Dual Intramolecular Charge-Transfer Fluorescence Derived from a

Article pubs.acs.org/JPCC

Dual Intramolecular Charge-Transfer Fluorescence Derived from a Phenothiazine-Triphenyltriazine Derivative Hiroyuki Tanaka,†,‡ Katsuyuki Shizu,†,‡ Hajime Nakanotani,‡,§ and Chihaya Adachi*,‡,§,∥ ‡

Center for Organic Photonics and Electronics Research (OPERA) and ∥International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan § Innovative Organic Device Laboratory, Institute of System, Information Technologies and Nano-technologies (ISIT), 744 Motooka, Nishi, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: A material containing a phenothiazine (PTZ) electron donor unit and 2,4,6-triphenyl-1,3,5-triazine (TRZ) electron acceptor unit, PTZ-TRZ, which exhibits thermally activated delayed fluorescence (TADF) was developed. Density functional theory calculations revealed the existence of two ground-state conformers with different energy gaps between the lowest singlet excited state and lowest triplet excited state (1.14 and 0.18 eV), which resulted from the distortion of PTZ, as confirmed by X-ray structure analysis. PTZ-TRZ in toluene solution showed two broad, structureless emissions, confirming the existence of two different excited states. From detailed analyses of the absorption and photoluminescence spectra, we determined that both emissions were intramolecular charge-transfer (ICT) fluorescence. Therefore, the excited-state conformers of PTZ-TRZ resulted in dual ICT fluorescence. Because previously reported dual fluorescence from single molecules involves locally excited and ICT fluorescence, the dual ICT fluorescence from PTZ-TRZ is novel. Temperaturedependence of transient PL spectra of a 2 wt % PTZ-TRZ-doped film in 3,3′-bis(N-carbazolyl)-1,1′-biphenyl measured by a streak camera revealed that the former and latter emissions were independent of and dependent on the film temperature, respectively. This confirms that the dual fluorescence involves TADF characteristics. An organic light-emitting diode containing PTZ-TRZ exhibited a maximum external quantum efficiency of 10.8 ± 0.5% with dual ICT fluorescence.

■

INTRODUCTION Control of intramolecular or intermolecular charge transfer (CT) is an important issue and sensitively affects the output from organic molecular systems in the research areas of photochemistry and organic photonics.1−10 Recently, we introduced a very efficient up-conversion mechanism of excitons from the spin-triplet charge-transfer excited state (3CT) to the spin-singlet charge-transfer excited state (1CT) in organic light-emitting diodes (OLEDs) to improve the efficiency of electroluminescence using conventional organic luminescent materials as an emitter.11 This process is called thermally activated delayed fluorescence (TADF) and enables all of the triplet excitons generated by electrical excitation to be converted into luminescence. Typical organic luminescent materials emit via fluorescence, which involves transition from a spin-singlet excited state (1(π,π*) or 1(n,π*)) to a spin-singlet ground state (S0). Therefore, under electrical excitation, typical fluorescent materials can only convert singlet excitons (statistically 25% of the excitons formed) to luminescence. TADF is realized in donor−acceptor (D-A) molecules with a small energy gap between the lowest spin-singlet excited state (S1) and lowest spin-triplet excited state (T1; ΔES‑T), which results from effective separation of the highest occupied molecular orbital © 2014 American Chemical Society

(HOMO) and lowest unoccupied molecular orbital (LUMO). Therefore, for efficient up-conversion of triplet excitons to the singlet excited state, the HOMO and LUMO in a single molecule need to be effectively separated. Because TADF materials possess HOMO−LUMO separation, intramolecular charge transfer (ICT) from a donor moiety to an acceptor moiety occurs in such molecules. TADF is an ICT-based fluorescence derived from the lowest spin-singlet charge-transfer excited state (1CT1) → S0 electronic transition that is facilitated by up-conversion of excitons from the lowest spin-triplet charge-transfer excited state (3CT1) to 1CT1 through reverse intersystem crossing. TADF can be classified as a luminescent mechanism that is different from that of typical fluorescent materials (Scheme 1). The performance of TADF materials is expected to surpass that of phosphorescent materials with respect to both luminescence efficiency and material design versatility without requiring expensive rare metals such as iridium and platinum. Using these advantages to produce OLED emitters, we have realized trailblazing developments of TADF materials.6,11−21 The performance of these Received: January 29, 2014 Revised: May 27, 2014 Published: July 1, 2014 15985

dx.doi.org/10.1021/jp501017f | J. Phys. Chem. C 2014, 118, 15985−15994

The Journal of Physical Chemistry C

Article

tional differences between the TADF molecules suggest that it is important to improve the molecular design strategy and achieve better understanding of TADF characteristics through systematic research on excited states. Analogous to PXZ, phenothiazine (PTZ) is a convenient electron donor in various applications such as dye-sensitized solar cells 28−37 and photogenerated charge separated states.38−41 In this article, we present a novel material with dual fluorescence, PTZ-TRZ, which possesses TADF characteristics (Figure 1). PTZ is a structural analogue of PXZ that has a

Scheme 1. Photoluminescent Processes of (a) General Fluorescence with Large ΔES‑T and (b) TADF with Small ΔES‑T

