Two-dimensional structural ordering in a chromophoric ionic liquid for triplet energy migration-based photon upconversion

Shota Hisamitsu a, Nobuhiro Yanai *ab, Hironori Kouno a, Eisuke Magome c, Masaya Matsuki a, Teppei Yamada ab, Angelo Monguzzi d and Nobuo Kimizuka *a
aDepartment of Chemistry and Biochemistry, Graduate School of Engineering, Center for Molecular Systems (CMS), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan. E-mail:;
bJST-PRESTO, Honcho 4-1-8, Kawaguchi, Saitama 332-0012, Japan
cKyushu Synchrotron Light Research Center, 8-7 Yayoigaoka, Tosu, Saga, 841-0005, Japan
dDipartimento di Scienza dei Materiali, Università Milano Bicocca, via R. Cozzi 55, 20125, Milano, Italy

Received 13th September 2017 , Accepted 19th October 2017

First published on 3rd November 2017

A novel chromophoric ionic liquid (IL) with two-dimensional (2D) nanostructural order is developed, and its structure–property relationship is investigated by harnessing photon upconversion based on triplet energy migration. An ion pair of 9,10-diphenylanthracene-2-sulphonate (DPAS) and asymmetric quaternary phosphonium ion exhibited both ionic crystal (IC) and supercooled IL phases at room temperature. Single crystal X-ray analysis of the IC phase showed an alternate alignment of polar (ionic) and non-polar (non-ionic) layers, and this layered structure was basically maintained even in the IL phase. The diffusion length of triplet excitons in the IL phase, obtained by the analysis of upconverted emission in succession to triplet–triplet annihilation (TTA), is larger than the domain size estimated from powder X-ray analysis. This suggests that triplet excitons in chromophoric ILs can diffuse over the nanostructured domains.


Contrary to the conventional picture of homogeneous liquid, the presence of bicontinuous nanostructures in ionic liquids (ILs) has been found in the last decade.1,2 This was first unveiled by Lopes et al. through the molecular dynamics (MD) simulation of 1-alkyl-3-methylimidazolium-based ILs, where polar ionic groups and nonpolar alkyl side chains show a nano-segregated structure.2 Triolo and co-workers reported the first experimental evidence for nanoscale segregation by X-ray diffraction measurements of 1-alkyl-3-methylimidazolium-based ILs.3 These studies showed that the ILs have structural heterogeneities in the order of a few nanometers that correspond to the distance between segregated ionic domains. The understanding of the nano-segregated structure in ILs has been enriched by a variety of theoretical and analytical studies.1–4 Although their unique nanostructures have been utilized for reaction media and shockwave absorption,1,5 the manifestation of structure–function relationships has been largely unexplored.

ILs composed of π-conjugated chromophoric ions have great potential for the construction of photofunctional materials due to their inherent optical transparency and their wide variety of applications including photoluminescence,6 chemical sensors,7 photo-patterning,8 molecular solar thermal fuels9 and photon upconversion (UC).10 While the unique bicontinuous nanostructure of ILs would be advantageous to the long-ranged arrangement of chromophoric groups, the example of its utilization for better transport of excitons and electrons has been scarce. We have previously prepared an IL composed of a 9,10-diphenylanthracene-2-sulfonate anion (DPAS) and a trihexyl(tetradecyl)phosphonium cation (P66614), and observed efficient UC based on the effective triplet energy migration and annihilation among the contiguous ionic chromophore arrays.10 However, there is no information regarding the nanoscale structure of the chromophoric IL, which is obviously required to understand the relationship between the structural order and its excitonic properties.

Here we shed light on this issue and scrutinized the excitonic properties of chromophoric ILs in correlation with the nanoscale structural order. A novel ion pair consisting of DPAS and a highly asymmetric cation, triethyl(tetradecyl)phosphonium (P22214), is synthesized (Fig. 1a). Due to the non-spherical asymmetric cation structure with a long alkyl chain, P22214DPAS gave an ionic crystal (IC) phase with a layered structure. Interestingly, this two-dimensional ordering was largely retained even in the supercooled IL phase at room temperature (Fig. 1b). A triplet diffusion length in the IL phase, obtained by analyzing a triplet–triplet annihilation (TTA) process, was found to be much larger than the domain size estimated by X-ray diffraction studies. This indicates that triplet excitons can migrate beyond the structural domains of ILs. This unique triplet exciton migration behaviour highlights the promising potential of nanostructured chromophoric ILs as engineered and tailored systems for energy transport.

image file: c7cp06266d-f1.tif
Fig. 1 (a) Chemical structure of P22214DPAS. (b) A schematic illustration of the phase transition of P22214DPAS between the supercooled ionic liquid (IL) phase and the ionic crystal (IC) phase.

