Revealing Internal Heavy Chalcogen Atom Effect on the Photophysics of Dibenzo[a,j]phenazine-Cored Donor-Acceptor-Donor Triad
A new twisted donor-acceptor-donor (D-A-D) multi-photofunctional organic molecule comprising of phenoselenazine as the electron-donors (Ds) and dibenzo[a,j]phenazine (DBPHZ) as the electron-acceptor (A) has been developed. The developed selenium-incorporated D-A-D compound is featured with multi-color polymorphism, distinct mechanochromic luminescence, chemically-stimulated luminochromism, thermally-activated delayed fluorescence, and roomtemperature phosphorescence. The internal heavy atom effect on the photophysical properties of the D-A-D system has been investigated through the comparison with the physicochemical properties of a previously developed sulfur analogue and a tellurium analogue.
Introduction<!>Results and Discussion<!>Steady-state photophysical properties in solutions.
<p>Selenium (Se) and Tellurium (Te), that are heavier congeners of Sulfur (S), have a very similar and lower electronegativity (cP) with Sulfur (Se: 2.55, Te: 2.10, S: 2.58), respectively, and they have slight larger covalent radius than that of sulfur (Se: 1.17 Å, Te: 1.38 Å, S: 1.04 Å). Owing to such electronic and structural perturbation, the replacement of the S atoms of an organosulfur compound with Se or Te atoms leads to a drastic change in reactivity and physicochemical property. Such "internal heavy atom effect" have fascinated scientists, and indeed a myriad of functional organo-selenium andtellurium compounds have been developed. Nowadays, they can find diverse applications in a range of fields such as biology, [1] medicinal chemistry, [1,2] chemical biology, [3] synthetic chemistry, [4] catalysis sciences, [5] polymer sciences, [6] and materials sciences. [7] In this context, planar p-conjugated molecules and polymers decorated with Se and/or Te at the main p-conjugated frameworks or at the peripherals of the p-systems have attracted considerable attention as functional materials for organic electronics. In addition to the intrinsic planar and rigid p-systems, the presence of Se/Te atoms results in not only strong intermolecular p-orbital interactions but also electrostatic interactions (i.e., chalcogen-bonding) in the solid states, [8] allowing for efficient charge carrier transport. Thus, some classes of compounds, such as seleno-/telluroarenes, seleno-/tellurophenes, and their oligomeric and polymeric derivatives, have emerged as promising organic semiconducting materials, and they find applications in organic field-effect transistors (OFETs), [9] organic photovoltaics (OPVs), [10] and organic thermoelectronics (OTEs). [11] In stark contrast, organoselenium and organotellurium compounds displaying luminescence in the solid states have been less developed, and thereby the optoelectronic applications thereof are in their infancy. [12] The incorporation of selenium and other heavy atoms such as halogens into a luminogenic scaffold often generates the pair of singlet excited state 1 (n,p*) (S1) and triplet excited state 3 (p,p*) (T1) or 1 (p,p*) and 3 (n,p*). In conjunction with the intrinsically large spin-orbit coupling (SOC) constant (zSO) of Se (ca. 5.07 kcal/mol) and Te (ca. 11.3 kcal/mol), [13] the SOC matrix element (HSOC) for the S1-T1 transition of a Se-or Te-containing p-conjugated compound becomes large, allowing the spin-forbidden S-T transitions. [14] Therefore, upon photo-excitation, the T1 state of an organoselenium/tellurium molecule is promptly populated through a rapid intersystem crossing (ISC) from the S1 state. [15] With a well-studied planar p-conjugated organoselenium/tellurium frameworks, the single-triplet energy splitting (DEST) is relatively large (>1 eV) comparable with other acene analogues, and as mentioned-above, the intermolecular interactions are strong in the solid states. As the results, the long-lived T1 state in the solids would be just depopulated through non-irradiative pathways such as thermal internal conversion (IC) via bond rotations and molecular vibrations, and energy diffusion.