Dual Emission through Thermally Activated Delayed Fluorescence

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Spectroscopy and Photochemistry; General Theory

Dual Emission Through Thermally Activated Delayed Fluorescence and Room-Temperature Phosphorescence, and Their Thermal Enhancement via Solid-State Structural Change in A Carbazole-Quinoline Conjugate Indranil Bhattacharjee, Nirmalya Acharya, Harsh Bhatia, and Debdas Ray J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00937 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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The Journal of Physical Chemistry Letters

Dual Emission through Thermally Activated Delayed Fluorescence and Room-Temperature Phosphorescence, and Their Thermal Enhancement via Solid-State Structural Change in A CarbazoleQuinoline Conjugate Indranil Bhattacharjee, Nirmalya Acharya, Harsh Bhatia, Debdas Ray* Department of Chemistry, School of Natural Sciences, NH-91, Tehsil Dadri, District Gautam Buddha Nagar, Uttar Pradesh, 201314, India. Corresponding Author * [email protected].

ABSTRACT. The emergence of single-component organic dual light emitters holds great promise for white light-emitting diodes (WLEDs) and biological detection due to the involvement of broad emission covering visible spectrum. Here we show experimental studies on dual emission of carbazole-quinoline conjugate (CQ) that exhibits both thermally activated delayed fluorescence (TADF) via reverse intersystem crossing (rISC) from the higher-lying triplet state (T2) to the singlet state (S1) and room-temperature phosphorescence (RTP) from the

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lowest triplet state (T1) due to low energy gap between T2 and S1, and energetic proximity of T1 with T2. We found in thermal effect that intensity of the dual features are enhanced with increasing temperatures up to 100 °C, which can be explained by a thermal-induced structural change (TISC) mechanism that compensates the emission losses due to non-radiative transitions at elevated temperatures. This property, in addition to its enhanced TADF and phosphorescence decay rates (~107 s-1and 101 s-1) at 100 °C, would have great promise for high-efficiency LEDs.

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Single-component organic dual light emitters (SCODLEs) have boosted an enormous interest in the white organic light-emitting diodes (WOLEDs)1-4, sensing5, and imaging applications6 underlying broad emission that cover visible spectrum. SCODLEs potentially offer advantages of better color reproducibility, stability, and easy fabrication over multi-component systems. Therefore, the search and use of new SCODLEs are of particular interest. However, SCODLEs can be mainly achieved by using excited state intramolecular proton transfer (ESIPT)1, a twisted intramolecular charge transfer state (TICT)7,8 equilibrated with the parent locally excited (LE) state, and two directly accessible excited states (e.g. S1 and S2), or states derived from them9. The materials with thermally activated delayed fluorescence (TADF) via reverse intersystem crossing (rISC) from T2 to S1 and room-temperature phosphorescence (RTP) from


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the radiative decay of the lowest triplet state (T1) that is populated via internal conversion from T2 or ISC from S1 to T1 (Figure 1a), are promising energy-efficient light sources. However, the studies of SCODLEs with such an approach remain limited and a challenging area of research in photophysics because a small energy gap between S1 and T2 states (∆ES1-T2) along with close proximity of T1 with the T2 (∆ET1-T2) and/or S1(∆ES1-T1) are important requisites. As well known, RTP10-12 materials, which are typically based on heavy-metal complexes13,14 that allow efficient phosphorescence with high decay rates (>104 s-1) from the triplet state through strong spin-orbit coupling (SOC)8,14, have attracted increasing attention over the past decades due to their potential applications in OLEDs15,16, bio-imaging17, and advanced security imaging.18 Since metal-free organic materials have typically slow intersystem crossing (ISC) from the S1 to the triplet manifold (Tn), and radiative decay rates (< 100 s-1)10 from T1 to the S0, RTP in organic semiconductor is rarely observed. In recent years, researchers have employed a variety of methods to study RTP, including crystalline materials19, carefully chosen rigid host20,21, polymer matrices22,23, aggregates21, micelles24, deuteration20,25 of molecular backbone, and special interactions 8,19,21,26. However, these studies are limiting their practical utility due to difficult processing, and have established that triplet energy, rigidity of the matrix and molecular oxygen play an important role in controlling the non-radiative deactivation channels (knr). In addition, to achieve thermally enhanced photoluminescence (TEPL)27,28, the chromophores need to compensate their emission losses due to non-radiative transitions at elevated temperatures. Because of these herculean pre-requisites, the study of materials with TADF and RTP features along with their TEPL properties becomes a tremendous challenge in photophysics without having a well-defined structure-property correlation. The major issue that limits such correlation is the lack of proper structural design and understanding of the origin of dual emission and their