Experimental section

Synthesis of P22214Br

Bromotetradecane (TCI, 2.0 g, 7.2 mmol) and triethyl phosphine (WAKO chemicals, 20 wt% in ethanol, 4.0 g, 6.8 mmol) were mixed and stirred for 70 h. The residue was evaporated to remove ethanol. The colourless solid product (0.273 g, yield: 10.1%) was collected by using silica gel column chromatography.

1H NMR (300 MHz, methanol-d4): δ (ppm) = 0.90–0.94 (t, 3H), 1.21–1.84 (m, 33H), 2.26–2.33 (m, 8H). Elemental analysis, calculated for C46H44BrP: C, 60.75; H, 11.22; N, 0.00; found: C, 60.52; H, 10.96; N, 0.04.

Synthesis of P22214DPAS

P22214Br (0.127 g, 0.322 mmol) and NaDPAS (0.278 g, 0.642 mmol)10 were dissolved in biphasic water/chloroform and stirred for several hours. Then the organic phase was filtered using a membrane filter. The obtained filtrate was washed with water and dried under reduced pressure. The collected yellow powder was purified by activated alumina column chromatography, giving P22214DPAS (0.127 g, yield: 54.5%).

1H NMR (300 MHz, methanol-d4): δ (ppm) = 0.90–0.94 (t, 3H), 1.21–1.63 (m, 33H), 2.16–2.32 (m, 8H), 7.38–7.50 (m, 6H), 7.60–7.78 (m, 10H), 8.28 (s, 1H). Elemental analysis, calculated for C46H61O3PS: C, 76.21; H, 8.48; N, 0.00; found: C, 76.07; H, 8.47; N, 0.05.

Optical measurements

UV-vis absorption spectra were recorded on JASCO V-670 and V-770 spectrophotometers. Luminescence spectra were measured by using a PerkinElmer LS 55 fluorescence spectrometer. Emission from samples was detected from a perpendicular angle to the excitation light. Neat samples were sandwiched between two quartz plates. Time-resolved fluorescence lifetime measurements were carried out by using a time-correlated single photon counting lifetime spectroscopy system, HAMAMATSU Quantaurus-Tau C11367-02 (for fluorescence lifetime)/C11567-01(for delayed luminescence lifetime). The quality of the fit has been judged by the fitting parameters such as χ2 (<1.2) as well as the visual inspection of the residuals. For fluorescence lifetime analysis, the excitation wavelength was set as 365 nm and detection wavelengths were 440 nm for the solution sample and 450 nm for neat samples. For TTA-UC emission lifetime analysis, excitation and detection wavelengths were set at 531 nm and 450 nm, respectively. The absolute fluorescence quantum yields were measured using a Hamamatsu C9920-02G instrument with a Xe excitation source and a monochromator, Hamamatsu A10080-01.

Samples for TTA-UC measurements were prepared by dissolving a triplet sensitizer (platinum octaethylporphyrin (PtOEP), Aldrich) into P22214DPAS, with the procedure described below. First, a CH2Cl2 solution of P22214DPAS and PtOEP was prepared with a DPAS[thin space (1/6-em)]:[thin space (1/6-em)]PtOEP molar ratio of 10[thin space (1/6-em)]000[thin space (1/6-em)]:[thin space (1/6-em)]1 or 1000[thin space (1/6-em)]:[thin space (1/6-em)]1. Dichloromethane was completely dried by using a rotary evaporator, and then samples were freeze-dried under vacuum conditions. They were further kept under vacuum for over 3 h at above 115 °C to complete crystallization. Then, the samples were sealed into quartz cells (optical path length: 0.05 mm) in an Ar-filled glove box (concentration of H2O and O2 < 0.1 ppm). The supercooled IL sample was prepared by heating at above 140 °C and cooling down to room temperature. For TTA-UC emission measurements, a diode laser (532 nm, 200 mW, RGB Photonics) was used as the excitation source. The laser power was controlled by combining the software (Ltune) and a variable neutral density filter, and measured using a PD300-UV photodiode sensor (OPHIR Photonics). The laser beam was focused on a sample using a lens. The diameter of laser beam (1/e2) was measured at the sample position using a CCD beam profiler SP620 (OPHIR Photonics). The typical laser spot area estimated by using the diameter was 1.26 × 10−3 cm2. The emitted light was collimated by an achromatic lens, the excitation light was removed using a notch filter (532 nm), and the emitted light was again focused by an achromatic lens to an optical fiber connected to a multichannel detector MCPD-9800 (Otsuka Electronics). Donor phosphorescence quantum yields were measured by using an absolute quantum yield measurement system. The sample was held in an integrating sphere and excited by the laser excitation source (532 nm, 200 mW, RGB Photonics). The scattered excitation light was removed using a 532 nm notch filter, and emitted light was monitored using a multichannel detector C10027-01 (Hamamatsu Photonics). The spectrometer was calibrated including the integration sphere and the notch filter by Hamamatsu Photonics.

