Enhancing photon upconversion properties in anthracene derivatives through meticulous control of excluded volume around π-systems

Abstract

Photon upconversion, which converts low-energy photons into high-energy photons, provides a strategy for overcoming the limitations imposed by excitation wavelengths in energy and biology applications. Recent advancement in triplet–triplet annihilation-based photon upconversion (TTA-UC) witnesses the importance of the bulkiness of the substituents in chromophores to avoid quenching of the singlet and triplet. One of the simplest bulky substituents, the tert-butyl (tBu) group, has been introduced into acene derivatives and shown both positive and negative effects on UC performance, underscoring the importance of further structure–property investigations. Here, we systematically synthesized anthracene derivatives of 9,10-bis(phenylethynyl)anthracene (BPEA) and 9,10-bis[(triisopropylsilyl)ethynyl]anthracene (TIPS-Ac), each bearing bulky tBu substituents. The relationship between the excluded volume imparted by these substituents and UC performance in both solution and the solid state was investigated. A moderate intermolecular distance effectively suppressed singlet and triplet quenching, yielding high UC quantum yields of approximately 15% in the solution state with both chromophores. A significant extension of the triplet lifetime was also observed in a donor–acceptor bilayer solid film, demonstrating the simple yet positive effects of tBu on the anthracene backbone, thereby boosting the UC performances in versatile material forms.

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Yoichi Sasaki

Yoichi Sasaki is an associate professor in the Department of Chemistry and Biochemistry at Kyushu University in Japan. He received his Ph.D. degree in engineering from Kyushu University in 2021 under the supervision of Prof. Nobuhiro Yanai and Prof. Nobuo Kimizuka. He was a visiting researcher at the University of Sheffield (Prof. Jenny Clark's group, 2019) and a postdoctoral fellow at the University of Strasbourg (Prof. Thomas Ebbesen's group, 2021–2022). His research focuses on photochemistry, physical chemistry, and material chemistry.

Introduction

Photon upconversion (UC) is a molecular-based photophysical process that converts low-energy photons into high-energy photons. This process has advanced to overcome limitations in excitation wavelength in fields such as energy, biology, materials chemistry, and more. In particular, triplet–triplet annihilation-based UC (TTA-UC), which exploits energy transfer processes involving long-lived excited triplets in molecules, can be driven even under non-coherent, low-excitation-intensity conditions, such as sunlight.^1–14 The ability to utilize a broad spectrum of light by applying appropriate molecular systems enables applications in solar energy harvesting,^15,16 biosensing,^17–19 and photocatalytic reactions.^20,21

As the TTA-UC process requires the collision of two triplets that are not populated by direct transitions from ground singlet states, triplet sensitizers (donors) showing highly efficient singlet-to-triplet intersystem crossing (ISC) are primarily combined with the UC chromophores (acceptors, Fig. 1). After the Dexter-type triplet energy transfer (TET) from the donor to the acceptor, two triplet-excited acceptors undergo TTA and generate one singlet-excited acceptor that emits upconverted photons. The key indices for TTA-UC materials are a high UC quantum yield (Φ_UC, theoretical maximum: 50%), a low threshold excitation intensity (I_th) above which the TTA process becomes efficient, and the wavelength shift from excitation to emission. The Φ_UC relies on each process of ISC, TET, TTA, fluorescence, and probability to obtain a singlet in TTA (f) as shown in eqn (1),^5,22

where Φ_ISC, Φ_TTA, and Φ_F represent the quantum yields of ISC, TTA, and acceptor fluorescence, respectively.

Fig. 1 Scheme of TTA-UC mechanism and excluded volume design on acceptor anthracene derivatives. D: donor.

While studies over the past two decades have optimized each process, the molecular design for achieving high f values has been unclear. The f values are affected not only by the spin-statistically predicted singlet generation efficiency from two excited triplets, but also by reverse intersystem crossing from higher-lying excited states^23–25 and the effect of excimer formation from singlet and triplet pair states.²⁶ Based on conventional spin-statistical probability, assuming that only singlet and triplet states are formed in a 1 : 3 ratio and that the triplets will be recycled to undergo TTA, the f value reaches 40%, as reported for several UC chromophores.²⁷ The effect of substituents has been investigated since the 2010s,^28,29 but with growing interest after the successful demonstration of efficient UC quantum yields by (triisopropylsilyl)ethynyl (TIPS)-substituted anthracene and naphthalene derivatives.^30,31

The attempts to reduce I_th are categorized into optimizations of sensitization and TTA processes. Based on the widely used TTA model in an isotropic system with almost unity ISC efficiency, I_th is expressed as follows,³²

where α is the donor absorption coefficient at the excitation wavelength, and D_T denotes the diffusion constant of acceptor triplets, a₀ denotes the interaction distance between acceptor triplets, and τ_T denotes the acceptor triplet lifetime.

