Luminescent Behavior of Gels and Sols Comprised of Molecular Gelators

Abstract

We present a brief review of some important conceptual and practical aspects for the design and properties of molecular luminescent gelators and their gels. Topics considered include structural and dynamic aspects of the gels, including factors important to their ability to emit radiation from electronically excited states.

Keywords: aggregation-induced emission enhancement (AIEE), quantum yield; excimer; fluorescence; gel; luminescence; molecular organic gelator (LMOG), hydrogel; phosphorescence.

Figure 1

Molecular structures of CAh and PySaG_n (where n = 2,3,4,6,7,8) gelators.

Figure 2

Fluorescence spectra (λ_ex = 350 nm; intensity normalized at 378 nm) of 1.5% (2.6 × 10⁻² M) PySaG₇ in acetonitrile in the gel (20 °C) and sol (80 °C; gel melting temperature, T_gel = 79 °C) phases. The inset shows the temperature-dependent wavelength maxima of the excimeric emission. Reprinted (adapted) with permission from [39]. Copyright (2013) American Chemical Society.

Figure 3

Molecular structure of (a) Pt_CD; (b) Pt_TEG; (c) and (d) formation of a Pt_CD-hydrogel and Pt_TEG-hydrogel and (e) photograph of the Pt_CD-hydrogel in daylight; scale bar = 1 mm. Republished from [42] with permission of Royal Society of Chemistry.

Figure 4

Molecular structures of 4PyCzBP, SiPhMe, PhAhFb and SA-Chol.

Figure 5

Conformational rotamers of SiPhMe. Republished from [55] with permission of Royal Society of Chemistry.

Figure 6

(a) Molecular structure of TPE-TPP; photo-luminescence spectra of TPE-TPP in solid and solution states. Inset: Photographs of a DMF solution (left) and a solid powder (right) of TPE-TPP taken under UV radiation. Concentration of TPE-TPP = 10 μM; λ_ex = 321 nm. Fluorescent images of CCCP (10 μM) treated HeLa cells stained with (b) MT (50 nM) for 15 min and (c) TPE-TPP (5 μM) for 30 min. λ_ex = 540−580 nm (for MT) and 330−385 nm (for TPE-TPP). Reprinted (adapted) with permission from [57]. Copyright (2013) American Chemical Society.

Figure 7

Molecular structures of TPEBe-I and PyFFK.

Figure 8

(a) Photographs of PyFFK gel-loaded wells of a multiwell plate in the presence or absence of different analytes under different lights. (b) Emission spectra of a gel sample in before and after treatment with picric acid (PA) solution (λ_ex = 337 nm). (c) The extent of quenching of the 477 nm band in the emission spectra of the gel samples in the presence of different analyte solutions. (d) Photographs of PyFFK gel-loaded wells of a multiwell plate after treatment with solutions of different concentrations of PA under different lighting conditions. (e) Extent of quenching of the emission at 477 nm for samples in (d). All experiments were performed at room temperature. Reprinted (adapted) with permission from [63]. Copyright (2019) American Chemical Society.

Figure 9

(a) Molecular structures of the EFK-bpy peptide and the structures of the EFK-bpy-Eu peptide. (b) Proposed schematic representation of the co-assembled hydrogel and the energy-transfer process. (c) The fluorescence emission spectrum of the EFK-bpy hydrogel (λ_ex = 300 nm; 4 mM), EuCl₃ (λ_ex = 300 nm; 1.33 mM) solution, and EFK-bpy-Eu hydrogel (λ_ex = 300 nm; 1.33 mM). (d) The fluorescence emission spectrum of the EFK-bpy hydrogel (λ_ex = 254 nm; 4 mM), EuCl₃ (λ_ex = 254 nm; 1.33 mM) solution, and EFK-bpy-Eu hydrogel (λ_ex = 254 nm; 1.33 mM). Republished with permission of Springer Nature, from [65].

Figure 10

Structure of naphthalene diimide (NDI) and derivatives of it with potential photophysical, self-assembly, and functional properties. Reprinted (adapted) with permission from [67]. Copyright (2016) American Chemical Society.

Figure 11

Molecular structure of NDIP.

Figure 12

(a) Fluorescence spectra of NDIP xerogels upon exposure to different volatile acid vapors. (b) Fluorescence "turn on" of NDIP xerogels upon exposure to HCl vapor. (c) Fluorescence on/off behavior by exposure and evaporation of acid vapor using a paper strip impregnated with NDIP. (d) Fluorescence on/off switching upon exposure to TFA and ammonia vapors of the paper strip device impregnated with NDIP. (e) Fluorescence on/off cycle of NDIP in the presence of acid and base vapors. (f) Fluorescence spectra of NDIP in milli-Q water in different amounts (ppm) of trifluoroacetic acid (TFA). From [32]. Copyright Wiley-VCH GmbH. Reproduced with permission.

Figure 13

Molecular structures of DDOA, DDOT and DHDOT.

Figure 14

(a) Simplified representation of a doped DDOA gel fiber (enlargement of three layers of sheets of DDOA) is represented, one excited donor (light blue) and one emissive acceptor (green), as well as energy transfer pathways (E.T., yellow arrows) for the direct process (dotted arrow) and several possibilities for exciton migration. Right: Chemical structures of DDOA and DDOT (b) Emission spectra of DDOA (2.0 × 10^–3 M) gels in DMSO at 293 K, λ_ex = 384 nm with increasing amounts of added DDOT (mol %). Reprinted (adapted) with permission from [76]. Copyright (2005) American Chemical Society.