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emission enhancement at elevated temperatures. It is imperative to design new organic systems, which would address these limitations. Here, we demonstrate four carbazole-quinoline based donor-acceptor (D−A) molecular systems, CQ, CQ-Me, CQ-OMe, and CQ-CN (Figure 1b) in which carbazole is covalently attached to the C8 position of the 2,4-aryl substituted quinoline framework (aryl = −Ph,

Figure 1. (a) Jablonski diagram for dual emission via TADF and phosphorescence in the proposed molecular design. (b) Molecular structures of the carbazole-quinoline conjugates. −PhOMe, −PhMe, and −PhCN) via a C−N bond. Inspired by the studies for the TADF that originates via rISC from the T1 to S1 state29-32, we hypothesized that the torsion between the donor and acceptor would play a crucial role to facilitate rISC from T2 to S1 state, thereby allowing RTP from T1 state. Our experimental studies revealed that only CQ in the solid state exhibits both TADF through rISC from T2 to S1 and RTP feature due to radiative decay of the T1 state. The theoretical calculation at the BLYP/6-31G(d) level of theory33,34 further shows that


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both T2 and S1 states are localized on quinolinyl fragment, while the carbazolyl ring contributes to T1, thus causing TADF from T2 to S1 and efficient RTP from T1. We also demonstrate that the CQ shows emission enhancement of dual features with increasing temperatures up to 100 °C. The CQ has the unprecedented ability to compensate the emission losses due to non-radiative transitions at elevated temperatures by a counter thermal-induced structural change (TISC) mechanism. The target chromophores, CQ, CQ-Me, CQ-OMe and CQ-CN were prepared by simple multicomponent condensation reaction35 (Scheme S1). All the conjugates were characterized by NMR spectroscopy, high-resolution mass spectrometry, and X-ray analysis (see the Supporting Information). We systematically investigated the UV-visible(vis) and photoluminescence (PL) properties in solution and solid state. CQ exhibits two absorption bands at 320 nm and 375 nm, respectively. The emergence of a hypsochromically shifted absorption band of λ375 with increasing polarity of the solvents confirms the charge transfer (CT) nature of the transition36 (Figure 2a). Likewise, steady-state emission shows positive solvatochromism37 effects and broad emission feature, suggesting CQ is less polar in the Franck-Condon (FC) region compared to the S1 minimum from which the emission occurs (Figure 2a). Moreover, the bi-exponential decay of the emission band in toluene (λem = 441 nm), showing prompt (τPF = 6.8 ns) and delayed components (τDF = 1.5 µs) in absence of molecular oxygen at RT and mono-exponential decay of the λ441 at 77 K in solution confirms a typical TADF dye29-32 (Figure 2b, Figure S8c).


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Figure 2. (a) UV-visible (vis) and steady-state emission spectra in solvents of disparate polarity. (b) Excited state decays at ambient and de-oxygenated conditions in toluene. (c) Steady state photoluminescence and phosphorescence spectra in solid state at ambient condition. (d) Excited state decay transients spectra measured at temperatures from 77 to 300 K. On the other hand, the steady state emission in the solid state at ambient conditions shows two emission peaks at 437 nm, and 517 nm, respectively, the later peak is accompanied with a broad shoulder at 545 nm (Figure 2c). The peak at 437 nm is assigned to the TADF character which is confirmed in the decay transient measurement, showing prompt and delayed components (τPF =9.48 ns, τDF = 7.55 µs) at RT and a continuous increase of lifetime upon lowering of temperature (300 K to 77 K) (Figure 2d). While the intense peak at 517


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nm is assigned as a purely RTP feature showing lifetime of 7.2 ms in the phosphorescence decay transient measurement at RT, the phosphorescence spectrum measured at 77 K (Figure S9) shows a FC type characteristic vibrational feature42 of a 3

LE state. Taking altogether, we believe that dual emission is occurring via TADF and

