Revisit Dual Intramolecular Charge-Transfer Fluorescence of

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Revisit Dual Intramolecular Charge-Transfer Fluorescence of Phenothiazine-Triphenyltriazine Derivative Deng-Gao Chen, Ta-Chun Lin, Yi-An Chen, Yi-Han Chen, Tzu-Chieh Lin, Yi-Ting Chen, and Pi-Tai Chou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04395 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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The Journal of Physical Chemistry

Revisit Dual Intramolecular Charge-Transfer Fluorescence of Phenothiazine-Triphenyltriazine Derivative Deng-Gao Chen, Ta-Chun Lin, Yi-An Chen, Yi-Han Chen, Tzu-Chieh Lin, Yi-Ting Chen and PiTai Chou* Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan

ABSTRACT. The photophysical properties of phenothiazine-triphenyl derivative, PTZ-TRZ, are reinvestigated. The results, in combination with the computational approaches, lead us to draw conclusion that the complicated excitation behavior in toluene (ref. 18), in part, is due to the UV absorption cut-off region for toluene where the < 315 nm excitation is greatly distorted by solvent absorption, i.e. the inner filter effect, in regular sample cuvette (1.0 cm path-length). Switching the solvent to cyclohexane where the UV cut-off wavelength at 235 nm simplifies the results. In cyclohexane, there exist two isomers for PTZ-TRZ in the ground state, quasi-axial and quasi-equatorial conformers. Upon electronic excitation, both quasi-axial and quasi- equatorial conformers undergo structural relaxation to an energy minimum state where the phenothiazine is in a planar configuration.

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Introduction Organic light-emitting diodes (OLEDs) have undergone significant advances and been applied in panel displays recently such as smart phones and televisions1-3. In general, the development of OLED materials and its mechanism design can be categorized into two major classes, namely, the fluorescent and phosphorescence materials, which harvest the singlet and triplet excitons with population ratio 1:3, respectively, according to spin statistic rule. This made phosphorescent materials, particularly those transition metal complexes, increasingly important4-6. However, the consumption of noble metals, such as Ir, Pt. etc.,2, 7 unavoidably increases the device cost as well as raises environmental concerns. Alternatively, organic materials employing thermally activated delayed fluorescence (TADF) have emerged as the next-generation OLEDs.5 TADF properties are realized in a connection with donor and acceptor (D-A) molecules,8-9 which exhibit a small energy gap (∆ES-T) between the lowest singlet excited state (S1) and lowest triplet state (T1), resulting from the effective separation of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).10-11 This leads to the advantage that statistically 75% population of the triplet state from electron-hole recombination can undergo reversible intersystem crossing back to the S1 state, greatly enhancing the emission efficiency in OLEDs. Among numerous TADF molecules 2,4,6-triphenyl-1,3,5-triazine (TRZ) is considered to be one of the most popular core moieties to act as the electron accepting site. One of the early prototype could be ascribed to compound PXZ-TRZ which links phenoxazine (PXZ) and TRZ as an electron donor and acceptor, respectively.12 The large twist angle between PXZ and TRZ moieties effectively separated HOMO and LUMO in PXZ-TRZ, resulting in the prominent TADF property.


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Scheme 1. Structures of PXZ-TRZ and PTZ-TRZ. Hitherto, TRZ has become a basic electron acceptor to design a D/A type TADF in which carbazoles and diphenylamines are commonly used as the electron donor groups. 10, 13-17 Among these, the dual emission of the PXZ-TRZ analogue, PTZ-TRZ, has received considerable attention. PTZ-TRZ consists of TRZ and phenothiazine (PTZ) as the electron acceptor and donor, respectively, in which PTZ simply replaces oxygen in PXZ by the sulfur atom. Tanaka et al.18 reported that compound PTZ-TRZ exhibited dual fluorescence bands maximized at 409 and 562 nm in toluene solution, originating from two isomeric forms, the quasi–axial (SA 0 ) and quasiequatorial (SE0 ) conformers, respectively. The rather complicated steady-state excitation spectral behavior and excitation-wavelength dependent multiple decayed kinetics (vide infra) led Tanaka et al. to propose the distinct difference in relaxation pathways between local excited (LE) and charge-transfer (CT) states in a single molecular identity. The CT excitation of the quasi-axial conformer underwent energy transfer to the quasi-axial conformer, resulting in both F1 and F2 bands. Also, the efficiency of energy transfer might be excitation energy dependent, as revealed by complicated excitation wavelength dependent decay dynamics of the F1 band (see Figure S2 in supporting information ).18 The absorption < 320 nm for PTZ-TRZ is ascribed to the highly


