An excimer to exciplex transition through realization of donor–acceptor interactions in luminescent solvent-free liquids

Vivek Chandrakant Wakchaure ab, Goudappagouda ab, Tamal Das bc, Sapna Ravindranathan bd and Sukumaran Santhosh Babu *ab
aOrganic Chemistry Division, National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune-411008, India. E-mail: sb.sukumaran@ncl.res.in
bAcademy of Scientific and Innovative Research (AcSIR), Ghaziabad-201 002, India
cPhysical and Materials Chemistry Division, National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune-411008, India
dCentral NMR Facility, National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune-411008, India

Received 8th April 2021 , Accepted 26th May 2021

First published on 27th May 2021


Luminescent solvent-free organic liquids are known for their enhanced quantum yield, color tunability, and availability of a matrix for other dopants to generate hybrid luminescent materials with improved features for newer applications. Herein, we report a donor–acceptor based luminescent “exciplex liquid” by utilizing the slightly different electron affinity of the acceptor molecules. A red-shifted broad exciplex emission exhibited by the donor–acceptor pair even at a lower concentration of the acceptor (0.001 equiv.) indicates high efficiency in the solvent-free state. A detailed NMR study revealed weak intermolecular interactions between the donor and acceptor in the solvent-free matrix that stabilizes the exciplex liquid. The failure of structurally similar solid counterparts to form an exciplex confirms the advantage of the available supportive liquid matrix. Besides, the luminescent exciplex liquid is found efficient in sensing application, which is unachievable by either the individual liquids or their solid counterparts. Here, a transition of a donor–acceptor pair from a solid to solvent-free liquid results in a new hybrid liquid that can be an alternative for solid sensor materials.


Solvent-free room temperature (RT) organic liquids are a relatively new area of research, which potentially generated impact in the areas of dye-sensitized solar cells, light-emitting diodes, optical lasers, and sensors, and as photon upconversion liquids, porous liquids, luminescent/phosphorescent liquids, etc.1–6 As an advantage, the presence of a liquid medium not only improves the processability but also enables doping with other molecules for tunable optical and/or optoelectronic properties also.7–9 However, doping with other components remains challenging to impart synergism; instead, it mostly leads to phase separation, and hence the concept of hybrid liquids remains less explored. To achieve the complete benefit of doping, new attempts such as liquid–liquid mixing are found to be successful.10–13 A recent report from our group demonstrated a highly efficient and stable charge transfer liquid (CTL) formed even at a donor–acceptor ratio of 1000[thin space (1/6-em)]:[thin space (1/6-em)]1.12 The advantages of a nonpolar liquid medium are beneficial for developing a CTL, and this method can emerge as a general strategy to prepare hybrid liquids by doping with compatible materials. In this work, we demonstrate the advantages of a transition from solid or solution-based materials to solvent-free liquids for achieving extraordinary optical features, which are otherwise unachievable using the former ones (Fig. 1a).
image file: d1nr02190g-f1.tif
Fig. 1 (a) Schematic of the physical interaction between the donor and acceptor molecules in the neat solid and solvent-free liquid states. (b) Chemical structures of D1, D2, N1, and N2 along with the corresponding photographs under visible light. Normalized (c) absorption and (d) emission of D1 and N1 in CH2Cl2 (solid line) and liquid state (dotted line) (λex = 350 nm).