RTP. The most exciting observation is the enhancement of intensity of all the emission bands (Figure 3a) with increasing temperatures from 25 °C to 100 °C, indicating a substantial reduction of non-radiative transitions by a counter mechanism, which is therefore able to sustain the emission with rising temperature. More importantly, a 3 nm red shift in the position of the peak at 437 nm is detected at 50 °C, while no further change in the peak position was detected above 50 °C, suggesting a structural change that modulates excited state energy levels. The decay transient measurement of λ437 shows a slight increase in lifetimes of the prompt component from 9.48 ns at RT to 9.76 ns at 100 °C and slight decrease of lifetimes of the delayed component from 7.55 µs at RT to 7.33 µs at 100 °C, which is in good agreement with the literature of TADF28 (Figure S10a and Table S3). Likewise, phosphorescence lifetimes of the λ517 decreases slightly from 7.2 ms at RT to 6.95 ms at 100 °C (Figure S10b and Table S3). On the other hand, an 8-10 nm shift is observed in the position of the λ517 towards longer wavelength for temperatures of 50 °C and 70 °C, while no appreciable change in the peak position was detected from 70° to 100 °C (Figure 3b). Further, these thermally perturbed spectra return to the original positions during cooling, indicating a structural change which is indeed fully retained (Figures S11a, S11b). The efficacy of the spectral change is further observed in the absorption and


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Figure 3. Steady-state PL (a) of CQ and (b) phosphorescence spectra upon heating from 25 °C to 100 °C. Inset showing change in steady-state intensity and lifetime upon heating. (c) Quantum yields of emission upon heating. (d) Rate constants upon heating. Error bar represents the maximum deviation over three independent measurements. (e) Phosphorescence intensity and lifetime cycles upon heating and cooling. (f) Temperature-dependent XRD pattern upon heating from 25° to 100 °C. excitation spectra (Figures S12 and S13). Further crystals of CQ manifest similar efficacy during heating and cooling (Figures S14c, S14d, S11c and S11d), thereby assigning the observed features to the possibility of structural change at elevated temperatures. We observed that the quantum yields of total emission (ΦE) and phosphorescence (ΦP) increases monotonically with increasing temperatures, leading to a two-fold increment (53.88%) of ΦE at 100 °C when compared to the absolute value of 26.2% at RT(Figure 3c, Table S4). In addition, a slight increase of quantum yields of the prompt fluorescence (ΦPF), and the triplet (ΦT) is observed, while a slight reduction of delayed fluorescence (ΦDF) is also seen. The results shown in Figure


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3d clearly demonstrate the effects of temperature on rate constants (Table S5), showing a noticeable decrease in the rate constant of rISC (krISC) due to increasing temperature from 25 °C to 100 °C, while non-radiative rate constant of prompt fluorescence (knrPF) and radiative rate constant of phosphorescence (krP) increase appreciably with a small increment in the nonradiative phosphorescence rate constant (knrP). To test the thermal reversibility of the RTP emission, we conducted four switching cycles by recording the PL measurement at 100 °C using the same excitation, followed by thermal relaxation at 25 °C (Figure 3e and Figure S15). This measurement shows that the phosphorescence feature is regenerated through subsequent heating and cooling. Ensuring high-thermal stability of the sample, we also tested the thermal stability of the RTP feature with time at a fixed temperature of 100 °C (Figure S16). These heating and cooling cycles do not lead to an appreciable change in intensity or lifetimes, which allows us to ensure that the RTP spectra are stable under the given experimental conditions. To elaborate upon our molecular understanding of structural changes associated with the PL, differential scanning calorimetry (DSC) and temperature-dependent PXRD measurements were performed. (Figure S17). DSC analysis shows that the phase transformations occur at Tα−β = 54 °C and Tβ−γ = 90 °C, which transform back to the original phases upon cooling. Hightemperature PXRD analysis at 50 °C shows that a new set of Bragg peaks appear at 23.26°, 23.7° and 28.22° (Figure 3f and Figure S18). Interestingly, these peaks disappear for the temperatures of 70 °C, 80 °C and 90 °C, while reappear again for further increase in temperature at 100 °C. These features in the Bragg peaks conclusively identify that a structural change has occurred at elevated temperatures.


