Polymorphism-Dependent Emission of Nonaromatic Luminophores

doi:10.6023/A20080368

Abstract

Recently, nonconventional luminophores have received increasing attention, owing to their fundamental importance, advantages in outstanding biocompatibility, easy preparation, environmental friendliness and potential applications in sensing, imaging, encryption, etc. In order to provide more information about relationship among molecular conformation, molecular packing and emission, and moreover, to guide the design of nonconventional luminophores, molecules with definite structures and explicit molecular packing are highly desired. In this contribution, we report two nonconventional luminophores, namely F-MA and F-MI, consisting of carbonyls (C=O), electron-rich heteroatoms (O/N), and unsaturated C=C subgroups. They are nonluminous in dilute solutions while being emissive in concentrated ones. Furthermore, through crystallization in different solvents, polymorphs of both compounds with various emission colors along with distinct room temperature phosphorescence (RTP) are successfully obtained. Under 312 nm UV irradiation, three polymorphs of F-MA emit bluish-violet, blue and white lights, accompanying photoluminescence (PL) and RTP quantum efficiencies ( Φ _c/ Φ _p) of 10.6%/1.8%, 9.4%/2.1% and 2.9%/1.7%, respectively. To acquire more efficient emission, hydrogen bonds are introduced via amidation of F-MA, leading to the target compound F-MI with strikingly improved PL performance. Notably, F-MI is also polymorphic, whose Φ _c and Φ _p are of up to 17.0% and 4.8%, respectively. Meanwhile, the RTP lifetimes of F-MI polymorphs are significantly prolonged by 10- to 56-fold, as compared with their corresponding F-MA counterparts. The above PL properties can well be rationalized by the clustering-triggered emission (CTE) mechanism, namely through-space electronic delocalization of π and n electrons among different molecules in concentrated solutions or crystals alongside with sufficiently rigidified conformations is accountable for the emission, which is also verified by single crystal analysis and theoretical calculation. Besides, the noticeable RTP emission should be ascribed to the presence of C=O and heteroatoms and the clustering of such subgroups, which are ready to enhance spin-orbit coupling (SOC) and subsequent intersystem crossing transitions with effective through-space conjugation. Surprisingly, subtle changes caused by trace solvents in molecular conformations and packing modes significantly impact on intra/intermolecular interactions, which alter the relative intensity of singlet (fluorescence) and triplet (RTP) emissions, thus resulting in polymorphism-dependent emission colors. For these unorthodox luminescent molecules, their PL properties of polymorphs will deepen the understanding of the relationship between subtle structure variation and emission, thus enlightening further luminescent mechanism understanding and future rational design of novel nonconventional luminophores.

Key words: nonaromatic luminophores, polymorphism, room temperature phosphorescence, clustering-triggered emission, through-space conjugation

Cite this article

Yueying Lai, Zihao Zhao, Shuyuan Zheng, Wang Zhang Yuan. Polymorphism-Dependent Emission of Nonaromatic Luminophores[J]. Acta Chimica Sinica, 2021, 79(1): 93-99.

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Fig. & Tab. 4

Figure 1. (a) Chemical structures, schematic diagram of the emission mechanism, and (b) photophysical parameters of the crystals for F-MA and F-MI. (c) Photographs and (d) emission spectra of F-MA/THF solutions at different concentrations with 312 nm UV irradiation. (e) Emission spectra of 0.4 mol/L F-MA/THF solutions at different λex. (f) Absorption spectra of F-MA/THF solutions with different concentrations

Figure 2. (a) Photographs under and after 312 nm UV irradiation at room temperature and 77 K, (b) prompt (solid line) and delayed (dash line) emission spectra and (c) emission decay profiles monitored at 546, 555 and 542 nm at room temperature of F-MA polymorphs. (d) The chromaticity coordinate at 77 K and room temperature, (e) emission spectra (λex=312 nm) and (d) decay profiles monitored at 600, 670, 551 and 560 nm of F-MA polymorphs at 77 K

Figure 3. (a) Photographs taken under and after ceasing the 312 nm UV irradiation, (b) prompt and delayed (td=0.1 ms) emission spectra (λex=312 nm), and (c) p-RTP decay profiles monitored at 517, 524 and 529 nm at room temperature of F-MI polymorphs

Figure 4. Crystal structures (298 K) with denoted intramolecular interactions and molecular packing with highlighted through-space conjugation for (a)βMA-A and (b) MI-A. (c) Single crystal data of crystals of F-MA and F-MI. (d) Electron density distributions of different orbital levels of monomer and dimer of F-MA

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