To achieve unity TET and TTA efficiency, it is ideal to use donors with high molar absorption coefficients, high ISC efficiency, and donors and acceptors with long excited-state lifetimes. In the solution state, the triplet diffusion constant depends on the molecular diffusion rate; thus, the use of acceptor molecules with a long triplet lifetimes is particularly important to reduce the I_th value. Albinsson et al. focused on substituted naphthalene derivatives to clarify the effect of bulkiness. They found a significant reduction in triplet lifetime at high acceptor concentrations, assigning the additional quenching pathway as triplet excimer formation.^31,33 With these works, control of intermolecular interaction with bulky substituents significantly suppresses the quenching of both singlets and triplets. However, bulkiness can reduce the triplet diffusion constant and TTA rates by increasing the net intermolecular distance^28,34–36. To elucidate optimal intermolecular distance and relative geometry that simultaneously affect UC properties, further systematic studies on the effect of steric bulkiness on UC chromophores are necessary.

In addition to optimizing solution-based TTA-UC systems, the development of efficient matrix-free solid-state UC systems has been pursued for practical applications. Developing efficient solid-state UC materials has been more challenging due to the tendency for strong chromophore interactions in the condensed state, which quench UC fluorescence.³⁷ In addition, such systems are sensitive to the compatibility within the mixed solid. Phase separation of the donor and acceptor significantly reduces the triplet energy transfer efficiency.⁵ To control the donor-dispersibility and triplet diffusion processes in the solid state, several side groups and molecular backbones have been proposed to manage molecular assembly.^28,37–44 One of the simplest substituents is the tert-butyl (tBu) group, composed of saturated sp³ carbons, which provides only a slight difference in π-conjugation. Several UC acceptors incorporating the tBu group have been proposed to date^26,42,43,45. The introduction of a tBu group into rubrene successfully reduced intermolecular interactions, thereby slowing detrimental singlet fission.⁴² Börjesson et al. successfully suppressed excimer formation in perylene with alkyl groups and improved the UC quantum yield from 5% to 10%.²⁸ Pun et al. investigated a diketopyrrolopyrrole derivative with tBu groups and found a decrease in the f value due to a reduction in the effective annihilation radius.⁴³ These works show that the effect of the bulky substituent is structure-dependent. Despite extensive research over the past two decades, the impact of the tBu group on the UC performance of anthracene derivatives, which are essential benchmarks, remains unclear.

In this work, to control intermolecular interactions in anthracene derivatives, tBu groups were introduced, and their TTA-UC properties were investigated in solution and in the solid state. Ethynylphenyl or triisopropylsilylethynyl (TIPS) groups were introduced to the anthracene skeleton, having one or two tBu groups,⁴⁶ and a series of 9,10-bis(phenylethynyl)anthracene (BPEA) or 9,10-bis[(triisopropylsilyl)ethynyl]anthracene (TIPS-Ac) derivatives with tBu groups were synthesized (Fig. 1, tBuBPEA,⁴⁷ (tBu)₂BPEA, and (tBu)₂TIPS-Ac). To clarify structure–UC profile relationships, TTA-UC properties of systems composed of palladium(II) tetraphenyl-tetrabenzoporphyrin (PdTPTBP) as a donor and the anthracene derivatives as an acceptor, were compared in the solution state. Furthermore, matrix-free donor–acceptor solid bilayer films were fabricated by vacuum deposition. In both systems, molecules bearing bulky substituents exhibited higher or comparable UC quantum yields and lower threshold excitation intensities than unsubstituted structures. The effects of tBu groups on the singlet and triplet dynamics were systematically investigated, and the potential quenching processes are discussed.

