Multiple Resonance Thermally Activated Delayed Fluorescence Sensitizers Enable Green-to-Ultraviolet Photon Upconversion: Application in Photochemical Transformations

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE7 Dec 2022Multiple Resonance Thermally Activated Delayed Fluorescence Sensitizers Enable Green-to-Ultraviolet Photon Upconversion: Application in Photochemical Transformations Yaxiong Wei, Ke Pan, Xiaosong Cao, Yuanming Li, Xiaoguo Zhou and Chuluo Yang Yaxiong Wei Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060 School of Physics and Electronic Information, Anhui Normal University, Wuhu, Anhui 241000 , Ke Pan Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060 , Xiaosong Cao *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060 , Yuanming Li Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026 , Xiaoguo Zhou Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026 and Chuluo Yang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060 https://doi.org/10.31635/ccschem.022.202101507 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Efficient visible-to-ultraviolet (UV) triplet–triplet annihilation upconversion (TTA-UC) with large anti-Stokes shift is highly promising for solar-powered and indoor applications. Nonetheless, the excitation wavelength is confined to the blue region (<450 nm), mainly due to large energy loss during triplet sensitization, resulting in reduced photon utilization efficiency in practical scenarios. Herein, a series of multiple resonance thermally activated delayed fluorescence (MR-TADF) compounds are developed as purely organic sensitizers for the purpose of energy-loss reduction, which also feature intense absorbance in the visible region, high intersystem crossing efficiencies, and long triplet lifetimes. By pairing the MR-TADF sensitizers with appropriate acceptors, green-to-UV TTA-UC systems were realized with an anti-Stokes shift up to 1.05 eV, upconversion quantum yield up to 8.6%, and threshold excitation intensity as low as 9.2 mW cm−2 in solution. The TTA-UC pairs were applied as internal or external sources of UV photons to trigger energy-demanding photopolymerization and photoligation reactions even under excitation of low-power-density green light-emitting diode light, revealing the broad utility of these molecular upconverters. This work unlocks the huge potential of MR-TADF-type sensitizers in upconversion applications. Download figure Download PowerPoint Introduction Triplet–triplet annihilation upconversion (TTA-UC), or triplet fusion, is an anti-Stokes shifting technique that converts low-energy incident light under weak irradiation intensity to high-energy photons.1,2 Compared to other photon upconversion methods, including two-photon absorption and rare-earth upconversion, TTA-UC enjoys a low power density requirement and a high upconversion quantum yield (ΦUC)3 and has attracted considerable attention in photovoltaics,4,5 photocatalysis,6–8 bioimaging,9,10 organic light-emitting diodes (OLEDs),11 circularly polarized luminescence,12 and photoinduced drug release.13 By pairwise selection of sensitizers and acceptors, the applicability of the TTA-UC system featuring adjustable excitation/emission wavelengths has been demonstrated.14,15 Notably, visible-to-ultraviolet (UV) upconversion is particularly valuable for enhancing the efficiency of photochemical systems, including H2 generation, CO2 reduction, and many organic transformations16,17 that always require a highly energetic excited state to induce challenging bond activation/formation reactions.18 Implementation of visible-to-UV TTA-UC with a concurrent large anti-Stokes emission shift, high ΦUC, and a low power density threshold (Ith) is essential for solar-powered and indoor applications but remains elusive.19 It is important to note that among all reported visible-to-UV TTA-UC systems, the energy gains are <0.92 eV, restricting the excitation wavelengths to the violet or blue regions (Scheme 1c and Supporting Information Figure S1 and Table S1). This