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excited states (Sn, n > 1). If the these states can undergo energy transfer that competes with the Sn→S1 relaxation, the mechanism would be extremely interesting. It disobeys the fundamental photophysics ubiquitously adopted in solution called Kasha’s rule, stating that the time constant of Sn(n > 1) → S1 internal conversion is very fast (~ps or less) and should be the predominant relaxation pathway, giving solely the S1 fluorescence.19-20 Due to its importance in gaining insight into the excited-state relaxation behavior, we herein revisited the photophysics of PTZ-TRZ. Our results, combining the experimental and computational approaches, on the one hand, affirm the existence of quasi-equatorial and quasiaxial conformer for PTZ-TRZ in solution. On the other hand, however, careful study indicates that the complication of spectroscopy and dynamics for PTZ-TRZ in toluene18 is due to the solvent interference. Particularly, the excitation wavelength around 280-315 nm is in the UV cutoff region of toluene where significant solvent absorption acts as an inner filter effect that strongly distorts the excitation spectrum and hence misleads the spectral assignments. The excited toluene also interferes with the relaxation dynamics, which, in part, accounts for the excitation wavelength-dependent emission decay components, especially at the short wavelength band of PTZ-TRZ. Alternatively, when we switched the solvent to cyclohexane where the UV cut-off region is < 230 nm, the steady-state emission and excitation spectra of PTZ-TRZ become fundamentally explainable. Further fluorescence up-conversion measurement, together with computational approach, reveals the excited-state relaxation kinetics of each conformer. Details of results and discussion are elaborated below. Methods section Photophysical Measurements.


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Steady-state absorption and emission spectra in cyclohexane and toluene solutions were recorded with a Hitachi (U-3310) spectrophotometer and an Edinburgh (FS-920) fluorimeter, respectively. Both population and TADF lifetimes were measured by multichannel plate coupled with either second or third harmonics of a femtosecond Ti-Sapphire laser (760-950 nm, ~70-100 fs) with 1 kHz repetition rate and used OB-900 lifetime spectrometer (Edinburgh) TCSPC system. This combination gives a systemresponse-time of ~20 ps. The early dynamic study was performed by a femtosecond photoluminescence up-conversion (uPL) system pumped at 380 nm. Because the delayed fluorescence has revealed slow relaxation kinetics up to several microseconds, the experiment performed by the use of high repetition rate femtosecond laser led to serious signal repeated accumulation. Alternatively, a low repetition rate 1 kHz femtosecond laser system was used as the pump-probe sources for the fluorescence up-conversion measurements which is generated by a mode-locked Ti sapphire laser (Tsunami, Spectra Physics, 82 MHz) then coupled to a regenerative amplifier that generated a ∼ 180 fs, 1 mJ pulse (760 nm, 1 kHz). In this measurement, fluorescence from a rotating sample cell was focused in a BBO crystal and its frequency was summed along with an interrogation gate pulse at a designated delay time with respect to the pump pulse. A half-wave plate was used to set the pump polarization at a magic angle (54.7°) with respect to the gate pulse to prevent the fluorescence anisotropy contributed by solute reorientation. Fluorescence up-conversion data were fitted to the sum of exponential functions convoluted with the instrument response function (IRF). The IRF was determined from the Raman scattering signal and its profile was fitted to a Gaussian function with a full width at half maximum of ∼150-170 fs. Computational method.


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The geometries of the ground state for the titled compounds were optimized by the density functional theory (DFT) method. The electronically excited-state structures with relevant photophysical properties were calculated by the time-dependent density functional theory (TDDFT) method with a BMK hybrid function. The 6-31+g (d, p) basis set was employed for all atoms. All theoretical calculations were performed using the Gaussian 09 program. Results and discussion Figure 1(a) shows the absorption and emission spectra of PTZ-TRZ in toluene. In this measurement, we used conventional 10 mm cuvette and prepared sample in toluene, in which the optical density of PTZ-TRZ is < 0.6 at > 270 nm. It is worth to note that the absorption spectrum at < 290 nm has been subject to sharp rising of the toluene absorbance. Therefore, the limitation of instrument dynamic range prohibits the absorbance readout at < 280 nm. Dual emissions were observed, consisting of a short (~410 nm, F1) and a long (~580 nm, F2) wavelength bands, which according to Tanaka et al.,18 were assigned to quasi-axial and quasi-equatorial conformers, respectively. Figure 1(b) shows the excitation spectra monitored at F1 and F2 bands. Using a regular 10 mm path-length cuvette the excitation spectrum monitored at the F2 band reveals a steeply falling at < 290 nm (Figure 1(b), gray circle line), which is in sharp contrast to the rising of the absorbance (cf. Figure 1 (a)). The decrease of excitation spectrum at < 290 nm (Figure 1 (b), orange circle line) is more pronounced upon monitoring at the F1 band. Our observation is consistent with that reported by Tanaka et al.18 (see Figure 5 of ref. 18). Tanaka et al.18 attempted to explain the results by the distinct difference in relaxation dynamics between local excitation (LE, < 320 nm) and charge-transfer (CT, > e.g., 320 nm) states, which might have different relaxation pathways. Furthermore, for the quasi-equatorial conformer, the CT excitation gave rise to the F2 band, while the CT excitation of the quasi-axial conformer underwent energy