Results and discussion

The effect of tBu groups on solution-state photophysical properties was investigated for BPEA and TIPS-Ac. The absorption and emission peaks of both compounds showed slight shifts, indicating hyperconjugation with the tBu group (Fig. 2).⁴⁸ This result is supported by the time-dependent density functional theory (TD-DFT) calculations (Fig. S6 and S7). All these molecules were found to satisfy the energetic conditions E(S₁) < E(2T₁) < E(T₂), which are ideal to maximize singlet formation via TTA. High fluorescence quantum yields (Φ_F > 90%) are retained after substitution, showing the tBu groups introduced in anthracene chromophores do not promote nonradiative deactivation (Table 1).⁴⁹

Fig. 2 (a–e) Normalized absorption (solid line) and fluorescence (filled in) spectra of BPEA and TIPS-Ac derivatives. (a) BPEA, (b) tBuBPEA, and (c) (tBu)₂BPEA in deaerated 100 µM THF. (d) TIPS-Ac and (e) (tBu)₂TIPS-Ac in deaerated 10 µM THF. (f) Normalized absorption spectrum of PdTPTBP in dearated 50 µM THF.

Table 1 Fluorescence peak wavelength (0–0 transition), quantum yield (BPEA derivatives: λ_ex = 425 nm, TIPS-Ac derivatives: λ_ex = 440 nm) and fluorescence lifetime (λ_ex = 405 nm) of anthracene derivatives in deaerated THF at a concentration of 100 µM. Calculated radiative and non-radiative rate constant are also shown

	λF (nm)	ΦF (%)	τF (ns)	kr (10^8 s^−1)	knr (10^6 s^−1)
BPEA	482	91	3.7	2.5	24
tBuBPEA	484	93	3.9	2.4	18
(tBu)2BPEA	486	93	3.9	2.4	17
TIPS-Ac	448	93	6.0	1.5	12
(tBu)2TIPS-Ac	457	94	6.4	1.5	8.5

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Upon irradiation with a 635 nm laser under Ar, THF solutions containing PdTPTBP and anthracene derivatives yielded blue-to-green upconverted emissions (Fig. 3a and 4a). The double logarithmic plots of the UC emission intensity as a function of the excitation intensity showed a transition from quadratic to linear dependence as the excitation intensity increased, confirming the TTA-UC-based mechanism (Fig. 3b and 4b). The excitation intensity at the intersection of the fitting lines of the quadratic and linear regimes is defined as I_th (eqn (2)). The impact of tBu substitution on the I_th was dependent on the core acceptor structure. For the TIPS-Ac derivatives, the introduction of the tert-butyl group led to a decrease in I_th from 45 mW cm⁻² to 24 mW cm⁻², while the BPEA derivatives showed the opposite trend from 6.9 mW cm⁻² to 19 mW cm⁻². The observed difference is likely due to a trade-off between the reduction in the TTA rate and the extension of the excited-state lifetime conferred by the tBu groups that reduce intermolecular interactions. The introduction of bulky substituents extended the triplet lifetimes τ_T of both chromophores (τ_T = 2τ_UC,^50,51 Fig. 3d and 4d), indicating the suppressed triplet quenching at high acceptor concentrations.³³ (tBu)₂TIPS-Ac showed longer τ_T than (tBu)₂BPEA, which is probably due to the rotation of phenyl rings in BEPA that can fluctuate energy levels and facilitate nonradiative decay.⁵² It is also considered that the bulkier TIPS group suppressed quenching in (tBu)₂TIPS-Ac.

Fig. 3 UC properties of BPEA derivatives in deaerated THF solution ([BPEA derivatives] : [PdTPTBP] = 1 mM : 50 µM, λ_ex = 635 nm). (a) UC emission spectra and (b) excitation intensity dependence of the upconverted emission intensity around emission peak wavelength (BPEA: 479.5 nm, tBuBPEA: 483 nm, (tBu)₂BPEA: 486 nm). The UCPL intensity is plotted in arbitrary units for clarity. (c) UC quantum yield (Φ_UC) at various excitation intensities. (d) UC emission decay curves of BPEA derivatives. Excitation light was removed with a 610 nm short-pass filter.

Fig. 4 UC properties of TIPS-Ac derivatives in deaerataed THF solution ([TIPS-Ac derivatives] : [PdTPTBP] = 10 mM : 50 µM, λ_ex = 635 nm). (a) UC emission spectra and (b) excitation intensity dependence of the upconverted emission intensity around emission peak wavelength (TIPS-Ac: 472 nm, (tBu)₂TIPS-Ac: 481 nm). The UCPL intensity is plotted in arbitrary units for clarity. (c) UC quantum yield (Φ_UC) at various excitation intensities. (d) UC emission decay curves of TIPS-Ac and (tBu)₂TIPS-Ac. Excitation light was removed with a 610 nm short-pass filter.

