A mechanistic investigation of mechanochromic luminescent organoboron materials

Abstract

Mechanochromic luminescence (ML) refers to the luminescence color and/or intensity change of solidstate materials induced by mechanical perturbations. For organic molecular solids, this phenomenon is related to the specific packing modes and orientations of individual fluorophores, which could give rise to different excited-state interactions. The molecular solids of difluoroboron dibenzoylmethane (BF 2 dbm) derivatives were previously found to exhibit reversible ML at room temperature and are promising as self-healing optical materials. In this report, we aim to shed some light on the mechanism of BF 2 dbm ML by trying to understand the excited-state interactions among solid-state BF 2 dbm molecules and elucidate how these interactions change upon mechanical stimulation. We first investigated the optical properties of monomeric, dimeric, and polymeric BF 2 dbm derivatives in optically dilute solutions and demonstrated unambiguously that BF 2 dbm moieties have a propensity to form H-aggregates. Next, we studied the physical properties of these boron complexes in the solid state including their crystal structures, fluorescence emissions, and mechanochromic luminescence. By correlating solution data with the solid-state characterization results, it was concluded that two coupled processes, force-induced emissive H-aggregate formation and energy transfer to the emissive Haggregates, are responsible for the observed BF 2 dbm ML in the solid state.

Mechanochromic Luminescent Properties of Difluoroboron β-Diketonate Dyes

“…3 All lifetimes were fit to multi-exponential decays as is typical for this family of solid-state dyes. 1,6,9 Such behavior is indicative of either excimer or ground state dimer formation, perhaps H-aggregates as proposed by Zhang et al [10][11][12][13] The H, F, Cl, and Br dyes all showed substantial increases in their pre-exponential weighted lifetimes (Table S2.2 Appendix A) after smearing while the I dye actually showed a slight decrease in this value. This may be attributed to increased intersystem crossing from S 1 to T 1 .…”

“…The blue-shifted emission observed in C 12 H 25 powders has been previously ascribed to the steric hindrance of the C 12 H 25 chain limiting dye-dye interactions that red shift emission. 13 In methoxysubstituted analogues, dye-dye interactions such as π-stacking and longer range effects that would red shift emission are more probable since the β-diketonate cores are closer together, undiluted by packed alkyl chain domains. 13,15 This result is analogous to a study performed by Nguyen et al wherein BF 2 dbm derivatives with long alkoxyl chains exhibited more blue-shifted solid-state emissions compared to shorter chain derivatives.…”

“…A previous study has shown that incorporation of an alkoxy chain longer than OCH 3 into difluoroboron dibenzoylmethane (dbm) fluorophores can grant ML properties. 13 Perhaps the ability of the alkoxy chains to break up closer intermolecular π-π interactions creates a greater variety of available states that may be accessed through different processing methods. In this example and others, [23][24][25] the addition of long alkoxy …”

“…3 Solvatochromic shifts are well understood and documented and serve as an excellent example of how a fluorophore can be used to probe its environment. 12,13 Different solvents can also either quench or enhance the fluorescence of their solutes and these phenomena most often must be evaluated on a case-by-case basis. In many cases, polar aprotic solvents may quench the emission of fluorophores with strong excited state dipoles.…”

“…As with previous systems, 110 °C was sufficient for annealing the H, C1, and C12 dyes. [1][2][3]13 However, the C5, C6, and C18 dyes required slightly higher temperatures to achieve the same blue-shifted maxima and narrow full widths at half maxima (FWHM). These temperatures were 150 °C for C5 and C6 and 120 °C for C18.…”