2.1.

Materials and Methods

All the solvents were used as received without further purification. DPA was purchased from Aldrich and purified by sublimation in order to minimize the amount of impurities, which may act as quencher in the solid system. PtOEP and CTAB were purchased from Aldrich and Wako chemical and used as received.

UV–visible absorption spectra were recorded on a JASCO V-670 spectrophotometer. Fluorescence spectra were measured by using a PerkinElmer LS 55 fluorescence spectrometer. Powder x-ray diffraction (PXRD) analyses were conducted on a BRUKER D2 PHASER with a Cu $\mathrm{K}\alpha$ source (${\lambda }_{ex}=1.5418\AA$). Scanning electron microscope (SEM) images were obtained by using a Hitachi S-5000. For SEM measurements, samples were collected by suction filtration using membrane filters with pore size of $0.2\mu \mathrm{m}$. These filters were directly used for SEM measurements after platinum sputtering with the thickness of ca. 2 nm. Time-resolved photoluminescence lifetime measurements were carried out by using a time-correlated single photon counting lifetime spectroscopy system, Hamamatsu Quantaurus-Tau C11567-01. Dynamic light scattering measurements were carried out by using Malvern Nano-ZS ZEN3600.

For TTA-UC measurements, the samples were sealed between quartz plates by using hot-melt adhesive in an Ar-filled glove box ($[{\mathrm{O}}_2]<0.1\mathrm{ppm}$). For TTA-UC emission spectra, a diode laser (532 nm, 200 mW, RGB Photonics) was used as an excitation source. The laser power was controlled by combining a 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 the laser beam ($1{/\mathrm{e}}^2$) was measured at the sample position using a CCD beam profiler SP620 (OPHIR Photonics). The typical area of laser irradiation spot estimated from the diameter was $2.9\times 10{}^{-4}{\mathrm{cm}}^2$. 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, which was supplied and calibrated by Otsuka Electronics and equipped with a CCD sensor for the detection of whole visible range with high sensitivity.

TTA-UC and donor phosphorescence quantum yields were measured by using an absolute quantum yield measurement system. The sample was held in an integration 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 with a multichannel detector C10027-01 (Hamamatsu Photonics). The spectrometer was calibrated including the integration sphere and notch filter by Hamamatsu Photonics.²²

2.2.

Sample Preparations

DPA and PtOEP were dissolved in THF at three different concentrations. As a moderately diluted condition, DPA (2 mM)–PtOEP($2\mu \mathrm{M}$) in THF was employed (condition 1). Separately, saturated solutions of DPA (140 mM) with two different PtOEP concentrations (140 and $14\mu \mathrm{M}$) were prepared for the rapid crystallization (conditions 2A and 2B). All the experiments of crystal growth and collection were carried out at room temperature (around 20°C). No preformed crystals in the 140 mM DPA solution were detected from dynamic light scattering measurements. 0.5 mL of DPA–PtOEP mixed THF solutions were rapidly injected into the aqueous CTAB (0.5 mM, 5 mL) at room temperature under 1000 rpm stirring. These mixtures were kept stirring for 3 min and then left to stand for 2 h. After the incubation for 2 h, the crystals were collected by centrifugation at 10,000 rpm for 5 min, washed with water for three times, and dried under vacuum at room temperature.

3.1.

Characterization of the Crystals

When the THF solution of DPA (2 mM)–PtOEP ($2\mu \mathrm{M}$) was injected into the aqueous CTAB (condition 1) under stirring, the immediately formed suspension turned into a uniform dispersion within 3 min of stirring. This specimen was then incubated for 2 h, during which crystalline particles were gradually formed. On the other hand, precipitates were immediately formed upon injection of the saturated DPA solutions into aqueous CTAB ($[\mathrm{DPA}]=140\mathrm{mM}$, $[\mathrm{PtOEP}]=140$ and $14\mu \mathrm{M}$, conditions 2A and 2B), which showed almost no changes during the standing for 2 h.

The composition of the obtained crystals was determined by dissolving the crystals in THF and measuring their absorption spectra. The DPA–PtOEP molar ratios were $2100:1$, $700:1$, and $7300:1$ for those obtained under the conditions 1, 2A, and 2B, respectively. The content of PtOEP in the obtained crystals was lower than the initial mixing ratio in THF for condition 1 ($1000:1$), whereas it was higher than those for conditions 2A ($1000:1$) and 2B ($10,000:1$). The enhanced accumulation of PtOEP in DPA crystals under conditions 2A and 2B suggests that the rapid growth of DPA microcrystals facilitate kinetic entrapment of PtOEP molecules in the interior.

