Visible-to-UV photon upconversion in metal-free molecular aggregates based on glassy diphenylnaphthalene derivatives

Abstract

Visible-to-ultraviolet photon upconversion (UC) based on triplet–triplet annihilation was demonstrated in metal-free glassy solid films consisting of an organoboron photosensitizer and diphenylnaphthalene-based emitter. Upon photoexcitation at 445 nm, UC emissions in the ultraviolet region (370–390 nm) were observed in binary solid films with high UC efficiencies of up to 2.6% and a threshold excitation intensity as low as 44 mW cm⁻².

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Photon upconversion (UC) is a versatile photophysical process that can convert lower-energy photons into higher-energy photons.¹ Triplet–triplet annihilation upconversion (TTA-UC), also known as triplet fusion UC, has recently attracted significant attention because of its advantages over other UC techniques, including low excitation intensity requirement, high UC quantum yield (Φ_UC), and tunable UC emission wavelength.^2–14 In particular, TTA-UC from visible to ultraviolet (UV) light has been applied to photocatalytic systems.^15–20 For further expansion of the applications, it is desirable to construct efficient solid-state TTA-UC systems that are completely different from those in conventional liquid media (diffusion-dependent systems). To date, various approaches to solid-like (or quasi-solid) TTA-UC have been attempted, including using polymer matrix,^21–36 gel matrix,^37–39 and solid film systems.^40–43 However, few reports exist on efficient solid-state visible-to-UV TTA-UC that can be driven with substantially low excitation power.^27,43

Here, we report a simple but efficient all-solid-state visible-to-UV TTA-UC system that does not require an additional host matrix nor a toxic metal-complex photosensitizer (Fig. 1). This novel TTA-UC system features metal-free binary solid mixtures of an organoboron photosensitizer (BBCz-SB-Br) doped into an amorphous organic emitter (SiDPN-1 and SiDPN-2). The photosensitizer absorbs incident light and subsequent intersystem crossing (ISC) forms the triplet (T₁) excitons, which are then transferred to the emitter via triplet energy transfer (TET); TTA between two T₁ excitons produces a higher-energy singlet (S₁) exciton on the emitter, resulting in UC emission (Fig. 1b). Unlike conventional solutions, our TTA-UC system does not require fluidic molecular diffusion and functions in the form of transparent glassy solid films.

Fig. 1 (a) Molecular structures of a photosensitizer and emitters used in solid-state TTA-UC systems. (b) Mechanistic diagram of TTA-UC in the BBCz-SB-Br:SiDPN-2 binary system.

BBCz-SB-Br,⁴⁴ which is employed as a photosensitizer in this study, is originally an organoboron-based multi-resonance thermally activated delayed fluorescence (MR-TADF) material,^45–47 that can populate its T₁ states with ∼100% intersystem crossing (ISC) quantum yield because of the small singlet–triplet energy gap (ΔE_ST, ∼0.17 eV) and ultrafast ISC (∼10⁹ s⁻¹) facilitated by the heavy atom effect of the Br group (ESI† for details). BBCz-SB-Br is also advantageous because it can suppress the reabsorption of UC emission in the range of 350–420 nm (Fig. 2) and reduce energetic losses associated with ISC due to its small ΔE_ST.^48–51 As UC emitters (also referred to as annihilators) functioning in the UV region, we developed SiDPN-1 and SiDPN-2 based on 1,4-diphenylnaphthalene (DPN). The basic photophysical properties of these emitters and sensitizer are presented in Fig. 2 and Table 1. Introducing a bulky tetraphenylsilane moiety allows the emitters to form thermodynamically stable glassy solids. SiDPN-2 is anticipated to perform better than SiDPN-1, especially in solid-state TTA-UC, because of the relatively high density of DPN subunits and potential for inter- and intramolecular TET and TTA.^32,52–56

Fig. 2 (a) UV-vis absorption, fluorescence (300 K), and phosphorescence (77 K) spectra of the BBCz-SB-Br photosensitizer and (b) fluorescence (300 K) and phosphorescence (77 K) spectra of DPN-based emitters in deaerated toluene solutions ([BBCz-SB-Br] = 100 μM, [DPN] = 20 mM, [SiDPN-1] = 20 mM, and [SiDPN-2] = 10 mM).

