
==== Front
J Mater Chem C Mater
J Mater Chem C Mater
TC
JMCCCX
Journal of Materials Chemistry. C
2050-7526
2050-7534
The Royal Society of Chemistry

d4tc00562g
10.1039/d4tc00562g
Chemistry
Ambient solid-state triplet–triplet annihilation upconversion in ureasil organic–inorganic hybrid hosts†
† Electronic supplementary information (ESI) available: Materials and methods; synthetic procedures; silica wt% calculations; additional data from: chromophore concentration determination studies, UV-vis absorption spectroscopy, steady-state PL spectroscopy, UC emission spectra, time-resolved emission lifetime fitting, dynamic mechanical analysis, FTIR deconvolution and fitting, XRD deconvolution and fitting, 4-point flexural tests, uniaxial tensile tests; photographs and video ambient TTA-UC activity in ureasils. See DOI: https://doi.org/10.1039/d4tc00562g

https://orcid.org/0000-0001-8073-1943
Collins Abigail R. a
https://orcid.org/0000-0002-2955-8409
J-8079-2015
Zhang Bolong a‡
https://orcid.org/0000-0003-3569-8931
Bennison Michael J. a
https://orcid.org/0000-0003-2956-4857
Evans Rachel C. a
a Department of Materials Science and Metallurgy, University of Cambridge 27 Charles Babbage Road Cambridge CB3 0FS UK rce26@cam.ac.uk

‡ Present address: State Key Laboratory of Functional Materials and Devices for Special Environmental Conditions; Xinjiang Key Laboratory of Electronic Information Materials and Devices; Xinjiang Technical Institute of Physics & Chemistry, CAS, 40-1 South Beijing Road, Urumqi 830011, China.

15 4 2024
2 5 2024
15 4 2024
12 17 63106318
9 2 2024
27 3 2024
This journal is © The Royal Society of Chemistry
2024
The Royal Society of Chemistry
https://creativecommons.org/licenses/by/3.0/ Triplet–triplet-annihilation upconversion (TTA-UC) has attracted significant attention as an approach to harvest low energy solar photons that cannot be captured by conventional photovoltaic devices. However, device integration requires the design of solid-state TTA-UC materials that combine high upconversion efficiency with long term stability. Herein, we report an efficient solid-state TTA-UC system based on organic–inorganic hybrid polymers known as ureasils as hosts for the archetypal sensitiser/emitter pair of palladium(ii) octaethylporphyrin and diphenylanthracene. The role of the ureasil structure on the TTA-UC performance was probed by varying the branching and molecular weight of the organic precursor to tune the structural, mechanical, and thermal properties. Solid-state green-to-blue UC quantum yields of up to 1.86% were observed under ambient conditions. Notably, depending on the ureasil structure, UC emission could be retained for >70 days without any special treatment, including deoxygenation. Detailed analysis of the structure-function trends revealed that while a low glass transition temperature is required to promote TTA-UC molecular collisions, a higher inorganic content is the primary factor that determines the UC efficiency and stability, due to the inherent oxygen barrier provided by the silica nanodomains.

Solid-state green-to-blue upconversion is shown in organic–inorganic hybrid ureasil hosts using the palladium(ii) octaethylporphyrin and diphenylanthracene sensitizer/emitter pair, with emission retained for >70 days without deoxygenation.

H2020 European Research Council 10.13039/100010663 818762 pubstatusPaginated Article
==== Body
pmcIntroduction

Triplet–triplet-annihilation upconversion (TTA-UC) is a photochemical process whereby two low energy photons are converted to a single higher energy photon,1,2 and has diverse applications in bioimaging,3,4 anti-counterfeiting,5 photocatalysis,6,7 sensing,8 and solar energy harvesting.9–11 As illustrated in Fig. 1, TTA-UC occurs between a chromophore pair, comprising a sensitiser with high intersystem crossing (ISC) efficiency, (e.g., palladium(ii) octaethylporphyrin (PdOEP)) and an emitter, such as diphenylanthracene (DPA).12 Since TTA-UC requires rapid energy transfer between the sensitiser and emitter, the physical properties of the host medium are also closely linked to the efficiency, quantified by the upconversion quantum yield (ΦUC, which is capped at 50%).13,14 Although high efficiencies (ΦUC ∼30%) can be achieved in liquid media,15–17 due to rapid molecular collisions and the ease of degassing to prevent oxygen quenching, this has limited practical use. In contrast, a solid-state system will not evaporate or leak and can offer mechanical stability for device integration.

Fig. 1 Triplet–triplet annihilation upconversion is a multi-step photophysical process between two chromophores. (a) Structures of the benchmark chromophore pair used in this study: PdOEP (sensitiser) and DPA (emitter). (b) Mechanism of TTA-UC. (i) The absorption of photons by the sensitiser PdOEP (green arrow) results in formation of the excited singlet state (S1), which relaxes by intersystem crossing (ISC, pink dashed arrow) to the excited triplet state (T1). (ii) Triplet sensitisers then collide with emitters to produce triplet emitters via triplet–triplet energy transfer (TTET, purple dashed arrow) if their energy levels overlap within the Dexter radius. (iii) Triplet emitters then collide with each other, leading to triplet–triplet annihilation (orange dashed arrow), whereby one molecule is excited to a higher energy level, and the second relaxes to the ground state. Radiative relaxation of the higher energy state leads to photon emission via fluorescence (blue arrow).

