figure 4

Fluorescence spectra, variation in relative emission intensity (I/I₀), and emission maxima wavelength of o-DBCNPy (a,d), m-DBCNPy (b,e), and p-DBCNPy (c,f) in THF–water mixtures with increasing f_w (concentration of compound: 1 × 10⁻⁵ M, λ_ex = 415 nm); size distributions of o-DBCNPy (g), m-DBCNPy (h), and p-DBCNPy (i) aggregates in THF–water mixtures (f_w = 95%) (insets are SEM images of the particles).

Figure 5

Optimized molecular geometries, electron density distributions, and energy levels of the HOMO and LUMO orbitals for the molecules at the B3LYP/6-311g(d) basis set.

Figure 6

Cyclic voltammograms of the compounds; scan rate, 50 mV s⁻¹.

Figure 7

Fluorescence emission spectra and XRD patterns of o-DBCNPy (a,d), m-DBCNPy (b,e), and p-DBCNPy (c,f) when solid at various states (pristine, ground, and fumed); insets are images of the each solid under 365 nm UV light.

Figure 8

Current density–voltage–luminance (J–V–L) (a) and EQE-luminance characteristics (b) for the devices (inset showed the EL spectra).