</p><p>Over the last few years, the internal heavy atom effects on the optoelectronic properties of triplet-harvesting organoselenium emitters based on twisted D-p-A [16,17] and D-p-D [18] scaffolds displaying thermally activated delayed fluorescence (TADF) [19] and room-temperature phosphorescence (RTP) [20] have been reported (Figure 1). In 2019, de Sa Pereira, Kukhta, and Lee investigated the effect of replacing Se for S atom of a D-p-A TADF emitter on the photophysics through a sophisticated time-resolved spectroscopic analysis of DSe-p-A type compound PSeZ-TRZ (Figure 1a) in comparison with that of its sulfur TADF emitter PTZ-TRZ. [16] The analysis of the complicated decays of PSeZ-TRZ in non-polar polymer matrix Zeonex ® rationally explained the independent irradiative pathways of two conformers, i.e., quasi-axial (ax) and quasi-equatorial (eq) (Figure 1a). Most importantly, replacing Se for S does not affect the reverse ISC (rISC) in significant degree but so does room-temperature phosphorescence (RTP) efficiency of the donor. On the other hand, Credgington and Seferos investigated the effect of replacing Se for S of a TADF compound TXO1-TPA on ISC and rISC processes by developing a D-p-ASe type compound SeXO1-TPA (Figure 1b). [17] The detailed kinetic analysis revealed that the Se affects both of ISC and rISC rates, enhancing the ISC more than 250 times and the rISC 22 time faster than those of the sulfur TADF emitter. Given these apparently contradicting results, the position of Se atom on p-electron systems significantly affect the photophysical properties of the selenium-embedded compounds. Most recently, Kim and Lee developed DSe-p-Dse type metal-free RTP compounds PSe1, PSe2, and PSe3 (Figure 1c) for OLEDs. The authors claimed that the n-p* transitions involving differentially-oriented p orbitals on the Se atom (pz→pxy) significantly enhance the spin-orbit coupling (SOC) element matrices between the S0 and T1, thereby accelerating the irradiation from the T1 state. [18] Zhao and Huang reported the effect of chalcogen atoms in the chalcogenophene-p-phenylcarbazoles on the optoelectronic properties and revealed dual emission behavior of fluorescence and RTP. [21] He and co-workers developed a D-ASe-D type organic phosphorescent compound for afterglow imaging and photodynamic therapy by taking the advantage of long-lived triplet excited states. [22] Therefore, investigation on the effect of replacing Se/Te for S atoms of TADF/RTP-active compounds on their photophysical properties would provide profound insights into understanding the design principles for balancing conflicting factors DEST and SOC to realize novel chalcogen-containing organic photofunctional materials.</p><p>Herein we disclose the development of novel twisted donor-acceptor-donor (D-A-D) organic emitters PSeZ-DBPHZ-PSeZ and PTeZ-DBPHZ-PTeZ based on a new organoselnium/tellurium scaffold (DSe-A-DSe and DTe-A-DTe, respectively) (Figure 1d). Most importantly, the investigation of their physicochemical properties revealed that PSeZ-DBPHZ-PSeZ is multi-photofunctional material featured with polymorphs-dependent multi emission colors, singlemolecular white emission (SMWE), and mechanochromic luminescence (MCL), chemical stimuliresponsive emission change, and distinct delayed emission in matrices and organic light-emitting diodes (OLEDs) (Figure 1d). The comparison of the experimental properties and quantum chemical calculation data of the series of D-A-D compounds clarified that the conformational preference arising from the "ax" and "eq" donors is strongly correlated with the chalcogen atoms and thus governs their photophyscical properties.</p><!><p>Design and Synthesis. Previously, we have developed multi-photofunctional organic materials displaying thermally activated delayed fluorescence (TADF), mechanochromic luminescence (MCL), [23] and room-temperature phosphorescence (RTP) properties, based on a donor-acceptor- donor (D-A-D) [24][25][26][27][28][29] or a D-A-D-A scaffold, [30] utilizing dibenzo[a,j]phenazine [31] as the key acceptor unit. In 2017, we reported the first example of TADF-active multi-color-changing MCL material (PTZ-DBPHZ-PTZ) based on the design concept of conformation-dictated regulation of luminescence properties, where the ax-and eq-conformer arising from the boat-chair geometry of phenothiazine plays the key role in giving divergent ground and excited state energies. [26a] To reveal the effect of replacing Se/Te for S of PTZ-DBPHZ-PTZ, D-A-D compounds on the photofunctionality, PSeZ-DBPHZ-PSeZ and PTeZ-DBPHZ-PTeZ were designed (Figure 1d). PSeZ-DBPHZ-PSeZ was successfully synthesized through a Pd-catalyzed Buchwald-Hartwig double amination of 3,11-dibromo-dibenzo[a,j]phenazine (Br2-DBPHZ) [31] with phenoselenazine (PSeZ) in a good yield (the upper equation in Scheme 1). The product was further purified through recrystallization from CH2Cl2/n-Hex solution to give yellow powdery solids. The product was fully characterized by 1 H, 13 C, & 77 Se NMR, X-ray crystallography, mass spectroscopy, IR spectroscopy, and elemental analysis (for the details, see the SI). PTeZ-DBPHZ-PTeZ was also