limitation is mainly caused by the inherent energy loss during S1*→S1 vibrational relaxation (VR) and the intersystem crossing (ISC) process of the sensitizers.20,21 As illustrated in the Jablonski diagram in Scheme 1a, the sum of the reorganization energy (λS*) and the gap between singlet and triplet excited states (ΔEST) is always >0.5 eV.19,22 To minimize energy loss for triplet sensitization, there have been recent advances by using direct S0-T1 absorption sensitizers, but low triplet energy (<1.8 eV) and the short triplet lifetime (∼102 ns) of these complexes circumvent their use in visible-to-UV TTA-UC.23–25 A more practical route is to adopt donor–acceptor (D–A) typed thermally activated delayed fluorescence (TADF) molecules owing to their energy degeneracy between excited states (ΔEST < 100 meV), which also allows efficient ISC courses from S1→T1 in the absence of noble metals.26 For instance, several groups have independently reported the use of carbazolyl dicyanobenzene-based TADF molecules to sensitize p-terophenyl or pyrene derivatives, and achieved anti-Stokes shifts up to 0.83 eV under photoirradiation below 450 nm.27–29 Nonetheless, the large structural flexibility of the charge-transfer-featured S1 state would result in substantial non-radiative energy loss and cause weak absorption in the visible range, leading to high excitation threshold that was not even close to solar irradiance.20 Scheme 1 | Mechanism of visible-to-UV photon upconversion employing (a) conventional sensitizer with large energy loss and (b) MR-TADF sensitizer with small energy loss during S1*→S1 VR and S1→T1 ISC process. λS* is the reorganization energy, ΔEST is the gap between singlet and triplet excited states. (c) Summary of reported visible-to-UV TTA-UC by using different types of sensitizers.7,19–21,27–35,55 Download figure Download PowerPoint In this context, new sensitizers with satisfactory molar extinction coefficients (ε) and absorption beyond the blue region (i.e., >500 nm) while maintaining high triplet energies (i.e., >2.2 eV), high ISC efficiency (ΦISC), and long triplet lifetimes becomes an emergent request. It is also noteworthy that the development of metal-free triplet sensitizers is an ongoing pursuit, considering the toxicity and poor energy-level tunability of precious- or rare-earth-metal-based complexes.36,37 In light of these stringent demands, we hereby report for the first time a highly efficient green-to-UV TTA-UC system operable with weak incident light by implementing multiresonance TADF (MR-TADF) compounds as photosensitizing species. The MR effect induced by the ortho-positioned, electron-rich N-atom and the electron-deficient B-atom in a rigid polycyclic aromatic hydrocarbon framework minimizes the VR of S1, and separates frontier molecular orbitals to ensure small ΔEST.38–40 Additionally, these compounds typically display a much more intense absorption band caused by short-range charge-transfer, which is a significant advantage compared to the D–A typed counterparts ( Supporting Information Scheme S1).41 These features render MR-TADF compounds superior candidates as triplet donor with increased sensitivity for weak incident light and broader choice of acceptors (Scheme 1b). To maximize the anti-Stokes shift, energy loss during triplet–triplet energy transfer (TTET) is further reduced by matching the MR-TADF sensitizers with the appropriate acceptors to give small or even negative triplet energy gaps, eventually leading to an unprecedentedly large energy gain of 1.05 eV among visible-to-UV TTA-UC materials. Ensured by the efficient UV-photon generation under weak excitation intensity, these systems can be successfully utilized to trigger energy-demanding photochemical reactions with low-power green light. Experimental Methods Instruments and general methods 1H NMR spectra were recorded with a Bruker AVANCE III 500 superconducting-magnet high-field NMR spectrometer (Bruker, Switzerland) at 500 MHz, where CDCl3 or CD2Cl2 was used as solvent and tetramethylsilane was the standard