Reversible Thermal Recording Media Using Time‐Dependent Persistent Room Temperature Phosphorescence

Thermoreversible on–off recording using time-dependent, persistent phosphorescence is demonstrated in air at room temperature using an organic film composed of an aromatic guest, phenol derivative, and steroid host. Reversible aggregation of guest molecules controlled by thermal hysteresis triggers large reversible changes in their time-dependent room temperature phosphorescence. Fluorescent imaging is an effective type of high resolution imaging.1 In particular, reversible aggregation changes of organic emitters and organic fluorescent dyes attached to photochromic compounds have realized unique thermoreversible2-5 and photoreversible6-11 fluorescence switching, respectively, which can be applied in high-level security devices,2, 4, 5 super-high-density optical memories,3, 6, 7, 11 and super-high resolution in vivo imaging.8-10 Although fluorescent imaging often provides highly sensitive signals in such applications, the quality of the information strongly depends on the background conditions. For example, in security and biosensing applications, signals from emissive substrates and impurities, respectively, decrease the signal-to-noise ratio of fluorescence. Although this ratio can be improved in fluorescent images by controlling the emission wavelength of a fluorescent probe so there is no overlap between its emission wavelength and background noise, this is difficult to achieve with organic fluorescent probes with broad emission wavelengths. Time-dependent luminescent information such as the intensity and color change of emission remaining after stopping excitation has potential for backgroundless highly sensitive emission imaging. In particular, persistent room temperature phosphorescence (RTP) with a lifetime longer than 1 s is a candidate for time-dependent luminescent information because it can be detected for several seconds after stopping excitation without using sensitive, high-speed detectors. Although inorganic metal oxides doped with a rare metal show persistent RTP in air,12-15 not only are they toxic and expensive, their time-dependent emission cannot be switched or recorded. Different from the inorganic materials with RTP, purely organic materials with RTP16, 17 have a possibility of a variety of functionalization. We recently found that purely organic amorphous materials composed of a deuterated aromatic guest and amorphous rigid steroidal host showed efficient RTP with lifetime longer than 1 s in air,18 which had not been realized previously. Because persistent RTP from organic materials is easily quenched by molecular aggregation of the deuterated guests, as its lifetime is much longer than that of fluorescence, we anticipated that reversible control of molecular aggregation of guest molecules could allow reversible on-off control of persistent RTP. Here we have developed time-dependent reversible thermal recording media using persistent RTP from organic materials in air. This reversible thermal recording material is composed of a secondary amino-substituted aromatic compound guest and a hydroxyl steroid host matrix, as shown in Figure 1(a). The recording medium was prepared by sandwiching the materials between two glass substrates. Recording material design and reversible thermal recording and erasing using persistent RTP. (a) Design of recording materials with thermal reversible persistent RTP. Inset shows the chemical structures of 1, THEB, and cholesterol. (b) Reversible thermal recording and erasing using persistent RTP. The recording material was composed of 1.0 wt% 1, 9.0 wt% THEB, and 90 wt% cholesterol. The recording material sandwiched between two glass slides was placed on printed white paper. (ix) and (x) are photographs of the recording medium under ambient lighting without exposure to excitation light in the recording and erasing states, respectively. (i) and (v) are photographs of the recording medium under excitation in the recording and erasing states, respectively. (ii)-(iv) and (vi)-(viii) are photographs of the recording medium after stopping excitation in the recording and erasing states, respectively. (i), (ii), (iii), (iv), and (ix) are photographs in the recording state. (v), (vi), (vii), (viii), and (x) are photographs in the erasing state. The power and wavelength of excitation light were 0.3 mW cm−2 and 345 nm, respectively. All photographs were taken at RT in air. Figure 1(b) shows a demonstration of reversible thermoresponsive on-off recording of temporal luminescent information in the host-guest material composed of 1.0 wt% d19 2-diethylamino-9H-fluorene (dye 1),19 9.0 wt% α,α,α′-tris(4-hydroxyphenyl)-1-ethyl-4-isopropylbenzene (THEB), and 90 wt% cholesterol. The first layer was in the recording state and showed blue-violet fluorescence from 1 under continuous excitation by ultraviolet (UV) light (Figure 1(b), (i)). When the excitation ceased, the blue-violet emission was changed to a blue-green one that lasted for several seconds. This delayed emission