It is typically challenging to achieve a high ΦUC in the solid-state,18,19 due to the potential problems of parasitic absorption, limited triplet-diffusion, and inhomogeneous chromophore distribution.20,21 To overcome these energy sinks, the ideal solid-state host must be transparent and allow good chromophore dissolution and mobility. In addition, it should provide barrier properties to reduce oxygen triplet quenching and a mechanical scaffold for practical integration. These traits are often contradictory and difficult to marry into one system.19 As such, a wide variety of materials and architectures have been investigated as solid-state UC hosts including gels,22–24 particles,25–28 metal–organic frameworks,29,30 micro or nanodroplet-containing polymers,31,32 and copolymers.33,34 Among these, polymer systems are particularly attractive due to their ease of processability and integration with thin film devices. Organic polymers with low glass transition temperature (Tg) such as polyurethanes have been demonstrated to facilitate molecular diffusion, but lack the mechanical stability and oxygen protection that inorganic systems provide.18,19,35 In high Tg polymers such as poly(methyl methacrylate),36,37 chromophores are immobilised within the rigid matrix,38–40 meaning the triplet–triplet energy transfer (TTET) rate relies entirely on homogenous blending to ensure triplet excitons transfer via an energy migration mechanism, leading to lower overall efficiencies.10,19,20

Organic–inorganic hybrid polymers may offer a solution to these limitations. However, to the best of our knowledge, hybrid polymers have not yet been deployed as TTA-UC hosts, despite offering highly favourable characteristics. For example, ORMOCERS® have been investigated extensively for use as food packaging materials due their excellent barrier abilities against oxygen, water vapor and other compounds.41–43 Moreover, with judicious selection of a low Tg organic precursor, it should be possible to design a hybrid polymer that provides a suitable environment to both solubilise embedded chromophores and facilitate molecular collisions. Herein, we explore this opportunity by demonstrating organic–inorganic hybrid ureasil polymers as chromophore hosts for solid-state TTA-UC for the first time. Ureasils consist of poly(oxyalkylene) chains covalently linked to a siliceous network via urea bridges (Fig. 2).44,45 The structure and organic-to-inorganic ratio can be easily tuned by altering the branching or molecular weight of the polymer backbone to form monoliths of varying Tg and flexibility.46–48 Ureasils have been used as waveguides for luminescent solar concentrators,47–49 electrochromic windows,50 and optical communications46,51 due to their ability to encapsulate organic and inorganic dopants. Moreover, both non-specific intermolecular forces such as hydrogen bonding, ionic or π–π interactions,52 or specific covalent grafting have been demonstrated as successful strategies to both solubilise and localise organic chromophores.53

Fig. 2 Reaction scheme for solid-state TTA-UC emitters based on ureasils. Two possible polyetheramine precursor branching structures are shown which yield the four ureasils studied - one lower and one higher molecular weight for each. The sensitiser PdOEP and emitter DPA are mixed with the intermediate before sol-gelation.

Using the benchmark DPA/PdOEP chromophore pair, we examine the effect the chain length and branching of the organic polymer backbone on the TTA-UC activity in ureasil hosts. While efficient green-to-blue upconversion was observed in all ureasils, the champion host DU(2000) exhibited a maximum ΦUC of 1.86 ± 0.05 at 1 W cm−2, which is comparable to other similar solid-state systems.19,54 Crucially, this efficiency is observed under ambient conditions (i.e., no oxygen removal) and measurable UC persists over 76 days. Moreover, both the UC efficiency and stability can be directly correlated with the organic-to-inorganic ratio in the ureasil host. Through detailed structural analysis, we tentatively link this to faster molecular diffusion for lower Tg systems, and improved oxygen resistance for those with higher silica content.

Results and discussion

Design of ureasil hosts

Ureasils are synthesised using a previously reported two-step process,46 in which a poly(oxyalkylene) is first reacted with the silica precursor isocyanatopropyltriethoxysilane (ICPTES) to form urea bridges, before the system is cross-linked at the silicate terminus using sol–gel chemistry (Fig. 2). By varying the length and branching of the poly(oxyalkylene), the ratio of organic to inorganic content in the final ureasil can be tuned. This ratio affects properties such as the Tg and flexibility of the host. Four poly(oxyalkylene) precursors were investigated and the resulting ureasils are given the following nomenclature according to literature convention: DU(2000), DU(4000), tU(3000), and tU(5000). The D or t terms refer to the branching, being di- or tri-branched respectively, and the number in brackets is the molecular weight (MW) of the poly(oxyalkylene).

Prior to gelation, ureasils were doped with the sensitiser-emitter pair: PdOEP (0.1 mM with respect to (w.r.t.) the dry monolith) and DPA (10 mM w.r.t. the dry monolith). These ratios were selected based on the UC performance of a set of test samples of tU(3000) loaded with different concentrations and ratios of DPA and PdOEP (see Section S3, Table S2 in ESI†). We note that while high, these sensitiser/emitter concentrations are in line with typical concentrations used in similar solid-state TTA-UC systems.31,35 For elevated emitter concentrations, competing relaxation pathways such as self-annihilation and/or reabsorption can occur.21,55 As some evidence of these processes was observed at higher (20 mM) emitter concentrations (reduced UC lifetime, decreased UC emission counts, Tables S2 and S3, ESI†), we selected DPA (10 mM) as the upper concentration limit. Reference samples containing just PdOEP or DPA (at equivalent loadings) or no lumophore at all were also prepared for comparison. Once formed, the gels were air-dried to evaporate residual solvent, yielding free-standing monoliths. We note that tri-branched ureasils shrink in size (relative to the initial solution volume) more than di-branched, as do lower molecular weight variants. All ureasils were characterised by powder X-ray diffraction (PXRD) and Fourier transform infrared (FTIR) spectroscopy and display structural features characteristic of ureasils. Full details of the synthetic methods and structural characterisation can be found in the ESI† (ESI,† see Sections S1.2 and S6). It is important to note that all samples were prepared under ambient conditions and not degassed at any point.