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Unequivocal evidence of conformational change was based on the single crystal X-ray diffraction (SCXRD) analysis (Figure 4). The X-ray data of CQ was collected at RT as well as 70 °C (supporting information). SCXRD analysis of CQ at RT showed that the carbazolyl and phenyl rings attached to C8 and C4 atoms of quinolinyl ring are deviated from planarity, and the

Figure 4. Oak Ridge thermal ellipsoid plots (50% probability ellipsoids) of (a) CQ at RT and (b) CQ at 70 °C. The protons are placed in calculated positions. (c) Superimposition of the two Xray structures (CQ-RT, orange; CQ-70 °C, green). The protons are removed for the sake of clarity. corresponding torsions are found to be 55.7(3)° and -127.4(3)° when viewed along the C(11A)– N(2)–C(8)–C(9) atoms and C(3)–C(4)–C(17)–C(18) atoms, respectively (Figure 4a and Figure S19). In addition, the phenyl ring attached to the C2 position of the quinolinyl moiety is slightly deviated (-2.9(4)°) when viewed along the atoms of N1−C2−C23−C28. Interestingly, these angles were tweaked upon heating to 105.4(2)°, 50.7(2)°, and 2.9(2)° respectively, when viewed along the same atoms in CQ (Figure 4b and Figure S20). Comparison of the SCXRD data of CQ at RT and 70 °C shows substantial differences in their torsions (Figure 4c and Tables S6, S7),


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which substantiate our previous arguments for TISC mechanism which compensates emission losses due to non-radiative transitions at elevated temperatures. To prove the mechanism of the observed PL feature of CQ, DFT and TD- DFT38 calculations were performed in GAUSSIAN 09 D.0139 at BLYP/6-31G(d) level of theory (see the supporting information). The calculated energy levels of S1, T1 and T2 states for the CQ crystal were assigned to be 1ππ*(CT), 3ππ*(LE), and 3ππ*(LE), respectively (Figure 5, Table S8). Comparison of the inverse solvent polarity effect of the λ375 and theoretical calculation suggest that S1 is of π−π∗ character with CT nature. The calculations also suggest that the isosurface of T1 (3ππ*) is localized on carbazole ring, while the corresponding isosurfaces of S1 1ππ*(CT) and T2 (3ππ*) states, which are quite similar, are delocalized over the quinolinyl ring and its phenyl substituent at C2 position, respectively, thereby enabling efficient ISC and rISC processes via 1CT → 3LE / 3

LE→1CT transition8 as T2 (3ππ*) lies just above the S1 (1ππ*(CT)) state (∆ES1-T2 = 0.144 eV).

Moreover, we observed that the calculated energy level of T1 (3ππ*) falls below the T2 (3ππ*) state (∆ET1-T2 = 0.507 eV), suggesting an additional phosphorescence channel operating from 3

LE(3ππ*). The small values of ∆ET1-T2 and ∆ES1-T1 (0.366 eV) therefore, play a pivotal role in

populating lower lying T1 (3ππ*), which might occur predominantly through internal conversion from the T2 (3ππ*) and/or ISC from S1 (Figure 1). Our results demonstrate that the torsion between the carbazolyl ring and quinolinyl moiety plays a crucial role and sets a benchmark for such intriguing photophysics. To test our hypothesis we synthesized CQ-Me, CQ-OMe and CQ-CN (see the supporting


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Figure 5. Isosurfaces of S0 (lower left image), S1 (upper left image), T2 (upper right image) and T1 (lower right image) of CQ calculated at the BLYP/6-31G(d) level; energy levels of CQ. information), in which the phenyl rings at the C2 and C4 positions of quinolinyl moiety are substituted by methyl, nitrile, and methoxy groups inducing electronic and steric effects. SCXRD analysis of CQ-Me and CQ-CN identifies a significant difference in their torsions associated with the phenyl at C4 position and carbazolyl rings at C8 position when we compare the data with that of CQ. Computational analysis shows that these substitutions drastically increase the energy gap between the S1 and T2 (Table S10), which eliminates the possibility of dual emission features through TADF that originates from rISC from T2 to S1, and RTP from T1. However, the spectroscopic measurements show that all the three compounds (CQ-Me, CQ-OMe and CQCN) show prompt fluorescence features via radiative decay of S1 (Figure S23 and Table S9) and quenching of their emission with increasing temperatures. These results indicate that the CQ is a rare one that can retain the pre-requisite of low energy gap between the S1 and T2 states (∆ES1-T2) along with the close proximity of T1 with the later (∆ET1-T2), and exhibit two independent emission channels through TADF and RTP even at 100 °C.