Table 1 Photophysical data of DPN-based emitters

Emitter	State^a	λ PL (nm)	Φ PL (%)	τ (ns)	k r (10^8 s^−1)	k nr (10^8 s^−1)	E S (eV)	E T (eV)
a Sol = deoxygenated toluene solution with a DPN subunit concentration of 20 mM; film = neat film of 1 μm thickness. b PL emission maximum. c Absolute PL quantum yield evaluated using an integrating sphere under N2. d PL lifetime. e Rate constant of fluorescence radiative decay (S1 → S0): kr = ΦPL/τ. f Rate constant of nonradiative decay: knr = (1 − ΦPL)/τ. g Lowest excited singlet (ES) and triplet (ET) energies estimated from onset wavelengths of the fluorescence and low-temperature phosphorescence spectra recorded at 300 and 77 K, respectively.
DPN	Sol	378	45	1.0	4.5	5.5	3.52	2.50
	Film	374	58	1.6	3.7	2.7	3.56	2.30
SiDPN-1	Sol	386	54	1.0	5.2	4.5	3.45	2.50
	Film	390	53	1.0	5.2	4.6	3.52	2.30
SiDPN-2	Sol	386	54	1.0	5.6	4.8	3.43	2.45
	Film	394	52	1.0	4.9	4.5	3.47	2.27

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To verify the intrinsic potential of the three UC emitters (DPN, SiDPN-1, and SiDPN-2), we first investigated their TTA-UC properties in deaerated toluene solutions in combination with the BBCz-SB-Br photosensitizer, which possesses the S₁ and T₁ excitation energies (E_S and E_T) of 2.66 and 2.53 eV, respectively (Fig. 3). All TTA-UC measurements in solution were performed under unified conditions with a photosensitizer concentration of 100 μM and an emitter DPN subunit concentration of 20 mM. Under excitation with a 445 nm laser, distinct UC emissions peaking at 370–390 nm in the UV region were observed for all three solution samples (Fig. 3a). As expected, the triplet-mediated UC emissions decayed in milliseconds with the lifetimes of 0.3–0.5 ms (Fig. 3b). The TTA-UC efficiencies (η_UC ≡ 2Φ_UC, standardized to 100% for the theoretical limit) of the SiDPN-1 and SiDPN-2 solutions reached 15.8% and 16.2%, respectively, which were marginally higher than that of the DPN solution (14.8%) measured at the same excitation intensities (Fig. 3c). This trend is consistent with the variation in photoluminescence quantum yield (Φ_PL) of the emitters in solution (Table 1). As shown in Fig. 3d, the excitation power dependence of the UC emission intensity for the three solution samples clearly demonstrated a quadratic-to-linear change in the correlation slopes; this behavior is a typical feature of TTA-UC.^57–59 The threshold excitation intensities (I_th), defined as the intersection of these two fitting lines, were estimated to be 35, 11, and 31 mW cm⁻² for DPN, SiDPN-1, and SiDPN-2, respectively, combined with BBCz-SB-Br.

Fig. 3 TTA-UC characteristics of deaerated toluene solutions containing BBCz-SB-Br photosensitizer and DPN-based emitters under photoexcitation at 445 nm ([BBCz-SB-Br] = 100 μM, [DPN] = 20 mM, [SiDPN-1] = 20 mM, and [SiDPN-2] = 10 mM). (a) UC emission spectra recorded at excitation power densities ranging from 0.1 mW cm⁻² to 68 W cm⁻² with a 425 nm shortpass filter, (b) UC emission decay curves, (c) UC efficiency (η_UC) as a function of excitation power density, and (d) double logarithmic plots of UC emission intensity versus excitation power density.

To study the Dexter-type TET behavior in TTA-UC, we performed Stern–Volmer quenching experiments (ESI†). Increasing the concentration of the emitters in each solution led to a gradual reduction in the delayed fluorescence lifetime of the BBCz-SB-Br photosensitizer. The TET quantum yields (Φ_TET) derived from the Stern–Volmer analysis were as high as ≥99% for the experimental concentration conditions depicted in Fig. 3, revealing highly efficient TET processes with negligible backward energy transfer. In general, η_UC is described by eqn (1), by considering the quantum efficiency of each photophysical process within the whole system:

η_UC = f·Φ_ISC·Φ_TET·Φ_TTA·Φ_PL (1)

where f is the spin statistical factor (or singlet generation efficiency), Φ_ISC is the ISC quantum yield of the photosensitizer, and Φ_TTA is the TTA quantum yield. For the present TTA-UC systems under excitation intensities above I_th, the Φ_ISC, Φ_TET, and Φ_TTA values can be assumed to be close to unity; the f values are therefore estimated to be 29% and 30% for SiDPN-1 and SiDPN-2, respectively (the theoretical maximum f = 40%). In the solution systems, in which molecular diffusion governs the TET processes, no significant difference is observed in the overall TTA-UC behavior between these two emitters at the same DPN subunit concentration ([DPN] = 20 mM).

To demonstrate all-solid-state TTA-UC, we fabricated and evaluated binary solid films with a thickness of ∼1 μm, in which a small amount of the BBCz-SB-Br photosensitizer (0.1–0.2 mol%) was dispersed in a host matrix of the DPN-based emitter (Fig. 4 and ESI†). Although the DPN-hosted film was polycrystalline, the SiDPN-1- and SiDPN-2-hosted films were amorphous and therefore, transparent (Fig. 4a).⁶⁰ Indeed, such thicker films retained >98% transmittance over the entire visible range, except for the narrow absorption band at ∼460 nm for the BBCz-SB-Br photosensitizer (ESI†). More importantly, DPN, SiDPN-1, and SiDPN-2 retained high Φ_PL values (52–58%) in the solid films, comparable to those in the foregoing solution states (Table 1). These notable features allowed the DPN derivatives to function not only as emitters but also as effective glassy matrices (without the need for an additional host) for solid-state TTA-UC.