synthesized through an amination of Br2-DBPHZ with phoenotellurazine (PTeZ) (the bottom equation in Scheme 1; for the details of synthesis and characterization, see the SI). In stark contrast to PSeZ, the known synthetic method for PTeZ is far from practical, due to the use of toxic mercury reagent. [32] Therefore, we decided to establish an up-dated synthetic method for PTeZ by modifying our method for phosphorous-bridged diarylamine (dihydrophenophosphanizine). [27] As the results, the cyclization of dilithiated N-Boc diarylamine with TeCl4 followed by the reduction of the resulting Te(IV) center successfully gave PTeZ (for the detail procedures, see the SI). Incidentally, during the preparation of this manuscript, the Patureau group also reported a robust synthetic method for PTeZ using Te(0) as the tellurium source. [33] It is noted that PTeZ-DBPHZ-PTeZ represents the first example of D-A type emissive compounds that have phenotellurazine (PTeZ) as the electron donor.  From the comparison with the packing structure of previously-reported sulfur-analogue PTZ-DBPHZ-PTZ in orange emitting single crystal (eq-ax conformation, lem 640 nm), [26a] it turned out that the packing structure of the selenium D-A-D compound in polymorph O is very similar to that of PTZ-DBPHZ-PTZ in the orange-emitting crystal, where the overlap of the p-unit in the dimeric pair and dinter (3.46 Å) is almost the same. Taken together, we can conclude that the difference in emission profiles among the polymorphs of PSeZ-DBPHZ-PSeZ are not only determined by the molecular conformations but also the intermolecular electronic interactions. [34] In contrast to the selenium D-A-D compound, any single crystals of tellurium containing compound PTeZ-DBPHZ-PTeZ suitable for X-ray crystallographic analysis were not obtained even after many attempts, due to powdery nature of the compound.</p><!><p>To clarify the single molecular photophysical properties of the D-A-D compounds, the steady-state photophysical properties of dilute solutions (c = 10 -5 M) in various solvents were investigated (Figure 3 and Table 1). Surprisingly, the Se-containing compound PSeZ-DBPHZ-PSeZ in dichloromethane displays a quite different absorption spectrum from the superposition of the individual spectra of D (PSeZ) and A (DBPHZ) (2D +A, Figure 3a).</p><p>The disappearance of the vibronic absorptions (ca. l 300 nm and 380-420 nm) derived from the acceptor unit indicates a strong electronic coupling between the D and A units (Figure 3a). This implies a quite different conformation of the Se compound from its sulfur analogue, where eq-eq conformer dominates and efficiently electron-decoupled absorption spectra displays. [26a] In a similar manner with our previously developed DBPHZ-cored D-A-D compounds, [25][26][27][28][29] an intramolecular charge-transfer (ICT) transition at around l 460 nm was observed (Figure 3a). However, when compared to the absorption spectra of the sulfur analogue, the molar coefficient (ca. e 25,000-28,000) of the ICT band of the selenium compound is much larger than that of sulfur compound (ca. e 10,000). [26a] A similar trend in the absorption spectra was observed for tellurium-containing compound PTeZ-DBPHZ-PTeZ, where more distinct vibronic structure in the lowest-energy absorption regime ranging from 420 to 480 nm was dominated than the selenium compound (Figure S4). This distinct difference caused by the Se and Te replacement for S atom suggests the ax-ax conformer is more dominated than sulfur analogue in solutions, which was supported by the theoretical calculations (vide infra). The comparison of the conformational population of the D-A-D compounds with each chalcogen-bridged diarylamine donor (S, Se, and Te) is illustrated in Figure 3b. The effect of Se replacement for S in the D-A-D scaffold includes the switching of the majority conformer from eq-eq (ca. 59% contribution for the sulfur compound) to ax-ax (ca. 55% contribution for the selenium compound; ca. 98% contribution for the tellurium compound, Tables S5 and S6). Importantly, energy decomposition analysis (EDA) [35] shows that the conformational preference results from an interplay between Pauli, electrostatic, and orbital relaxation factors (Table S10). All these terms are strongly coupled to the equilibrium geometry. Nevertheless, substituting S with Se and Te in the D-A-D scaffold at a fixed geometry always leads to the relative stabilization of the axial conformer by Pauli repulsion and its destabilization by electrostatic and orbital relaxation. Given that the destabilizing Pauli energy accounts for electronic repulsion, the thermodynamic preference of ax-ax conformers over eq-eq conformers in the selenium and tellurium-containing D-A-D compounds would be rationalized by less transannular electronic repulsion between the lone pairs on the N and the chalcogen atoms in the boat-chair donor unit.