OLEDs has attracted considerable attention for TADF material as third-generation OLED emitters. As traditional luminescent materials that display ICT, 4aminobenzonitrile (ABN) derivatives have been experimentally and theoretically investigated by many researchers to date.22−27 The ABN derivatives are a basic model of a D-A molecular system and have a large ΔES‑T between the 1CT1 and 3CT1 states. By introducing different substituents onto the amino group, the ABN derivatives can exhibit dual fluorescence derived from both the 1(π,π*) → S0 transition from the locally excited (LE) state and the 1CT → S0 transition from the ICT excited state, which indicates that the luminescence behavior of such materials strongly depends on the relationship between molecular conformation and excited states. With respect to the luminescence from the ICT excited state, it is believed that TADF materials have similar properties to the ABN derivatives. Many dual-fluorescent materials have been studied in solution with a focus on solvatochromic effects in various solvents of different polarity.22−27 Although TADF materials display ICT like the ABN derivatives, simple dialkylamino groups have been avoided as the donor unit because the molecular design strategy needs to realize small ΔES‑T between 1CT1 and 3CT1 for efficient up-conversion of triplet excitons. Recently, we reported PXZ-TRZ-type TADF emitters that contain phenoxazine (PXZ) as a donor unit and 2,4,6-triphenyl1,3,5-triazine (TRZ) as an acceptor unit.15,21 PXZ-TRZ has a large twist angle between the PXZ and TRZ moieties, which induces effective separation of HOMO and LUMO, resulting in a small ΔES‑T. Therefore, PXZ-TRZ is a basic model of a TADF material and shows broad CT absorption and emission derived from the twisted ICT state and positive solvatochromic effect. These properties resulted from the introduction of the PXZ donor unit with a six-membered fused ring into the D-A molecular system, which confirmed that the formation of a twisted ICT state was effective to yield TADF.15,19,21 We have also reported TADF molecules without a large twist angle between the donor and acceptor moieties compared with that of TADF molecules with PXZ; these molecules possess a simple donor unit such as carbazole or diphenylamine.17,20 Therefore, a suitable selection of the donor and acceptor units enables us to achieve TADF materials that have either a large or small twist angle, which differs from the ABN derivatives that exhibit only LE fluorescence from the molecular conformation with a small twist angle. Although simple comparison of PXZdonor-based TADF molecules with those with other donors does not provide definitive information about the relationship between molecular conformation and TADF, the conforma-

Figure 1. (a) Molecular structures and (b, c) HOMO and LUMO of the ground-state quasi-equatorial and quasi-axial conformers of PTZTRZ calculated at the CAM-B3LYP/cc-pVDZ level, respectively.

sulfur atom instead of an oxygen atom as in PXZ. Therefore, use of the PTZ donor unit into D-A molecular systems produces specific molecular conformations that differ from the typical twisted D-A structures found with the PXZ donor unit. As an example of a D-A molecular system containing the PTZ donor unit, Daub and Schneider et al.42−44 reported PTZpyrene D-A dyads. These dyads exhibit dual LE and CT fluorescence in solution states derived from the conformational heterogeneity caused by the rotational dynamics of the donor, bridge, and acceptor around the single bonds linking them.42−44 The observed dual fluorescence from these dyads indicates that the PTZ donor unit is a suitable component to create dualfluorescent D-A molecules. With respect to the molecular design of TADF materials, the differences between PXZ and PTZ as donor units are important not only to gain a better understanding of the relationship between the molecular conformation and ICT excited state of TADF materials but also to develop novel TADF materials. Indeed, PTZ-TRZ exhibits dual ICT fluorescence with TADF characteristics in both the solution state and solid-state films, which is composed of the direct 1CT1 → S0 transition with a large ΔES‑T between the 1CT1 and 3CT1 states and a different 1 CT1 → S0 transition allowed via the successive 1CT1 ← 3CT1 up-conversion followed by reverse intersystem crossing with a small ΔES‑T between the 1CT1 and 3CT1 states. This dual ICT fluorescence derived from excited-state conformational hetero15986



Article

geneity of PTZ-TRZ is different from the traditional dual LE and CT fluorescence observed for other materials. In this study, we present a dual ICT fluorescent molecule, PTZ-TRZ, with TADF characteristics and the complex emission mechanism that contains direct and indirect ICT transitions derived from the conformational heterogeneity. On the basis of density functional theory (DFT) calculations, X-ray structural analysis, photophysical properties in both the solution and solid states, and OLED characteristics, we examine the relationship between the steady-state excited-state conformers and photophysical properties of PTZ-TRZ to reveal the origin of the dual ICT fluorescence with TADF characteristics.

■

RESULTS DFT Calculation. To investigate the electronic states of PTZ-TRZ, we performed DFT calculations using Gaussian 09 software.45 Absorption and emission wavelengths for PTZ-TRZ in toluene solution were computed with time-dependent density functional theory (TD-DFT)46,47 using the CAMB3LYP/cc-pVDZ method48,49 with the polarizable continuum model.50−52 Like in previous studies,42−44 the DFT calculations predicted two ground-state conformations, “quasi-axial (SA0 )” and “quasi-equatorial (SE0 )” conformers, which were calculated by DFT calculation using the same functional and basis set (denoted as CAM-B3LYP/cc-pVDZ; Figure S1). The groundstate quasi-axial conformer was more stable than the groundstate quasi-equatorial one by 0.02 eV. Therefore, two kinds of ground-state conformers can exist in almost equal proportions in toluene solution. Figure 1b shows the HOMO and LUMO of the ground-state quasi-equatorial conformer of PTZ-TRZ in toluene solution. The central six-membered ring of the PTZ moiety is not planar because of the presence of an S atom; in the case of PXZ-TRZ, the central ring is almost planar. As shown in Figure 1b, the HOMO is mainly distributed on the PTZ moiety, while the LUMO is localized on the TRZ moiety. There is a small overlap between HOMO and LUMO in the phenyl ring connecting PTZ and TRZ. The HOMO and LUMO distribution patterns were similar to those of PXZ-TRZ. The HOMO and LUMO of the ground-state quasi-axial conformer differed from those of the quasi-equatorial conformer, as shown in Figure 1c. As expected from the effective separation of HOMO and LUMO on the quasi-equatorial conformer, its ΔES‑T was estimated to be 0.18 eV, which is sufficiently small to enable the 1 CT1 ← 3CT1 up-conversion of triplet excitons through reverse intersystem crossing. In contrast, ΔES‑T of the quasi-axial conformer was estimated to be 1.14 eV, which is too large to induce TADF, suggesting that the luminescence from the quasiaxial conformer is only derived from the normal S1 → S0 transition in which the S1 state is directly excited. X-ray Crystal Structure Analysis. To verify the optimized structures obtained from the DFT calculations, we carried out single-crystal X-ray structure analysis of PTZ-TRZ. Figure 2 shows ORTEP drawings of PTZ-TRZ. The drawing in this figure shows that the molecular conformation was the quasi-axial conformer, which was consistent with the relationship between the energy levels of the two ground-state conformers. The TRZ moiety was almost coplanar, while the PTZ moiety had a nonplanar structure, in which the nitrogen and sulfur atoms were displaced from the two phenyl rings of PTZ in the same direction, as shown in the lower image in Figure 2.