In general, a quantum yield is defined as the ratio of absorbed photons to emitted photons, and thus the maximum yield (ΦUC) of the bimolecular TTA-UC process is 50%. However, many reports multiply this value by 2 to set the maximum quantum yield at 100%. To avoid the confusion between these different definitions, the UC quantum yield is written as ΦUC′ (= 2ΦUC) when the maximum efficiency is normalized to be 100%. The ΦUC′ of PtOEP-doped IL P22214DPAS was determined relative to a standard (Nile red in the IL P22214DPAS) according to the following equation:11

image file: c7cp06266d-t1.tif(1)
where Φ, A, F, I, and η represent quantum yield, absorbance at the excitation wavelength, integrated photoluminescence spectral profile, excitation intensity, and the refractive index of the IL, respectively. The absolute fluorescence quantum yield of the standard, Nile red in the IL P22214DPAS (Nile red/P22214DPAS = 0.01 mol%), was determined as 0.56.

Other characterizations

Elemental analysis was conducted at the Elemental Analysis Center, Kyushu University. Differential scanning calorimetry (DSC) traces were obtained by using a DSC1 STARe system (METTLER TOLEDO) under a N2 atmosphere. Before performing measurements, samples were completely dried under vacuum for at least 3 h at 115 °C. The scanning rate was 2 K min−1. Single crystal X-ray data were collected on a CCD diffractometer (Rigaku Saturn VariMax) with graphite-monochromated Mo Kα radiation (λ = 0.71070 Å).

Synchrotron PXRD was performed at BL15 beamline, Kyushu Synchrotron Light Research Center. Incident X-ray energy was adjusted to 15 keV, and diffraction data were obtained using the Debye–Scherrer method with imaging plates. Samples were sealed in capillaries made of Kapton or glass and kept in a He atmosphere to suppress background scattering. Domain sizes in IL and IC samples were estimated by using Scherrer's equation, D = /((βγ)cos[thin space (1/6-em)]θ), where K is Scherrer's constant (0.94 with an assumption of cubic crystallites), λ is the wavelength of X-rays (0.8265 Å), β is full-width at half maximum (FWHM) of the diffraction peak, γ is the register constant for FWHM estimated using crystalline Si and θ is the peak position. FWHM of diffraction peaks was estimated using a split-Pearson VII function. The contribution of the measurement systems upon peak broadening was estimated by measuring Si powder with a known domain size in the capillary.

Results and discussion

A novel fluorescent IL P22214DPAS was synthesized by the ion-exchange between P22214Br and NaDPAS. P22214Br was synthesized by quaternization reaction of triethylphosphine with bromotetradecane. NaDPAS was synthesized according to the previously reported method.10 P22214Br and NaDPAS were mixed into biphasic water/chloroform, and the hydrophobic P22214DPAS was extracted to the organic phase. The organic phase was washed with water and dried under reduced pressure. Subsequent column chromatography gave P22214DPAS. The purity of P22214DPAS was confirmed by elemental analysis and 1H NMR measurements. While P22214DPAS was obtained as a liquid, it underwent crystallization after standing for several days at room temperature (Fig. 2a). A similar behaviour is often observed for supercooled IL materials due to entropic and kinetic reasons. Long alkyl chains assume various conformations and high viscosity of ILs stabilizes the metastable supercooled IL phase.12 The rate of crystallization can be increased by applying mechanical stimuli or by thermal energy to decrease their viscosity. In the case of P22214DPAS, after standing for several hours (>3 h) at 115 °C, the liquid sample underwent complete solidification.
image file: c7cp06266d-f2.tif
Fig. 2 (a) Photographs and polarized microscopic images of P22214DPAS in the supercooled IL phase (left) and the IC phase (right). The microscopic image of the IL was taken at room temperature after heating at 160 °C and keeping at 25 °C for 2 h. (b) DSC trace of P22214DPAS under a N2 atmosphere. Scanning rate was 2 K min−1. P22214DPAS in the IC state was used as the initial sample.