The parent ureasils show high optical transmittance (>90%) in the visible range, with a cut-off around 250–300 nm (see Fig. 3a for DU(2000), and Fig. S2 for all other ureasils, ESI†). This cut-off can provide UV-B protection to reduce chromophore photobleaching.56 When doped, the ureasils still show high transmittance >600 nm, but as expected absorb strongly in the spectral regions of the chromophores (∼490–560 nm for PdOEP, 300–420 nm for DPA, Fig. 3b). The absorption and emission spectra of DPA (λex = 375 nm) and PdOEP (λex = 532 nm) in THF solution are shown in Fig. 3c for reference. Solid-state emission measurements on doped ureasils under selective excitation of either the sensitiser or emitter further confirmed successful integration into the ureasil host (Fig. S4, ESI†).

Fig. 3 Optical properties of ureasils. (a) % Transmittance (T) of parent DU(2000); (b) %T of DU(2000) doped with 10 mM DPA and 0.1 mM PdOEP; (c) normalised absorbance (solid lines) and emission (dashed lines) of DPA (blue, λex = 365 nm, 10−2 mM) and PdOEP (pink, λex = 532 nm, 10−4 mM) in THF solution.

TTA-UC activity in ureasils

Doped ureasils show green-to-blue upconversion in ambient conditions when irradiated with a green laser pointer (see Fig. 4a and Fig. S4, Video S1, ESI†). No UC emission is observed in the absence of PdOEP, which supports the sensitised mechanism. These results were confirmed by steady-state photoluminescence spectroscopy (Fig. 4b, and Fig. S5, ESI†). Upon excitation at 532 nm (in the PdOEP absorption band), a broad emission between 400–500 nm is observed, which increases with excitation intensity. We note subtle differences in structure of the UC emission band compared to direct excitation of DPA (at 375 nm in dilute solution, see Fig. 3c), namely the absence of the first vibronic band at 408 nm and the red-shift of the second peak maximum to 440 nm. This behaviour is typical of strong self-absorption due to the high emitter concentration. The photoluminescence quantum yield (ΦPL) upon direct excitation at 375 nm is >95% for all DPA-ureasils (Table S4, ESI†), which is comparable to the near-unity ΦPL of DPA in solution (95% in ethanol).57,58 This demonstrates that the ureasil does not affect the performance of DPA as an emitter. In presence of PdOEP, the ΦPL (λex = 375 nm) decreases, but remains high (78–87%, Table 2). This may be due in part to parasitic absorption from the Soret band of the sensitiser (which cannot be isolated from the DPA absorption in the ΦPL measurement), along with the presence of new non-radiative relaxation pathways. In both cases, it is noted that the ΦPL is higher for DU(2000) and tU(3000) than their higher MW counterparts, and higher in tri-branched than di-branched ureasils. While ureasils are known to emit weakly between 390–600 nm upon excitation at 375 nm (see Fig. S6, ESI†)47,59 the ΦPL of undoped ureasils (2–7%, with the exception of DU(2000), Table S4, ESI†) is significantly lower than that of DPA. More importantly, no detectable ureasil emission is observed upon excitation at 532 nm, meaning the ureasil host should not offer competing photophysical pathways to the TTA-UC process.

Fig. 4 TTA-UC in ureasils. (a) Photograph of chromophore-doped DU(2000) irradiated with a 532 nm laser, which turns visibly blue as it passes through the ureasil. (b) Upconverted DPA emission in DU(2000) with increasing excitation intensity from 8 mW to 5 W cm−2. Full-width-half maximum = 40 nm. (c) Upconversion quantum yield of DU(2000) and tU(3000) with increasing excitation intensity. (d) Log–log plot of the excitation intensity dependence of UC emission intensity. The solid lines correspond to linear fits of slopes 2.0 and 1.0 in the low and high excitation intensity regimes.

To quantitatively compare the TTA-UC activity in different ureasils, the upconversion quantum yield was measured (see Section S1.5, ESI†). Since the process requires two lower energy photons to emit one photon of higher energy, the maximum ΦUC cannot exceed 50%,18 and is expressed by:1

where ΦISC, ΦTTET, ΦTTA and ΦPL are the quantum yields for ISC, TTET, TTA, and emitter photoluminescence, respectively. The factor f represents the probability that a singlet state is produced per TTA step, rather than a triplet or quintet excited state which do not radiatively decay as desired.60 Some authors report ΦUC as a normalised yield (doubled, up to 100%), however we use the conventional description for reporting ΦUC, which is capped at 50%.13 DU(2000) and tU(3000) both show a ΦUC of 1.86 ± 0.05% and 1.67 ± 0.18%, respectively, at a power density of 1 W cm−2 of laser excitation at 532 nm. These values are highly efficient compared to similar systems prepared and measured in entirely ambient conditions (i.e., the host system has not been degassed or encapsulated from molecular oxygen).19 One of the most efficient of these was a crosslinked epoxy polymer doped with DPA/PtOEP, which was notably prepared and measured in air, achieved a ΦUC of 1.9%, 40 mW cm−2 threshold intensity, and 17.8 ms UC lifetime.39 While higher ΦUC's have been achieved in other solid-state media, oxygen removal was employed through use of a physical barrier or addition of scavengers.35,61,62