Figure 2. (Top) Structural representation of PTZ-TRZ. The thermal ellipsoids are drawn at the 50% probability level. (Bottom) Alternative view of PTZ-TRZ, showing the distortion of the PTZ moiety and the quasi-planarity of the TRZ moiety.

This conformation is attributed to the difference of the bond lengths between the C−N and C−S bonds in the PTZ moiety. From the crystal structure analysis, the bond lengths of the C− N and C−S bonds were estimated to be 1.44−1.45 and 1.76− 1.77 Å, respectively. Therefore, the conformation of the PTZ moiety differs from that of the PXZ moiety, which has an anthracene-like planar structure.15,19,21 PXZ-TRZ has a large twist (dihedral angle of 74.9°) between the PXZ and TRZ moieties, while PTZ-TRZ shows a bent conformation at the nitrogen atom of the PTZ moiety, that is, the quasi-axial conformer, which minimizes the steric repulsion between the PTZ and TRZ moieties through the distortion of the PTZ moiety. Therefore, PTZ-TRZ possesses the rotational dynamics of the PTZ donor, bridge (sp3 N−C bond), and TRZ acceptor around the single bond linking them like the PTZ-pyrene dyads mentioned above. These rotational dynamics induce two kinds of conformers, quasi-axial and quasi-equatorial, in the ground and excited states, as predicted from the DFT calculations. Photophysical Properties in Solutions. The UV−vis absorption and photoluminescence (PL) spectra of PTZ-TRZ in toluene solution are presented in Figure 3. For the absorption spectrum (black line), a weak onset was observed at about 460 nm, which gradually increased in intensity until about 390 nm. Then, another broad and structureless absorption was observed from 390 to 310 nm. Both absorptions show features of ICT. The spectrum also exhibited a sharp, intense absorption around 282 nm, which is assigned to the 1 (π,π*) ← S0 absorption derived from the TRZ acceptor moiety of PTZ-TRZ (Figure S3). For the PL spectrum (blue line), excitation at 340 nm generated two emission signals with peaks around 409 and 562 nm. The second emission at around 562 nm was intense with a large Stokes shift and wide half-width of 105 nm, so it was fully assigned to ICT emission (1CT1 → S0 transition), similar to that of PXZ-TRZ.15,21 Furthermore, the spatial distributions of HOMO and LUMO of the quasi-equatorial conformer of PTZTRZ were similar to those of PXZ-TRZ. Therefore, we tentatively assigned the origin of this emission around 562 nm to the excited-state quasi-equatorial conformer of PTZ-TRZ 15987



Article

intensity of λEPL indicates the existence of another excitation pathway other than via direct 1CT ← S0 transition. The details of this excitation pathway are discussed below. Then, the PL intensity increased markedly as the excitation wavelength increased from 310 to 360 nm, which synchronized with the increase of the absorption intensity, as shown in Figure S5 (upper). For excitation wavelengths from 360 to 440 nm, a large decrease of PL intensity was observed from 360 to 400 nm followed by a small decrease from 400 to 440 nm, as illustrated in Figure S5 (lower). In particular, the excitation wavelengths from 400 and 440 nm mainly showed emission at λEPL, which indicates that the absorption from 400 to 440 nm is the 1CT ← S0 transition caused by the more stable excited-state conformer. Therefore, these results reveal that the origin of λEPL is the weak absorption starting from about 460 nm (λEAbs, Figure 3). Transient PL Decay Behaviors in Solution and Doped Film. To confirm the up-conversion of triplet excitons necessary to produce TADF, transient PL decay curves can give evidence of the up-conversion of triplet excitons generated by a photoexcitation and intersystem crossing (S0 → 1CT → 3 CT1). As already reported by our group, the triplet excitons of TADF materials are strongly deactivated by triplet oxygen molecules (3O2).6,11−21 Therefore, decay curves containing the up-conversion of triplet excitons were measured after deoxygenation of PTZ-TRZ in toluene solution by bubbling with nitrogen. The observed transient PL decay curves of λEPL (monitored at 562 nm) following excitation at 280, 340, 365, and 405 nm are shown in Figures S7−10. To investigate the thermal activation of the observed delayed components derived from the up-conversion of triplet excitons, we measured the temperature dependence of the transient PL decay curves of a 2 wt % doped film of PTZ-TRZ in 3,3′-bis(Ncarbazolyl)-1,1′-biphenyl (mCBP) as a host material using a streak camera (see Supporting Information, Figures S11−14). The energy gap of PTZ-TRZ was estimated to be 3.1 eV from the onset position of λAAbs. Therefore, mCBP is suitable as a host material to confine the generated excitons on the two conformers in the emitting layer (the HOMO−LUMO energy gap of mCBP was estimated to be about 3.5 eV by other groups).53,54 The streak camera used here can monitor the wavelength region from 200 to 850 nm to give time-resolved fluorescence measurements. A nitrogen gas laser with an excitation wavelength of 337 nm was used in these measurements, which was suitable for the wavelength region of λAAbs. Figure S11 shows the temperature dependence of the streak images and PL spectra of the prompt and delayed components of the film, in which the former was assigned to the image around 0 ms and the latter was assigned to the image at later times. The observed spectra of the prompt components at T = 300, 200, 100, and 10 K are shown in Figure S11 (center). Evaluation of OLED Device Performance. We fabricated an OLED containing PTZ-TRZ as a dual ICT fluorescent emitter with the structure of indium tin oxide (ITO)/N,N′diphenyl-N,N′-bis(1-naphthyl)-1,10-biphenyl-4,4′-diamine (αNPD) (35 nm)/2 wt % PTZ-TRZ:mCBP (15 nm)/1,3,5tris(2-N-phenylbenzimidazolyl)benzene (TPBi) (65 nm)/LiF (0.8 nm)/Al (80 nm). The HOMO and LUMO levels of PTZTRZ were estimated to be 5.5 and 3.0 eV, respectively, from the UV−vis absorption spectrum of a neat film and a work function measurement (AC-2).55 The external quantum efficiency (EQE) versus current density plot of the OLED exhibited a maximum EQE (EQEmax) of 10.8 ± 0.5%, as shown in Figure 4.

