Naphthalonitriles featuring efficient emission in solution and in the solid state

Abstract

In this work, a series of γ-substituted diphenylnaphthalonitriles were synthesized and characterized. They show efficient emission in solution and in the aggregated state and their environment responsiveness is based on having variable substituents at the para-position of the two phenyl moieties. The excited state properties were fully investigated in tetrahydrofuran (THF) solutions and in THF/H₂O mixtures. The size of the aggregates in aqueous media were measured by dynamic light scattering (DLS). The steady-state and time-resolved photoluminescence spectroscopy studies revealed that all the molecules show intense fluorescence both in solution and in the aggregated state. In THF solutions, a blue emission was observed for the unsubstituted (H), methyl- (Me) and tert-butyl- (t-Bu) substituted γ-diphenylnaphthalonitriles, which can be attributed to a weak π-donor capability of these groups. On the other hand, the methoxy- (OMe), methylsulfanyl- (SMe) and dimethylamino- (NMe₂) substituted compounds exhibit a progressive red-shift in emission compared to H, Me and t-Bu due to a growing π-electron donating capability. Interestingly, upon aggregation in water-containing media, H, Me and t-Bu show a slight red-shift of the emission and a blue-shift is observed for OMe, SMe and NMe₂. The crystal structure of Me allowed a detailed discussion of the structure-property relationship. Clearly, N-containing substituents such as NMe₂ possess more electron-donating ability than the S-based moieties such as SMe. Moreover, it was found that NMe₂ showed higher luminescence quantum yields (Φ_F) in comparison to SMe, indicating that N-substituted groups could enhance the fluorescence intensity. Therefore, the π-donor nature of the substituents on the phenyl ring constitutes the main parameter that influences the photophysical properties, such as excited state lifetimes and photoluminescence quantum yields. Hence, a series of highly luminescent materials from deep blue to red emission depending on substitution and environment is reported with potential applications in sensing, bioimaging and optoelectronics.

Keywords: aggregation caused quenching (ACQ); aggregation-induced emission enhancement (AIEE); naphthalonitriles (NCNs); solution and solid state emitters (SSSE); solvent quenching (SQ).

Scheme 1

A) Structures of the six investigated 2,3-disubstituted-6,7-diphenylnaphthalene derivatives with varying distal groups. B) Schematic depiction of the D–π–A system.

Scheme 2

Synthesis of p-phenyl-6,7-disubstituted naphthalene-2,3-dicarbonitrile.

Figure 1

Molecular structure in the crystal of Me as obtained by X-ray diffractometric analysis. Thermal displacement ellipsoids are shown with 50% probability. Color code: black = carbon, grey = hydrogen and blue = nitrogen.

Figure 2

A) Intermolecular CH^…π interactions for compound Me. B) Weak intermolecular π–π stacking interactions. C) Packing of compound Me along the a-axis. Color code: black = carbon, grey = hydrogen and blue = nitrogen. The unit cell is shown in Supporting Information File 1, Figure S32.

Figure 3

Normalized absorption spectra of the evaluated compounds in fluid THF at rt. All solutions were optically diluted (A < 0.1).

Figure 4

Normalized emission spectra of the evaluated compounds in fluid THF at rt (left) and in a frozen glassy matrices of 2-methyl-THF at 77 K; λ_ex = 320 nm (H, Me, t-Bu); λ_ex = 340 nm (OMe, SMe); λ_ex = 350 nm (NMe₂).

Figure 5

A) Photographs of H at different THF/H₂O ratios under UV excitation (λ = 365 nm). B) Photoluminescence spectra of H at different THF/water ratios. C) Emission wavelength and Φ_F vs water content for H. D) Photographs of OMe at different THF/H₂O ratios under UV excitation (λ = 365 nm). E) Photoluminescence spectra of OMe at different THF/water ratios. F) Emission wavelength and Φ_F vs water content of OMe. G) Photographs of SMe at different THF/H₂O ratios under UV excitation (λ = 365 nm). H) Photoluminescence spectra of SMe at different THF/water ratios. I) Emission wavelength and Φ_F vs water content of SMe. J) Photographs of NMe₂ at different THF/H₂O ratios under UV excitation (λ = 365 nm). K) Photoluminescence spectra of NMe₂ at different THF/water ratios. L) Emission wavelength and Φ_F vs water content of NMe₂. Concentrations in all cases: 10 µM.