The UC parameters obtained from the experiments are summarized in Table 2. The introduction of bulky substituents was found to boost the maximum upconversion (UC) quantum yield (Φ_UC, max 50%). The ISC efficiency was taken as 97%, following previously reported values.⁵³ The donor phosphorescence decays within several microseconds in the presence of the acceptor, suggesting high TET efficiency (Φ_TET ≈ 100%, Table 2 and Fig. S9). The donor's phosphorescence quantum yield also confirms high TET efficiency (Table S2). Although the Stern–Volmer plot indicated a reduction in the donor–acceptor energy transfer rate due to the excluded volume of bulky substituents in acceptors (Fig. S10 and Table S3), the estimated TET efficiencies were unaffected, thanks to the long excited-state lifetime of PdTPTBP (τ_T = 139 μs, Fig. S8).

Table 2 Summary of UC properties in deoxidized THF ([BPEA derivatives] : [PdTPTBP] = 1 mM : 50 µM, [TIPS-Ac derivatives] : [PdTPTBP] = 10 mM : 50 µM)

	ΦUC (%)	f	ΦTET (%)	ΦF (%)	Ith (mW cm^−2)	τT (ms)
BPEA	4.5	0.10	≈100	87	6.9	0.52
tBuBPEA	5.7	0.13	≈100	88	14	0.70
(tBu)2BPEA	15	0.34	≈100	91	19	0.84
TIPS-Ac	12	0.31	≈100	81	45	0.83
(tBu)2TIPS-Ac	14	0.32	≈100	93	24	2.0

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The suppressed intermolecular interaction improved Φ_F of TIPS-Ac from 81% to 93% in 10 mM THF solution (Table S1), which resulted in the higher Φ_UC of 14% in the (tBu)₂TIPS-Ac/PdTPTBP system (Fig. 4c). By assuming that the TTA efficiency Φ_TTA approaches 100% at higher excitation intensity, the f value of TIPS-Ac and (tBu)₂TIPS-Ac were calculated as 0.31 and 0.32, respectively. The negligible change in the f value indicates that the bulkier TIPS group dominantly contributes to the net intermolecular interactions. In contrast, the f value for BPEA derivatives increased from 0.10 to 0.34 as the number of tBu groups increased. A previous report explained the low f value of BPEA as due to rotation of the phenyl groups, which fluctuate the T₁ energy,⁵² and discussed the contributions of the T₁ and T₂ energy gaps to the f value.^23,25 The small differences in the calculated T₁ and T₂ energy levels for the current BPEA derivatives indicate that the improvement in the f value is ascribable to moderate intermolecular interactions and reduced nonradiative deactivation channels by the bulky tBu groups. The similar f value of (tBu)₂BPEA as TIPS-Ac, together with the reported improvements in Φ_UC with TIPS substituents,^{30,31,33,35,36,45,54–58} indicates that the steric effect is crucial for achieving high f values regardless of the skeleton.^42,43 Therefore, the effect of bulky substituents improves the UC performance of anthracene derivatives in solution through multiple benefits from the moderately reduced intermolecular interactions: an enhancement of key photophysical parameters, such as the fluorescence quantum yield and f value, and an extension of the triplet lifetime.