Figure 3. Normalized absorption (black line) and PL (blue line) spectra of PTZ-TRZ in toluene solution with a concentration of 1.0 × 10−5 M. The PL spectrum was obtained by excitation at 340 nm.

and hereafter refer to it as λEPL. On the other hand, the first emission at around 409 nm was weak and possessed a small Stokes shift. We tentatively assign the emission at 409 nm to the excited-state quasi-axial conformer of PTZ-TRZ, λAPL. Table 1 contains a list of all the energy levels, emission peaks, and absorption ranges defined here and later. Table 1. Energy Levels of the Ground-State and ExcitedState Quasi-Axial and Quasi-Equatorial Conformers and the Wavelengths of the CT Absorptions and CT Emissionsa

conformation

energy level (eV; ground state)

CT absorption

quasi-axial

SA0 = 1.05

quasiequatorial

SE0 = 1.25

energy level (eV; lowest excited state)

CT emission peak

λAAbs = 320−390 nm

1

λEAbs = 400−460 nm

CTE1 = 3.46

λAPL ∼ 409 nm λEAbs ∼ 562 nm

CTA1 = 4.08

a

The energy levels of the excited-state quasi-axial and quasi equatorial conformers of PTZ-TRZ were estimated by TD-DFT calculations using the TD-CAM-B3LYP/cc-pVDZ method.

To reveal the origin of the dual emission from PTZ-TRZ in detail, we investigated the dependence of its PL spectrum in toluene solution on excitation wavelength, which gave us useful information that could not be gained from excitation spectra in toluene solution. Figure S5 shows the PL spectra obtained following excitation at wavelengths of 280 to 440 nm in increments of 10 nm. For λAPL, the PL intensities obtained for excitation wavelengths from 280 to 310 nm were very weak. As the excitation wavelength increased from 320 to 360 nm, the PL intensity gradually increased. The increase of the PL intensity showed an inflection at 320 nm (Figure S6), which agreed with the point of the increase of the absorption from 320 to 360 nm. This result indicates that λAPL is gradually enhanced by the absorption of PTZ-TRZ from 320 to 360 nm. Then, the PL intensity synchronously decreased as the absorption intensity decreased, as shown in Figure S5 (lower). These results reveal that the origin of λAPL is the broad absorption from 320 to 390 nm (λAAbs). For λEPL, the PL intensity abruptly increased at 290 nm and continuously maintained its intensity until 310 nm. Because the absorption spectrum from 290 to 310 nm did not contain obvious CT absorption, the abnormal increase of the PL 15988



Article

obviously differed from that of PTZ-TRZ. The emission from TRZ in toluene solution exhibited weak PL intensity that had bad signal-to-noise ratio. These spectral characteristics of PTZPh and TRZ differ from that of λAPL (409 nm), as shown in Figure S4, which indicates that the origin of λAPL differs from the LE states formed on PTZ-Ph or TRZ. ICT Excited State Generated by Photoinduced Electron Transfer. Figure 5 shows the dependence of the

Figure 4. Dependence of EQE on current density for an OLED with the structure ITO/α-NPD (35 nm)/2 wt % PTZ-TRZ:mCBP (15 nm)/TPBi (65 nm)/LiF (0.8 nm)/Al (80 nm). Inset: energy diagram of this OLED.

This value substantially surpasses the theoretical limit of general fluorescent materials (5−7% assuming an outcoupling efficiency of 0.2−0.3). This high EQEmax confirms the additional formation of singlet excitons derived from the successive up-conversion of triplet excitons through reverse intersystem crossing, which also indicates that the electroluminescence (EL) from this device contains TADF derived from the excited-state quasi-equatorial conformer that has a small ΔES‑T.

Figure 5. Dependence of the maximum PL intensities of the two emissions of PTZ-TRZ at around 409 (blue dashed line with circles) and 562 nm (red dashed line with squares) on excitation wavelength in toluene solution with a concentration of 1.0 × 10−5 M. The UV−vis absorption spectrum of PTZ-TRZ observed under the same conditions is shown as a black solid line.