</p><p>The absorption spectra of PSeZ-DBPHZ-PSeZ were not affected by solvent polarity, suggesting the almost constant distribution of the conformers (dotted lines, Figure 3c). The weak effect of solvent polarity on the conformer distribution is also confirmed by theoretical calculations as shown in Table S7. The difference in the absorption spectra between S, Se, and Te-containing D-A-D compounds is also fully explained by different conformer stability. The theoretical spectra calculated by weighting contributions from all the conformers agree with the experiments in terms of both relative band positions and their intensities (Figure S16).</p><p>Not only the absorption, the impact of the Se replacement was drastic in photoluminescence (PL) spectra as well. Most importantly, the toluene solution of PSeZ-DBPHZ-PSeZ displayed a dual emission peaked at l 480 nm and 644 nm, thereby giving white emission (the photograph in Figure 3c). This is a remarkable contrast with the sulfur compound: it only gives a broad Gaussian-type CT emission at around l 657 nm in toluene. [26a] This should be associated with the dominance of the orthogonally-structured conformers (eq-eq and eq-ax) in the sulfur compound, which generates distinctly charge-separated CT excited states and relaxed in a polar solvent. The Se replacement affects the thermodynamic stability of the conformers, and thereby the admixture of the greenish LE emission at around l 480 nm from the dominant ax-ax conformer and the CT emissions from eq-ax (~38%) and eq-eq (~8%) conformers should yield white emission. Recently, single molecular white emitters (SMWEs) have emerged as promising materials for optoelectronic and sensor applications. [36] Nevertheless, the rational design of SMWEs is difficult, due to the energy transfer problems, for instance. Therefore, the introduction of PSeZ donors in D-A-D scaffold allows for strategic design of SMWEs by mixing ax-ax and eq-based conformers to balance energetically-different emissions. As the polarity of solvent increases, the LE emission at around 480 nm in toluene gradually red-shifted, while the CT emission at around 650 nm in toluene was totally vanished (Figure 3c). This polaritydependence would support the significant difference in CT natures of the two emissions observed in toluene: the bluer emission is LE with a weak CT nature yielded from ax-ax conformer, while redder emission is derived from a stabilized CT excited state with a highly twisted structure (twisted intramolecular charge-transfer: TICT state) from eq-ax and eq-eq conformers. Due to the distinct charge separation, the redder emission is highly susceptible to solvent polarity, and in more polar solvent than toluene, the lower CT excited state is dissipated through atomic vibrations and rotations, following the energy gap law. Theoretical calculations reveal that strong stabilization of the S1 state in eq-ax and eq-eq conformers is due to the planarization of the equatorial donor unit in the excited state (Figure S15). Such CT state is further stabilized by the dielectric response of the solvent; the calculated emission wavelength in toluene is 558 nm (2.22 eV) for eq-ax and 608 nm for eq-eq (2.04 eV) (Table S8). In a polar THF solvent, the CT emission are predicted at 656 nm (1.89 eV) and 720 nm (1.72 eV), respectively, but these states are most likely relaxing non-radiatively.</p><p>Tellurium-incorporated D-A-D compound PTeZ-DBPHZ-PTeZ shows fluorescence with a low quantum yield (FPL 0.01-0.15) in diluted organic solutions, showing positive solvatochromic luminochromism (Figure S4 and Table S4). In contrast to PSeZ-DBPHZ-PSeZ, the tellurium compound did not show dual emission in toluene, which is in good agreement with the domination of ax-ax conformer in this solvent (Figure 3b) as suggested by the theoretical calculation (vide infra).  [a] Determined with an integrating sphere.</p><p>Mechanochromic Luminescence (MCL) properties. The photoluminescence behavior of PSeZ-DBPHZ-PSeZ under external stimuli was then investigated (Figure 4). The as-prepared sample is a powdery crystalline sample, which shows sharp diffraction patterns in PXRD measurement (Figure S6a). Upon the irradiation of UV light, the solid emits yellow PL (lem 576 nm, FPL 0.04) (the inset photograph, Figure 4a). Upon grinding the solid with a pestle and a mortar, the emission color drastically turned to deep-red, displaying a broad Gaussian emission spectrum (lem 671 nm, FPL 0.09).