Effect of Polymer Composition on the Optical Properties of a New Aggregation-Induced Emission Fluorophore: A Combined Experimental and Computational Approach

Abstract

Nowadays, fluorophores with a tetraphenylethylene (TPE) core are considered interesting due to the aggregation-induced emission (AIE) behavior that enables their effective use in polymer films. We propose a novel TPE fluorophore (TPE-BPAN) bearing two dimethylamino push and a 4-biphenylacetonitrile pull moieties with the typical AIE characteristics in solution and in the solid state, as rationalized by DFT calculations. Five different host polymer matrices with different polarity have been selected: two homopolymers of poly(methylmethacrylate) (PMMA) and poly(cyclohexyl methacrylate) (PCHMA) and three copolymers at different compositions (P(MMA-co-CHMA) 75:25, 50:50, and 25:75 mol%). The less polar comonomer of CHMA appeared to enhance TPE-BPAN emission with the highest quantum yield (QY) of about 40% measured in P(MMA-co-CHMA) 75:25. Further reduction in polymer polarity lowered QY and decreased the film stability and adhesion to the glass surface. LSC performances were not significantly affected by the matrix's polarity and resulted in around one-third of the state-of-the-art due to the reduced QY of TPE-BPAN. The theoretical investigation based on density functional theory (DFT) calculations clarified the origin of the observed AIE and the role played by the environment in modulating the photophysical behavior.

Keywords: aggregation-induced emission; density functional theory investigations; luminescent solar concentrator; methacrylate copolymers; optical and device efficiencies; push–pull tetraphenylethylene fluorophore; quantum yield (TPE).

Figure 1

Schematic structure of TPE-BPAN highlighting the donor (green) and acceptor (red) substituent groups introduced in the TPE core. A1 and A2 represent the amino groups; P1 and P2 are the aromatic rings in the biphenyl group.

Figure 2

(a) Emission spectra of TPE-BPAN dissolved in THF/H₂O mixtures at different compositions (% of H₂O labeled, concentration: 5 × 10⁻⁵ M); (b) Picture showing emission features of TPE-BPAN in various THF/H₂O mixtures under UV irradiation (366 nm).

Figure 3

(a) NTO orbital associated with the S₀–S₁ transition of TPE-BPAN. (isodensity = 0.025 a.u.). (b) Graphical representation of the D_CT: barycenters of hole and electron charge distribution are in violet.

Figure 4

Schematic representation of the normal modes related to the largest HR factors of TPE-BPAN.

Figure 5

(a) Absorption spectra of TPE-BPAN in the five polymeric matrices under study at 2.0 wt.% concentration. Peak positions are marked with red vertical dashes. (b) Emission spectra of TPE-BPAN in the five polymeric matrices under study in the concentration of 0.4 wt.% (dashed) and 2.0 wt.% (solid). Peak positions are shown with red vertical dashes.

Figure 6

(a) Quantum yield (QY %) vs. fluorophore concentration for the five TPE-BPAN-containing thin films. Error bars are shown only with reference to the 0.4 wt.% concentration for clarity (see all data from Figures S28 to S32); (b) internal photonic efficiency and (c) external photonic efficiency vs. TPE-BPAN concentration in PMMA and P(MMA-co-CHMA) 75:25 films. (d) Electrical device efficiency vs. TPE-BPAN concentration in PMMA and P(MMA-co-CHMA) 75:25 films.