DU(4000) and tU(5000) show lower but still very respectable solid-state ΦUC's of 0.71 ± 0.06% and 1.14 ± 0.14%, respectively. This was an unexpected result - structurally, the higher MW ureasils were expected to provide a more liquid-like environment, which should enable fast molecular diffusion for efficient triplet transfer, resulting in a higher ΦUC, but surprisingly the lower MW ureasils out-perform them. Threshold intensity measurements were carried out on the two leading systems, DU(2000) and tU(3000). The power density threshold (Ith) is the excitation at which the ΦTTA is 50% of the maximum value, and the conversion yield is maximised.63 The Ith is a result of a myriad of intrinsic and external factors: a high ΦISC, long sensitiser triplet lifetime and efficient TTET will all lower the threshold.63 The Ith of tU(3000) was 27 mW cm−2, and UC emission is observed at sub-solar excitation densities as low as 1.5 mW cm−2 meaning it can be activated by sunlight (Fig. 5b). Systems with a very low Ith are highly desired for solar applications as sunlight is a low density light source (100 mW cm−2 across solar spectrum, 1.6 mW cm−2 at 532  ±  5 nm).64,65 DU(2000) had a higher threshold at 166 mW cm−2 which could be due its shorter triplet lifetime (Fig. 6), but UC is still seen from the low power density 2.3 mW cm−2.

Fig. 5 Emission decay studies on TTA-UC ureasils. (a) Upconversion decay curves of PdOEP (0.1 mM)/DPA (10 mM)-doped ureasils DU(2000), DU(4000), tU(3000) and tU(5000) obtained from excitation at 532 nm and detection at 440 nm (with 550 nm short-pass filter). (b) Corresponding phosphorescence decays obtained on excitation at 532 nm and detection at 665 nm (with 550 nm long-pass filter). All measurements were performed under ambient conditions.

Fig. 6 Effect of the ureasil host on the magnitude and stability of the UC quantum yield over time. λex = 532 nm, λem = 440 nm.

The ΦTTET was also evaluated by comparing the phosphorescence intensity of ureasils loaded with only sensitiser only (0.1 mM PdOEP) against those with the sensitiser/emitter pair (0.1 mM PdOEP/10 mM DPA) – see Fig. S8 (ESI†). Lower MW ureasils showed more intense phosphorescence under ambient conditions. This potentially suggests there is increased oxygen protection in this system. Notably all ureasils demonstrate a high ΦTTET, especially tU(3000) and DU(2000), both with above 95% (Table 1).

Key photophysical properties of ureasil TTA-UC systems

Ureasil	Φ UC a (%)	I th b (mW cm−2)	Φ PL d (%)	Φ TTET e (%)	〈τUC〉f (ms)	〈τPHOS〉g (with DPA) (μs)	〈τPHOS〉g (no DPA) (μs)	
DU(2000)	1.86 ± 0.05	166	81.4 ± 2.1	97 ± 2	16 ± 0.5	102	25	
DU(4000)	0.71 ± 0.06	—c	78.4 ± 0.8	84 ± 3	10 ± 0.3	327	9	
tU(3000)	1.67 ± 0.18	27	86.6 ± 2.0	95 ± 2	22 ± 2.5	59	25	
tU(5000)	1.14 ± 0.14	—c	83.9 ± 1.3	92 ± 3	8 ± 0.6	182	8	
a Upconversion quantum yield at excitation intensity of 1 W cm−2 (532 nm).

b Threshold intensity determined from logarithmic plot of integrated UC emission intensity at varying 532 nm laser power.

c Not measured.

d Photoluminescence quantum yield of the emitter determined by the integrating sphere method (λex = 375 nm).

e TTET calculated from average phosphorescence emission intensity with and without emitter present (see eqn S3 (ESI), λex = 532 nm, λem = 600–800 nm).

f Average upconversion lifetime (λex = 532 nm, λem = 440 nm).

g Average phosphorescence lifetime in absence or presence of DPA (λex = 532 nm, λem = 665 nm). All measurements performed under ambient conditions. Errors are the standard deviation of two measurements.

Due to the bimolecular nature of TTA, the rate of TTA will show a quadratic dependence on the concentration of excited triplet emitters (and thus excitation intensity). In diffusion-controlled ideal solutions, a clear distinction between low (quadratic) and high (linear) excitation intensity regimes can often be discerned.21 However, in (semi)solid-state systems, a more gradual shift between the two regimes is more commonly observed,21,66 as is the case here (Fig. 4d). Transition to linear dependence with excitation intensity implies that the concentration of triplet excited emitters is sufficient to ensure a high probability of annihilation collisions within their excited state lifetime, leading to near-unity triplet relaxation by TTA. However, in solid-state systems, sensitiser/emitter pairs are under non-ideal diffusion-control: the mean free path of both chromophores is likely to be more restricted compared to fluid solution, which may lead to less random collisions. As such, the change in slope from linear to quadratic may be less well-defined, as idealised TTET behaviour competes with alternative relaxation processes such as sensitiser self-annihilation.67

Lifetime studies

We had expected the lower Tg's of the higher MW organic precursors (Table 2) would lead to a higher rate of molecular diffusion and therefore improved UC performance; yet we see the opposite trend. To explore why this occurs, we next investigated the UC decay kinetics. The UC emission is very long-lived (milliseconds) and display a rise-time that cannot easily be reconvoluted from the instrument response function (see Fig. S9, ESI†). As such, the decay lifetimes were determined by tail fits (Fig. 5a).67 Full details of the fitting parameters are available in Table S5 (ESI†). Two exponential decay components were required to fit the decays: a shorter-lived, minor component (f1 = 18–34%, τ1 = 5–11 ms) attributed to partially oxygen-quenched UC emission, and a longer-lived component attributed to the natural UC emission (f2 = 66–82%, τ2 = 11–22 ms). The average upconversion lifetime 〈τUC〉 follows the trend tU3000 > DU2000 > tU5000 = DU4000, which mirrors the calculated inorganic silica content (Table 2 and Table S4, ESI†). 〈τUC〉 decreases with increasing polymer host MW and reduced branching. Previous studies on poly(ethylene)-poly(ethylene glycol)-SiO2 block copolymers observed that the oxygen transfer rate (OTR) was reduced when the ratio of inorganic component was increased.68 Moreover, polymers with a lower MW organic component showed a lower OTR due to reduced chain mobility.69 It is therefore highly probable that these combined factors also determine the observed trends in the ureasil system. We note that while it has been reported that DPA can form an endoperoxide bridge with singlet molecular oxygen, providing a certain degree of intrinsic oxygen protection,70 this cannot explain the differences that arise here since the DPA concentration is identical in all ureasils under investigation.