</p><p>(the inset photograph, Figure 4a). The ground sample was found amorphous from the PXRD analysis (Figure S6a). At a glance, this mechanochromic luminescence (MCL) behavior is very similar to that of its sulfur analogue. [26a] However, the response against thermal and vapor treatment of the ground Se-compound was found quite different from its sulfur atom (Figure 4a). Upon heating the red-emitting ground sample on a hot plate at 200 °C for 15 min, the emission color reverted back to yellow, which was clearly seen on the PL spectra (Figure 4a). The PXRD patterns of the heated sample were found almost superimposed with those of the as-prepared powder sample (Figure S6a), confirming the recovery of the initial state. The fuming of the ground sample with organic vapor such as AcOEt and CHCl3 also caused the similar recovery of yellow emission (Figure S5). The PXRD analysis supported the recovery of the initial state (Figure S6a). The deep-red emitting amorphous state was reproduced by grinding the heated and fumed samples. In contrast, the amorphous state of the sulfur analogue PTZ-DBPHZ-PTZ shows a much smaller blue-shift for heating (621 cm -1 ) and fuming (1920 cm -1 ) than that for selenium-compound (2458 cm -1 ), and the deep-red emitting (ground sample) sate does not revert back to yellow-emitting state. [26a] Therefore, the Se replacement for S atoms allows for high reversibility of the two-emitting states. To obtain deeper insights on this unique phenomenon, we compared the PXRD patters of the yellow-emitting crystalline sample with those predicted from the single-crystals of G, Y, and O (Figures S6b). The comparison suggested that the eq-ax conformer would be the conformer contained in the yellow-emitting powder solids. Differential scanning calorimetry (DSC) of each solid sample revealed that the only the ground orange-emitting sample</p><p>shows exothermic peaks at around 159 and 198 °C (Figure S7), suggesting the ground sample is metastable state. Therefore, we can conclude that the enhanced reversibility of the initial state would be associated with the thermodynamic stability of the eq-ax conformer over eq-eq conformer in the aggregate states. Although this thermodynamic conformational preference appears to be inconsistence with that in solutions (Figure 3b), there could be many factors controlling the thermodynamic preference of conformers in the solid states such as packing forces and intermolecular interactions.</p><p>In addition to the reversible MCL behavior in response to various stimuli, the selenium compound shows response to chemical stimuli. For example, the photoluminescence (PL) of the as-prepared</p><p>PSeZ-DBPHZ-PSeZ turns off upon the treatment of vapor of trifluoroacetic acid (TFA), while the yellow PL turns on by fuming the solid with basic triethylamine (TEA) vapor (Figure S8a). This turn/on-off cycle is reversible and repeatable (Figure S8b). The selenium compound also shows redoxresponsive emission color change through the reversible manipulation of the oxidation state of the Se atoms (i.e., Ar2Se ⇄ Ar2Se=O) (Figure 4b, for the details of the redox procedures, see the SI). It is worth mentioning that such Se-oxidation caused drastic change in absorption spectra, where the vibronic absorptions ascribed to the acceptor unit (ca. labs 390 and 420 nm) are manifested and the CT absorption at around 450 nm is decreased (Figure 4b) when compared to those of PSeZ-DBPHZ-PSeZ (Figure 3c). The photoluminescence of the oxidized form (PSeOZ-DBPHZ-PSeOZ) displays more CT character showing larger positive solvatochromic shift (Figure 4b) than that of Se(II) species (Figure 3c), although the electronic donating ability of the phenoselenazine unit should be decreased.</p><p>A possible rational for explaining these contradicting spectra change would that the effect of the Seoxidation caused drastic change in conformational preference from ax-ax to eq-based conformers, where the D and A units are almost completely decoupled, and thereby the charge-separated excited states are stabilized by the perpendicular D-A-D scaffold. In contrast to the behavior of the selenium compound, PTeZ-DBPHZ-PTeZ did not show photoluminescence in the solid state, probably due to the rapid ISC and IC accelerated by larger SOC in the condensed state. Although the D-A-D tellurium compound does not show PL in the aggregated solid states, the compound exhibits blue-greenish PL in matrices with a long lifetime, indicating the room-temperature