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In summary, we have demonstrated an unique organic conjugate showing dual emission via RTP and TADF, and its emission enhancement at elevated temperatures. Spectroscopic measurements tracking TISC, which is therefore highlighting the predominant role of torsion between the donor and acceptor, provide unequivocal evidence for TEPL, particularly, phosphorescence with increasing temperatures. Compared to the traditional molecular systems, this design principle seems to be most predominant one for high-efficiency (LEDs) because the conjugate provides stable PL generating capability when tested under repeated heating and cooling cycles. ASSOCIATED CONTENT Supporting Information. Experimental procedures, NMR spectra of key compounds, Single crystal X-ray diffraction analysis, UV/vis absorption spectra, Steady-state emission and excitation spectra, temperature-dependent absorption and emission, time correlated single photon counting (TCSPC) measurement, phosphorescence measurement, temperature-dependent quantum yields and rate parameters, and Computational details. AUTHOR INFORMATION Corresponding Author *[email protected] ORCHIDId. 0000-0002-6169-8823 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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This paper is dedicated to Prof. Parimal K. Bharadwaj on the occasion of his 65th birthday. D. R. is grateful to the Science & Engineering Research Board (SERB) (File No: SB/EMEQ-004/2014 & YSS/2014/000923), DST, and Shiv Nadar University (SNU) for generous support. I. B., H. B and N. A thanks SNU for fellowship. We thankfully acknowledge the use of the MAGUS supercomputing system in SNU. REFERENCES (1) Tang, K. C.; Chang, M. J.; Lin, T. Y.; Pan, H. A.; Fang, T. C.; Chen, K. Y.; Hung, W. Y.; Hsu, Y. H.; Chou, P. T. Fine tuning the energetics of excited-state intramolecular proton transfer (ESIPT): white light generation in a single ESIPT system. J. Am. Chem. Soc. 2011, 133, 17738−17745. (2) Yang, Q. Y.; Lehn, J. M. Bright White‐Light Emission from a Single Organic Compound in the Solid State. Angew. Chem. Int. Ed. 2014, 53, 4572−4577. (3) Zhang, Y.; Miao, Y.; Song, X.; Gao, Y.; Zhang, Z.; Ye, K.; Wang, Y. Single-Molecule-based White-Light Emissive Organic Solids with Molecular-Packing-Dependent Thermally Activated Delayed Fluorescence. J. Phys. Chem. Lett. 2017, 8, 4808−4813. (4) He, Z.; Zhao, W.; Lam, J. W.; Peng, Q.; Ma, H.; Liang, G.; Shuai, Z.; Tang, B. Z White light emission from a single organic molecule with dual phosphorescence at room temperature. Nat. Comm. 2017, 8, 416. (5) Carter, K. P.; Young, A. M.; Palmer, A. E. Fluorescent sensors for measuring metal ions in living systems. Chem. Rev. 2014, 114, 4564−4601. (6) Zhang, G.; Palmer, G. M.; Dewhirst, M. W.; Fraser, C. L. A dual-emissive-materials design concept enables tumour hypoxia imaging. Nat. Mater. 2009, 8, 747−751. (7) Qian, Y.; Cai, M.; Zhou, X.; Gao, Z.; Wang, X.; Zhao, Y.; Yan, X.; Wei, W.; Xie, L.; Huang, W. More than restriction of twisted intramolecular charge transfer: three-dimensional expanded#-shaped cross-molecular packing for emission enhancement in aggregates. J. Am. Chem. Soc. 2012, 6, 12187−12195. (8) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Principles of molecular photochemistry; University Science Books: Sausalito, California; 2009. (9) Pigliucci, A.; Nikolov, P.; Rehaman, A.; Gagliardi, L.; Cramer, C. J.; Vauthey, E. Early Excited State Dynamics of 6-Styryl-Substituted Pyrylium Salts Exhibiting Dual Fluorescence J. Phys. Chem. A, 2006, 110, 9988−9994. (10) Baldo, M. A., O'brien, D. F., You, Y., Shoustikov, A., Sibley, S. "Highly efficient phosphorescent emission from organic electroluminescent devices. Nature, 1998, 395, 151−154. (11) Adachi, C.; Baldo; M. A.; Thompson, M. E.; Forrest, S. R. Nearly 100% internal phosphorescence efficiency in an organic light-emitting device. J. Appl. Phys. 2001, 90, 5048−5051.


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Dual Emission through Thermally Activated Delayed Fluorescence

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