Physical properties of undoped ureasil samples

Ureasil	Silica contenta (wt%)	Coherence length, Lb (Å)	Structural unit distance, db (Å)	Shrinkagec (%)	T g (°C)	Elastic modulus (MPa)	Bending modulus (MPa)	
DU(2000)	4.1	17.28	4.30	55.5	−70	1.16 ± 0.002	4.93 ± 0.20	
DU(4000)	2.1	15.55	4.34	50.3	−73	0.53 ± 0.001	1.95 ± 0.21	
tU(3000)	4.1	17.39	4.28	49.7	−64	1.90 ± 0.004	9.39 ± 0.25	
tU(5000)	2.5	15.64	4.35	40.8	−82	0.97 ± 0.002	3.67 ± 0.66	
a Determined from the molecular weight of inorganic silica with respect to the MW of the ureasil (see ESI Section S1.2 and Table S1).

b From PXRD (see ESI, Section S5.3).

c Determined from the volume of the sol before casting and the final area of the solid ureasil when dried (see ESI, Section S1.2).

The phosphorescence decays in air were also measured for sensitiser/emitter and sensitiser-only ureasils (Fig. 5b and Fig. S9b, Table S6 (ESI†), PdOEP = 0.1 mM in both systems). This system is unusual as the lifetime with the emitter present is longer – the opposite of the usual case. This is thought to be due to back-transfer of triplet from the DPA emitter extending the lifetime of phosphorescence.71 The shorter lifetimes in PdOEP doped ureasils could also be due to relatively high concentrations of PdOEP leading to self-annihilation due to TTA between excited PdOEP triplets at high excitation intensities.67

Long-term UC stability

Air-stability is a huge challenge in the field of TTA-UC and the demand for materials with inherent oxygen protection is high. However, the long term ΦUC over time in air is seldom reported. Fig. 6 shows the fluctuation in ΦUC for samples measured over nearly 3 months. Notably, DU(2000) and tU(3000) retained their activity over a significantly longer period (up to 76–78 days) than the samples with the lower %wt silica content. In contrast, ΦUC for DU(4000) and tU(5000) dropped off rapidly and was negligible after 24 days. Air-stability decreases with increasing polymer host molecular weight and reduced branching.

For all samples, the loss in efficiency is likely due to two factors: atmospheric oxygen ingress into the ureasil leading to triplet quenching, and photobleaching of the chromophores over time. Fig. S10 (ESI†) shows the UV/Vis absorption spectra over time (up to 23 days post gelation). While the PdOEP absorption remains effectively constant in all ureasils, the DPA absorption decreases with time, suggesting photobleaching may be the primary factor. We note that ureasils themselves are typically photostable, with accelerated aging studies using intense UV irradiation (24 hours) leading to no change in optical performance.48

Effect of host structure on TTA-UC performance

Our photophysical studies demonstrate that the relationship between the ureasil host structure and the TTA-UC performance is non-trivial. If we consider the di- or tri-branched series individually, a lower MW and higher silica content leads to ureasils with higher ΦUC and increased stability. However, if we compare analogues between the two series, the trends are less clear.

Table 2 summarises the key structural, thermal, and mechanical properties of the undoped ureasils. All ureasils have a Tg well below 0 °C, as determined by dynamic mechanical analysis (DMA), indicating that at room temperature the polymer backbone should be in a viscous liquid or rubbery state with individual polymer chains moving freely (see also Fig. S12 and Table S7, ESI†). DMA measures viscoelasticity and gives a direct order of viscosity between ureasil samples, which is used as a proxy for chromophore diffusion rates. This below-ambient Tg suggests that all ureasils investigated should offer good chromophore solubility and a reasonable degree of mobility to facilitate the essential molecular collisions that for TTET. For both series, increasing the precursor chain length leads to a decrease in Tg, which is attributed to both an increase in chain flexibility68 and a decrease in the number of urea-silica linkages. This effect is more significant for the tri-branched series, for which the incidence of cross-linkages is inherently higher (Fig. 7a).

Fig. 7 Structural features of ureasil hosts. (a) Schematic representation of the effect of changing the MW and branching of the polymer precursor on the organic-to-inorganic ratio within the overall ureasil structure. The wavy black line represents organic polymer, and the blue circles represent inorganic silica, which is then crosslinked. (b) Hydrogen-bonding associations and corresponding peak assignments in the amide I region of the FTIR spectra of ureasils. R and R′ represent the inorganic and organic portion of the structure, respectively.