is ascribed to persistent RTP from 1. The time-dependent photoluminescence behavior after excitation finishes is presented in Figure 1(b), (ii)→(iv). The blue-green persistent RTP was completely quenched by heating the mixture up to 90 °C (the crystallization temperature of the host composed of 9.0 wt% THEB and 90 wt% cholesterol) for 1 min, and then cooling to RT, which is the erasing process. Although blue–violet fluorescence was observed from 1 under continuous exposure to UV light after heating to 90 °C (Figure 1(b), (v)), no time-dependent blue-green persistent RTP remained after stopping excitation (Figure 1(b), (vi)→(viii)). The recording state could be completely recovered by heating the material to 150 °C, which is above its melting point (Mp), for a few seconds and then cooling to RT. By repeating these recording and erasing processes, thermoresponsive on–off recording of a time-dependent emission signal is realized (see Movie S1 in Supporting Information). While the recording region is not recognized on white paper without UV irradiation and is not photocopied by a conventional scanner and copier (Figure 1(b), (ix) and (x)), information was recorded through the time-dependent emission of persistent RTP on white paper with high contrast after stopping excitation (Figure 1(b), (xi)). In contrast, materials with a reversible fluorescent recording function do not provide high contrast information on white paper, because the white paper also emits efficiently under UV excitation. Therefore, imaging technology using time-dependent emission such as persistent RTP is potentially useful for highly sensitive imaging that is not affected by background conditions, and secret media such as money, passports, and tickets. Emission spectral changes in the recording and erasing states of the recording materials are shown in Figure 2. Blue–violet emission at 365 nm under excitation at 345 nm20 in the recording state (fine red line in Figure 2(a)) is fluorescence from 1. The fluorescence quantum yield at RT (ΦF(RT)) in the recording state is 19.1 ± 2.1%. The shoulder around 480 nm is ascribed to persistent RTP from 1. Although the fluorescence at 365 nm disappeared completely after stopping excitation in the recording state, blue-green persistent RTP around 480 nm remained for several seconds, as shown in Figure 2(b), (i). The phosphorescence quantum yield at RT (ΦP(RT)) and phosphorescence lifetime at RT (τ(RT)) were 3.1 ± 0.4% and 1.40 ± 0.04 s in air, respectively. Conversely, the fluorescence at 365 nm in the erasing state is half the intensity of that in the recording state (fine blue line in Figure 2(a)). In addition, the persistent RTP at 480 nm in the erasing state is 10 times less intense than that in the recording state (ΦP(RT) = 0.30 ± 0.04%), so the time-dependent signal is hardly observed in the erasing state (Figure 2(b), (ii)). The partially quenched blue-violet fluorescence and strongly quenched blue-green persistent RTP in the erasing state were recovered to their initial intensities in the recording state after heating to 150 °C for a few and cooling to RT (green dotted line in Figure 2(a)). Thermoreversible changes in emission intensity of a recording medium in recording, erasing, and second cycle of recording states. The recording material was composed of 1.0 wt% dye 1, 9.0 wt% THEB, and 90 wt% cholesterol. The recording state was produced by heating to 150 °C for a few seconds and then quenching to RT. The erasing state was produced by heating to 90 °C for 1 min and then cooling to RT. The recording material was excited by light of 350 nm at a power of 0.3 mW cm−2 at RT in air. (a) Changes in emission intensity under excitation. Fine red, fine blue, and dashed green lines show emission spectra in recording, erasing, and second cycle of recording states, respectively. (b) Changes in intensity of persistent RTP in the recording (i) and erasing states (ii). The integration of persistent RTP intensity from 0 to 0.05 s after stopping excitation in the erasing state was normalized to 1.0. The inset figures in (i) and (ii) show changes in emission intensity after stopping excitation at 0 s. The inset photographs in (i) and (ii) represent changes in intensity of persistent RTP after stopping excitation at 0 s. The power and wavelength of the excitation light were 0.3 mW cm−2 and 345 nm, respectively. We investigated the mechanism of the appearance of persistent RTP in the recording state. The A in Figure 3(a), (i) indicates that Mp of the recording material is 150 °C, so 1 and THEB dissolve in the melted cholesterol when the recording material is heated above the Mp of cholesterol (state A in Figure 3(b)). In this state, a homogeneous dispersion of 1 in the melted steroidal matrix does not exhibit persistent phosphorescence (A in Figure 3(a), (iii)), because the active thermal motion of the melted host matrix readily quenches long-lived triplet excitons of 1. Polarized microscope observation in B of Figure 3(a), (i) indicates that the recording material is vitrified to a disordered state by