for which δ = 0.00 ppm. High-resolution mass spectra were measured using an Agilent 7250 (Agilent, United States) and JEOL-JMS-T100LP AccuTOF (JEOL, Japan). UV–vis absorption spectra were recorded with a Shimadzu UV-2700 spectrophotometer (Shimadzu, Japan) at 25 °C. Steady-state photoluminescence (PL) spectra were measured with a fluorescence spectrophotometer (F-7100, Hitachi, Japan). The transient PL decay curves were obtained by FluoTime 300 (PicoQuant GmbH, Germany) with a Picosecond Pulsed LASTER (LASTER480) as the excitation source. The fluorescence quantum yields were measured on a Hamamatsu UV-NIR absolute PL quantum yield spectrometer (C13534, Hamamatsu Photonics, Japan) equipped with a calibrated integrating sphere, and the integrating sphere was purged with dry argon to maintain an inert atmoshphere. The general procedure for the synthesis of sensitizers and acceptors, calculation methods of TTA-UC quantum yields, and TTET efficiency are listed in the Supporting Information. TTA-UC spectra TTA-UC spectra were recorded using a homemade fluorescence emission spectrometer. A semiconductor laser (517 or 532 nm) was selected as the excitation light source. The diameter of the laser spot was ∼5 mm. In the TTA upconversion experiments, the solutions mixing sensitizer and acceptor were prepared in a glove box and kept in a quartz cuvette (10 mm × 1 mm). In experiments, the upconverted fluorescence of acceptors was collected and detected with a commercial fiber-optic spectrometer (ULS2048-2-USB2, AvaSpec, Avantes, Netherlands), under photoexcitation at 517 nm (or 532 nm). Computational methods Geometries of the compounds were optimized using density functional theory (DFT) with the B3LYP function and 6-31G(d) basis set. The spin-density surfaces of the compounds and the energy gaps between ground state and lowest triplet state were calculated with the time-dependent DFT (TD-DFT) level using the same basis set. The vertical excitation energies were directly compared with absorption spectra, and the corresponding electronic transitions were identified subsequently. The polarized continuum model (PCM) model was applied to evaluate the solvent effect. All these calculations were performed with the Gaussian 09W program package. The spin-orbital couplings (SOCs) between S1 and Tn (n = 1, 2, 3) states were calculated with PySOC by considering that the three Tn substrates (m = 1, 0, −1) are degenerate, that is, ⟨ S 1 | H ^ SOC | T 1 ⟩ = Σ m = 0 , ± 1 ⟨ S 1 | H ^ SOC | T 1 m ⟩ 2 , where H ^ soc represents the interaction of the SOC. All SOCs were obtained at the TD-DFT level of theory using the B3LYP functional and the 6-31G(d) basis set. Results and Discussion The advent of MR-TADF molecules has brought exciting opportunities for the fabrication of OLEDs with extraordinary efficiency and color purity,26,42 yet their potential as sensitizers remains underexplored. An important issue to be addressed is the insufficient ΦISC of MR-TADF compounds compared to conventional (D–A typed) TADF fluorophore, as also reflected by the smaller contribution of long-lived delayed components of the photoluminescence quantum yields (ΦPLs).43 This characterization is related to the pseudo spin-forbidden nature of the S1→T1 transition with slightly enlarged energy differences (typically in the range of 100–200 meV).41 To overcome this obstacle, we established two MR-TADF compounds (BN-2Cz and BN-2Cz-tBu, Scheme 2a) with twisted geometry as highly promising heavy-element-free sensitizers ( Supporting Information Schemes S2 and S3 and Figures S2–S12, see Supporting Information for detailed synthetic procedures and characterizations). Importantly, the electronic donors (carbazole or 3,6-di-tert-butylcarbazole) orthogonally linked to the parent B,N-skeleton not only shifted the absorbance into the green region but also guaranteed high ΦISC. Scheme 2 | Molecular structures of (a) the MR-TADF sensitizers and (b) ethynyl naphthalene-based acceptors. Download figure Download PowerPoint Characterization of MR-TADF sensitizers