Herein, we study the donor–acceptor interactions between dialkoxynaphthalene (DAN) and naphthalenemonoimide (NMI) derivatives in the solvent-free liquid state (Fig. 1b). The synthesis of both DAN and NMI derivatives is shown in Scheme S1. All compounds were obtained in good yields and were unambiguously characterized by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, and high-resolution mass spectrometry and their purity was confirmed by high performance liquid chromatography (HPLC). As discussed in our earlier report,12D1 is a colourless free-flowing liquid at RT (30 °C) and exhibits a glass transition temperature (Tg, offset) around −60 °C. However, N1 is a low melting solid with a melting point (Tm) around 42 °C (Fig. S1). The rheology experiments revealed that both D1 and N1 exhibit a viscous to elastic transition from low to high angular frequency (0.1 to 200 rad s−1) with a modulus intersection point Gi in the intermediate frequency region of ∼20 rad s−1 (Fig. S2). An increase of frequency gradually transforms the solvent-free liquid to a semi-solid material indicating the characteristics of shear-hardening (Fig. S2). In thermogravimetric analysis (TGA), 5% weight loss at 234 and 283 °C was noticed for D1 and N1, respectively (Fig. S3). Absorption spectra indicate that all the derivatives absorb in a range of 300 to 400 nm both in solution and in the solvent-free liquid state (Fig. 1c and S4). Donor molecules D1 and D2 show an enhanced deep blue emission ranging from 350 to 450 nm (Fig. 1d and S5). However, both N1 and N2 show a broad emission from 400 to 600 nm in solution, solvent-free liquid (N1) and thin-films (N2). Interestingly, the red-shifted emission (∼50 nm) for both N1 and N2 is due to the formation of an excimer, which is further confirmed by the excitation spectrum and emission lifetime decay profile (Fig. 1d and S6, S7). In comparison, the lifetime of N1 is 2.88 ns (28.9%) in solution and 17.56 ns (9.48%) in solvent-free liquid, while for N2, the lifetimes varied as 2.99 ns (35.5%) and 26.97 ns (21.77%) in solution and thin-films, respectively (Tables S1 and S2).

Our next attempt was to study the optical properties to understand the effect of guest molecules on the ground and excited-state features of the donor–acceptor hybrid liquid N1[thin space (1/6-em)]:[thin space (1/6-em)]D1. An increasing amount of N1 resulted in a charge transfer absorption band around 450 nm in dichloromethane (polar solvent) and n-hexane (nonpolar solvent) for N1[thin space (1/6-em)]:[thin space (1/6-em)]D1 (Fig. S8). At the same time, the donor emission was quenched entirely along with a decrease in the emission lifetime upon increasing the acceptor equivalence (Fig. S9 and Table S3). In contrast, an aggregation-induced exciplex emission was exhibited by D1[thin space (1/6-em)]:[thin space (1/6-em)]N1 (1[thin space (1/6-em)]:[thin space (1/6-em)]1) in an acetonitrile[thin space (1/6-em)]:[thin space (1/6-em)]water mixture (Fig. S10). A steady decrease in emission intensity (375 nm) and lifetime of the donor followed by an increase in emission intensity and lifetime of the exciplex emission peak at 530 nm was observed with increasing water content (Fig. S10 and Tables S4, S5). Surprisingly, no charge transfer band was formed when the absorbance of D1 and N1 in the solvent-free liquid state was monitored. However, a slight broadening of the absorption spectra between 380 and 450 nm due to an increasing amount of the acceptor was noticed (Fig. S11). The drastically different optical properties of D1[thin space (1/6-em)]:[thin space (1/6-em)]N1 under solution and solvent-free conditions point to the impact of the solvent and its polarity in the former case. Hence, an understanding of the optical properties in the absence of solvents, i.e., a solvent-free liquid state, is highly required.

When the deep blue emissive D1 was mixed with bluish-green emissive N1, the emission spectrum of the resulting mixture showed a red-shifted broad peak (Fig. 2a). The emission spectrum of D1 with λmax = 375 nm underwent an immediate decrease in intensity on varying the D[thin space (1/6-em)]:[thin space (1/6-em)]A ratio with a concomitant new red-shifted broad emission peak centered at around 530 nm (Fig. 2a). Furthermore, this red-shifted emission is confirmed as the exciplex emission from the hybrid liquid by the excitation spectrum and emission lifetime (Fig. S12–S14 and Tables S6, S7). Hence, we address the hybrid liquid D1[thin space (1/6-em)]:[thin space (1/6-em)]N1 as the exciplex liquid (EL). We noticed that the exciplex peak started to appear even with 0.001 equivalents of the acceptor (Fig. S15a). A clear transition from excimer to exciplex formation was seen when we recorded the emission spectra by varying the donor D1 in the acceptor N1 (Fig. S15b). The observation of EL emission at such a low loading of N1 was very much surprising and hence demanded a more detailed investigation. A further increase in the acceptor loading enhanced the exciplex emission intensity along with the corresponding lifetime decay component (Fig. S14 and Table S8). Surprisingly, when the solid derivatives D2 and N2 were mixed in neat and in solution, an emission quenching of D2 with an increase in the amount of N2 along with an enhanced emission of the acceptor N2 was noticed (Fig. 2b and S16, S17). The photographs of EL and D2[thin space (1/6-em)]:[thin space (1/6-em)]N2 under UV light (365 nm) demonstrate the difference in their optical properties (Fig. 2c and d). EL has been used as paint to coat on filter paper strips and glass (10 × 10 cm) with no change in optical features to show the practical utility of EL (Fig. 2e and f and S17). The uniform distribution of donor and acceptor molecules in the hybrid liquid, and its stability are confirmed by DSC and TGA analyses and temperature-dependent emission measurements (Fig. 2g and h and S18).