Fig. 4 All-solid-state TTA-UC characteristics. (a) Photos of 1 μm-thick DPN, SiDPN-1, and SiDPN-2 films (15 mm × 15 mm) containing BBCz-SB-Br photosensitizer ([BBCz-SB-Br] = 0.1 mol% for DPN and SiDPN-1; 0.2 mol% for SiDPN-2) taken under room light. (b) UC emission spectra under photoexcitation at 445 nm and power densities ranging from 0.8 mW cm⁻² to 67 W cm⁻², (c) UC emission decay curves, (d) UC efficiency (η_UC) as a function of excitation power density, and (e) double logarithmic plots of UC emission intensity versus excitation power density.

UC emissions in the same UV region were clearly observed even in the binary solid films upon excitation at 445 nm (Fig. 4b) similar to those in the solution states. While the polycrystalline DPN-hosted film displayed a relatively short UC emission lifetime (τ_UC) of ∼0.1 ms, those for the amorphous SiDPN-1- and SiDPN-2-hosted films were significantly prolonged to 2.8 and 1.4 ms, respectively (Fig. 4c). The I_th for the SiDPN-1- and SiDPN-2-hosted films were as low as 49 and 44 mW cm⁻², respectively, and their η_UC values, determined by the absolute method, exceeded 2% (Fig. 4d and e). By contrast, the DPN-hosted film exhibited a relatively higher I_th of 134 mW cm⁻² and a lower η_UC of 1.6%. The lower TTA-UC performance of the DPN-hosted film can presumably be partially attributed to agglomeration of the photosensitizer and/or inhomogeneous (discontinuous) grain formation due to emitter crystallization. Thus far, a high η_UC of 8.6% has been reported for solid-state visible-to-UV TTA-UC combining 3,3′-carbonylbis(7-diethylaminocoumarin) (CBDAC) as a sensitizer and 2,5-diphenyloxazole (PPO) as an emitter.⁴³ Although our present system is somewhat less efficient, it enables UC emissions in the form of transparent solid films.

To gain insight into the impact of the molecular structures of the emitter on triplet energy migration and TTA, we further estimated the triplet exciton diffusion constants (D_T) from the experimental I_th, according to eqn (2) and (3).⁵⁷

I_th = (α·Φ_TET·γ_TT)⁻¹·(τ_T)⁻² (2)

γ_TT = 8πa₀D_T (3)

where α is the absorption coefficient of the photosensitizer at the excitation wavelength (α = 780 and 1700 cm⁻¹ for the SiDPN-1-and SiDPN-2-hosted films, respectively), γ_TT is the second-order annihilation constant, τ_T is the emitter triplet lifetime (τ_T ≈ 2τ_UC),⁶¹ and a₀ is the annihilation distance of triplets (assumed to be ∼0.9 nm).^57,62 Here, Φ_TET ≈ 1 is assumed because the intermolecular distances between the photosensitizer and emitters are considered close enough to TET in these solid films. Consequently, the D_T value of twin SiDPN-2 (3.6 × 10⁻¹⁰ cm² s⁻¹) was twice as much that of SiDPN-1 (1.8 × 10⁻¹⁰ cm² s⁻¹). The larger D_T and shorter τ_UC of the SiDPN-2 film than the SiDPN-1 film can be primarily attributed to the relatively higher density of DPN subunits in the films. In the SiDPN-2 film, another additional possibility is the contribution from intramolecular TET and TTA^32,52–56 in addition to the common intermolecular processes; the proximity of the two DPN units can allow for faster TTA, accelerating the UC emission decay (Fig. 4c).

In summary, using a judiciously selected combination of DPN-based glass-forming emitters and an MR-TADF-type photosensitizer, we demonstrated metal-free, solid-state visible-to-UV TTA-UC with high processability and transparency. The photophysical analyses revealed that the design strategy of twin emitters such as SiDPN-2 is particularly useful for the development of efficient solid-state TTA-UC systems. We believe that the findings obtained in this study will contribute to further improving the efficiency of solid-state visible-to-UV TTA-UC and developing their practical applications.

Author contributions

T. Y. conceptualized the project; S. W. synthesized the materials; S. W. and K. M. performed spectroscopic measurements and analysis; S. W. and T. Y. wrote the manuscript; N. K. and T. Y. supervised the entire research project.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported in part by Grant-in-Aid for JSPS KAKENHI (Grant No. JP21H04694 and JP20H05676) and JST CREST (Grant No. JPMJCR21O5). The authors are grateful for the support provided by the Cooperative Research Program of “Network Joint Center for Materials and Devices”.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc01820f

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