Multi‐Responsive Thermally Activated Delayed Fluorescence Materials: Optical ZnCl2 Sensors and Efficient Green to Deep‐Red OLEDs

Thermally activated delayed fluorescence (TADF) is an emission mechanism whereby both singlet and triplet excitons can be harvested to produce light. Significant attention is devoted to developing TADF materials for organic light‐emitting diodes (OLEDs), while their use in other organic electronics applications such as sensors, has lagged. A family of TADF emitters, TPAPyAP, TPAPyBP, and TPAPyBPN containing a triphenylamine (TPA) donor and differing nitrogen‐containing heterocyclic pyrazine‐based acceptors is developed and systematically studied. Depending on the acceptor strength, these three compounds emit with photoluminescence maxima (λPL), of 516, 550, and 575 nm in toluene. Notably, all three compounds show a strong and selective spectral response to the presence of ZnCl2, making them the first optical TADF sensors for this analyte. It is demonstrated that these three emitters can be used in vacuum‐deposited OLEDs, which show moderate efficiencies. Of note, the device with TPAPyBPN in 2,8‐bis(diphenyl‐phoshporyl)‐dibenzo[b,d]thiophene (PPT) host emits at 657 nm and shows a maximum external quantum efficiency (EQEmax) of 12.5%. This electroluminescence is significantly red‐shifted yet shows comparable efficiency compared to a device fabricated in 4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl (CBP) host (λEL = 596 nm, EQEmax = 13.6%).


