Solvatochromism of new tetraphenylethene luminogens: integration of aggregation-induced emission and conjugation-induced rigidity for emitting strongly in both solid and solution state

Abstract

In this study, we synthesized and characterized four tetraphenylethene (TPE) analogs, investigated their photophysical properties, and conducted quantum chemical calculations. Some molecules exhibited aggregation-induced emission enhancement behavior and showed efficient emission in both solid and solution states. Solvatochromism was observed in particular derivatives, with solvent polarity influencing either a bathochromic or hypsochromic shift, indicating the occurrence of photoinduced intramolecular charge transfer (ICT) processes. Quantum chemical calculations confirmed that variations in molecular packing and rigidity among the TPE analogs contributed to their diverse behavior. The study showcases aggregation in luminophores without significant impact on the excited state and highlights how minor alterations in terminal substituents can lead to unconventional behavior. These findings have implications for the development of luminescent materials. Furthermore, the synthesized compounds exhibited biocompatibility, suggesting their potential for cell imaging applications.

Scheme 1. Synthesis routes to chromophores 1, 2, 3, and 4.

Fig. 1. (a) Absorption spectra of solutions (5 × 10⁻⁵ mol L⁻¹). (b) Normalized emission spectra of solutions (λ_ex = 391, 420, 400, and 409 nm for 1, 2, 3, and 4, respectively, 5 × 10⁻⁵ mol L⁻¹). (c) Normalized emission spectra of solids (λ_ex = 391, 420, 400, and 409 nm for 1, 2, 3, and 4 respectively). (d–g) Photos taken under UV irradiation for solids of compounds 1, 2, 3, and 4, respectively {insets: optical images of the solutions in dioxane}.

Fig. 2. Emission spectra at λ_ex = 391 and 420 nm for 1 and 2, respectively; (a) 1 and (b) 2 solutions (5 × 10⁻⁵ mol L⁻¹) in DMSO with various H₂O fractions. Variation of emission intensity of (c) 1 and (d) 2 {insets: optical images of the solutions with varied water content under UV light (365 nm)}.

Fig. 3. Emission spectra at λ_ex = 400 and 409 nm for 3 and 4, respectively; (a) 3 and (b) 4 solutions (5 × 10⁻⁵ mol L⁻¹) in DMSO with various H₂O fractions. Variation of emission intensity of (c) 3 and (d) 4 {insets: optical images of the solutions with varied water content under UV light (365 nm)}.

Fig. 4. Normalized emission spectra at λ_ex = 391, 420, 400, and 409 nm for 1, 2, 3, and 4, respectively; (a) 1, (b) 2, (c) 3, and (d) 4 in solvents of different polarity (5 × 10⁻⁵ mol L⁻¹).

Fig. 5. The HOMO and LUMO of (a) 1, (b) 2, (c) 3, and (d) 4, along with their energies calculated at the B3LYP-D3BJ/6-31G(d) level. The molecular electrostatic potential of (e) 1, (f) 2, (g) 3, and (h) 4 at the B3LYP-D3BJ/6-31G(d) level. Color: red is the most negative (nucleophilic atoms), and blue is the most negative (electrophilic atoms).

Fig. 6. Critical changes in structural parameters between the ground and excited states of (a) 1 and (b) 3 were calculated at the B3LYP-D3BJ/6-31G(d) level. Color: grey = C; blue = N; red = O. (c) Key changes in structural parameters between the monomer and dimer of molecule 3 calculated at the B3LYP-D3BJ/6-31G(d) level.

Fig. 7. The geometry of the most stable dimers of molecules (a) 1, (b) 2, (c) 3, and (d) 4 computed at the B3LYP-D3BJ/6-31G(d) level. The intermolecular interactions in the dimers of molecules (e) 1 and (f) 3 obtained by the Multiwfn program on geometries optimized at the B3LYP-D3BJ/6-31G(d) level. Color code: blue indicates hydrogen bonding (absent here), red indicates repulsion (steric effect), and green indicates van der Waals interactions.