The phase behaviour of P22214DPAS was monitored by DSC measurements (Fig. 2b). The IC phase of P22214DPAS was used as the initial sample. During the heating process in the first cycle, an endothermic peak was observed at 141 °C, which is identified as a melting point of the IC phase. During the subsequent cooling process, no exothermic peaks due to crystallization were observed. Instead, changes in baseline were observed in the succeeding cooling and heating process at around 15 °C, which is therefore ascribed to glass transition. Another endothermic peak was repeatedly observed at around 90 °C only in the heating process. The origin of this transition is not clear and probably related to a further structural relaxation of the IL phase. These results indicate that cooling of the IL P22214DPAS state gives a supercooled IL phase which is stable at ambient temperature.

The melting behaviour of IC phase was also observed under a polarizing microscope. The birefringence observed for the IC sample (Fig. 2a) completely disappeared by heating to 146 °C. Upon subsequent cooling to room temperature, the birefringence did not recover from the optically isotropic supercooled IL phase (Fig. 2a). Further cooling below the glass transition temperature to 0 °C gave the glassy state without any birefringence (Fig. S1, ESI). In the following experiments in the IL phase of P22214DPAS, the supercooled IL was prepared by heating the sample above its melting point for enough time for complete liquefaction.

To obtain insights into the IL structure, we conducted a single crystal structural analysis of the IC sample and powder X-ray diffraction (PXRD) measurements for the IC and IL samples. Fig. 3a shows a single crystal structure of IC P22214DPAS.13 The space group was assigned to the P21/c monoclinic system. The IC phase shows a lamellar structure consisting of ionic and non-ionic layers. The distance between two ionic layers of P22214DPAS is 23.1 Å. In the ionic layers, Coulomb interactions operate between the cationic phosphonium and anionic sulfonate moieties, while long-tail alkyl chains and DPA moieties are packed in the non-ionic layers through van der Waals and CH–π interactions. This lamellar structure is reminiscent of the previously reported 1-alkyl-3-methylimidazolium-based ICs.14 In solid ion pairs having long alkyl-chained cations, the lamellar structure is considered to be enthalpically more stable than the amorphous structures where all ionic species are present without translational periodicity. In our system, although the anion contains a bulky chromophore group, a similar layered structure was obtained. The centre-to-centre intermolecular distance between anthracene units along the b axis is 9.2 Å, which is much larger than that in the single crystalline anthracene and its derivatives.15 It is to note that the long alkyl chains between DPA units prevent the strong π–π stacking (Fig. 3a, inset). The shortest distance between two DPA units is 4.3 Å, but there is no π stacking in between. Consequently, the DPA moieties seem to have no strong inter-chromophore interactions in the IC phase, as further demonstrated by the optical absorption data discussed below.

image file: c7cp06266d-f3.tif
Fig. 3 (a) Crystal structure of IC P22214DPAS viewed along the b-axis, showing a lamellar structure consisting of ionic and non-ionic layers. Inset: the structure viewed along the c-axis to see a long tail alkyl chain sandwiched between two anthracene rings. P, orange; S, yellow; O, red; C, grey. Hydrogen atoms are omitted for clarity. (b) PXRD patterns of P22214DPAS in the supercooled IL phase (red) and the IC phase (blue) at room temperature. A simulated pattern from the single crystal structure of P22214DPAS is also shown (orange).