Further insight into the local organic structure can be obtained by examining the FTIR spectra in the amide I region (1600–1800 cm−1), which reveals the extent of hydrogen bonding (see Fig. 7b, Fig. S12 and Table S8, ESI†).44 Ureasils with a lower precursor MW have a lower proportion of free unbound urea groups (Peak 4, Fig. 7b and Fig. S12, ESI†) and weak, disordered hydrogen bonds (Peak 3, Fig. 7, and Fig. S12, ESI†) and a higher fraction of direct, ordered hydrogen- bonding from N–H to C <svg xmlns="http://www.w3.org/2000/svg" version="1.0" width="13.200000pt" height="16.000000pt" viewBox="0 0 13.200000 16.000000" preserveAspectRatio="xMidYMid meet"><metadata> Created by potrace 1.16, written by Peter Selinger 2001-2019 </metadata><g transform="translate(1.000000,15.000000) scale(0.017500,-0.017500)" fill="currentColor" stroke="none"><path d="M0 440 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z M0 280 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z"/></g></svg> O on neighbouring chains (Peak 1). A higher contribution from ordered hydrogen-bonding peaks is linked with a higher Tg and more closely packed structure.44,47 Moreover, increased levels of hydrogen-bonding order have also been shown to decrease oxygen penetration in related organic–inorganic polymers.69

DU(2000) and tU(3000) showed the highest ΦUC – these samples also have the lower MW chain for the di-and tri-branched series, respectively. However, tri-branched samples will contain a more condensed silica network than di-branched samples of comparable MW. The silica content (wt%) in the sample is therefore a good indirect measure of the combined effects of branching and chain length. For both di- and tri-branched precursors, the lower MW leads to a higher silica content (both ca. 4.1%), due to the shorter distance between chain ends as illustrated schematically in Fig. 7a. This can be observed at the macroscopic level: ureasils with the lowest wt% silica are more flexible and bendy (Fig. S13, ESI†), and therefore have higher elastic and bending moduli (see Table 2, and Fig. S13, S14, ESI†). Similarly, tri-branched structures have higher bending moduli than their di-branched counterparts.

PXRD provides insight into the size and coherence of the silica domains and the distance between them (see Fig. S15 and Table S9, ESI†) at the sub-nanometer scale.53 Interestingly, while the structural unit distance is consistent across all samples (d ∼4.3 Å, from Bragg's law), the coherence length, L, over which the structural unit survives (i.e. the crystallite size) is longer for both DU(2000) and tU(3000). This is somewhat counterintuitive, since a more persistent and condensed silica network would be expected to reduce lumophore mobility. This suggests that the molecular collisions responsible for TTA-UC are primarily localised to the organic domains (which is reasonable based on solubility grounds), and that as long as the Tg is below room temperature, there will be sufficient mobility for these to occur.

To examine the effect of oxygen exclusion on the UC performance, a preliminary test was conducted on DPA (10 mM)/PdOEP (0.1 mM)-tU(3000) in which the precursor solution was degassed by nitrogen bubbling for 1 hr before gelation. A slight increase in 〈τUC〉 and large increase in 〈τPHOS〉 was observed (to 289 μs, Table S10, ESI†), and ΦUC increased to 2.39 ± 0.30% at 1 W cm−2. This indicates that while the ureasil structure provides some intrinsic oxygen barrier (as shown by the long stability), higher TTET rates and ΦUC could possibly be achieved with additional oxygen removal.

Conclusions

We have demonstrated ureasils as highly effective host materials for solid-state TTA-UC, achieving ΦUC up to 1.86% (at 1 W cm−2 excitation density) in ambient conditions. The UC efficiency varies with the ureasil host used. All ureasils studied inherit high optical transparency, mechanical strength, and oxygen protection from the inorganic silica nanodomains, whilst retaining the chromophore solubility, low Tg, flexibility and processability of organic polymers. However, it is the balance between the organic-to-inorganic ratio and the persistence length of the silica domains that determines both the absolute efficiency and long-term stability of the UC emission. Of the four ureasils studied, DU(2000) and tU(3000) contain the highest silica content and domain coherence length. We postulate that these structural features help provide increased oxygen protection, as shown by the enhanced triplet lifetime and ΦUC. Impressively, measurable UC emission is retained for both samples for >70 days without any special treatment or storage, including deoxygenation. This ability to operate in ambient conditions due to inherent oxygen protection is highly desirable for almost all TTA-UC applications. We are currently undertaking detailed studies of the O2 transmission rates and local diffusivity in these ureasils and structurally-related analogues to fully identify the key structure-function characteristics. The underpinning knowledge obtained through this investigation will inform the design of next generation hybrid host structures, with the goal of further enhancing the efficiency and long-term stability of TTA-UC in the solid-state.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