cooling to RT from the melted state. In the recording state (B in Figure 3(b)), 1 shows strong persistent RTP (B in Figure 3(a), (iii)) because of weak concentration quenching of 1 and weak thermal motion of the host matrix. The quantum efficiency of the persistent phosphorescence was similar at –200 °C and RT because of the weak thermal motion of the host matrix (from B to C in Figure 3(a), (iii)).18 The high oxygen barrier properties of a host matrix composed of cholesterol and THEB also contributes to the appearance of persistent RTP in air because oxygen quenches RT triplet excitons of 1 within 1 ms in conventional organic matrices.18 For example, no persistent RTP is observed in air when 1 is dispersed homogeneously in other conventional amorphous matrices such as polymer host poly(methyl methacrylate) and low molecular weight host 1, 3-bis(N-carbazolyl)benzene because other matrices possess active thermal motion at RT and poor oxygen barrier properties. Mechanism of thermoreversible on-off recording and changes in physical properties as a function of temperature. The recording material was composed of 1.0 wt% dye 1, 9.0 wt% THEB, and 90 wt% cholesterol. (a) Changes in physical properties as a function of temperature when the recording material is heated at a rate of 5 K min−1. The recording state was produced by heating to 150 °C for a few seconds and then cooling to RT. The erasing state was produced by heating to 90 °C for 1 min and then cooling to RT. Red and blue represent data when the recording and erasing states were used at starting point of the measurement, respectively. (i) DSC trace; the inset shows optical microscope images obtained under crossed nicols. Dotted red line represents 5 times enlarged view of the red fine line from 0 to 52 °C. Changes in (ii) fluorescence intensity, (iii) persistent phosphorescence intensity, and (iv) phosphorescence lifetime. F→A→B and B→D→E→F processes correspond to recording and erasing processes, respectively. The recording material was excited by light of 350 nm with a power of 0.3 mW cm−2. (b) Schematic illustration showing the mechanism of thermoreversible on-off recording using persistent RTP of the material. Next, we determine the mechanism of the disappearance of persistent RTP in the erasing state. D in Figure 3(a), (iii) shows that the persistent phosphorescence was readily quenched when the recording material was heated over its glass transition temperature, Tg = 40 °C (C in Figure 3(a), (i)), and below the crystallization temperature (Tc) of cholesterol. D in Figure 3(a), (ii) indicates that the fluorescence intensity of the recording material hardly decreases, because there is no concentration quenching of 1 as the recording material is not crystallized and 1 does not aggregate in the matrix at 40 °C. In D in Figure 3(b), the thermal motion of cholesterol increases rapidly, quenching the persistent RT triplet excitons of 1 dispersed in the matrix. The intensity of fluorescence of 1 decreased as the recording material crystallized during heating to Tc from 60 to 125 °C (E in Figure 3(a), (i) and (ii)). In state E in Figure 3(b), 1 and THEB are segregated by the crystallization of cholesterol in the film. This phase separation under heat treatment caused 1 to aggregate, which in turn triggered concentration quenching. Consequently, the persistent phosphorescence decreases significantly (E in Figure 3(b)). This crystallinity is preserved as the erasing state after cooling the recording material to RT (state F in Figure 3(b)). In the erasing state (F in Figure 3(b)), the intensity of persistent RTP is ten times lower than that in the disordered state because of aggregation of 1 (F in Figure 3(a), (iii)). The remaining persistent RTP is ascribed to a small amount of 1 isolated in the crystallized matrix because the phosphorescence lifetime of the weak persistent RTP in the erasing state (1.08 ± 0.05 s) is similar to that in the recording state, as shown in B and F of Figure 3(a), (iv). Aggregation of 1 because of crystallization of the matrix causes little quenching of violet–blue fluorescence but significantly quenches the blue–green persistent RTP, as shown in the difference between processes B–E in Figure 3(a), (ii) and processes B–E in Figure 3(a), (iii), respectively. The persistent RTP is quenched much more strongly than the fluorescence, because the phosphorescence lifetime of 1 is at least 108 times longer than that of fluorescence. This recording material realizes not only good reversible recording endurance and stable recording information at RT, but also nondestructive readout of persistent RTP information. Recorded images are retained at RT for at least half a year without deteriorating (Figure 4(a)), whereas they were removed completely within a minute upon heating above Tg and below MP. Furthermore, the on–off function of persistent RTP can be reversibly changed more than 20 times without any decrease in the intensity of persistent RTP, as shown in Figure 4(b). Recorded information was not destroyed even under excitation by UV light (Figure 4(c)). Reversible endurance and stability of recorded information in a recording medium. The recording material was composed of 1.0 wt% dye 1, 9.0 wt% THEB, and 90 wt% cholesterol. The wavelength and power of the excitation light was 350 nm and 0.1 mW cm−2, respectively. All experiments were measured at RT in air. (a) Changes in persistent RTP intensity in the recording state (filled circles) and erasing state (open circles) when the recording medium was excited at the moment of readout. (b) Changes in reversible persistent RTP intensity upon cycling between the recording and erasing states many times by thermal hysteresis. Open and filled circles indicate changes in phosphorescence intensity changes and contrast of persistent RTP, respectively. (c) Changes in persistent RTP intensity in the recording state (filled circles) and erasing state (open circles) when the recording medium was continuously excited. We demonstrated a reversible thermal recording medium using time-dependent persistent RTP from a host-guest material composed of a secondary amino-substituted aromatic compound as a guest and a phenol derivative and steroidal compound as the host matrix. The reversible amorphous-crystalline transition of the steroidal matrix changed the aggregation of the guest, leading to strong concentration quenching of long-lived triplet excitons of the guest at RT. The reversible amorphous-crystalline transition provided the reversible on–off recording function of time-dependent emission changes such as phosphorescence lifetime and color of the persistent RTP after stopping excitation. The time-dependent information was stable at RT under visible light and nondestructive readout was obtained. We also achieved thermoreversible on-off recording using the color change of persistent RTP (see supporting information). This novel organic recording material with reversible persistent RTP controlled by thermal input is a good candidate for a highly-advanced secret media, because imaging technology using time-dependent emission is not affected by background conditions, resulting in highly sensitive imaging. This research is an important step towards the next generation of advanced recording and imaging technology. 1 was synthesized according to a previous report.18, 191, THEB, and cholesterol were mixed at 150 °C and cooled to RT to prepare a recording material. Characteristics of differential scanning calorimetry (DSC) and polarization optical microscopy (POM) in the recording material were measured by a Rigaku Thermo Plus DSC8230 instrument and an Olympus BX50 microscope, respectively. The recording medium was fabricated as follows: A small amount of the cooled recording material was spread between two glass slides separated by about 10 ± 0.5 μm and kept at 150 °C on a hotplate. The substrate on the hotplate was then cooled to RT. The recording and erasing processes were performed as follows: recording was carried out by heating to 90 °C for 1 min and then cooling to RT. Erasing was carried out by heating to 150 °C for 10 s and then quenching to RT. UV-vis and fluorescence spectra were measured on a UV-vis spectrophotometer (Jasco V-560) and fluorescence spectrophotometer (Jasco FP-6500), respectively. ΦP(RT), ΦP(RT), and τ(RT) of the host-guest films in recording and erasing states were measured by the method as shown in a previous report18 using absolute photoluminescence quantum yield measurement system (Hamamatsu, C9920–02). The temperature dependence of fluorescence intensity, phosphorescence intensity, and phosphorescence lifetime was measured using time-resolved charge-coupled device (Otsuka Electronics, MCPD-7000) and an excitation unit attached to the spectrophotometer (Jasco, FP-6500).from –200 to 180 °C by changing the temperature of the samples with a temperature controller attached to a cryostat (Oxford Ltd. Optistat DN-V). ΦF(T) and ΦP(T) of the host–guest films were calculated using the change in the ratios of fluorescence and phosphorescence intensity at T °C to fluorescence and phosphorescence intensity at RT, and ΦP(RT) and ΦP(RT), respectively. This research was supported by an International training program to Pre-Tenure-Track Young Researchers in “Nano-Materials” at the Tokyo University of Agriculture and Technology supported by the Japan Society for the Promotion of Science (JSPS) International Training Program (ITP), a Grant-in-Aid for Young Scientists (B) (22750132), an Adaptable and Seamless Technology Transfer Program through Target-driven R&D (AS231Z01236B), a Grant-in-Aid for Challenging Exploratory Research (24655175), and a Grant-in-aid from the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST). S. H. designed the materials and S. H. synthesized them. S. H., and H. K. performed deuteration experiments. S. H., and K. T. measured photophysical characteristics, collected data, and analyzed it. S. H., M. V., T. W., and C. A. drafted the manuscript. All authors discussed the progress of research and reviewed the manuscript. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.