PXRD measurements were performed on both of IC and IL phases of P22214DPAS using a synchrotron X-ray source (Fig. 3b). The PXRD pattern of the IC phase showed a good match with the pattern simulated from the crystal structure (Fig. 3b). A sharp (100) peak at 2.08° corresponds to the distance between the ionic layers. Significantly, such a low-angle peak was also clearly observed at 1.63° in the IL phase, indicating the presence of ordered arrangement of ionic layers. In addition, the broad peaks around 5° and 10° of the IL phase seem to reflect the peaks of the IC phase. These results indicate that the nanosegregated layer structure is maintained even in the IL phase. The inter-layer distance was larger for the IL phase (29.1 Å) than the IC phase (23.1 Å), probably due to volume expansion by disordering of long alkyl chains in cations.16 We additionally carried out the PXRD measurement for a trihexyl(tetradecyl)phosphonium (P66614)-based IL (P66614DPAS) with longer alkyl chains (Fig. S2, ESI).10 The IL P66614DPAS also showed a peak at 2.6° (18.2 Å). To our interest, in spite of the longer trihexyl chains, the peak of the P66614DPAS IL appeared at a wider angle than that of the P22214DPAS IL. This comparison supports that the more asymmetrically shaped P22214 cation results in the formation of anisotropic layered structure. We suppose that the P66614DPAS IL forms a nano-segregated structure directed by ionic chain formation with the aid of limited miscibility between the ionic and non-ionic layers, which will be investigated by future MD simulations.

Diameters of structured domains in P22214DPAS were estimated using Scherrer's equation by using full width at half maximum (FWHM) of the peaks at 2.08° of the IC phase and at 1.63° of the IL phase. The domain size was estimated as 418.9 nm and 9.3 nm for the IC and IL phases, respectively. While small in domain size, it is worth pointing out that there are well-ordered nanostructures in transparent and macroscopically-isotropic liquid.

In order to obtain insight into the inter-chromophore arrangements, we performed absorption and fluorescence spectroscopy studies. A 0.1 mM methanol solution of P22214DPAS showed π–π* absorption peaks at 357, 374, and 394 nm (Fig. 4). These peaks showed slight red shifts in IL (358 nm, 376 nm, and 397 nm) and IC (360 nm, 376 nm, and 396 nm) phases. The observed slight red shift implies the presence of weak J-like inter-chromophore interactions between transition moments along the short axis of the anthracene units in the neat IL and IC states.17 The intensity of the 0–0 vibronic band at 397 nm was slightly higher for the IC phase than the IL phase due to the restricted molecular vibration in crystals. The peak positions are almost the same for both phases, which suggests the similarity in the molecular environment around DPAS units between these two phases.

image file: c7cp06266d-f4.tif
Fig. 4 UV-vis absorption spectra (solid lines) and emission spectra (broken lines) of P22214DPAS in 0.1 mM methanol solution (black), IL phase (red), and IC phase (blue).

The presence of inter-chromophore interactions in the IL and IC phases was also supported by red-shifted fluorescence bands as compared with that of the methanol solution (Fig. 4). The IL phase showed a larger red shift (458 nm) than the IC phase (454 nm), suggesting the presence of emission sites with stronger intermolecular interactions. The emission spectra did not show any excitation wavelength dependence, indicating that all excited energy is transferred to the lower-energy aggregating sites (Fig. S3, ESI). Considering that there was almost no difference in the position of absorption bands between the IL and IC phases, such fluorescence may have emitted from the strongly interacted sites which rather present as minor species in the IL state. Singlet energy migration and transfer to such strongly interacting trap sites would be possible in the chromophoric IL.

The presence of inter-chromophore interactions in the IL and IC phases was also supported by fluorescence lifetime and quantum yield (ΦFL) measurements. The time-resolved fluorescence measurement of P22214DPAS in methanol (0.01 mM) showed a single-exponential decay with a lifetime of 7.3 ns. Meanwhile, the decays of IC and IL phases can be fitted with two components of 3.4 ns (19.9%) and 10.5 ns (80.1%) and of 3.0 ns (18.7%) and 12.2 ns (81.3%), respectively (Fig. S4, ESI). Averaged lifetimes were 9.1 ns and 10.5 ns for IC and IL phases, respectively. The ΦFL value in the methanol solution was 70%, and those in IC and IL phases showed slightly lower values of 69% and 65%, respectively, showing the minor effect of aggregation-caused quenching. Although the IL state showed the red-shifted emission from strongly interacting sites, these seem to scarcely work as quenching sites of singlet exciton because of less decrease of the quantum yield of the IL.