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transfer to the quasi-axial conformer, resulting in both F1 and F2 bands. However, careful studies indicate that the complicated spectroscopy, especially the excitation spectra and the corresponding relaxation dynamics for PTZ-TRZ measured in toluene, originates from the spectral distortion caused by the solvent interference. In a conventional 10 mm optical-path cuvette pure toluene has absorbance of 0.05 at 315 nm and is as high as 1.0 at 280 nm, followed by a very steep rise at < 280 nm. Therefore, the dominant absorbance for toluene around 280 nm causes serious inner filter effect as well as complicated decay dynamics due to the solvent excitation. Direct support of this viewpoint is given by the use of sample cell made by two quartz disks spaced by 0.01 cm, from which the emission is acquired through a front-face excitation. As a result, upon monitoring at F1 and F2 emission bands, instead of falling excitation intensity at < 290 nm observed using 1.0 cm cuvette, the rise of excitation intensity is apparent at < 290 nm (see Figure 1(b)), which is maximized at ~270-275 nm. Therefore, the excitation spectra follow the same trend as the absorption spectrum at < 290 nm. In other words, excitation at either LE or CT absorption band should result in the same origin of emission. As a result, either quasi–axial or quasi-equatorial conformers of PTZ-TRZ should be treated as a single molecular entity. Upon excitation to the highly excited state (e.g. LE band), fast relaxation (~ps) takes place, which follows common Kasha’s rule19-21, giving rise to the emission from the respective lowest lying states.


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Figure 1. (a) The absorption (red) and emission (black) spectra of PTZ-TRZ in toluene solution. The excitation wavelength is 350 nm for emission. (b) The excitation spectra of PTZTRZ in toluene monitored at 410 nm in 1 cm (orange circle line) and 0.01 cm (red solid line), path-length sample cell as well as at 600 nm in 1 cm (gray circle line) and 0.01 cm (black solid line) path-length sample cell. The inner filter effect and simultaneous excitation of toluene make inappropriate the assignment of spectral characterization and corresponding relaxation dynamics. To gain more clear insight into the photophysics of PTZ-TRZ, we alternatively performed the measurement in cyclohexane. Cyclohexane provides optically clear window down to 230 nm. Therefore, the


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corresponding spectroscopy and relaxation dynamics of PTZ-TRZ should be free from solvent interference. Figure 2 (a) shows the absorption and emission spectra of PTZ-TRZ in cyclohexane, for which the absorption spectrum clearly shows a higher lying transition maximized at the region of 260-270 nm. Dual emissions were observed, consisting of a short (~385 nm, F1) and a long (~525 nm, F2) wavelength bands, in which the F2 band reveals TADF behavior, evidenced by the drastic quenching of its emission in the aerated solution. The overall photo luminescence quantum yield (PLQY) is increased from 3.5% in aerated solution to nearly 58% in the degassed solution. These results resemble emission spectra observed in toluene excepted for blue shift of both emission bands due to smaller degree of solvent polarization relaxation in cyclohexane.