Introduction
[3][4] An important class of optical sensors uses fluorescent compounds as the basis for their detection mechanism thanks to their numerous benefits including high specificity, low detection limits, fast response time, and technical simplicity. [5]Fluorescent sensors typically work by exhibiting a change in their emission, such as fluorescence intensity, emission wavelength, or lifetime, in response to interactions with specific analytes or environmental changes. [6,7]Organic fluorescent compounds such as rhodamines, [8,9] fluoresceins, [10] cyanine, [11] BODIPY, [12,13] and coumarin dyes [14] have long been used in optical sensing.Phosphorescent complexes have also been explored as sensors in oxygen sensing, [15] metal ion detection, [16] biomolecule detection, [17] and temperature sensing. [18]Indeed, both oxygen and temperature sensing rely in particular on accessible triplet excited states of the sensor.
[21] They have garnered much attention due to their capacity to harvest both singlet and triplet excitons to produce light in electroluminescent devices such as organic light-emitting diodes (OLEDs). [20,21][27] The first reported example employed a TADF compound, acridine yellow (Figure 1a), as a temperature sensor. [28]Steinegger et al. subsequently reported a series of carbazole-substituted dicyanobenzene and diphenylaminesubstituted anthraquinone donor-acceptor (D-A) TADF emitters, such as compound 3 (Figure 1a), for use as oxygen and temperature sensors. [29]In doped films, these dyes exhibit a temperature sensitivity in the investigated temperature range (278-323 K), showing a 1.4 to 3.7% K −1 change of the delayed lifetime, compared to that at 298 K. [29] Tonge et al. disclosed a TADF polymer, PTZ-ODA (Figure 1a), which acts as a singlecomponent ratiometric oxygen sensor. [30]In addition to oxygen and temperature sensors, Li et al. developed a sensor for solvent polarity based on compound 3 (Figure 1b), which shows dual emissions at 332 nm (strong locally-excited, LE, fluorescence) and 435 nm (weak charge-transfer, CT, TADF) in DCM under air.Using the solvent-invariant LE fluorescence as an internal reference, the ratio of the intensities of the LE and CT bands as well as the ratios of the prompt and delayed emission lifetimes were used to calibrate against solvent polarity. [31]Recently, Yin et al. reported a TADF turn-on chemosensor, DCF-MPYM-lev (Figure 1c), for sulfite ion SO 3 2− detection.The fluorescence intensity of DCM-MPYM-lev solution in CH 3 CN/PBS buffer (1/1) significantly increased and dual emissions at 535 and 640 nm were observed after the addition of SO 3 2− .DCF-MPYM-lev was also used to monitor exogenous SO 3 2− in living cells. [32]Qiu et al. reported the carbazole-triazine-based donoracceptor TADF emitter PhTRZ-OCHO (Figure 1c) as a fluorescence turn-off/fluorescence quenching sensor for the detection of Na + , Mg 2+, and Fe 3+ ions. [26]The emission intensity at 470 nm of PhTRZ-OCHO decreased with the addition of many of the metal ions tested (Ba + , Ca + , Cd 2+ , Co 2+ , Cr 2+ , Cu 2+ , Fe 3+ , Hg 2+ , K + , Mg 2+ , Mn 2+ , Na + , Ni 2+ , Pb + ), the strongest emission quenching occurred in the presence of Na + , Mg 2+ , and Fe 3+ .The remarkable fluorescence quenching behavior was attributed to the metal-binding aldehyde group present in PhTRZ-OCHO where, in the presence of these ions, the CT state is destabilized and non-emissive.
