Elsevier

Tetrahedron

Volume 69, Issue 29, 22 July 2013, Pages 5818-5822
Tetrahedron

Solid-state fluorescence properties and mechanofluorochromism of D–π-A pyridinium dyes bearing various counter anions

https://doi.org/10.1016/j.tet.2013.05.047Get rights and content

Abstract

A pronounced bathochromic shift-type mechanofluorochromism (MFC) is found for D–π-A pyridinium dyes OD1OD3 having diphenylamino group as an electron donor and pyridinium ring as an electron acceptor linked by carbazole as a π-conjugated bridge, and bearing various counter anions (X=Cl, Br, or I). Interestingly, both absorption and fluorescence peaks of as-recrystallized dyes show bathochromic shift by grinding, and the degrees of the bathochromic shift depend on the type of counter anion of OD1OD3. We propose that the use of various counter anions for D–π-A pyridinium dyes can be one of the most effective strategies, not only to express a pronounced MFC, but also to control changes of fluorescence properties by the grinding and heating, resulting in fine-tuning of fluorescent color change by the MFC.

Introduction

Recently, mechanofluorochromic and piezofluorochromic dyes, which can exhibit a change in fluorescent color by external mechanical and pressure stimuli, respectively, to the solids, being accompanied by a reversion to the original fluorescent color by heating, have attracted much attention from chemists, physicists, and engineers because of enormous scientific interest in the fundamental research fields of photochemistry and photophysics and interesting smart materials for next generation optoelectronic devices.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 Some research groups reported that symmetrical donor–acceptor–donor (D–A–D) dyes,6, 7, 8, 9, 10, 11 difluoroboron β-diketone complexes,12, 13, 14, 15 and amide-substituted phenylpyrene derivatives16, 17, 18, 19 exhibit mechanofluorochromism (MFC) or piezofluorochromism (PFC). The MFC and PFC of these dyes so far reported are attributed to a reversible switching between crystalline and amorphous states, between stable and metastable liquid crystalline phases, or between the two different crystalline phases with changes of intermolecular interactions, such as π–π interaction, dipole–dipole interaction, and hydrogen bonding before and after grinding of solids. Therefore, to control changes of fluorescence color and fluorescence quantum yield by the MFC and PFC, much effort has been made to modify the molecular structures by the introduction of the substituents onto the fluorophore skeletons. However, the number of mechanofluorochromic and piezofluorochromic dyes is still limited.

On the other hand, we have found that newly developed unsymmetrical heteropolycyclic donor–acceptor π-conjugated (D–π-A) fluorescent dyes with strong electron-withdrawing substituents (e.g., cyano group) as acceptor and thus large dipole moments (ca. 5 Debye) show a bathochromic shift-type MFC; grinding of as-recrystallized dyes induces a bathochromic shift of fluorescent maximum wavelength, followed by a reversion to the original fluorescent maximum wavelength by heating.24, 25, 26, 27 We have revealed that the MFC of D–π-A fluorescent dyes is attributed to a reversible switching between crystalline and amorphous states with changes of dipole–dipole interaction and intermolecular π–π interaction. Moreover, our results suggest that larger dipole moment (ca. 8.0 Debye) leads to reduce the MFC properties of D–π-A fluorescent dyes, that is, the strong dipole–dipole interaction between the dyes might inhibit a switching between crystalline and amorphous states on grinding.28 Thus, not only to express a pronounced MFC, but also to control changes of fluorescence properties by the MFC, it is necessary to provide a new direction in the molecular design for effective development of mechanofluorochromic dyes.

Herein, we report that a pronounced bathochromic shift-type MFC is found for D–π-A pyridinium dyes OD1OD329 (Scheme 1) having diphenylamino group as an electron donor and pyridinium ring as an electron acceptor linked by carbazole as a π-conjugated bridge, and bearing various counter anions (X=Cl, Br, or I). Interestingly, both absorption and fluorescence peaks of as-recrystallized dyes show bathochromic shift by grinding, and the degrees of the bathochromic shift depend on the type of counter anion of OD1OD3. We have demonstrated that the type of counter anion of D–π-A pyridinium dyes can certainly control the MFC, that is, the type of counter anion can conveniently tune fluorescent color change by the grinding and heating.

Section snippets

Results and discussion

The absorption and fluorescence spectra of OD1OD3 in 1,4-dioxane are shown in Fig. 1. The three dyes show absorption band at around 440 nm, which is assigned to the intramolecular charge-transfer (ICT) excitation from electron donor moiety (diphenylamino group) to electron acceptor moiety (pyridinium ring). The corresponding fluorescence maxima (λmaxfl) for the three dyes occur at around 590 nm, and the fluorescence quantum yield (Φfl) increase in the order OD3 (0.02)<OD2 (0.16)<OD1 (0.27),

Conclusions

We found that D–π-A pyridinium dyes bearing various counter anions show a pronounced bathochromic shift-type MFC. We have demonstrated that the type of counter anion of D–π-A pyridinium dyes can certainly control the MFC. We propose that the use of various counter anions for D–π-A pyridinium dyes can be one of the most effective strategies, not only to express a pronounced MFC, but also to control changes of fluorescence properties by the grinding and heating, resulting in fine-tuning of

General

Absorption spectra were observed with a Hitachi U-2910 spectrophotometer and fluorescence spectra were measured with a Hitachi F-4500 spectrophotometer. Absorption spectra of the solids were observed with a Shimadzu UV-3150 spectrophotometer by using a calibrated integrating sphere system. The fluorescence quantum yields in solution and in the solid state were determined by a Hamamatsu C9920-01 equipped with CCD by using a calibrated integrating sphere system. Fluorescence lifetimes were

Acknowledgements

This work was supported by Grants-in-Aid for Scientific Research (B) (23350097) and (C) (24550225) from the Japan Society for the Promotion of Science (JSPS). Y.O. also acknowledges the DIC Award in Synthetic Organic Chemistry, Japan.

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