■

PL intensity of λAPL and λEPL on excitation wavelength along with the absorption spectrum of PTZ-TRZ. This dependence of λAPL (blue dashed line with circles in Figure 5) was obviously A consistent with the fluctuation of λAbs . In contrast, the E dependence of λPL (red dashed line with squares in Figure 5) showed two large increases. However, the origin of λEPL was assigned to λEAbs earlier. Therefore, the dependence of λEPL on excitation wavelength indicates the existence of two indirect pathways for the population of the 1CT1 state that lead to the 1 CT1 → S0 transition of λEPL that differ from the direct 1CT ← S0 transition derived from λEAbs. The large increase of the PL intensity of λEPL in the excitation wavelength region from 280 to 310 nm cannot be explained by either λAAbs. or λEAbs. This result indicates that the origin of the other excitation pathway leading to λEPL (1CT1 → S0 transition on the quasi-equatorial conformer) differs from CT absorption. Generally, CT emission is generated from the excitons formed by the 1CT ← S0 transition. A key to resolve the origin of the excitation pathway in this region is in the research on the previously reported dual fluorescence composed of LE and ICT fluorescence.42−44 To investigate the origin of the large increase of the PL intensity of λEPL in detail, we compared the absorption spectrum of PTZ-TRZ in toluene solution with those of the resolved molecules PTZ-Ph and TRZ (Figure S17). The absorption spectrum of TRZ in toluene solution showed a single sharp absorption at around 282 nm, which agreed with the absorption at around 280 nm observed for PTZ-TRZ. Even through overlap between HOMO and LUMO on the TRZ acceptor moiety, as shown in Figure 1c, was calculated for the quasi-axial conformer of PTZ-TRZ, an obvious influence of the HOMO− LUMO overlap on the absorption derived from the TRZ

DISCUSSION Origin of Emission Peak at 409 nm. The photophysical properties of TADF materials that show ICT exhibit solvatochromism, in which the emission wavelength displays a large red shift as the polarity of the solvent increases.21 To further resolve the character of λAPL, we investigated the photophysical properties of PTZ-TRZ in various solvents. The emission maxima exhibited a gradual red shift as the polarity of the solvent increased (Figure S15). The LE fluorescence from previously reported dual-fluorescent materials were derived from the 1LE → S0 transition, in which the 1LE state forms on a localized moiety of the luminescent molecules and shows weak solvatochromism. Therefore, the emission wavelengths of the LE fluorescence were almost the same as that of the resolved molecules.42−44 Unlike the quasi-equatorial conformer, the quasi-axial conformer had an overlap between HOMO and LUMO on the TRZ acceptor moiety (see Figure 1c), which resulted in large oscillator strength (f = 1.144). Therefore, the small Stokes shift of λAPL can be attributed to the effect of the π−π* character derived from the HOMO−LUMO overlap of the quasi-axial conformer. From the small Stokes shift, λAPL appears to be the 1(π,π*) → S0 transition. To resolve the character of λAPL, we compared λAPL obtained from the toluene solution (as shown in Figure 3) with the emissions from the resolved molecules PTZ-Ph and TRZ (see Scheme S1) under the same measurement conditions (see Supporting Information, Figures S2−4). The wavelengths of the maximum PL intensities of PTZ-Ph and TRZ were 445 and 391 nm, respectively. The emission from PTZ-Ph in toluene solution showed two shoulder peaks that were attributed to the rotational dynamics derived from the PTZ donor moiety, which 15989



Article

moiety was not observed in the absorption spectrum of PTZTRZ. This result indicates that the components of the absorption spectrum corresponding to the π−π* transitions (1LE ← SA0 ) of the quasi-axial conformer of PTZ-TRZ can also be found in the absorption spectra of TRZ and PTZ-Ph as well as that of the quasi-equatorial conformer of PTZ-TRZ. In contrast, the absorption spectrum of PTZ-Ph in toluene solution showed a single broad, structureless absorption at around 322 nm, which is indicative of the influence of conformational changes of the PTZ moiety.43 Thus, simply combining the spectra of TRZ and PTZ-Ph can reproduce the absorption spectrum of PTZ-TRZ, which contains contributions from both the quasi-axial and quasiequatorial conformers, in toluene solution except for the parts of the spectrum related to λAAbs and λEAbs (Figure S17). This indicates that the photophysical properties of both TRZ and PTZ-Ph are essentially maintained in PTZ-TRZ, which can be attributed to the sufficient separation of the spatial distributions of HOMO and LUMO in both the quasi-axial and quasiequatorial conformers of PTZ-TRZ to maintain the respective spectral characteristics of TRZ and PTZ-Ph moieties in PTZTRZ. However, the emission properties of PTZ-TRZ, which has a D-A structure, differ markedly from those of both TRZ (the acceptor moiety only) and PTZ-Ph (the donor moiety only), as mentioned above. This indicates that one of the indirect ICT excitation pathways is by conversion of LE states to the ICT excited state, which was proposed by Hasegawa et al. and is called photoinduced electron transfer.56,57 From the comparison of the absorption spectra and dependence of the PL intensity of λEPL on excitation wavelength, the LE state (1LE (PTZ) ←SE0 ) transition) of the PTZ donor moiety of the quasiequatorial conformer of PTZ-TRZ, which has the similar spatial distribution of HOMO as that of PTZ-Ph, can be attributed to the origin of the one indirect excitation pathway because of the small oscillator strength (f ≈ 0) of the ground-state quasiequatorial conformer of PTZ-TRZ. The part from 280 to 310 nm of the excitation spectrum obtained following emission at 562 nm was almost consistent with the absorption of PTZ-Ph (Figure S18). This confirms that the absorption of the PTZ donor moiety is the origin of the large increase of the PL intensity. Because the emission observed in the excitation region from 280 to 310 nm was λEPL, we considered that the origin of this indirect pathway is the formation of the 1CT1 state on the quasi-equatorial conformer via absorption to excite the 1LE state (unoccupied MO ← occupied MO transition) of the PTZ donor moiety in the quasi-equatorial conformer followed by electron transfer from the 1LE state of the PTZ donor moiety to the LUMO of the TRZ acceptor moiety, as shown in Figure 6. To verify this indirect pathway from the low-lying excited states of the quasi-equatorial conformer, we investigated the excited-state properties and frontier molecular orbitals of the quasi-equatorial conformer of PTZ-TRZ in detail (Figure 7). For the S1 ← S0 transition, the HOMO → LUMO transition was dominant. Because the HOMO and LUMO are localized on the PTZ donor and TRZ acceptor moieties, respectively (Figures 1 and 7), the HOMO → LUMO transition was assigned to the CT absorption (λAAbs). For the S2 ← S0 transition, the HOMO → LUMO+2, LUMO+3 and LUMO+4 transitions were considered. The electron density distributions of LUMO+2 and LUMO+3 are delocalized over the donor and acceptor moieties. In contrast,