and acceptors The UV–vis absorption spectra, fluorescence, and phosphorescence (recorded at 77 K) emission spectra were measured and are shown in Figure 1. BN-2Cz displayed a strong absorption band at 490 nm (ε = 2.37 × 105 M−1 cm−1) in toluene, attributable to the multiple resonance charge transfer transition (Figure 1a). The intense mirroring fluorescence emission was located at 522 nm (ΦPL = 70%), followed by the phosphorescence emission peaking at 554 nm (2.24 eV). Attaching tert-butyl (t-Bu) moieties to the 3,6-position of the carbazole unit decreased the delocalization energy of electronic states and led to a moderate bathochromic shift of the absorption peak (504 nm, ε = 2.95 × 105 M−1 cm−1). Meanwhile, the fluorescence and phosphorescence emission peaks were also shifted to 543 and 569 nm (2.20 eV), respectively, with ΦPL as high as 84%. Accordingly, the total energy losses were estimated to be only 0.29 and 0.26 eV during triplet sensitization for BN-2Cz and BN-2Cz-tBu (Figure 1c), respectively. These results also demonstrated that delicate manipulation of excited state energy levels are possible via structural modification.26 Figure 1 | Normalized UV–vis absorption (Abs.), fluorescence emission (Fl.), and phosphorescence (Ph.) spectra of (a) sensitizers and (b) acceptors in toluene (c = 0.01 mM). (c) Singlet and triplet energy levels of sensitizers and acceptors, estimates based on (a) and (b). Download figure Download PowerPoint Time-resolved PL measurements (λex = 480 nm) unveiled distinct double-exponential decay profiles for both compounds in deoxygenated toluene solution, characteristic of TADF properties. The prompt fluorescence lifetimes (τps) and delayed lifetimes (τds) were determined to be 4.5 ns (24.9%) and 21.8 μs (75.1%) for BN-2Cz, 5.2 ns (40.5%) and 17.8 μs (59.5%) for BN-2Cz-tBu, respectively ( Supporting Information Figures S13 and S14). The long-lived components for both compounds were strongly quenched in air-saturated solution, indicating the origination of delayed fluorescence from triplet states. Based on the ΦPL values and transient PL decay characteristics, the ISC rates (kISCs) and ΦISCs were deduced to be 1.1 × 108 s−1 and 60% for BN-2Cz-tBu, 1.7 × 108 s−1 and 75% for BN-2Cz (detailed calculation in Supporting Information Table S2), respectively, suggestive of an efficient ISC process for triplet sensitization (Table 1). In stark contrast, the parent molecule BN (BN-2Cz-tBu without orthogonally linked substituents) exhibited similar ΔEST (0.15 eV) value but a feeble delayed fluorescence signal, reflective of weak ISC ( Supporting Information Figure S15). To uncover the inherent factors that govern these excited-state properties, we carried out TD-DFT calculations and predicted the SOC matrix elements (ξ) between excited states for BN-2Cz-tBu (ξS1-T1 = 0.112 cm−1, ξS1-T2 = 0.653 cm−1), BN-2Cz (ξS1-T1 = 0.116 cm−1, ξS1-T2 = 0.802 cm−1, Supporting Information Table S3) and BN (ξS1-T1 = 0.054 cm−1, ξS1-T2 = 0.131 cm−1). Since ΔES1-T2s were small enough in all cases (<0.16 eV, Supporting Information Table S4), these results hinted at the transition between S1/T2 could be a more efficient ISC pathway compared to S1/T1 with nearly identical orbital parentages, in line with El-Sayed rules.44–46 Importantly, the introduction of auxiliary donating groups to form a twisted D–A geometry was the key to induce a much larger orbital difference between S1 and T2 in BN-2Cz-tBu and BN-2Cz ( Supporting Information Figures S16 and S17),47 which consequentially facilitated the spin-flip transition and made them more suitable sensitizer candidates. Table 1 | Photophysical Parameters of Sensitizers and Acceptors Compound λabsa λflb λphc ΦPLd ΦISCe kTTETf ΦUC′g BN-2Cz 490 (2.37) 522 554 70 75 1.34 (0.95) 8.6 (6.9) BN-2Cz-tBu 504 (2.95) 543 563 84 60 0.25 (0.08) 3.7 (3.4) 1,5-DTNA 326 (1.95); 343 (1.74) 350/366 554/599 73 – – – 1,4-DTNA 333 (2.85); 350 (3.34) 357/373 588/638 81 – – – aMaximum absorption peaks, nm (molar extinction coefficient, 105 M−1 cm−1). bFluorescence