ZnCl 2 is a versatile Lewis acid used widely in chemical manufacturing as a dehydrating agent, catalyst, and in materials preparation. [33]ZnCl 2 is also used in the textile industry as a mordant.Monitoring its levels is essential for both industrial process control and environmental regulation. [34]Additionally, while zinc is vital for biological processes, ZnCl 2 can be toxic and corrosive at high concentrations, making it important to monitor its presence for public health and safety reasons. [35]Although there are plenty of studies on the detection of Zn 2+ , motivated by its importance in various biological processes, [36,37] there have been few reports of an optical sensor specifically designed for the detection of ZnCl 2 .Manandhar et al. reported a pyrene-based triazole receptor (pyrene-derived molecule), which formed self-assembled induced excimers upon the addition of ZnCl 2 .The Pyrene-derived molecule showed two distinct emission bands emanating from monomers and excimers. [38]This compound, however, provided a spectral response for other Zn 2+ salts and was not specific for the detection of ZnCl 2 .Sabarinathan et al. reported selective colorimetric sensing of ZnCl 2 •2H 2 O by the polyoxometalatesalt (POM-salt). [39]The addition of ZnCl 2 •2H 2 O into a mixture of POM-salt in DMSO-H 2 O resulted in the formation of a blue color; notably, anhydrous ZnCl 2 did not produce the color change under the same conditions.To the best of our knowledge, these are the only two optical sensors for ZnCl 2 that have been reported to date.
Here, we report three new TADF donor-acceptor emitters with a triphenylamine (TPA) donor and nitrogen-containing heterocyclic pyrazine-based acceptor, 4 [1,10]phenanthrolin-12-yl)aniline (TPAPyBPN) (Figure 1d).Theoretical and experimental results demonstrate that the electron-withdrawing strength of the acceptor increases with both the increased conjugation of the acceptor and the number of nitrogen atoms contained within, leading to a red shift of the emission within the series.These nitrogen atoms can also act as ligands for metal binding and the resulting change in photophysics can be exploited in metal ion sensing. [40]We found that these compounds exhibited a stark spectral response to the detection of ZnCl 2 , due to the formation of zinc chloride complexes.Of these three emitters, TPAPyBP showed the most dramatic and fast fluorescence response toward ZnCl 2 by shifting emission from green (550 nm) to deep red (680 nm).We separately explored these compounds as emitters in OLEDs and documented a rather large host polarity-induced shift in the emission from films doped in 4,4′-bis(N-carbazolyl)−1,1′-biphenyl (CBP) to 2,8-bis(diphenyl-phosphoryl)-dibenzo[b,d]thiophene (PPT).In particular, the OLEDs with TPAPyBPN in PPT emitted at 657 nm and showed an EQE max of 12.5%.This electroluminescence was 61 nm red-shifted in comparison to a device fabricated in a CBP host ( EL = 596 nm, EQE max = 13.6%),without significant loss in efficiency.The devices with TPAPyAP and TPAPyBP doped in CBP emitted at  EL = 526 nm with EQE max = 7.6% and  EL = 558 nm with EQE max = 9.1%, respectively.