Furthermore, we observed UC emission from bilayer solid UC films fabricated by sequential thermal deposition of the donor and the acceptors TIPS-Ac or (tBu)₂TIPS-Ac onto a quartz substrate (Fig. 5, S12, S13 and Table S4). In the solid bilayer architecture, which separates the donor and acceptor layers, potentially suppresses back energy transfer from the acceptor to the donor when triplet diffusion in acceptors is fast.⁶⁰ In both films, blue UC emission was observed under the excitation at 635 nm (Fig. 5a). The (tBu)₂TIPS-Ac showed one order of magnitude longer triplet lifetime of 1.1 ms (Fig. 5c), resulting in a low threshold excitation light intensity of 12 mW cm⁻². This significant improvement underscores the importance of a controlled excluded-volume concept that reduces radiationless quenching by introducing bulky substituents around the π-electron systems to moderately isolate them, while maintaining the appropriate intermolecular distance (<1.0 nm) for triplet energy migration within solid acceptor assemblies. To estimate TET efficiency, we compared phosphorescence quantum yield (Φ_P). However, we did not observe clear difference between the PdTPTBP film (1.9%) and PdTPTBP/TIPS-Ac (1.6%) and PdTPTBP/(tBu)₂TIPS-Ac (2.2%), which indicates the TET is not efficient in the bilayer systems. The double-logarithmic plots of phosphorescence intensity versus excitation intensity yielded slopes of 0.93 for PdTPTBP/TIPS-Ac and 0.80 for PdTPTBP/(tBu)₂ TIPS-Ac (Fig. S1). These sub-linear slopes indicate the presence of additional quenching pathways or exciton saturation within the sensitizer thin films. By accounting for this excitation intensity dependence of the reference sensitizer's phosphorescence quantum yield, the relative Φ_UC values were determined to be 0.41% and 0.20%, respectively (Fig. 5d). In addition to the low TET efficiency, singlet back energy transfer (BET) from acceptor to donor potentially reduces Φ_UC. Based on the fluorescence decay profile, the singlet BET efficiency (Φ_BET = 1 − τ_F/τ_F,0) of TIPS-Ac/PdTPTBP and (tBu)₂TIPS-Ac/PdTPTBP were estimated to be 49% and 58%, respectively (Fig. S16 and Table S5). Therefore, suppression of the BET using singlet energy collectors with properly designed molecular assemblies is necessary for further improvement.^61,62 The reduction of the Φ_UC under high excitation light intensity is attributed to the triplet back energy transfer from the acceptor to the donor.⁵ A remaining challenge in the bilayer system is that TET is limited to the donor–acceptor interface, which can lead to the loss of a fraction of absorbed photons via competing decay pathways such as phosphorescence. The need remains to resolve challenges through integrated structure design and energy collectors.

Fig. 5 UC properties of TIPS-Ac derivatives in the solid-state (λ_ex = 635 nm). (a) UC emission spectra and (b) excitation intensity dependence of the upconverted emission intensity around emission peak wavelength (TIPS-Ac: 473.5 nm, (tBu)₂TIPS-Ac: 459.5 nm). I_th values were estimated by theoretical fitting equation reported by Murakami et al. (SI, section S10).⁵⁹ (c) UC emission decay curves of TIPS-Ac (green) and (tBu)₂TIPS-Ac (red). Excitation light was removed with a 590 nm short-pass filter. (d) UC quantum yield (Φ_UC) at various excitation intensities calculated with relative method (standard: PdTPTBP phosphorescence).

Conclusions

In conclusion, the introduction of a bulky tBu group to the anthracene backbone enhances TTA-UC performance in solution and the solid state with minimal change in energy levels. Both (tBu)₂BPEA and (tBu)₂TIPS-Ac showed longer triplet lifetimes, consistent with previous studies with naphthalene derivatives.³³ The threshold excitation intensities decrease or increase with substitution, indicating the trade-off between the reduction in the TTA rate and the extension of the excited-state lifetime. Furthermore, controlled steric hindrance enhances the UC quantum yield, with a maximum of around 15%. The increase in triplet lifetime after tBu substitution was more apparent in the solid state, where a significant decrease in the threshold intensity was observed in (tBu)₂TIPS-Ac. These results demonstrate that control of intermolecular interactions with sp³ carbon-based excluded-volume engineering provides an effective molecular design strategy for extending triplet lifetimes, particularly in the solid state, thereby achieving low threshold intensities.

Author contributions

Y. S. conceived the project using sp³ carbon-based bulky substituents. M. M. synthesized and characterized the compound. M. M. conducted the UC measurements with K. Mizukami and K. Matsumoto. M. M., Y. S., and N. K. wrote the manuscript with input from the other authors. Y. S. and N. K. supervised the project. All authors contributed to and have approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article are included in the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5nr05481h.

The data underlying the figures and conclusions in this publication will be made available in the Zenodo data repository after publication of the manuscript.

Acknowledgements

This work was supported by JSPS KAKENHI (grant numbers JP20H05676, JP24K17745) and Japan Science and Technology Agency (JST, grant numbers JPMJAX24D8, JPMJAP2508). The authors thank the Service Center for Elementary Analysis of Organic Compounds, Faculty of Science, Kyushu University, for the elementary analysis.

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Footnote

† Present address: RIKEN Center for Emergent Matter Science, Wako, Saitama, Japan.

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