To investigate the triplet exciton diffusion in P22214DPAS, the delayed fluorescence through triplet–triplet annihilation-based photon upconversion (TTA-UC, Fig. S5, ESI)10,11,18,19 was measured. In TTA-UC, acceptor (DPAS) triplet excitons, sensitized by a photo-excited triplet donor (PtOEP), diffuse and collide, followed by TTA and delayed fluorescence from acceptor excited singlets. By analyzing the TTA-UC emission, it is possible to obtain the information of triplet exciton dynamics.19,20 Samples for TTA-UC measurements were made by doping 0.01 mol% of the triplet sensitizer PtOEP into the IL and IC materials (see the Experimental section for details). To evaluate the dispersibility of the donor, the Q-band absorption of PtOEP in the samples was compared to that of PtOEP in chloroform solution and bulk solid (Fig. S6, ESI). In solution, PtOEP showed a sharp absorption peak at 536 nm, whereas bulk PtOEP showed a broadened and red-shifted absorption band at around 552 nm due to aggregation. In the IL and IC of P22214DPAS, the PtOEP molecules showed absorption bands at 536 nm without peak broadening, indicating complete dispersion of the donor into the IL and IC samples.

To estimate the donor-to-acceptor triplet–triplet energy transfer (TTET) efficiency ΦTTET, we compared the PtOEP phosphorescence quantum yields between PtOEP in P22214DPAS and PtOEP in a control IC trietyl(tetradecyl)phosphonium bromide (P22214Br) that possesses no triplet acceptor unit. No aggregation of PtOEP in P22214Br was confirmed by absorption measurements (Fig. S7, ESI). The phosphorescence quantum yields of PtOEP in P22214DPAS IL, P22214DPAS IC, and P22214Br IC were 0.8%, 1.0%, and 28%, respectively. The ΦTTET values were estimated by using the following equation,

image file: c7cp06266d-t2.tif(2)
where ΦP and ΦP0 represent the phosphorescence quantum yields of PtOEP in P22214DPAS and P22214Br, respectively. We obtained high ΦTTET values of 97% and 96% for PtOEP in the IL and IC phases of P22214DPAS, respectively. As PtOEP is a non-ionic molecule, it is natural to assume that they are preferentially located in the non-ionic portion of P22214DPAS. The observed high ΦTTET is explainable by efficient triplet energy transfer from PtOEP in the nonionic layer to the neighbouring DPAS through the electron-exchange Dexter mechanism. In our previous study, there was no information about the location of sensitizer molecules in IL P66614DPAS.10 The current findings on the presence of order and preferred location of triplet donors in the chromophoric ILs provide important guidelines for the rational design of IL materials showing TTA-UC.

Under excitation at λex = 532 nm, both of the PtOEP-doped IC and IL samples showed clear TTA-UC emissions at 450–470 nm (Fig. 5a). The triplet lifetime (τT) of P22214DPAS in the IL and IC phases was estimated as 1.9 ms and 5.0 ms, respectively, from the UC emission decays (Fig. S8, ESI). Although the obtained τT value in the IL P22214DPAS is on the order of ms and thus sufficiently long for achieving TTA-UC, it was slightly shorter than that observed for the PtOEP-doped IL P66614DPAS (τT, 2.5 ms).10 This is probably because of the strongly interacting DPAS units in the P22214DPAS IL that increased the probability of forming quenching centres of triplet exciton. This consideration is supported by the observed larger fluorescence red-shift for P22214DPAS (458 nm) as compared to that of P66614DPAS (454 nm), and by the slightly lower ΦFL of the IL P22214DPAS (65%) than that in the P66614DPAS (70%). These also give grounds for the lower ΦUC′ value observed for PtOEP-doped P22214DPAS (0.42%) as compared with that for PtOEP-doped P66614DPAS (11.2%). These differences may stem from the higher local concentration of DPAS units in P22214DPAS with shorter-chained triethyl groups. This comparison suggests the importance to finely tune the local chromophore concentration for balancing high UC efficiency and effective triplet energy migration.

image file: c7cp06266d-f5.tif
Fig. 5 (a) TTA-UC emission spectra of P22214DPAS doped with 0.01 mol% PtOEP in the IL (red) and IC (blue) phases. The excitation wavelength was 532 nm and the scattered excitation light was cut by a notch filter. (b) Double logarithm plots of TTA-UC emission intensity against incident laser intensity for the IL (red) and IC (blue) doped with 0.01 mol% PtOEP.