image file: d1nr02190g-f2.tif
Fig. 2 (a) Variation of steady-state emission of D1 with an increasing equivalence of N1 in the solvent-free liquid and (b) variation of steady-state emission of D2 with a gradual increase in the equivalence of N2 in the solid-state. Photographs of (c) D1, N1, and D1[thin space (1/6-em)]:[thin space (1/6-em)]N1 (1[thin space (1/6-em)]:[thin space (1/6-em)]1) in the solvent-free liquid state and (d) D2, N2, and D2[thin space (1/6-em)]:[thin space (1/6-em)]N2 (1[thin space (1/6-em)]:[thin space (1/6-em)]1) in the solid-state under UV light (λex = 365 nm). Photographs of (e) D1[thin space (1/6-em)]:[thin space (1/6-em)]N1 (1[thin space (1/6-em)]:[thin space (1/6-em)]1) coated on Whatman filter paper strips from dichloromethane solution (1 mM) and (f) a large-area coating of D1[thin space (1/6-em)]:[thin space (1/6-em)]N1 (1[thin space (1/6-em)]:[thin space (1/6-em)]1) on glass under UV light (365 nm). Comparison of (g) DSC and (h) TGA of D1, N1, and D1[thin space (1/6-em)]:[thin space (1/6-em)]N1 (1[thin space (1/6-em)]:[thin space (1/6-em)]1) in the solvent-free liquid state.

NMR studies were carried out for a detailed understanding of the molecular level interaction between D1 and N1 in EL. 1H NMR shows a considerable upfield shift of the aromatic signals for both D1 and N1 in the solvent-free state with respect to their solution state spectra (Fig. S19). Hence, we monitored the variation of the 1H NMR signals of D1 on increasing the equivalents of N1 (Fig. 3a). All aromatic protons of acceptor N1 showed an upfield shift in the presence of donor D1, indicating π–π stacking interactions between D1 and N1. Acceptor proton signals approached the peak position of pure N1 with an increasing ratio of N1 to D1, while the corresponding chemical shift changes were induced in the donor protons as well. The –OCH2 (Hd) proton signal of D1 undergoes a downfield shift when N1 is added. A maximum downfield shift of 0.3 ppm is estimated for the Hd proton with respect to the signal position in the “neat” donor based on the analysis of chemical shift variation as a function of acceptor concentration. We analysed the chemical shift variation of Hd to estimate an “interaction constant”, K = 0.61 ML−1, and a significantly lower value than that of the CTL indicates the comparatively weaker donor–acceptor interaction in EL (Fig. S20).12


image file: d1nr02190g-f3.tif
Fig. 3 (a) Chemical structures of D1 and N1 with labelled protons and shift of the 1H NMR signals of D1 with increasing equivalents of N1 (0.03 to 1.45) at 318 K. 2D-NMR (b) NOESY and (c) ROESY spectra at 100 and 250 ms mixing time, respectively, showing intermolecular cross-peaks in the EL. (d) DFT optimized dimer of D1[thin space (1/6-em)]:[thin space (1/6-em)]N1.