phosphorescence (RTP) (for the details, see the following time-resolved spectroscopy section). More interestingly, the phenotellurazine donor (PTeZ) shows yellow (lem 556 nm, FPL 0.09) and orange (lem 591 nm, FPL 0.19) photoluminescence (PL) in the solid states (Figure S9a) and Zeonex ® matrix (Figure S9b), respectively. The unexpected orange emission in the matrix was characterized with a long lifetime (tavr 11.7 µs under vacuum, Figure S9c), which becomes shorter under the effect of air (tavr 5.7 µs, Figure S9c). Given this emission peak almost matches with that of phosphorescence spectra acquired at 77 K (Figure S9b), the orange emission from PTeZ should be RTP. Although the details of the origin of this unique RTP awaits further analysis, the large SOC, which was supported by the theoretical calculation (<S0|HSOC|T1> 13.51 cm -1 , Table S13), should accelerate the irradiation from the T1 to the S0 state. It should be noted that PTeZ represents a new entry to organotellurium RTP family. [37] Time-resolved Spectroscopic Analysis. Initial steady-state photoluminescence analysis revealed the possibility of room-temperature phosphorescence (RTP) of both D-A-D compounds. Nevertheless, the full time-resolved analysis was indispensable to fully proof the photophysical mechanism. For that purpose, we investigated the photophysical properties of the solid-state layers of PSeZ-DBPHZ-PSeZ</p><p>and PTeZ-DBPHZ-PTeZ in both a non-polar polymeric host Zeonex ® and small molecular host CBP [4,4'-bis(N-carbazolyl)-1,1'-biphenyl] (Figure 5). As supporting work, the analysis of pure donors (PSeZ and PTeZ) in Zeonex ® matrix was also investigated (Figure S14). The layers were deposited on sapphire substrate and analyzed in broad range of temperatures from 10 K up to 300 K. The shortlived emission of PSeZ-DBPHZ-PSeZ in Zeonex ® matrix observed at 5.1 ns could be associated with emission from S1 ( 1 LE) state and is similar at low (80 K, black line in Figure 5a) and room-temperature (300 K, blue line in Figure 5a). At further time delay, the emission is shifted to lower energies, and the phosphorescence is observed at low temperature (red line in Figure 5a). As the temperature rises, the delayed emission is hypsochromically shifted, and the spectrum becomes broader (green and purple lines in Figure 5a). Based on theoretical calculation, the ΔEST gap of PSeZ-DBPHZ-PSeZ in an axax form is larger (490 meV) than the gap between T1 and T2 states (ΔET1-T2 = 400 meV) (Figure 7).</p><p>Given these energy alignments and the SOC between the S0-T2 states (1.56 cm -1 ), the temperature activated process (Figure 5a and e) could be related with the processes comprising of ISC (from S1 to T1) followed by thermal reverse internal conversion (from T1 to T2) and the photo-irradiation from the T1 and/or T2 states. [29] Given the PSeZ-DBPHZ-PSeZ is admixrue of ax-ax and eq-ax conforms</p><p>(Figure 3b), the energy transfer from the triplet excited state of ax-ax conformer to eq-ax conformer</p><p>follwed by TADF emission from the 1 CT state would also likely to occur.</p><p>Slightly different and complex behavior was observed for the PTeZ-DBPHZ-PTeZ in Zeonex ® (Figure 5b). The emissions at short delay times (in the ns region) are related with the emission from the S1 (black and blue lines in Figure 5b). As for the long-delayed emission (phosphorescence), the emission slightly shifted to the higher energy regime with the rise of temperature (red and green lines in Figure 5b). Interestingly, while the energy level and the spectrum shape of the phosphorescence from T1 of PSeZ-DBPHZ-PSeZ are similar to those of previously studied DBPHZ-cored D-A-D compounds, [24][25][26][27][28][29] the phosphorescence of PTeZ-DBPHZ-PTeZ appears to be much more similar to the phosphorescence of donor (PTeZ) (Figure S14b). But, the calculation suggests that similarity is coincidental. The T1 state of the dominant ax-ax conformer of PTeZ-DBPHZ-PTeZ is localized on the acceptor ( 3 LEA), and the donor retains its bent structure (Figure 7f), while the T1 state of PTeZ has a planarized geometry which stabilizes the excited state (Figure S17). In the ax-ax conformer of the D-A-D compound, such planarization is not possible, due to the steric repulsion between D and A units. As the results, the energies of both triplet emissions of D and D-A-D compounds appear to be very close to each other.