TC-012-D4TC00562G-s001

TC-012-D4TC00562G-s002

This work was funded by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (Grant Agreement No. 818762 - SPECTRACON).
==== Refs
Parker C. A. Hatchard C. G. Proc. Chem. Soc. 1962 269 574 584
Parker C. A. Hatchard C. G. Joyce T. A. Nature 1965 205 1282 1284 10.1038/2051282a0
Askes S. H. C. Leeuwenburgh V. C. Pomp W. Arjmandi-Tash H. Tanase S. Schmidt T. Bonnet S. ACS Biomater. Sci. Eng. 2017 3 322 334 10.1021/acsbiomaterials.6b00678 28317022
Dou Q. Jiang L. Kai D. Owh C. Loh X. J. Drug Discovery Today 2017 22 1400 1411 10.1016/j.drudis.2017.04.003 28433535
van Son M. H. C. Berghuis A. M. Eisenreich F. de Waal B. Vantomme G. Gómez Rivas J. Meijer E. W. Adv. Mater. 2020 32 2004775 10.1002/adma.202004775 33118197
Yu T. Liu Y. Zeng Y. Chen J. Yang G. Li Y. Chem. – Eur. J. 2019 25 16270 16276 10.1002/chem.201904025 31587399
Uji M. Zähringer T. J. B. Kerzig C. Yanai N. Angew. Chem., Int. Ed. 2023 e202301506
Yildiz D. Baumann C. Mikosch A. Kuehne A. J. C. Herrmann A. Göstl R. Angew. Chem., Int. Ed. 2019 58 12919 12923 10.1002/anie.201907436 31265744
Tayebjee M. J. Y. McCamey D. R. Schmidt T. W. J. Phys. Chem. Lett. 2015 6 2367 2378 10.1021/acs.jpclett.5b00716 26266619
Pedrini J. Monguzzi A. J. Photonics Energy 2017 8 1
Beery D. Schmidt T. W. Hanson K. ACS Appl. Mater. Interfaces 2021 13 32601 32605 10.1021/acsami.1c08159 34228449
Simon Y. C. Weder C. J. Mater. Chem. 2012 22 20817 10.1039/C2JM33654E
Zhou Y. Castellano F. N. Schmidt T. W. Hanson K. ACS Energy Lett. 2020 5 2322 2326 10.1021/acsenergylett.0c01150
Yanai N. Suzuki K. Ogawa T. Sasaki Y. Harada N. Kimizuka N. J. Phys. Chem. A 2019 123 10197 10203 10.1021/acs.jpca.9b08636 31663744
Singh-Rachford T. N. Castellano F. N. Coord. Chem. Rev. 2010 254 2560 2573 10.1016/j.ccr.2010.01.003
DECC, 2011, 83–86
Baluschev S. Miteva T. Yakutkin V. Nelles G. Yasuda A. Wegner G. Phys. Rev. Lett. 2006 97 143903 10.1103/PhysRevLett.97.143903 17155253
Islangulov R. R. Lott J. Weder C. Castellano F. N. J. Am. Chem. Soc. 2007 129 12652 12653 10.1021/ja075014k 17900120
Bennison M. J. Collins A. R. Zhang B. Evans R. C. Macromolecules 2021 54 5287 5303 10.1021/acs.macromol.1c00133 34176961
Joarder B. Yanai N. Kimizuka N. J. Phys. Chem. Lett. 2018 9 4613 4624 10.1021/acs.jpclett.8b02172 30059619
Gray V. Moth-Poulsen K. Albinsson B. Abrahamsson M. Coord. Chem. Rev. 2018 362 54 71 10.1016/j.ccr.2018.02.011
Bharmoria P. Yanai N. Kimizuka N. Gels 2019 5 18 10.3390/gels5010018 30917611
Vadrucci R. Weder C. Simon Y. C. Mater. Horiz. 2015 2 120 124 10.1039/C4MH00168K
Zhou H. Lin J. Wan S. Lu W. Phys. Chem. Chem. Phys. 2022 24 29151 29158 10.1039/D2CP04532J 36444712
Huang L. Kakadiaris E. Vaneckova T. Huang K. Vaculovicova M. Han G. Biomaterials 2019 201 77 86 10.1016/j.biomaterials.2019.02.008 30802685
Nishikiori H. Takeshita M. Komatsu Y. Satozono H. Teshima K. ACS Omega 2018 3 8529 8536 10.1021/acsomega.8b01107 31458982
Schloemer T. Narayanan P. Zhou Q. Belliveau E. Seitz M. Congreve D. N. ACS Nano 2023 17 3259 3288 10.1021/acsnano.3c00543 36800310
Zhang B. Richards K. D. Jones B. E. Collins A. R. Sanders R. Needham S. R. Qian P. Mahadevegowda A. Ducati C. Botchway S. W. Evans R. C. Angew. Chem. 2023 135 e202308602 10.1002/ange.202308602
Simon Y. C. Weder C. Nat. Mater. 2015 14 864 865 10.1038/nmat4375 26237126
Healy C. Hermanspahn L. Kruger P. E. Coord. Chem. Rev. 2021 432 213756 10.1016/j.ccr.2020.213756
Vadrucci R. Monguzzi A. Saenz F. Wilts B. D. Simon Y. C. Weder C. Adv. Mater. 2017 29 1702992 10.1002/adma.201702992 28898468
Bharmoria P. Hisamitsu S. Sasaki Y. Kang T. S. Morikawa M. A. Joarder B. Moth-Poulsen K. Bildirir H. Mårtensson A. Yanai N. Kimizuka N. J. Mater. Chem. C 2021 9 11655 11661 10.1039/D1TC00287B
Singh-Rachford T. N. Lott J. Weder C. Castellano F. N. J. Am. Chem. Soc. 2009 131 12007 12014 10.1021/ja904696n 19650646
Yang H. Guo S. Jin B. Luo Y. Li X. Polym. Chem. 2022 13 4887 4894 10.1039/D2PY00596D
Monguzzi A. Mauri M. Bianchi A. Dibbanti M. K. Simonutti R. Meinardi F. J. Phys. Chem. C 2016 120 2609 2614 10.1021/acs.jpcc.6b00223
Lee S. H. Lott J. R. Simon Y. C. Weder C. J. Mater. Chem. C 2013 1 5142 5148 10.1039/C3TC30878B
Karpicz R. Puzinas S. Gulbinas V. Vakhnin A. Kadashchuk A. Rand B. P. Chem. Phys. 2014 429 57 62 10.1016/j.chemphys.2013.12.004