2D nuclear Overhauser effect spectroscopy (NOESY) and rotating frame Overhauser effect spectroscopy (ROESY) experiments were carried out for a detailed understanding of D1N1 interactions in EL (D1[thin space (1/6-em)]:[thin space (1/6-em)]N1 ratio 1[thin space (1/6-em)]:[thin space (1/6-em)]1) at 318 K (Fig. 3b and c). NOESY and ROESY spectra recorded with 100 and 250 ms mixing times, respectively, show intra- and intermolecular cross-peaks, and the latter indicates the close spatial proximity of D1 and N1. The intermolecular cross-peaks in ROESY are free from spin diffusion effects, thus unambiguously verifying that D1 and N1 approach each other closely within 5 Å. From an analysis of the intermolecular cross-peak intensities in the ROESY spectrum, the estimated aromatic ring separation between D1 and N1 is 4.0–4.5 Å on average. After correcting for spin-lock irradiation offset effects, the intermolecular cross peak intensities in the ROESY spectrum varied as H1/Ha: 2.8, H1/Hb: 3.1, H1/Hc: 3.3, H3/Hb: 2.7, H3/Hc: 2.1, and H4/Ha: 0.58, H4/Hb: 0.35, H4/Hc: 0.37, supporting a mutually stacked orientation for D1 and N1 in the EL. The cross-peaks involving Hb/Hc and N1 protons are of similar intensity, while that involving Ha is relatively different. The relative intensities of the cross-peaks between the different aromatic proton pairs suggest that the aromatic rings are not stacked face to face with the ring planes parallel to each other; however, a tilt of one aromatic plane relative to that of the other is involved. The possibility of a relative tilt in the aromatic ring planes of the D1N1 pairs is also supported by the downfield shift observed for the aromatic protons of D1 due to the local magnetic field augmenting ring current effects. In the neat liquid, the D1N1 pairs may further stack in a staggered configuration, with the long alkyl chain of N1 pointing in opposite directions to avoid crowding. The stacked dimers were visualized by density functional theory (DFT) optimization, and the results matched well with the 2D-NMR results (Fig. 3d and S20).

From the above discussion, it is clear that the lower equivalence of the acceptor molecule is efficient at forming an emissive EL with reasonably good stability (Fig. S15 and Table S9). Hence, we planned to use EL for the detection of explosives in direct contact mode. The detection and differentiation of trinitrotoluene (TNT) from other nitroaromatic compounds are very tedious due to the similarity in responses to fluorescence signals.14 Even though the currently available fluorescence-based sensors differentiate TNT from other nitroaromatic molecules, an exclusive distinction is still challenging.15 In a typical experiment, acetone solution of TNT (10 μL, 1 mM) or solid TNT (0.1 mg) is mixed with EL (10 mg), and in the case of TNT solution, the solvent is allowed to evaporate for 10 min. A visible colour change due to a strong interaction between EL and TNT is noticed (Fig. 4aand S21). Fig. 4b shows the corresponding variation in absorbance spectra when EL is mixed with various amounts of TNT. It has been found that the broad spectrum from 400 to 550 nm is due to CT between the EL and TNT. The visible color change of EL (10 mg in 0.5 cm2) from light yellow to orange was observed in the presence of TNT, and the intensity of color increased with an increasing amount of TNT from 1 to 1000 μg (Fig. 4c). The fluorescence of EL underwent a complete quenching under UV light excitation (365 nm) (Fig. 4d). A total of 90.8% intensity of EL was quenched in the presence of 1 wt% of TNT (Fig. 4e and S22). The fluorescence lifetime of EL exhibited a significant decrease in the presence of TNT (Fig. S23 and Tables S10, S11). The initial lifetimes of 1.63 and 15.42 ns decreased to 0.11, 7.71 ns, and 0.098, 3.11 ns in the presence of 1 and 10 weight % TNT, respectively. Such a drastic luminescence lifetime decrease in the presence of an acceptor indicates an efficient CT interaction between EL and TNT, which enables selective detection of TNT. Hence, EL can be directly used for dual-mode, colorimetric and fluorometric detection of TNT.


image file: d1nr02190g-f4.tif
Fig. 4 (a) Photographs of EL showing a visible colour change (top) and fluorescence quenching with TNT in the dark (bottom). (b) Variation of absorbance spectra and (c) the corresponding photograph of EL with various amounts of TNT. (d) Variation of steady-state emission intensity and (e) emission quenching % of EL with different quantities of TNT (λex = 350 nm) (3% error bar). (f) Photographs of EL in the presence of various acceptors and explosives showing the selectivity towards TNT.