</p><p>More difficult processes to evaluate are involved in the photophysical properties of the investigated compounds inside CBP host, which was used for the fabrication and evaluation of OLED devices (Figure 5c, d, g, and h). The emissions of both D-A-D compounds at shortest times are similar to those in Zeonex ® matrix and should be ascribed to the localized excited singlet state ( 1 LE) in both low (80 K, black lines in Figure 5c and d) and room temperature (300 K, blue lines in Figure 5c and d).</p><p>With both compounds, at low temperatures and long delay times, the phosphorescence is observed in the lower energy region than those in for Zeonex ® layers, but the intensity is not that significant, suggesting it could be the emission from the lowest CT state. On one hand, the thermally activated emission processes are on the same energy levels like the process from the lowest 3 LE state, but the spectra are broader like the emission related to charge transfer process (Figure 5c, d, g, and h). Such complex behavior is not usually observed, but as mentioned before, we probably observe the emission from T1 and a higher triplet state (T2). [29] The change of the host more affects the CT excited state energies to the lower level than LE states, thereby in CBP host both D-A-D compounds display almost the same energy level. OLED fabrication and characterization. The electrochemical behavior of the donors and D-A-D compounds in dichloromethane was investigated with cyclic voltammetry (CV) (Figure S12). The cyclic voltammogram of PSeZ-DBPHZ-PSeZ displayed a reversible oxidation and reduction curves at ox Eonset = +0.31 V and red Eonset = -1.71 V (vs. Fc/Fc + redox couple), respectively. This electrochemical behavior looks typical for DBPHZ-cored D-A-D compounds (Figure S12a). [24][25][26][27][28][29] On the other hand, the tellurium compound PTeZ-DBPHZ-PTeZ showed irreversible electrochemical redox behavior (Figure S12b), indicating the less electrochemical instability than the sulfur [26a] and selenium analogues. Such tendency in the electrochemical analysis was previously reported in thiophene, selenophene, and tellurophene analogues, where the decomposition of the tellurophene ring was observed during the oxidation process. [38] When compared to the donors (PSeZ and PTeZ), distinct positive shift in the oxidation potential for the both D-A-D compounds by >0.3 V was observed (Figure S12). The ionization potential (IP) and electron affinity (EA) for the PSeZ-DBPHZ-PSeZ and PTeZ-DBPHZ-PTeZ estimated from the CV experiment were estimated to be 5.41/3.39 eV and 5.38/3.35 eV, respectively. [39] When compared to the IP/EA of the sulfur analogue PTZ-DBPHZ-PTZ</p><p>(5.33 eV/3.38 eV) [26a] and oxygen analogue POZ-DBPHZ-POZ (5.36 eV/3.38 eV), [25] the Se/Te replacement effect for S atoms in the D-A-D scaffold on electrochemical redox characters is very small. This can be explained by the kinetic process: the conformer with the lowest oxidation/reduction potential (eq-eq) is faster electrochemically oxidized/reduced on the electrode than other conformers (eq-ax and ax-ax) with higher oxidation/reduction potentials. What is also interesting, both D-A-D compounds undergo oxidative electropolymerization process after the third oxidation peak (Figure S13a and b). It is also noted that only PTeZ donor forms polymer on the electrode (Figure S13c). The performances and photophysical results of the fabricated OLED devices were investigated and compared in order to evaluate emissive pathways that boost the efficiency properties. Most importantly, dual-emissive property of the emitters was manifested in both OLED devices (Figure 6a). Such behavior is not usual, but taking into the time-resolved spectroscopic analysis data (Figure 5), the higher-energy emission at around lem 500 nm in both cases would be related with the electroluminescence from the S1 ( 1 LE) state and the lower-energy emission at around lem 600 nm with the RTP from the 3 LE state. In other words, fluorescence and room-temperature phosphorescence (RTP) processes are involved in the electroluminescence. [40] In both cases, the RTP contribution in overall emission, which was estimated from the comparing the area of emission spectra acquired in aerated and degassed conditions, is higher than 90%, and therefore the mostly related factor for the efficiency of the device should be the phosphorescence process. The highest external quantum efficiency (EQE) of the OLEDs was obtained with PSeZ-DBPHZ-PSeZ (4.35%), where the device with tellurium derivative PTeZ-DBPHZ-PTeZ showed a lower EQE of 1.89% (Figure 6c). These EQE values may not be that high, when compared with the OLED devices with existing heavy-atomcontaining RTP organic emitters. But, if we assume that the overall PLQY for those compounds is less than 10%, it would suggest we obtained as highest