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Figure 2. (a) Absorption (red) and emission spectra of PTZ-TRZ in aerated (blue) and degassed (black) cyclohexane solution. λex = 350 nm. Note: the emissions in aerated and degassed cyclohexane are normalized at 525 nm. (b) Excitation spectra of PTZ-TRZ in cyclohexane solution monitored at 400 nm (purple) and 570 nm (blue). For comparison, also shown in (b) is the absorption spectrum of PTZ-TRZ (red) in cyclohexane. The corresponding excitation spectra, which were previously baffled by UV cut-off of toluene, now become clear and interpretable in cyclohexane (Figure 2 (b)). Upon monitoring at the F2 band, the corresponding excitation spectral profile maximizes at 265 nm, 350 nm and a nonnegligible shoulder tailing down to ~450 nm. Note that the absorption spectrum also shows 389450 nm shoulder/tail in both toluene and cyclohexane (see Figures 1 (a) and 2 (a)). The excitation spectrum monitored at the F1 band, on the other hand, reveals peak intensity at ~265 nm and 355 nm. Despite similar peak positions for the excitation spectra of F1 and F2 bands, several prominent differences can be promptly pointed out: (1) The 380-450 nm shoulder/tail is not observed in the excitation spectrum monitored at the F1 band. (2) The intensity ratio for ~350 nm versus 265 nm bands is quite different between the excitation spectra of F1 and F2 bands. (3) Upon normalizing the 350 nm band, the excitation intensity at 300-340 nm for the F2


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band is apparently higher than that for the F1 band (see Figure 2 (b)). The results of steady-state measurement, on the one hand, indicate that F1 and F2 bands may possibly originate from two different ground-state species, namely the quasi–axial and quasi-equatorial conformers, respectively. On the other hand, the F1 and F2 bands may act as precursor and successor, respectively, incorporating the conversion from quasi–axial to quasi-equatorial conformers in the excited state.

Figure 3. The emission decay diagram of PTZ-TRZ in cyclohexane solution monitored at 550 nm under the degassed condition (red line), fitted to be 7.5 ns and 5.5 µs (pink line). Inset: under the aerated condition (blue line) with the fitting curve (cyan line). λex: 310 nm. We then applied a time-correlated single photon counting (TCSPC) technique coupled with multichannel plates and a femtosecond light source (310 nm, ~90 fs, 1 kHz) to perform the corresponding pico-nanoseconds relaxation dynamics. This combination gives a system response time of ~20 ps. In the degassed cyclohexane, upon monitoring at the F2 band (550 nm), the results shown in Figure 3 reveal two single exponential decay components fitted to be 7.5 ns and 5.5 µs. Both decay components are quenched significantly upon aeration (see inset of Figure 3), reaffirming a typical TADF behavior.18 Upon monitoring at the decay of the F1 band (405 nm)


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by TCSPC, as shown in Figure 4 (a) inset, the emission show a predominant, system-responselimited decay component (< 20 ps), accompanied by a very small amplitude (< 1% at t = 0), long decay component fitted to be 1.2 ns. It is noteworthy that the crude PTZ-TRZ exhibits a trace impure fluorescence band maximized at ~425 nm with a lifetime of ~1.2 ns (see Figure S3 in SI). Further purification of PTZ-TRZ reduced the 425 nm emission intensity and hence the decrease of amplitude of the 1.2 ns decay component. The result leads us to conclude that the 1.2 ns decay component very plausibly originates from the impurity. Unfortunately, repetitive purification by recrystallization could not rid of the impurity completely.


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Figure 4. The fluorescence up-conversion measurement of PTZ-TRZ in cyclohexane solution monitored at (a) F1 band (405 nm) and (b) the F2 band (600 nm) with λex: 380 nm within 10 picoseconds. Inset of (a): The emission decay curve of PTZ-TRZ in cyclohexane solution monitored at 405 nm measured by TCSPC (see text). To gain more insight into the early relaxation dynamics, we further probe the emission by the femtosecond fluorescence up-conversion technique. In this study, because of the rather slow TADF kinetics, a femtosecond laser (~100 fs) with low repetition rate of 1 kHz was applied as the pump-probe source to avoid repetitive accumulation (see Methods section for experimental details). Figures 4 and Table S1 show the relaxation dynamics of PTZ-TRZ at different emission wavelengths in cyclohexane. Upon monitoring at 405 nm (λex = 380 nm) of the F1 band, the relaxation dynamics consist of a resolvable fast decay component fitted to be 1.8 ps (see Figure 4(a)). Accordingly, the 1.8 ps decay component is responsible for the systemresponse-limited fast decay observed in the TCSPC (see Figure 4 (a) inset). Figure 4 (b) shows the relaxation dynamics of PTZ-TRZ upon monitoring at ~600 nm (λex = 380 nm), the emission relaxation dynamics consist of dual rise components of 0.40 ps and 1.75 ps (see also Table S1), followed by a constant intensity up to a 10 ps that has been resolved to be 7.5 ns by TCSPC measurement. This kinetic pattern remains similar up to the monitored wavelength at 600 nm. The 1.75 ps rise component of the F2 band, within the experimental uncertainty, seems to be on the same order of the magnitude (1.83 ps) as the decay of the F1 band. It is thus believed that the quasi-axial isomer, upon excitation, may undergo structural relaxation, giving rise the F2 band. On the other hand, the shorter 0.4 ps rise component of the F2 band may refer to the fast structural relaxation of the quasi-equatorial isomer. Support of this proposed mechanism is given by monitoring the 600 nm emission under 420 nm excitation where only quasi-equatorial