X-Ray Diffraction Analysis of TPAPyBP and TPAPyBPN
Single crystals of TPAPyBP and TPAPyBPN were obtained by slow evaporation of a saturated toluene solution at room temperature.The structure and packing mode of both molecules in the solid state are shown in Figure 2 and the crystallographic data are shown in Table S1 (Supporting Information).The phenylene bridge is near coplanar with the adjacent ring of the acceptor in both compounds, except in one independent molecule of TPAPyBP, where it is more noticeably out of plane (TPAPyBP: 2.79 (Figure 2a) and 31.03°(FigureS14a,c, Supporting Information), TPAPyBPN: 4.36°(Figure 2b).TPA-PyBP packs as arrays of co-planar compounds along the b-axis, the donor groups of alternate molecules oriented to opposite sides to avoid a steric clash.These arrays are held together by slipped - stacking interactions, with adjacent molecules 3.24 and 3.49 Å apart, centroid•••centroid distances of 3.522(2) to 3.753(2) Å (Figure 2a; Figure S14, Supporting Information).In addition to these, CH••• interactions occur both to help further link adjacent molecules within the stacks (H•••centroid distances of 2.78 Å), and also to link adjacent stacks together (two independent H•••centroid distances of 2.92 Å).TPAPyBPN, also adopts a -stacked arrangement, however, these arrays form along the a-axis, and adjacent molecules adopt an alternating head-to-tail

Theoretical Calculations
The ground-state (S 0 ) geometries of TPAPyAP, TPAPyBP, and TPAPyBPN were optimized using density functional theory (DFT) at the PBE0 [41] /6-31G(d,p) [42] level of theory in the gas phase starting from a geometry generated in Chem3D. [43]At the optimized S 0 geometries, the dihedral angles between the bridging phenylene of the TPA and acceptor moieties are ≈ 31°for TPAPyAP, 39°for TPAPyBP, and 41°for TPAPyBPN (Figure S16, Supporting Information), slightly larger than those found in the crystal structures of the latter two (Figure 2).The calculated energy levels of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are shown in Figure 3; Figure S17 (Supporting Information) and the results are summarized in Table S2 (Supporting Information).The HOMOs are localized on the TPA donor, with some minor contribution to the proximal pyridine ring of the acceptor moiety.The LUMOs of all three compounds are localized on the acceptor group, with some contribution also located on the bridging phenylene of the TPA donor.As the acceptor strength increases along the series from TPAPyAP to TPAPyBP and TPAPyBPN both the HOMO and LUMO are stabilized, with the stabilization more significant for the latter.The HOMO-LUMO gap, ΔE HOMO-LUMO , thus decreases from 3.21 eV for TPAPyAP to 3.00 eV for TPAPyBP and 2.90 eV for TPAPyBPN (Figure 3a).The excited-state properties were calculated using time-dependent density functional theory (TD-DFT) within the Tamm-Dancoff approximation (TDA-DFT) based on the optimized ground-state geometries. [44,45]The oscillator strength, f, for the S 0 →S 1 transition is high at 0.47, 0.39, and 0.36 for TPAPyAP, TPAPyBP, and TPAPyBPN, respectively, reflecting a significant overlap of the electron density between the HOMO and LUMO, a result of the relatively small torsions that exist between the TPA and the acceptor moieties.The S 1 energies are 2.82 eV for TPAPyAP to 2.59 eV for TPAPyBP and 2.48 eV for TPAPyBPN, while the T 1 energies likewise decrease from 2.44, 2.25, and 2.17 eV, respectively, following a similar trend to that observed for ΔE HOMO-LUMO .The degree of spatial separation of the frontier orbitals in TPAPyBPN is reflected in a ΔE ST of 0.31 eV, while the larger overlap between HOMO and LUMO for TPA-PyAP and TPAPyBP lead to ΔE ST values that are slightly larger at 0.37 and 0.34 eV, respectively.
Natural transition orbital (NTO) analyses at the optimized S 1 and T 1 geometries calculated at the TDA-DFT-PBE0/6-31G(d,p) level are shown in Figure 3c,d, respectively.For all three compounds, the S 1 states are of CT character from the TPA donor to the acceptor.However, the T 1 states possess mixed CT and locally excited (LE) characters on the acceptor.At the relaxed S 1 geometry, there is a decreasing S 1 -T 1 spin-orbit coupling matrix element (SOCME) from 0.27 cm −1 in TPAPyAP to 0.21 cm −1 in TPAPyBP and 0.16 cm −1 in TPAPyBPN (Figure 3a), while at the relaxed T 1 geometry, the T 1 -S 1 SOCME are 0.17, 0.22, 0.22 cm −1 for TPAPyAP, TPAPyBP and TPAPyBPN, respectively.