The crystallinity of obtained samples was confirmed by using PXRD measurements (Fig. 2). The diffraction patterns of the crystals formed under these conditions showed good agreements with the PXRD pattern of bulk DPA. On the other hand, the diffraction peaks of bulk PtOEP, for example, the peak at $2\theta =9.5\mathrm{deg}$, were hardly observed from those of the composite crystals, which would be due to the high dispersibility of the donor molecules or the too small amount of PtOEP for detection. It is confirmed that the basic arrangement of DPA molecules is almost independent of the crystallization conditions.

Fig. 2

PXRD patterns of samples prepared by using (a) condition 1, (b) condition 2A, and (c) condition 2B. PXRD patterns of (d) bulk DPA and (e) bulk PtOEP.

To get insights into the crystal growth processes, the crystals were collected by suction filtration before and after the 2 h of incubation, and their morphology was observed by scanning electron microscopy. In the case of the condition 1 sample, partly aggregated irregular nanorods were observed before incubation. After standing for 2 h, a few $\mu \mathrm{m}$-sized rods and a few tens of $\mu \mathrm{m}$-sized sheets appeared [Fig. 3(a)]. The observed morphological changes suggest that the ordered crystals gradually grew in the course of incubation. On the other hand, samples obtained under the conditions 2A and 2B gave larger spherical microstructures with similar size before and after the incubation process [Figs. 3(b) and 3(c)]. It is to be noted that the morphology of crystals differs considerably depending on the preparative conditions, i.e., kinetic parameters determine both of the nucleation and growth processes. When the same preparation procedure was carried out for PtOEP without DPA, only a few tens of nm-sized nanocrystals were observed, and thus the above-mentioned $\mu \mathrm{m}$-sized objects cannot be the crystals consisting of pure PtOEP.

Fig. 3

SEM images of crystals prepared by using (a) condition 1, (b) condition 2A, and (c) condition 2B (left) before and (right) after 2-h incubation.

3.2.

Photophysical Properties of the Crystals

To investigate the dispersed state of PtOEP molecules in acceptor DPA crystals, UV–vis absorption spectra of PtOEP were measured (Fig. 4). A THF solution of PtOEP $([\mathrm{PtOEP}]=10\mu \mathrm{M}$) showed a $Q(\mathrm{0,0})$ band at 534 nm, whereas this band is red-shifted to 552 nm in the cast solid sample due to aggregation.⁴¹ Interestingly, absorption spectra obtained for PtOEP in DPA–PtOEP composite crystals showed blue shifts compared to that of the neat cast solid. Notably, the peaks of the condition 2A (537 nm) and 2B (536 nm) samples are more blue shifted compared to that of the crystals obtained under condition 1 (541 nm). It is to note that the main peak of the condition 2B sample is close to that observed for the diluted THF solution, with a suppressed shoulder component at around 547 nm. This shoulder component reflects the presence of interchromophore interactions among PtOEP molecules. Apparently, the sample prepared under the condition 2B showed the spectrum revealing the most isolated PtOEP chromophores, showing the highest dispersibility of PtOEP is achieved under the rapid crystal growth condition with the high DPA:PtOEP molar ratio. We confirmed that the DPA–PtOEP crystals prepared in the absence of CTAB showed a pronounced aggregate-shoulder peak, indicating that CTAB significantly influenced the crystallization kinetics in the aqueous mixtures.

Fig. 4

UV–vis absorption spectra of PtOEP in several conditions. Solid lines represent the absorption of the DPA–PtOEP composite crystals. Dashed lines show the absorption spectra of a diluted THF solution of PtOEP ($10\mu \mathrm{M}$) and a bulk PtOEP solid.

The fluorescence quantum yields with direct excitation of DPA (${\mathrm{\varphi }}_A$, ${\lambda }_{ex}=365\mathrm{nm}$) were 42%, 28%, and 54% for powdery samples obtained under conditions 1, 2A, and 2B, respectively. Since the higher ${\mathrm{\varphi }}_A$ values were observed for crystals with lower PtOEP contents, it is possible that the DPA-to-PtOEP singlet–singlet energy transfer and/or the reabsorption of the DPA fluorescence by the donor molecules takes place.