emission peaks, nm. cPhosphorescence emission peaks, nm. dAbsolute fluorescence quantum yield in Ar, %, measured with excitation wavelength at 475 nm (BN-2Cz), 480 nm (BN-2Cz-tBu), 327 nm (1,5-DTNA), and 333 nm (1,4-DTNA). eISC efficiency, %. fBimolecular quenching rate constant with 1,4-DTNA as acceptor (1,5-DTNA as acceptor), 109 M−1 s−1. gΦUC′ values with 1,4-DTNA as acceptor (1,5-DTNA as acceptor), %, c[sensitizer] = 0.01 mM, λex = 517 nm. The tiny Stokes shifts and high triplet energy levels (2.20–2.24 eV) of BN-2Cz and BN-2Cz-tBu allowed them to pair with UV-emissive triplet acceptors. Referring to a recent report by Yanai et al.,19 we expected that 1,4-bis((triisopropylsilyl)ethynyl)naphthalene (1,4-DTNA, Scheme 2b) would be a suitable acceptor due to its appropriate triplet energy (T1 = 2.11 eV) and high statistical probability to generate the singlet excited state (f = 32%). It was also anticipated that chemical structure modification of ethynyl naphthalene derivatives would allow simultaneous manipulation of both singlet and triplet excited states, thus minimizing enthalpic energy loss in TTET and maximizing the anti-Stokes shift value. Based on computational results for a series of isomers ( Supporting Information Figure S18), a new acceptor 1,5-bis((triisopropylsilyl)ethynyl)naphthalene (1,5-DTNA) was also constructed with a smaller π-conjugated structure and slightly higher triplet energy. Correspondingly, toluene solution of 1,5-DTNA (0.01 mM) displayed a hypsochromic shifted 0–0 absorption peak at 343 nm (ε = 1.74 × 105 M−1 cm−1) compared to that of 1,4-DTNA (350 nm, ε = 3.34 × 105 M−1 cm−1) (Figure 1b and Table 1). Following a similar trend, the 0–0 vibronic fluorescence and phosphorescence peaks were located at 350 nm, 554 nm (2.24 eV) for 1,5-DTNA, and 357 nm, 588 nm (2.11 eV) for 1,4-DTNA. The ΦPL of 1,5-DTNA (73.3% in Ar) was only slightly lower than that of 1,4-DTNA (80.5% in Ar). Based on the energy relationships (Figure 1c), the sensitizer/acceptor pairs were separated into two categories: BN-2Cz/1,4-DTNA, BN-2Cz/1,5-DTNA, and BN-2Cz-tBu/1,4-DTNA as exothermic/isoenergetic systems, and BN-2Cz-tBu/1,5-DTNA as the energy transfer process in the was the energy was small eV) to allow thermally activated to the long triplet lifetime of the sensitizer was also to the triplet energy transfer which is a cause of low efficiency in It is expected that pairing a sensitizer and acceptor with a negative energy difference would be in to energy gain without the TTA-UC TTET process between sensitizers and acceptors The TTET process was between sensitizers and acceptors (Figure and Supporting Information Figure For instance, the delayed fluorescence was quenched of the BN-2Cz solution with indicating the of TTET from the sensitizer to the acceptor (Figure The quenching rate constant values from the (Figure 2b) were determined to be 1.34 × 109 M−1 s−1 for and × 109 M−1 s−1 for BN-2Cz/1,5-DTNA, and allowed the use of low of acceptors = 0.01 mM, = 1 mM) to satisfactory TTET efficiency Supporting Information Table By the sensitizer with BN-2Cz-tBu, the energy transfer process was to of 0.25 × 109 M−1 s−1 for This was with the spin-density of by the groups that the probability between energy transfer pairs ( Supporting Information Figure to its BN-2Cz-tBu/1,5-DTNA even smaller of × 109 M−1 s−1. To the TTET using BN-2Cz-tBu as the we optimized to in the TTA-UC measurements to the high value of for BN-2Cz-tBu/1,4-DTNA and for BN-2Cz-tBu/1,5-DTNA ( Supporting Information Table Notably, is also for the process and an in For BN-2Cz-tBu/1,5-DTNA = eV) the effect of on TTA-UC could be with optimized based on the × = × s−1 × = × where the was estimated to be × 109 M−1 s−1 from the = TTET × ( T T Figure 2 | (a) of delayed fluorescence lifetime of BN-2Cz with different of the acceptor 1,4-DTNA. (b) The quenching rate from the delayed fluorescence lifetime quenching as the c[sensitizer] = 0.01 mM, λex = 480 nm. Download figure Download PowerPoint TTA-UC in solution TTA-UC employing the MR-TADF