efficiency as possible. On the other hand, the devices exhibit quite high luminance as for selenium and tellurium derivatives with up to 10,000 cd/m 2 for PSeZ-DBPHZ-PSeZ and more than 4,000 cd/m 2 for PTeZ-DBPHZ-PTeZ-based device. Theoretical Calculations. To obtain deeper insights into the developed emitters, theoretical calculations were performed at the density functional theory level for the PSeZ-DBPHZ-PSeZ and PTeZ-DBPHZ-PTeZ compounds (see SI for details). Ground state geometry optimizations of the different conformers associated with a weighting procedure using Boltzmann factors at 300 K allowed us to conclude that all the three PSeZ-DBPHZ-PSeZ conformers (i.e., ax-ax, ax-eq, and eq-eq) should coexist at room temperature under vacuum, with the ax-ax conformation being the most stable (55% probability), followed by eq-ax (38%) and eq-eq (7%). In contrast, only two PTeZ-DBPHZ-PTeZ conformers are expected to be present at room temperature, namely ax-ax (98%) and eq-ax (2%). As such, we notice that one of the effects of heavy atom substitution from S to Se and Te is a progressive dominance of ax-ax conformations (Figure 3b), due to the deepening of the potential energy surface in this particular geometry. Importantly, conformational preference is not significantly affected by solvents, as shown in Table S7.</p><p>Excited state calculations in toluene revealed that for all PSeZ-DBPHZ-PSeZ conformers the T1 states correspond to a localized excitation ( 3 LEA) state of the acceptor unit. On the other hand, S1 states correspond to charge transfer (CT) states in all but the ax-ax conformer, as evidenced by the natural transition orbitals (NTOs) presented in Figure 7a, b, and c. However, Figure 7d, e, and f show that for eq-ax and ax-ax PTeZ-DBPHZ-PTeZ conformers, both S1 and T1 states correspond to localized excitations in the acceptor portion of the molecule, whereas the eq-eq conformer presents an S1 state with CT character and a T1 state that is localized in the donor unit. In particular, this difference is reflected in the significantly higher oscillator strength associated with S0-S1 transitions of the PTeZ-DBPHZ-PTeZ molecule. In fact, the S0-S1 transitions for both the eq-eq and eq-ax conformations of PSeZ-DBPHZ-PSeZ are predicted to be forbidden (zero oscillator strength). These characteristics, in addition to the aforementioned population of the different conformers, account for the observed differences in the absorption spectrum of the S, Se and Te-containing D-A-D compounds.</p><p>Calculated emission energies from the S1 state in the ax-ax conformers of both compounds predict fluorescence at around 2.5 eV, in agreement with experimental results. A lower energy band in PSeZ-DBPHZ-PSeZ fluorescence spectrum is also predicted at 2.2 eV to result from CT emission from the eq-ax conformer. Such second band is not observed for PTeZ-DBPHZ-PTeZ, however. The behavior stems from the fact that the donor unit of the eq-ax Se compound undergoes planarization in the S1 state (Figure S15), resulting in a lower energy CT state that is further strongly stabilized by a polarizable environment. [41] . Such planarization is not seen in the Te compounds, for which the S1 emission energy in the eq-ax conformer is much larger (2.81 eV, Figure 7e).</p><p>Phosphorescence energies calculated for both D-A-D compounds are also very similar and predicted to lie around 2.0 eV. This is due to the fact that T1 states in the ax-ax conformation of both molecules correspond to localized excitations in the acceptor unit. For the pure PSeZ and PTeZ donor fragments, phosphorescence energies are predicted at 2.3 eV and 1.9 eV, respectively (Table S13). As such, the experimentally observed similarity in phosphorescence energies between the Te-containing D-A-D compound and the PTeZ fragment does not necessarily imply that phosphorescence in PTeZ-DBPHZ-PTeZ stems from the donor unit.</p><p>Spin-orbit coupling calculations have also been performed for all molecules in toluene using the T1 geometry. Results are shown in Figure 7. These calculations reveal that S0-T1 spin-orbit couplings are affected mostly by substitution of the Se atoms for Te rather than by conformation. This can be seen from the overall larger couplings calculated for PTeZ-DBPHZ-PTeZ conformers when compared to PSeZ-DBPHZ-PSeZ. For the ax-ax conformation, which is the most likely to be present, S1-T1 couplings are found to be around 0.7 cm -1 for both molecules. The S1-T1 transition in this conformer is allowed, partially due to the fact that it corresponds to a transition between two different LE states (n-p* and p-p*).</p>