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conformer is excited. The results shown in Figure 5 (b) reveal a single rise component fitted to be 0.4 ps. Further confirmation is provided by the observation of a 0.4 ps decay component upon monitoring at 450 nm (Figure 5 (a)) that is at the onset of the F2 band upon excitation at the quasi-equatorial conformer only (λex = 420 nm, also, see Table S1). We therefore conclude that both quasi-axial and quasi-equatorial conformers, upon excitation, undergo structural relaxation, for which the planarization of phenothiazine moiety plausibly plays a major role to rationalize the large Stoke-shifted emission even in the nonpolar cyclohexane. Moreover, the only one population decay rate for the F2 band leads us to conclude that both conformers may relax to the same energy minimum state in the excited state.


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Figure 5. The fluorescence up-conversion measurement of PTZ-TRZ in cyclohexane solution monitored at (a) 500 nm and (b) 600 nm with λex: 420 nm within 10 picoseconds. We then performed computational approach in attempts to gain more insight into the photophysical properties for PTZ-TRZ in cyclohexane solution, which were assessed with timedependent density functional theory (TD-DFT) using the BMK/6-31+g* method/basis set. Similar to the previous report18, the results predict two ground-state conformers, namely quasiaxial conformer and quasi–equatorial conformer (Figure 6 and Table S2), in which the quasiequatorial conformer was more stable than the quasi-axial conformer by 0.6 kcal/mol in cyclohexane, showing good agreement with the previous result (0.5 kcal/mol) calculated by CAM-B3LYP/cc-pVDZ method in toluene.18 In both conformers, it seems that the large sulfur atom makes the phenothiazine moiety twisted to release partially steric effect. As reported by Tanaka et al.,18 such a puckered arrangement did not take place for the phenoxazine moiety in PXZ-TRZ.12 The calculated HOMO and LUMO properties for both isomers are depicted in Figure S4 and S5. The degree of charge-transfer and HOMO/LUMO spatial separation to rationalize TADF (F2 band) versus non-TADF (F1 band) properties, which have been well discussed by Tanaka et al.,18 are not focusing in this study. Instead, we then attempted to construct the potential energy surface (PES) (Figure 7) for both isomers along the structural relaxation in the ground (S0) and excited (S1) states. One of the primary interests is to probe the structural relaxation for both quasi-axial and quasi-equatorial conformers.


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Figure 6. DFT optimized geometry of (a) quasi-axial conformer (θa = 13°, θb = 131°) and (b) quasi-equatorial conformer (θa = 90°, θb = 149°) in the ground state (S0) (c) Both conformers relax to the same configuration in the first excited state (S1) (θa = 90°, θb = 180°). Realizing that the torsional angle θa, which is defined as the dihedral angle between the planes C(14)-C(13)-C(15) and C(7)-N(4)-C(4), i.e., between phenothiazine and 2,4,6-triphenyl-1,3,5triazine moieties and the bending angle θb which is defined as the dihedral angle between the planes C(1)-C(4)-C(3)-C(6) and C(7)-C(8)-C(11)-C(10) of the phenothiazine moiety play key roles for the corresponding structural relaxation (see Figure 6), we thus recorded the geometries


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properties accordingly. The quasi-axial conformer shows the geometry properties θa in 13o, and θb in 131o (see Figure 6 (a)). Compared to the quasi-axial conformer, the quasi-equatorial conformer shows 90o in θa, 149o in θb in the ground state (see Figure 6 (b)). Upon vertical excitation for either conformer, followed by full geometry optimization, interestingly, we obtain the same structure shown in Figure 6(c), for which θa and θb are in 90 o and 180o, respectively. This calculation supports our experimental results, concluding that both conformers give the same F2 band after relaxation. In addition, the calculated global minimum S1 state reveals a vertical transition at 514 nm (see Figure 7), which is in good agreement with the emission onset (520 nm) of the F2 emission.