Figure 15

Syntheses of QAh and NAh. Republished from [77] with permission from the Royal Society of Chemistry.

Figure 16

Synthesis of PaCzBph. Reprinted from [78] with permission from Elsevier.

Figure 17

Synthesis of DODA [44]. Copyright John Wiley and Sons. Reproduced with permission.

Figure 18

Synopsis of the synthetic route to DtBuCZBP.

Figure 19

(a) Molecular structures of QA and ethynylpyrenyl derivatives of QA (P₃QA, P_3,8QA and P_3,6QA). (b) Normalized fluorescence emission spectra of P_3,6QA in the gel state (CGC concentration) in the absence of Zn²⁺ (red curve) and in the presence of 0.5 equiv. of Zn²⁺ (black curve) in acetone. Republished from [28] with permission of The Royal Society of Chemistry.

Figure 20

(a) Molecular structures of NDI-ala and amines, n-decylamine (C10) or n-dodecylamine (C12). (b) Tentative model (based on data from Fourier transform infrared (FTIR), PXRD, and SAXS) for the molecular packing arrangement of the two-component co-assembled system, NDI-ala12. The molecular length of C12 in its extended conformation was used to estimate the total length of NDI-ala12. Reprinted (adapted) with permission from [84]. Copyright (2016) American Chemical Society.

Figure 21

(a) Molecular structure of Azo-KC. (b) Photographs showing methyl orange removal from the H-gel upon irradiation at 365 nm and induced syneresis. (c) UV-vis spectra of methyl orange and the expelled water from (b). (d) Maximum concentrations of different dyes, which can be removed completely during syneresis of the hydrogel (1.15%). All measurements were carried out at room temperature. Republished from [85] with permission of The Royal Society of Chemistry.

Figure 22

Molecular structures of PBA, PDA and KS.

Figure 23

(a) Excitation and (b) emission spectra of a 5 wt% DODA in 1-octanol gel at 25 °C (solid line, λ_em = 458 nm, λ_ex = 400 nm), sol at 45 °C (dashed line, λ_em = 481 nm, λ_ex = 425 nm), and melt-solidified DODA at 25 °C (dotted line, λ_em = 458 nm, λ_ex = 400 nm). (c) Dependence of the excitation wavenumber maxima on the inter-carbonyl dihedral angle of a series of α-diketo containing molecules at 25 °C in solution (solid squares; republished from [90] with permission of The Royal Society of Chemistry); a 5 wt% DODA in 1-octanol gel at 25 °C (open circle); a 5 wt % DODA in 1-octanol sol at 45 °C (open triangle). The range of multiple excitation maxima of the 5 wt % DODA in 1-octanol sol is indicated by the curve between the two open triangles. (d) excitation, and (e) emission spectra at 25 °C of gels of 5 wt % DODA in 1-octanol after incubating the sols at 0 °C (dotted line, λ_em = 458 nm, λ_ex = 400 nm), 30 °C (solid line, λ_em = 458 nm, λ_ex = 400 nm), and 45 °C (dashed line, λ_em = 481 nm, λ_ex = 425 nm). Reproduced with permission from [44]. Copyright John Wiley and Sons.

Figure 24

Structures of salts examined as metallo gelators derived from DODA. Republished from [91] with permission of The Royal Society of Chemistry.

Figure 25

(a) 2D packing diagram of DODA viewed perpendicular to the polymethylene chains; the red dashed lines indicate the positions of H-bonds; (b) a 2D packing diagram of DODA viewed along the axis of the polymethylene chains; the blue dashed lines indicate dipole–dipole interactions. Reproduced from [44] with permission. Copyright John Wiley and Sons. (c) Schematic representation of proposed molecular packing arrangements and proposed mechanisms for mechano-destruction and reformation of self-assembled fibrillar networks in the DODA-Ni·H₂O gels. Republished from [91] with permission of The Royal Society of Chemistry.

Figure 26

(a) Structure of TPE-Cho_n (n = 1,4,5,6). (b) UV−vis absorption spectrum of 1 × 10⁻⁴ M TPE-Cho₅ in dichloromethane. (c) Emission spectra (λ_ex = 355 nm) of 1 × 10⁻⁴ M TPE-Cho₅ in different acetone:water mixtures. (d) Images under 365 nm radiation of TPE-Cho₅ in acetone:water mixtures, with different water fractions, and SEM images of the aggregates at water contents equal to (e) 30%, (f) 90% and (g) 99%. Reproduced from [92] with permission. Copyright Wiley-VCH GmbH.

Figure 27

Molecular structures of CAC, CAMC and CAB.

Figure 28

(a) Molecular structure of CNC; (b) Avrami plot of fluorescence data according to the Avrami equation (slope = 1.08 (R² = 1.00)). Inset: Plot of emission intensity (λ_em = 375 nm; λ_ex = 318 nm) from a 1.0 wt % CNC in n-octane sample incubated at 1.1 °C. Reprinted (adapted) with permission from [95]. Copyright (2005) American Chemical Society.

Figure 29

Molecular structure of PyPheAla.

Figure 30

Molecular structure of C_nOBC and Fmoc-AA-Car.