Electrochemistry
The electrochemical behavior of TPAPyAP, TPAPyBP, and TPA-PyBPN was studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in degassed dichloromethane (DCM) with tetra-n-butylammonium hexafluorophosphate ([ n Bu 4 N]PF 6 ) as the supporting electrolyte.Voltammograms are referenced versus F c /F c + and the data are reported versus a saturated calomel electrode (SCE) and collated in Table S3 (Supporting

Photophysical Properties in Solution
The UV-vis absorption spectra of the three emitters in dilute toluene are shown in Figure 4b and the photophysical properties are summarized in Table 1.All three compounds exhibit strong absorption bands at ≈ 320 nm, which are assigned to locally excited (LE) - * transitions of the donors and acceptor moieties based on the TD-DFT predicted transitions (Figure S19, Supporting Information).A strong and broad absorption band  is observed at 427 nm ( = 31×10 3 M −1 cm −1 ) for TPAPyAP, 456 nm ( = 37×10 3 M −1 cm −1 ) for TPAPyBP and 469 nm ( = 19 ×10 3 M −1 cm −1 ) for TPAPyBPN, which is assigned in each case to an intramolecular charge transfer (ICT) transition from the TPA donor to the acceptor moiety.The molar absorption coefficient of the ICT band at 427 nm of TPAPyAP is higher than that of the ICT band at 469 nm of TPAPyBPN, which aligns with the TD-DFT calculated oscillator strength (f = 0.47 for TPAPyAP and f = 0.36 for TPAPyBPN, Figure 3b), while TPAPyBP exhibits the highest  at 456 nm (f of 0.39).The ICT absorption bands of these three compounds also expectedly shift to lower energies as the acceptor strength increases.All compounds show unstructured and broad photoluminescence (PL) spectra in toluene (Figure 4b), indicative of an excited state of ICT character, with peak maxima,  PL , at 513, 550, and 575 nm for TPAPyAP, TPAPyBP, and TPAPyBPN, respectively.Positive solvatochromism is observed for all three compounds (Figure 4b; Table S4, Supporting Information), which is consistent with the ICT nature of the emissive excited state.The optical bandgaps, E g , calculated from the intersection point of the normalized absorption and emission spectra, are 2.62, 2.46, and 2.38 eV for TPAPyAP, TPAPyBP, and TPAPyBPN, respectively (Figure S20, Supporting Information).The photoluminescence quantum yields, Φ PL , in a degassed toluene solution of TPAPyAP,  c) Quinine sulfate in H 2 SO 4 (aq) was used as the reference (Φ PL = 54.6%, exc = 360 nm) for the solution-state measurements. [ 58]Values quoted are in degassed solutions, which were prepared by three freeze-pump-thaw cycles.Values in parentheses are for aerated solutions, which were prepared by bubbling air for 10 min; d) Thin films of CBP and PPT were prepared as spin-coated films.The Φ PL of the thin films were determined using an integrating sphere ( exc = 305 or 340 nm) under a N 2 atmosphere at 298 K.
Values quoted inside the parentheses are in the air.Average lifetime  avg = ΣA i  2 i ∕ΣA i  i , where A i is the pre-exponential for lifetime  i .Prompt and delayed emissions were measured by TCSPC and MCS, respectively ( exc = 379 nm).
The PL decays of the three emitters in toluene under degassed and aerated conditions were measured using time-correlated single-photon counting (TCSPC, Figure S21, Supporting Information).There is only a single decay component (monoexponential) observed for all three compounds, with lifetimes,  p , of 4.6 ns for TPAPyAP, 5.6 ns for TPAPyBP, and 7.2 ns for TPAPyBPN.[49][50] The S 1 and T 1 energies of the three emitters were elucidated from the onsets of the respective fluorescence and phosphorescence spectra determined in frozen toluene at 77 K (Figure 4c; Table 1).The S 1 energies of TPAPyAP, TPAPyBP, and TPAPyBPN, are 2.64, 2.43, and 2.38 eV, while the T 1 energies are 2.34, 2.23, and 2.21 eV, respectively.The phosphorescence spectra of all three compounds are structured, and each is assigned from the TDA-DFT calculations as a mixed locally-excited triplet ( 3 LE) state of the acceptor and 3 ICT state (Figure 3).The ΔE ST values of TPAPyAP, TPA-PyBP, and TPAPyBPN are 0.30, 0.20, and 0.17 eV, respectively, which, though smaller than the calculated values, nonetheless mirror the trend predicted from the theoretical study.Similar to the other TPA-based TADF emitters, [48,51] these three compounds also have large ΔE ST in solution, yet TADF is observed in the solid state.