sensitizers and ethynyl naphthalene-based acceptors were under 517 nm fluorescence in the range of nm that with the emission of the acceptors could be detected in a toluene solution of BN-2Cz (0.01 mM) and (Figure by a fluorescence lifetimes = μs for μs for the long-lived ( Supporting Information Figure strong delayed fluorescence in the UV region could also be obtained by using BN-2Cz-tBu (Figure and Supporting Information Figure In all these systems, anti-Stokes shifts from eV 517 nm) for 1,4-DTNA as acceptor to eV 517 nm) for 1,5-DTNA as acceptor were deduced by the energy difference between excitation wavelengths and the emission the absorption band of BN-2Cz-tBu to nm, photoexcitation of the BN-2Cz-tBu/1,5-DTNA system with a 532 nm laser was also to intense emission ( Supporting Information Figure a anti-Stokes shift up to 1.05 eV 532 nm). To the of this is the first reported to efficient low-power green-to-UV with anti-Stokes shift values for visible-to-UV upconversion systems ( Supporting Information Table S1 and Scheme Figure | (a) fluorescence emission spectra of sensitizers and acceptors, c[sensitizer] = 0.01 (b) of the upconverted fluorescence intensity as a function of excitation power (c) of as a function of excitation power line the intensity of mW cm−2 for BN-2Cz, Supporting Information Figure by upconversion intensity at nm under irradiation with a 517 nm laser at = mW as the c[sensitizer] = Download figure Download PowerPoint The upconversion quantum yield values with the to be Supporting Information Table deduced by the quantum yield of BN-2Cz (0.01 mM) were up to and pairing with 1,4-DTNA and 1,5-DTNA, respectively, under 517 nm excitation ( Supporting Information Figures and The higher values by using 1,4-DTNA were in line with the superior absolute fluorescence quantum the sensitizer to BN-2Cz-tBu (0.01 mM) led to decreased for 1,4-DTNA as for 1,5-DTNA as acceptor), to the ISC rate and TTET of the low of BN-2Cz-tBu, the effect of was to be as by only the long-lived delayed induced by of BN-2Cz-tBu = Supporting Information Figure see Supporting Information for detailed To the values of systems under 532 nm excitation could with 1,4-DTNA as acceptor and with 1,5-DTNA ( Supporting Information Figure and Table These results are among the for purely organic visible-to-UV upconversion systems (Scheme 1c and Supporting Information Table S1). It is that the could be further by structural of the MR-TADF sensitizer to the ΦISC (i.e., elements as The threshold excitation power density where a of upconversion intensity on excitation power is key to evaluate TTA-UC systems wavelengths and to the = 2 ( 0 T 2 ( TTET , where the 0 T is the triplet state decay rate of the is the decay rate for and is the extinction of the this directly with the absorbance of sensitizer and the of sensitizer would to smaller value for the same TTA-UC As illustrated in Figure the system a from 2 to 1 in the of emission the laser power and the was decreased from mW cm−2 ( Supporting Information Figures and to mW cm−2 by the of BN-2Cz from 0.01 to to a larger molar extinction of BN-2Cz-tBu at 517 nm, was further reduced to 9.2 mW cm−2 in BN-2Cz-tBu a value lower than that of the reported TTA-UC system conventional TADF sensitizer mW to Figure the values were as high as for these even with increased of sensitizer indicating that the photon and singlet energy transfer is not This was also by only a of fluorescence emission intensity in the of sensitizer (λex = nm, Supporting Information Figure the obtained were close to the solar of mW cm−2 for BN-2Cz and mW cm−2 for BN-2Cz-tBu mM, = 1 Supporting Information Figure the values could at the solar For 1,5-DTNA as the values were to mW cm−2 for BN-2Cz and mW cm−2 for BN-2Cz-tBu ( Supporting Information Figure As an important for practical the inherent of the upconversion systems was by the emission intensity at nm under 517 nm photoexcitation with a power density of mW 1 the upconversion intensity of systems of

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