Figure 6. Schematic diagram for the one indirect excitation pathway of λEPL.

Figure 7. Schematic diagram of the molecular orbitals of the excited states of the quasi-equatorial conformer of PTZ-TRZ calculated at the TD-CAM-B3LYP/cc-pVDZ level.

LUMO+4 is localized on the PTZ moiety and phenyl ring, as shown in Figure 7. Thus, we assign the S2 states to a localized excited state, 1LE, originating mainly from the HOMO → LUMO+4 transition. Therefore, one of the indirect pathways for excitation leading to λEPL can be attributed to transitions to the 1CT1 state on the quasi-equatorial conformer caused by the formation of the 1LE state followed by photoinduced electron transfer (HOMO → LUMO+4 → LUMO), as shown in Figure 6. In this indirect pathway, the ground-state quasi-equatorial conformer had the small oscillator strength as mentioned earlier. However, the excited-state quasi-equatorial conformer generated by the indirect pathway exhibited the sufficiently observable ICT emission as λEPL. This indicates that the relationship between the radiative decay rate from the S1 state and the oscillator strength of the state cannot be adapted to all luminescent materials.19 Emission Pathways of the Dual ICT Fluorescence. As mentioned previously, the absorption spectrum of PTZ-TRZ in toluene solution except for the parts of λAAbs and λEAbs was reproduced by simply combining spectra of TRZ and PTZ-Ph, which confirms that the parts of the absorption spectrum derived from the non-CT transitions are composed of those of the component molecules. Therefore, the parts of the 15990



Article

absorption spectrum of PTZ-TRZ that could not be reproduced by those of the resolved molecules, λAAbs and λEAbs, can be inevitably attributed to the absorption of the CTs because the only additional property of PTZ-TRZ is the CT character (Figure 8). Thus, we finally determine that λAPL is

Figure 9. Schematic energy level diagram of PTZ-TRZ in a toluene solution exhibiting dual ICT fluorescence.

is expected. Therefore, the observed large increase of the PL intensity of λEPL by the excitation of λAAbs (as seen in Figure 5) is attributed to the energy transfer from 1CTA to 1CTE. The consistency between the excitation spectrum for emission at 562 nm and λAAbs confirmed that λEPL strongly depends on the formation of excitons on the quasi-axial conformer (1CTA ← SA0 transition; Figure S18). Therefore, the other indirect (not 1CTE ← SE0 transition) excitation pathway leading to the emission of λEPL is assigned to the energy transfer from 1CTA to 1CTE, as shown in Figure 9. Thus, the dual ICT emission mechanism for the PL spectrum of PTZ-TRZ in toluene solution was constructed by the existence of the steady-state excited-state conformers and energy transfer between the respective excited states (1CTA, 1 CTE, and 1LE), which is a new mechanism that differs from those of traditional dual emission and results from the rotational dynamics in PTZ-TRZ and execution of HOMO− LUMO separation for the molecular design of TADF materials. Transient PL Decay Characteristics in the Doped Film. Here, we discuss the existence of the quasi-axial and quasiequatorial conformers in the doped film. The two observed emissions obviously mean that the excited-state quasi-axial and quasi-equatorial conformers of PTZ-TRZ coexisted in the doped film, as shown in Figure S11 (center). The first was assigned to λAPL (1CTE1 → SA0 transition) like that of the PL spectra of PTZ-TRZ in toluene solution. In the doped film at 10 K, it is thought that two kinds of molecular conformations were constant. Therefore, the second can be attributed to λEPL derived from the indirect excitation pathway (1CTA → 1CTE1 → SE0 transition) caused by the energy transfer of the excitons from the excited-state quasi-axial conformers of PTZ-TRZ, in which energy transfer is caused by the Förster-type dipole− dipole interaction between the quasi-axial and quasi-equatorial conformers because the conformers are diluted by the host molecules (mCBP).58,59 This result strongly supports the indirect excitation pathway of λEPL derived from the energy transfer of the excitons from 1CTA to 1CTE in the toluene solution. As the temperature was gradually increased from 10 to 300 K, the PL intensity of λEPL increased strongly, while that of λAPL was independent of the film temperature. In addition, the spectra of the delayed component from 10 to 300 K showed λEPL only around 520 nm (Figure S11 (right)). These results confirm that λAPL is the normal ICT fluorescence and λEPL has TADF characteristics that results from energy transfer of the excitons from the quasi-axial to the quasi-equatorial conformer

Figure 8. Distinction of the CT absorption derived from PTZ-TRZ from the absorptions derived from PTZ-Ph and TRZ.