Fluorescence Sensing of Lewis Acids
Recognizing that the acceptors contain Lewis basic nitrogen atoms of differing number and strength, we decided to assess the potential of these compounds to act as selective optical sensors of Lewis acids.Although TADF luminophores have shown great potential as sensors, [23,52] such as for oxygen, [53][54][55] as temperature probes, [27,29,56] and for acid-base sensing. [57]There is to date no report on the use of TADF luminophores for Lewis acid sensing.We first investigated the optical sensing responses of TPAPyBP (1.3×10 −4 M) toward different metal ions.There is a quenching of the PL intensity of TPAPyBP at 550 nm with varying degrees of efficiency upon addition of excess of various metal salts (NaCl, NiCl 2 , Ni(OAc) 2 , CuI, Cu(OAc) 2 , CoCl 2 , CuCl, CuCl 2 , ZnCl 2 , SnCl 2 , Zn(BF 4 ) 2 , FeCl 3 , and AlCl 3 ) in an ethanol/toluene(1/99, v/v) solvent mixture Figure 5a,e).Remarkably, there is a significant emission response upon the addition of either ZnCl 2 or SnCl 2 , in both cases there is the emergence of new emission bands at ≈ 680 nm (Figure 5b; Figure S22a, Supporting Information).However, the emission intensity at 680 nm upon the addition of SnCl 2 is much lower than for the addition of ZnCl 2 and also lower than the emission of pristine TPAPyBP, (Figure S22b, Supporting Information), indicating that TPAPyBP acts as a more responsive fluorimetric sensor for ZnCl 2 than for SnCl 2 .So, it is noteworthy that only the addition of ZnCl 2 to the TPA-PyBP toluene solution resulted in a distinct intense red emission (Figure 5c).As shown in Figure 5d, new, strong absorption bands were observed for ZnCl 2 , SnCl 2 , Zn(BF 4 ) 2 , FeCl 3 and AlCl 3 .Similarly, the spectral response of TPAPyAP and TPAPyBPN also revealed a binding selectivity toward ZnCl 2 , showing a new, red-shifted emission band at 650 and 655 nm, respectively (Figure S23, Supporting Information).The Job plot for both compounds indicates the same 1:1 binding stoichiometry as that observed for TPAPyBP (Figure S23, Supporting Information).Given the more distinct and stronger optical response using TPAPyBP compared to TPAPyAP and TPAPyBPN, here we only focused on TPAPyBP.
The intriguing observation of this selective ZnCl 2 sensing prompted us to explore the underlying mechanism.We first investigated the detection limit of ZnCl 2 , which is correlated with the concentration of the emitter.As shown in Figure S24 (Supporting Information), the fluorescence spectra of different concentrations of TPAPyBP in a mixture of ethanol and toluene (0.0012/1 v/v) upon addition of 1 equivalent of ZnCl 2 were measured.As the concentration of TPAPyBP: ZnCl 2 (1:1 equiv.)increases, the fluorescence intensity at 555 nm increases until the concentration reaches 1.3×10 −5 M. When the concentration increases further, the intensity of the 555 nm emission band decreases while concomitantly a new emission band at 680 nm emerges and gradually becomes the principal emission band, reflecting the observed color change from green to deep red (Figure S24c,e, Supporting Information).As expected, the corresponding absorption spectrum exhibits a new band at 505 nm, which increases in intensity as the concentration of TPAPyBP: ZnCl 2 (1:1 equiv.)increases.As shown in Figure S24f (Supporting Information), the detection limit of ZnCl 2 is ≈ 5.0 ×10 −5 M: at this concentration, the presence of the 1:1 adduct with TPAPyBP can be confirmed.Furthermore, we highlight the fast reaction time, which occurs within several seconds (ESI Video S1, Supporting Information).This rapid response is highly desirable for sensing applications.
We then systematically investigated the PL response of TPA-PyBP (1.3×10 −4 M) in toluene upon the gradual addition of ZnCl 2 (0.10 M) in ethanol.As shown in Figure 6a-c, the PL intensity of TPAPyBP at 550 nm decreases progressively upon the addition of ZnCl 2 with a concomitant increase of a new emission band at 680 nm.This leads to a stark spectral response where the emission changes from greenish yellow to deep-red Figure 6b, with corresponding the Commission International de L'Éclairage (CIE) coordinates from (0.44, 0.55) to (0.61, 0.38), Figure 6c.The time-resolved photoluminescence (TRPL) of TPAPyBP with 10 equiv. of ZnCl 2 still shows monoexponential decay kinetics; however, the lifetime is shorter at 2.9 ns compared to 4.9 ns in the absence of ZnCl 2 (Figure S25, Supporting Information).Similarly, there are distinct spectral changes in the UV/vis absorption spectrum whereupon gradual addition of ZnCl 2 , the absorption band at 338 nm was bathochromically shifted to 358 nm while a new CT band appeared at 505 nm, probably due to the formation of a Zn complex (Figure 6d).An isosbestic point at 487 nm and the 1:1 stoichiometry identified in the Job plot indicate that only a single ZnCl 2 is coordinated to TPAPyBP (Figure 6e).Single crystals were grown by slow evaporation of a saturated toluene solution of the complex at room temperature.The structure of Zn(TPAPyBP)Cl 2 is shown in Figure 6f and reveals that the zinc ion adopts a distorted tetrahedral geometry, coordinated through the pyridyl nitrogen of TPAPyBP (N1), two chloride ligands and a molecule of ethanol solvent (N-Zn-Cl bond angle of 107.6(2) and 116.0(2)°andN─Zn─O bond angle of 96.5(2)°).This, or a structurally related tetrahedral complex, is the likely putative species in solution.The 1 H NMR spectrum of TPAPyBP with increasing    decrease in the ΔE HOMO-LUMO from 2.95 to 2.81 eV (Figure 6g).As excepted, the S 1 energy decreases to 2.38 eV for Zn(TPAPyBP)Cl 2 from 2.55 eV of TPAPyBP, corresponding to a large red-shift of both the CT band of the absorption and the emission of TPAPyBP upon addition of ZnCl 2 (Figure 5) .