Fig. 5b shows double logarithm plots of TTA-UC emission intensity against incident laser intensity. Generally, TTA-UC systems show that the slope changes from 2 to 1. A key parameter Ith is determined as the cross section of fitting lines in the quadratic and linear regimes, where the quantum efficiency of TTA (ΦTTA) becomes 50%.21 The Ith values of 308 mW cm−2 and 23 mW cm−2 were observed for PtOEP-doped IL and IC of P22214DPAS.

To estimate the triplet diffusion length, the Ith value is described for a three-dimensional diffusion system by the following equation:19

Ith = (αΦTTETDTa0)−1(τT)−2(3)
where α is the absorption coefficient at the excitation wavelength, DT is the diffusion constant of acceptor triplet, and a0 is the annihilation distance between acceptor triplets (0.91 nm between two DPA molecules in the triplet excited state). According to this equation, the DT value in the IL P22214DPAS was estimated as 2.33 × 10−9 cm2 s−1. Determination of the exact DT value in the IC phase was difficult because the strong light scattering hampers the precise measurement of the absorption coefficient. The triplet diffusion length image file: c7cp06266d-t3.tif was estimated as 21.1 nm in the IL phase.19 Notably, this LD value is larger than the domain size obtained from the X-ray diffraction result (9.3 nm). This result indicates that long-lived triplet excitons can migrate beyond domains in the IL phase.

Finally, in order to demonstrate the advantage of chromophoric ILs as a structured soft material, we increased the concentration of PtOEP in P22214DPAS up to 0.1 mol%. In the IC phase, a broadened absorption band of PtOEP with a shoulder peak at around 550 nm was observed, indicating the formation of PtOEP aggregates (Fig. S9, ESI). In contrast, no aggregation of PtOEP was observed in the IL (Fig. 6a). Thanks to the higher ability to molecularly dissolve PtOEP, the prepared sample of the IL P22214DPAS with 0.1 mol% PtOEP showed a low Ith value of 31 mW cm−2 (Fig. 6b). Thus, the chromophoric ILs show order in the liquid phase but their soft and flexible nature allow us to disperse dopants uniformly in the IL phase even at high concentration without losing its nanostructures and their derived functions.

image file: c7cp06266d-f6.tif
Fig. 6 (a) UV-vis absorption spectra of a 0.1 mM chloroform solution PtOEP (black), bulk PtOEP solid (red), IL P22214DPAS with 0.01 mol% PtOEP (blue), and IL P22214DPAS with 0.1 mol% PtOEP (green). (b) Double logarithm plot of TTA-UC emission intensity against incident laser intensity for the IL P22214DPAS doped with 0.1 mol% PtOEP.


The presence of nanoscale structural order in the chromophoric ILs and their relevance to triplet diffusion behaviours have been demonstrated for the first time. The newly synthesized IL P22214DPAS showed the X-ray diffraction peak at a low angle, which was correlated with the two-dimensional ordering of the layered ionic/non-ionic structures in macroscopically isotropic liquid. The efficient donor-to-acceptor TTET was rationalized by the preferential accommodation of non-ionic triplet sensitizers in the non-polar layers containing acceptor moieties. The chromophoric ILs allow high density of DPAS units, whereas the strongly interacting DPAS species can be generated in the dynamic system, which work as the energy trapping sites for the singlet and triplet excitons. More future studies are necessary for the detailed understanding of inter-chromophore arrangements by experimental and theoretical approaches. Nevertheless, we found that the triplet diffusion length in ILs was much larger (21.1 nm) than the domain size estimated from the X-ray diffraction peak width (9.3 nm). These observations suggest that triplet excitons can diffuse beyond the domain boundaries in chromophoric ILs. The concept of domain-free energy transportation in IL materials gives a new perspective in IL materials, which may bring out a new research area of functional ILs.

Conflicts of interest

There are no conflicts to declare.


This work was partly supported by JSPS KAKENHI grant number JP25220805, JP17H04799, JP16H06513 (Coordination Asymmetry), JP16H00844 (Soft Molecular Systems), PRESTO program on “Molecular Technology and Creation of New Functions” from JST (JPMJPR14KE) and the Asahi Glass Foundation.


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Electronic supplementary information (ESI) available: XRPD patterns, fluorescence and UC emission decays, TTA-UC mechanism, absorption spectra. CCDC 1574167. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7cp06266d

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