The TTA-UC characteristics of each composite crystals were then evaluated. Under excitation with a 532-nm laser, upconverted emission was clearly observed for each sample with the maximum intensity at around 440 nm (Fig. 5). It is to be noted that these TTA-UC behaviors observed for the present DPA–PtOEP crystals prepared by the CTAB-assisted colloid technique is significantly improved as compared to that previously reported for the DPA single crystal doped with PtOEP.⁴

Fig. 5

Photoluminescence spectra of DPA-based crystals prepared under (a) condition 1, (b) condition 2A, and (c) condition 2B under Ar atmosphere with various excitation intensities (${\lambda }_{ex}=532\mathrm{nm}$). Scattered incident light was removed by using a 532 nm notch filter.

The donor phosphorescence at 650 nm was much weaker than the UC emission, and the phosphorescence quantum yields of all the three samples were less than 0.1%. These results suggest that the triplet energy of the donor molecules is efficiently transferred to the surrounding acceptor molecules or thermally dissipated in the donor aggregates.⁴¹

TTA-UC quantum yields for samples obtained under each condition were determined by the absolute method using the integrating sphere and the laser excitation source, to avoid inaccuracy that could arise from the strong light scattering of the crystals. In general, the quantum yield is defined as the ratio of absorbed photons to emitted photons, and thus the maximum yield (${\mathrm{\varphi }}_{UC}$) 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 ${\mathrm{\varphi }}_{UC}^{\prime }$ ($=2{\mathrm{\varphi }}_{\mathrm{UC}}$) when the maximum efficiency is normalized to 100%. The ${\mathrm{\varphi }}_{UC}^{\prime }$ value detemined for the condition 1 sample was as low as $0.044\pm 0.011\%$ (Fig. 6). On the other hand, much higher ${\mathrm{\varphi }}_{UC}^{\prime }$ values were observed for crystals prepared under the conditions 2A ($0.44\pm 0.022\%$) and 2B ($2.0\pm 0.17\%$).

Fig. 6

TTA-UC quantum yield as a function of the excitation intensity of 532-nm laser for DPA–PtOEP composite crystals prepared with three different conditions. A series of ${\mathrm{\varphi }}_{UC}^{\prime }$ data for each sample were obtained from the same specimen, and each points show average values of measurements conducted for more than five times. Error bars are smaller than the dot sizes for most points.

To understand the observed difference, factors affecting to the ${\mathrm{\varphi }}_{UC}^{\prime }$ value need to be considered. ${\mathrm{\varphi }}_{UC}^{\prime }$ is represented by the following equation:

We would like to call attention to the complexity of the condensed systems when the donor distribution is not homogeneous in acceptor crystals. Generally, TTA-UC emission intensity shows a quadratic dependence with the incident light intensity at low excitation intensity, where the thermal deactivation of the triplet states is governed by the main deactivation pathway. The quadratic-to-linear transition occurs by increasing the excitation intensity, and the transition point gives a threshold excitation intensity ($I_{th}$).^42–44 Above $I_{th}$, the TTA becomes the main deactivation channel for the acceptor triplets. The quadratic-to-linear transitions were observed for all the three conditions (Fig. 7). Despite the considerable difference in the UC quantum yield (Fig. 6), these three samples showed rather similar $I_{th}$ values in the range of 326 to $531\mathrm{mW}{\mathrm{cm}}^{-2}$.

Fig. 7

Double logarithmic plots of the UC photoluminescence intensity as a function of the excitation intensity for DPA–PtOEP composite crystals prepared with three different conditions. The linear fits with slope 2 and 1 in the lower and higher excitation intensity regimes are shown.

This discrepancy might be explained by the following hypothesis. In the condition 1 sample, not all donor molecules form aggregates and some donor molecules well dispersed in acceptor crystals contribute to the excitation intensity dependence with $I_{th}$ value similar to the condition 2B sample. However, as we described above, most of the donor molecules are present as aggregates in the condition 1 sample, which hinder the donor-to-acceptor TET and resulted in the observed low UC quantum yield. It is therefore essential to characterize the UC characteristics in a comprehensive manner, and the determination of UC quantum yields is prerequisite for the evaluation of solid upconverters.