Photophysical Properties in the Solid-State
We next measured the photophysical properties of all three compounds in an OLED-relevant nonpolar host (4,4′-bis(Ncarbazolyl)−1,1′-biphenyl (CBP)) at different weight concentrations ranging from 2 to 10 wt% (Figure S28, Supporting Information).The 2 wt% doped CBP films of TPAPyAP, TPAPyBP, and TPAPyBPN emit at  PL of 537, 560, and 585 nm, respectively, corresponding to the emission in dilute toluene solutions.The Φ PL of the 2 wt% CBP doped films of TPAPyAP TPAPyBP, and TPAPyBPN are 62, 60 and 62%, respectively (Table S5, Supporting Information).As the doping concentration increased, all compounds showed a red-shifted emission accompanied by a decrease in Φ PL .While the 10 wt% TPAPyBPN doped film in CBP exhibited a more pronounced red-shifted emission at  PL of 605 nm and a high Φ PL of 56%; thus, this doping concentration was chosen for the following characterization studies.As shown in Figure S28 (Supporting Information), all three compounds show unstructured ICT-based emission at room temperature.Similar to that observed in toluene at 77 K, the prompt fluorescence of 2 wt% TPAPyAP, TPAPyBP, and TPAPyBPN doped in CBP film at 77 K are structureless, with associated S 1 energies of 2.50, 2.33 and 2.23 eV, respectively.As expected, the phosphorescence spectra of all three compounds are structured, with T 1 values of 2.18, 2.18, and 2.17 eV, matching well with the TDA-DFT calculations as a mixed 3 LE/ICT state.The ΔE ST of these films of TPAPyAP, TPAPyBP, and TPAPyBPN are 0.32, 0.15, and 0.06 eV, respectively (Figure S28b, Supporting Information).As shown in Figure S29 (Supporting Information), TPA-PyBP, and TPAPyBPN each showed multiexponential decay kinetics at room temperature, with average prompt fluorescence lifetimes,  p , of 10.0 and 15.0 ns, respectively (Figure S29, Supporting Information), and average delayed emission lifetimes,  d , of 2.3 and 2.1 ms, respectively.The relative intensity of the delayed PL increases with increasing temperature from 100 to 300 K for both compounds, thereby corroborating the TADF nature of the emission of these three compounds in the CBP films.However, TPAPyAP showed monoexponential decay kinetics with a fluorescence lifetime of 8.4 ns (Figure S29a, Supporting Information), which can be explained by the large ΔE ST and inefficient TADF in the doped CBP film.We also explored the photophysical properties of the three emitters in a higher polarity host, PPT (Figure S30, Supporting Information).TPAPyBPN exhibited the most red-shifted emission of 53 nm compared to that in TPAPyAP (42 nm) and TPAPyAP (48 nm).The larger red-shift in TPAPyBPN can be attributed to it having the largest dipole moment of 5.7 D. The Φ PL values of TPAPyAP, TPAPyBP, and TPAPyBPN doped in nonpolar CBP are 62, 60 and 62%, respectively, and they remain high, at 75, 63 and 60%, in polar PPT, respectively in 2 wt% doped films (Table S5).As shown in Figure S31 (Supporting Information), the doped PPT films of TPAPyAP, TPAPyBP, and TPAPyBPN all show multiexponential decay kinetics with average  p of 6.8, 9.7, and 14.0 ns and average  d of 1.4, 0.68 and 0.11 ms at room temperature, respectively.Temperature-dependent time-resolved PL decays evidence of the TADF nature of the emission in the PPT-doped films (Figure S31, Supporting Information).The S 1 levels of TPAPyAP, TPAPyBP, and TPAPyBPN are stabilized modestly from 2.48 to 2.41 eV, 2.33 to 2.31 eV, 2.22 to 2.11 eV, respectively, in the PPT host compared to that in CBP host.The corresponding ∆E ST values decrease (Table 1; Figure S32, Supporting Information), leading to a shorter  d in PPT than in CBP.The -stacking interactions in these three compounds were also discussed in ESI (Figure S33, Supporting Information).
The performance of the OLEDs is summarized in Table 2.The EQE-luminance, current density-voltage-luminance (J-V-L) curves, and electroluminescence spectra (EL) are given in Figure 7c-e.Initially, we fabricated devices using device structure A and observed that each EL spectrum is similar to that of the corresponding PL spectrum in the CBP doped thin film, with EL maxima,  EL , of 526 nm for TPAPyAP, 558 nm for TPAPyBP and 597 nm for TPAPyBPN, with corresponding Commission International de l'Éclairage, CIE, coordinates of (0.317, 0.578), (0.434, 0.547) and (0.565, 0.433), respectively (Figure 7e).The EQE max of the TPAPyAP-based device is 7.6% while that of the TPAPyBP-based device is 9.1% and that of the TPAPyBPN-based device is 13.6% (Table 2; Figures S34 and S35, Supporting Information).Devices of TPAPyAP and TPAPyBP showed similar, moderate efficiency roll-off, with the EQE at 100 cd m −2 (EQE 100 ) at 4.9%, and the EQE at 1,000 cd m −2 (EQE 1000 ) at 4.3%; however, the TPAPyBPN-based device showed a more severe efficiency roll-off with EQE 100 at 4.6% and EQE 1000 at 3.2%.The theoretical EQE max is 13.9% for TPAPyBPN in CBP when considering an outcoupling efficiency of  out ≈ 25% that assumes that the film is isotropic.We next fabricated device B with an EML containing TPAPyBPN doped into the PPT host at the same 10 wt% doping concentration as that in CBP.As expected, the  EL is red-shifted to 657 nm [CIE coordinates (0.651, 0.348)], close to the  PL for the 10 wt% doped film in PPT (Figure S30, Supporting Information).The EQE max of TPAPyBPN-based device B was 12.5%, close to that for the TPAPyBPN-based device A (in CBP), and is also close to the theoretical EQE max = 14.2%.However, the TPAPyBPNbased device B showed much higher efficiency roll-off, despite the short  d and small ΔE ST of the TPAPyBPN doped film in PPT.