indeed ICT emission derived from the excited-state quasi-axial conformer generated by λAAbs. Therefore, the dual emission from PTZ-TRZ originates from two different ICT states, which indicates that PTZ-TRZ possesses two kinds of steady-state ICT excited states made possible by its rotational dynamics and HOMO−LUMO separations. As mentioned earlier, we assigned the origins of λAPL and λEPL to λAAbs and λEAbs, respectively. Here, we define the CT excited states corresponding to these emission and absorption as CTA and CTE. To verify the emission mechanism of the observed dual ICT fluorescence of PTZ-TRZ, we investigated the lowlying excited states of the two conformers in toluene solution by TD-DFT calculations using the TD-CAM-B3LYP/cc-pVDZ method. The excitation energies of the lowest singlet CT excited states of the quasi-axial and quasi-equatorial conformers were estimated to be 4.08 (1CTA1 ) and 3.46 eV (1CTE1 ), respectively. Because the emission wavelengths of λAPL and λEPL (409 and 562 nm) correspond to the 1CTA1 → SA0 and 1CTE1 → SE0 transitions, respectively, the energy levels (SA0 and SE0 ) of the ground-state quasi-axial and quasi-equatorial conformers were calculated to be 1.05 and 1.25 eV, respectively. From the DFT calculations and observed spectra, the calculated energy gap between the two ground-state conformers (ΔEGS) was 0.20 eV. The energy levels and wavelengths are summarized in Table 1. Figure 9 shows a schematic energy level diagram of PTZTRZ in toluene solution. Emission λAPL and λEPL result from the excited-state quasi-axial (1CTE1 ) and quasi-equatorial (1CTE1 ) conformers, respectively. Therefore, the origins of λAAbs and λEAbs can be attributed to the 1CTA ← SA0 and 1CTE ← SE0 transitions, respectively. The respective intensities of λAAbs and λEAbs reflect the oscillator strengths of the ground-state quasi-axial (f = 1.144) and quasi-equatorial conformers ( f ≈ 0), as shown in Figures 3 and 8. Considering the relationship between the energy levels of the excited-state conformers, the energy transfer of the excitons from 1CTA to 1CTE with/without the conformational change (the energy barrier derived from a repulsive hydrogen−hydrogen short contact) between the quasi-axial and quasi-equatorial conformers in toluene solution 15991



Article

(1CTA → 1CTE) and up-conversion (1CTE1 ← 3CTE1 ) followed by reverse intersystem crossing. To experimentally confirm the small ΔES‑T of the excitedstate quasi-equatorial conformer of PTZ-TRZ that emitted TADF, we performed low temperature PL measurements for the prompt and delayed components (Figure S14). A significant overlap between the prompt and delayed spectra was observed, which clearly indicates that the ΔES‑T of the excited-state quasi-equatorial conformer is very small as well as those of PXZ-TRZ-type TADF materials.15,21 These streak images and time-resolved fluorescence measurements reveal that PTZ-TRZ is a new type of TADF material with the dual ICT fluorescence characteristics that result from conformational heterogeneity derived from its rotational dynamics. Dual EL Emission from the OLED Containing PTZ-TRZ. To investigate the electroluminescence processes of two kinds of excited-state conformers in the OLED containing PTZ-TRZ as a dual ICT fluorescent emitter in greater detail, we measured EL spectra at different current densities, as shown in Figure 10.

materials as OLED emitters began recently in our group. In particular, the intentional control of ΔES‑T between the spinsinglet and triplet CT excited states of D-A molecular systems is an original strategy and a novel scientific approach for luminescent materials. The obtained dual ICT fluorescence with TADF characteristics was achieved by the combination of the control of ΔES‑T and the conformational heterogeneity that results from the introduction of a PTZ donor unit, reconfirming the importance of structural chemical control in TADF materials.

■

ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures and supplementary photophysical properties and OLED characteristics. CCDC 948737 for PTZTRZ. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

These authors contributed equally to this study (H.T. and K.S.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

We thank Dr. K. Tokumaru and Dr. W. Potscavage for fruitful discussion. This research was supported by the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program) and the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology. We acknowledge Ms. H. Nomura and Ms. N. Nakamura for performing thermal analysis (TG-DTA and DSC) and sublimation, and Mr. T. Matsumoto for singlecrystal X-ray crystal structure analysis. Computations were partly carried out using the computer facilities at the Research Institute for Information Technology, Kyushu University.

Figure 10. Dependence of the dual EL spectrum obtained from the OLED containing PTZ-TRZ on current density.

The relationship between the EL intensity and current density gives us significant information for the carrier recombination in the emitting layer. At low current density (3.6 mA/cm2), the EL spectrum only showed a single EL emission around 532 nm (Figure 10, blue), which was attributed to λEPL from the quasiequatorial conformer. Because the EQE at low current density were high because of the up-conversion of triplet excitons, as shown in Figure 4, the emission pathway in this region involves the quasi-equatorial conformers in the emitting layer of the OLED. As the current density increased, the EL intensity of λEPL showed a monotonic increase, and a new EL emission appeared at around 393 nm, as shown in Figure 10. The new EL emission was attributed to λAPL from the quasi-axial conformer like that of the PL spectra for the doped film. Therefore, the EL emission observed at high current density is a dual ICT fluorescence emission. This dual EL emission is the first example of such emission obtained from the conformational heterogeneity of a single luminescent molecule.