Raišys S. Juršėnas S. Simon Y. C. Weder C. Kazlauskas K. Chem. Sci. 2018 9 6796 6802 10.1039/C8SC02151A 30310612
Kashino T. Hosoyamada M. Haruki R. Harada N. Yanai N. Kimizuka N. ACS Appl. Mater. Interfaces 2021 13 13676 13683 10.1021/acsami.0c23121 33656328
Kashino T. Haruki R. Uji M. Harada N. Hosoyamada M. Yanai N. Kimizuka N. ACS Appl. Mater. Interfaces 2022 14 22771 22780 10.1021/acsami.1c17021 35014267
Amberg-Schwab S. Hoffmann M. Bader H. Gessler M. J. Sol-Gel Sci. Technol. 1998 13 141 146 10.1023/A:1008628029870
Amberg-Schwab S. Weber U. Burger A. Nique S. Xalter R. Monatsh. Chem. 2006 137 657 666 10.1007/s00706-006-0466-4
Sakka S. , in Handbook of Sol–gel Science and Technology, Kluwer Academic Publishers, 2005, p. 455
de Zea Bermudez V. Carlos L. D. Alcácer L. Chem. Mater. 1999 11 569 580 10.1021/cm980372v
Carlos L. D. De Zea Bermudez V. Sá Ferreira R. A. Marques L. Assunção M. Chem. Mater. 1999 11 581 588 10.1021/cm980373n
Collins A. , Southern T. , Lyu G. , Bennison M. J. and Evans R. C. , in Photosensitive Materials and their Applications, ed. R. R. McLeod, Y. Tomita, J. T. Sheridan and I. Pascual Villalobos, SPIE, 2020, vol. 11367, p. 32
Meazzini I. Blayo C. Arlt J. Marques A. T. Scherf U. Burrows H. D. Evans R. C. Mater. Chem. Front. 2017 1 2271 2282 10.1039/C7QM00264E
Kaniyoor A. Mckenna B. Comby S. Evans R. C. Adv. Opt. Mater. 2016 4 444 456 10.1002/adom.201500412
Nolasco M. M. Vaz P. M. Freitas V. T. Lima P. P. André P. S. Ferreira R. A. S. Vaz P. D. Ribeiro-Claro P. Carlos L. D. J. Mater. Chem. A 2013 1 7339 7350 10.1039/C3TA11463E
Fernandes M. and de Zea Bermudez V. , in Chemical Solution Synthesis for Materials Design and Thin Film Device Applications, Elsevier, 2021, pp. 439–475
Bastos A. McKenna B. Lima M. André P. S. Carlos L. D. Evans R. C. Ferreira R. A. S. ACS Omega 2018 3 13772 13781 10.1021/acsomega.8b01726 31458077
Lyu G. Southern T. J. F. Charles B. L. Roger M. Gerbier P. Clément S. Evans R. C. J. Mater. Chem. C 2021 9 13914 13925 10.1039/D1TC02794H 34745631
Meazzini I. Willis-Fox N. Blayo C. Arlt J. Clément S. Evans R. C. J. Mater. Chem. C 2016 4 4049 4059 10.1039/C5TC03952E
Kim J.-H. Deng F. Castellano F. N. Kim J.-H. Chem. Mater. 2012 24 2250 2252 10.1021/cm3012414
Naimovičius L. Bharmoria P. Moth-Poulsen K. Mater. Chem. Front. 2023 7 2297 2315 10.1039/D3QM00069A 37313216
Molina E. F. Marçal L. Pereira De Carvalho H. W. Nassar E. J. Ciuffi K. J. Polym. Chem. 2013 4 1575 1582 10.1039/C2PY21049E
Morris J. V. Mahaney M. A. Huber J. R. J. Phys. Chem. 1976 80 969 974 10.1021/j100550a010
Congrave D. G. Drummond B. H. Gray V. Bond A. D. Rao A. Friend R. H. Bronstein H. Polym. Chem. 2021 12 1830 1836 10.1039/D1PY00118C
Fu L. Sá Ferreira R. A. Fernandes M. Nunes S. C. de Zea Bermudez V. Hungerford G. Rocha J. Carlos L. D. Opt. Mater. 2008 30 1058 1064 10.1016/j.optmat.2007.05.011
Monguzzi A. Tubino R. Hoseinkhani S. Campione M. Meinardi F. Phys. Chem. Chem. Phys. 2012 14 4322 10.1039/C2CP23900K 22370856
Wei L. Fan C. Rao M. Gao F. He C. Sun Y. Zhu S. He Q. Yang C. Wu W. Mater. Horiz. 2022 9 3048 3056 10.1039/D2MH00887D 36213984
Saenz F. Ronchi A. Mauri M. Vadrucci R. Meinardi F. Monguzzi A. Weder C. Adv. Funct. Mater. 2021 31 2004495 10.1002/adfm.202004495
Durandin N. A. Isokuortti J. Efimov A. Vuorimaa-Laukkanen E. Tkachenko N. V. Laaksonen T. J. Phys. Chem. C 2019 123 22865 22872 10.1021/acs.jpcc.9b08026
Carrod A. J. Gray V. Börjesson K. Energy Environ. Sci. 2022 15 4982 5016 10.1039/D2EE01600A
Ogawa T. Yanai N. Monguzzi A. Kimizuka N. Sci. Rep. 2015 5 1 9
Bharmoria P. Bildirir H. Moth-Poulsen K. Chem. Soc. Rev. 2020 49 6529 6554 10.1039/D0CS00257G 32955529
Edhborg F. Olesund A. Albinsson B. Photochem. Photobiol. Sci. 2022 21 1143 1158 10.1007/s43630-022-00219-x 35441266
Minelli M. De Angelis M. G. Doghieri F. Marini M. Toselli M. Pilati F. Eur. Polym. J. 2008 44 2581 2588 10.1016/j.eurpolymj.2008.06.006
Toselli M. Pilati F. Marini M. Doghieri F. De Angelis M. G. Minelli M. Eur. Polym. J. 2008 44 3256 3263 10.1016/j.eurpolymj.2008.07.037
Drews W. Shimidt R. Brauer H. D. Chem. Phys. Lett. 1980 70 84 88 10.1016/0009-2614(80)80065-0
Kinoshita M. Sasaki Y. Amemori S. Harada N. Hu Z. Liu Z. Ono L. K. Qi Y. Yanai N. Kimizuka N. ChemPhotoChem 2020 4 5271 5278 10.1002/cptc.202000143