Conclusion
A family of TPA derivatives, TPAPyAP, TPAPyBP, and TPA-PyBPN, shows progressively red-shifted emission in toluene as a function of the increasing number of nitrogen atoms in the heterocyclic pyrazine-based acceptors.All three compounds exhibit a spectral response to the detection of ZnCl 2 in toluene, with the most notable being for TPAPyBP, where the emission rapidly changed from green ( PL = 550 nm) to deep red ( PL = 680 nm), which is distinct from the typical response of most Zn 2+ or ZnCl 2 sensors that only rely on changes in emission intensity.We also investigated the potential of these compounds as emitters in OLEDs.Both TPAPyBP and TPAPyBPN emit in the deep red in PPT, while TPAPyAP exhibits a smaller red-shift from green emission in CBP to yellow emission in PPT compared to the other two compounds.The OLEDs showed moderate efficiencies, with the device with TPAPyBPN doped in PPT emitting at  EL = 657 nm and showing an EQE max 12.5%.This electroluminescence was red-shifted by 61 nm compared to device the with CBP as the host ( EL = 596 nm, EQE max = 13.6%), a reflection of the impact of solid-state solvatochromism.

Figure 1 .
Figure 1.Reported TADF emitters' structures for a) temperature and oxygen sensors; b) environmental polarity sensors; c) anion and cation sensing; d) fluorescent sensor for Zn 2+ ions; e) This work: Multi-responsive TADF emitters based on planar and rich N-type acceptors.

Figure 2 .
Figure 2. Thermal ellipsoid plot (ellipsoids are drawn at the 50% probability level), view of the spacing between adjacent -stacked molecules, and view showing interactions between adjacent molecules of (a) TPAPyBP (only one independent molecule shown in the ellipsoid plot) and (b) TPAPyBPN, respectively.
concertation of ZnCl 2 in CDCl 3 revealed that the resonances at positions 1 and 3 (FigureS26, Supporting Information) of TPA-PyBP were the most perturbed upon the addition of ZnCl 2 , suggesting a possible coordination of Zn 2+ ion through pyrido[3,4- b]pyrazine core of the acceptor (Figure6f).Furthermore, HRMS of TPAPyBP with excess ZnCl 2 confirms the formation of a complex with a 1:1 stoichiometry (FigureS27, Supporting Information).

For
more insights into the origin of new deep red emission in solution, the HOMOs and LUMOs of TPAPyBP and Zn(TPAPyBP)Cl 2 calculated at the PBE0/6-31G(d,p) level (based on the structure obtained from the single crystal X-ray diffraction study) are shown in Figure 6f.The energy levels of both the LUMO (−2.82 eV) and the HOMO (−5.63 eV) for Zn(TPAPyBP)Cl 2 are significantly stabilized compared to those of TPAPyBP (LUMO: −2.40 eV, HOMO: −5.35 eV), leading to a

Figure 6 .
Figure 6.a) PL measurements of TPAPyBP (1.3×10 −4 m) with the addition of ZnCl 2 from 0 to 2.0 equiv.( exc = 487 nm); b) Samples in daylight and excited by UV torch ( exc = 360 nm) of TPAPyBP ZnCl 2 from 0 to 2.0 equiv.; c) The corresponding CIE coordinates of TPAPyBP (1.3×10 −4 m) with the addition of ZnCl 2 from 0 to 2.0 equiv.;(d) UV−vis absorption spectra obtained from TPAPyBP (1.3×10 −4 m) with the addition of ZnCl 2 from 0 to 2.0 equiv.; e) Job plot of absorbance ( abs = 510 nm) for the determination of binding stoichiometry between TPAPyBP and ZnCl 2 .f) Thermal ellipsoid plot of the single crystal structure of Zn(TPAPyBP)Cl 2 with partial atomic numbering (Ellipsoids are drawn at the 50% probability level, toluene solvent and minor component of disorder in the coordinated EtOH are omitted) g) Combined view of the single crystal structures of TPAPyBP and Zn(TPAPyBP)Cl 2 , and the corresponding frontier molecular orbitals (isovalue: 0.02) calculated using single crystal geometry in the gas phase at the PBE0/6-31G(d,p) level.

Table 1 . Photophysical properties of TPAPyAP, TPAPyBP, and TPAPyBPN in
solution and the solid-state.
a) At 298 K, values quoted are in degassed toluene solutions prepared by three freeze-pump-thaw cycles: for  PL the  exc = 340 nm; for lifetime  exc = 379 nm; b) Obtained from the onset of the prompt fluorescence (time window: 1 -100 ns) and phosphorescence spectra (time window: 1 -8.5 ms) measured in 2-MeTHF glass at 77 K,  exc = 343 nm;

Table 2 .
Electroluminescence data for the devices.