Selectivity is significant in any sensing, especially in the case of nitroaromatic explosives. In most of the existing sensors the differentiation between TNT and DNT, even TNT and NT, has been rarely achieved.16 Hence to check the selectivity of EL towards TNT, we reviewed the response with different nitroaromatic and other nitro groups containing explosives and found that EL is highly selective towards TNT (Fig. 4f and S21–S25). As a control experiment, we checked the dual-mode sensing selectivity of nitroaromatics with D1 and N1 and noticed that the donor or acceptor alone is not selective in the sensing of TNT (Fig. S26 and S27). Besides, D2 or N2 individually and D2[thin space (1/6-em)]:[thin space (1/6-em)]N2 combinations also failed to exhibit any selectivity towards TNT (Fig. S28). By using DFT calculations, the HOMO–LUMO energy levels of DANs, NMIs, and corresponding D–A complexes (EL and D2[thin space (1/6-em)]:[thin space (1/6-em)]N2), as well as D–A complexes with TNT were calculated (Tables S12 and S13). A reduced HOMO–LUMO gap and binding energy showed that EL + TNT is a more stable complex than other combinations, thereby supporting selective dual-mode sensing (Table S14). The same selectivity and efficiency exhibited by EL coated paper strips and a large area coating point to the availability of multiple platforms of EL, which enhances its utility as an alternative sensor material (Fig. S29). The interaction between EL and TNT was examined by detailed NMR experiments (Fig. S30 and S31). NMR spectra recorded in the presence of TNT shows evidence of interactions between EL (D1[thin space (1/6-em)]:[thin space (1/6-em)]N1 = 1[thin space (1/6-em)]:[thin space (1/6-em)]0.5) and TNT (Fig. S30). In 1H spectra, the aromatic proton signals of D1 undergo an upfield shift, while those of N1 shift downfield in the presence of TNT. This implies that the mutual stacking of D1 and N1 is altered in the presence of TNT. ROESY and NOESY spectra reveal strong cross peaks between the aromatic protons of D1 and TNT but no cross peaks to the aromatic protons of N1 (Fig. S31). A comparison of ROESY spectra in the presence and absence of TNT clearly shows the weakening of D1N1 interactions in the presence of TNT (Fig. S31). The methyl protons of TNT also show ROESY cross peaks to D1 aromatic protons; however, these are of lower intensity compared to the cross peaks involving the aromatic protons of TNT. The only ROESY crosspeak connecting N1 to TNT is between the H4 proton and the aromatic proton of TNT (Fig. S31). Even though D1N1 interactions are weaker in the presence of TNT and not readily observed in ROESY spectra, NOESY spectra show cross peaks between N1 protons and the methyl protons of TNT, which could arise from spin diffusion effects. However, these are much weaker when compared to NOESY cross peaks between D1 protons and methyl protons of TNT (Fig. S31). This result indicates that TNT is located closer to the D1 in EL. The data indicate that in the presence of TNT, the relative orientations of the aromatic planes of D1 and N1 are probably staggered further apart and the TNT ring stacks close to the D1 aromatic ring. The reusability and sensitivity are confirmed by the sensing experiments using recovered EL by column chromatography purification.

In conclusion, an emissive exciplex liquid has been obtained by mixing two donor and acceptor solvent-free liquids. 2D-NMR studies have been carried out for a detailed understanding of intermolecular donor–acceptor interactions. Compared to that of the solution, the intermolecular interaction between the donor and acceptor molecules in the solvent-free state resulted in an emissive exciplex liquid with enhanced quantum yield and lifetime. A transition from the solid to solvent-free liquid state resulted in the exclusive formation of an exciplex liquid, which is unachievable by its solid counterparts. The exciplex liquid has been effectively used for the exclusive dual-mode, colorimetric and fluorometric sensing of TNT. We believe that our demonstration will urge the design of new organic liquid hybrids for various sensing applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

VCW and Goudappagouda acknowledge the University Grants Commission (UGC), India, for the fellowship. This work is supported by DRDO, ERIP/ER/DG-ACE/991115201/M/01/1676. Director, High Energy Materials Research Laboratory, Pune is acknowledged for explosive samples.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d1nr02190g

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