Figure 7. The potential energy surface diagram of PTZ-TRZ in cyclohexane (a) for the quasiaxial conformer as a function of torsional angle θa in the first excited state (S1 (blue ▲)), and ground state (red ●), (b) calculated PES scanned along the change of bending angle θb, in which S0 (red ●) denotes ground state optimized state and S1 (blue ▲) shows the excited state optimized state at the same θb. Open data symbols black □ and blue △ mark the global (or local)


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△), respectively. Black △ mark the Franck-Condon minimum of the PES for S0 (□) and S1 (△ absorption point. Above results are calculated by TD-DFT/BMK /6-31+g* method. As for the quasi-axial conformer, due to large changes of θa along the relaxation (cf. Figure 6(a) and (c)) we then scan θa in every 10-degree decrement, followed by full geometry optimization at each step. The result shown in Figure 7 (a) indicates that θa requires a rather small barrier (at θa~15o) of 0.3 kcal/mol perhaps due to the steric hindrance encountered along the θa coordinate. We also scan the phenothiazine moiety along the bending angle θb for both quasi-axial (see Figure S6) and quasi-equatorial conformers (see Figure 7(b)) in every 10degree increment, followed by full geometry optimization, to θb = 180o, i.e. a planar geometry of phenothiazine. The results shown in Figure 7 (b) and Figure S6 indicate that the planarization of the phenothiazine moiety is nearly barrierless up to 180o for both quasi-axial and quasiequatorial conformers in the excited state. Figure 8 depicts a schematic energy level diagram of the excited-state structural relaxation for PTZ-TRZ in cyclohexane. Although the structural relaxation for both quasi-axial and quasi-equatorial conformers has negligibly small barrier, the associated large-amplitude motion may be subject to the viscosity restrain in cyclohexane, giving picoseconds relaxation time constant in this study.22-25 It is also worthy to note that varying θa or θb is only one of the dimensions along the structural relaxation. More sophisticated, multiple dimensional approaches are required to cope with simultaneous structural relaxation, which are currently formidable due to the limitation of computation capacity.


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Figure 8. Schematic energy level diagram of PTZ-TRZ in cyclohexane solution exhibiting excited-state structural relaxation. Conclusion In conclusion, the complicated spectra and corresponding dynamics of the TADF-modeling compound, PTZ-TRZ in toluene lead us to revisit its associated photophysics. Our results are summarized into several remarks: (1) The toluene UV-absorption cut-off strongly distorts the excitation spectra and interferes the associated relaxation dynamics, making the interpretation of PTZ-TRZ complicated. (2) In cyclohexane, the results become clear in which the existence of the quasi–axial and quasi-equatorial conformers is reaffirmed. Upon electronic excitation, planarization of phenothiazine, accompanied by the relaxation of the framework, takes place for both quasi-axial and quasi-equatorial conformers, giving the same energy minimum state that exhibits F2 band emission. For the quasi-axial conformer, the weak F1 band can be resolved due to its far separation from the F2 band, while the F1 band of the quasi-equatorial conformer is


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obscure due to its even weaker (0.4 ps decay) emission and closer emission spectrum to the F2 band. We believe that similar structural relaxation should take place in the low viscous polar solvents. However, the relaxation kinetics should be more complicated due to the charge transfer effect and hence the solvent dipole relaxation, which adds an additional coordinate that channels into the relaxation dynamics.

Associated content Supporting Information. Additional computational and spectroscopic data are provided in Supporting Information.

Notes The authors declare no competing financial interests. Corresponding Author Pi-Tai Chou, E-mail: [email protected] Acknowledgment We thank the ministry of science and technology (MOST 106-2628-M-002-003), Taiwan, for generous supports. Reference (1) Tang, C. W.; VanSlyke, S. A., Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913-915. (2) Endo, A.; Ogasawara, M.; Takahashi, A.; Yokoyama, D.; Kato, Y.; Adachi, C., Thermally Activated Delayed Fluorescence from Sn4+–Porphyrin Complexes and Their Application to Organic Light-Emitting Diodes — a Novel Mechanism for Electroluminescence. Adv. Mater. 2009, 21, 4802-4806. (3) Tao, Y.; Yuan, K.; Chen, T.; Xu, P.; Li, H.; Chen, R.; Zheng, C.; Zhang, L.; Huang, W., Thermally Activated Delayed Fluorescence Materials Towards the Breakthrough of Organoelectronics. Adv. Mater. 2014, 26, 7931-7958.


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