(1) Veldman, D.; Iṗ ek, ö.; Meskers, S. C. J.; Sweelssen, J.; Koetse, M. M.; Veenstra, S. C.; Kroon, J. M.; van Bavel, S. S.; Loos, J.; Janssen, R. A. J. Compositional and Electric Field Dependence of the Dissociation of Charge Transfer Excitons in Alternating Polyfluorene Copolymer/ Fullerene Blends. J. Am. Chem. Soc. 2008, 130, 7721−7735. (2) Vilmercati, P.; Castellarin-Cudia, C.; Gebauer, R.; Ghosh, P.; Lizzit, S.; Petaccia, L.; Cepek, C.; Larciprete, R.; Verdini, A.; Floreano, L.; et al. Mesoscopic Donor−Acceptor Multilayer by UltrahighVacuum Codeposition of Zn-Tetraphenyl-Porphyrin and C70. J. Am. Chem. Soc. 2009, 131, 644−652. (3) Venkataraman, D.; Yurt, S.; Harihara Venkatraman, B.; Gavvalapalli, N. Role of Molecular Architecture in Organic Photovoltaic Cells. J. Phys. Chem. Lett. 2010, 1, 947−958. (4) Martinez-Ferrero, E.; Albero, J.; Palomares, E. Materials, Nanomorphology, and Interfacial Charge Transfer Reactions in Quantum Dot/Polymer Solar Cell Devices. J. Phys. Chem. Lett. 2010, 1, 3039−3045. (5) Goushi, K.; Yoshida, K.; Sato, K.; Adachi, C. Organic LightEmitting Diodes Employing Efficient Reverse Intersystem Crossing for Triplet-to-Singlet State Conversion. Nat. Photonics 2012, 6, 253−258.

■

CONCLUSION In this article, we presented PTZ-TRZ, which shows TADF characteristics, as a material that falls into a new category of dual ICT fluorescent materials. Detailed research on TADF 15992



Article

(6) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234−238. (7) Rao, A.; Chow, P. C. Y.; Gélinas, S.; Schlenker, C. W.; Li, C.-Z.; Yip, H.-L.; Jen, A. K.-Y.; Ginger, D. S.; Friend, R. H. The Role of Spin in the Kinetic Control of Recombination in Organic Photovoltaics. Nature 2013, 500, 435−439. (8) Nayak, P. K.; Narasimhan, K. L.; Cahen, D. Separating Charges at Organic Interfaces: Effects of Disorder, Hot State, and Electric Field. J. Phys. Chem. Lett. 2013, 4, 1707−1717. (9) Yang, L.; Yan, L.; You, W. Organic Solar Cells beyond One Pair of Donor−Acceptor: Ternary Blends and More. J. Phys. Chem. Lett. 2013, 4, 1802−1810. (10) Pastore, M.; Fantacci, S.; De Angelis, F. Modeling Excited States and Alignment of Energy Levels in Dye-Sensitized Solar Cells: Successes, Failures, and Challenges. J. Phys. Chem. C 2013, 117, 3685− 3700. (11) Endo, A.; Ogasawara, M.; Takahashi, A.; Yokoyama, D.; Kato, Y.; Adachi, C. TADF from Sn4+−Porphyrin Complexes and Their Application to Organic Light-Emitting Diodes  A Novel Mechanism for Electroluminescence. Adv. Mater. 2009, 21, 4802−4806. (12) Endo, A.; Sato, K.; Yoshimura, K.; Kai, T.; Kawada, A.; Miyazaki, H.; Adachi, C. Efficient Up-Conversion of Triplet Excitons into a Singlet State and its Application for Organic Light Emitting Diodes. Appl. Phys. Lett. 2011, 98, 083302. (13) Lee, S. Y.; Yasuda, T.; Nomura, H.; Adachi, C. High-Efficiency Organic Light-Emitting Diodes Utilizing TADF from Triazine-Based Donor−Acceptor Hybrid Molecules. Appl. Phys. Lett. 2012, 101, 093306. (14) Nakagawa, T.; Ku, S.-Y.; Wong, K.-T.; Adachi, C. Electroluminescence Based on TADF Generated by a Spirobifluorene Donor−Acceptor Structure. Chem. Commun. 2012, 48, 9580−9582. (15) Tanaka, H.; Shizu, K.; Miyazaki, H.; Adachi, C. Efficient Green TADF (TADF) from a Phenoxazine−Triphenyltriazine (PXZ−TRZ) Derivative. Chem. Commun. 2012, 48, 11392−11394. (16) Mehes, G.; Nomura, H.; Zhang, Q.; Nakagawa, T.; Adachi, C. Enhanced Electroluminescence Efficiency in a Spiro-Acridine Derivative through TADF. Angew. Chem., Int. Ed. 2012, 51, 11311− 11315. (17) Zhang, Q.; Li, J.; Shizu, K.; Huang, S.; Hirata, S.; Miyazaki, H.; Adachi, C. Design of Efficient TADF Materials for Pure Blue Organic Light Emitting Diodes. J. Am. Chem. Soc. 2012, 134, 14706−14709. (18) Sato, K.; Shizu, K.; Yoshimura, K.; Kawada, A.; Miyazaki, H.; Adachi, C. Organic Luminescent Molecule with Energetically Equivalent Singlet and Triplet Excited States for Organic LightEmitting Diodes. Phys. Rev. Lett. 2013, 110, 247401. (19) Lee, J.; Shizu, K.; Tanaka, H.; Nomura, H.; Yasuda, T.; Adachi, C. Oxadiazole- and Triazole-Based Highly-Efficient TADF Emitters for Organic Light-Emitting Diodes. J. Mater. Chem. C 2013, 1, 4599− 4604. (20) Li, J.; Nakagawa, T.; MacDonald, J.; Zhang, Q.; Nomura, H.; Miyazaki, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes Based on a Hidden TADF Channel in a Heptazine Derivative. Adv. Mater. 2013, 25, 3319−3323. (21) Tanaka, H.; Shizu, K.; Nakanotani, H.; Adachi, C. Twisted Intramolecular Charge Transfer State for Long-Wavelength TADF. Chem. Mater. 2013, 25, 3766−3771. (22) Lippert, E.; Lüder, W.; Boos, H. In Advanced Molecular Spectroscopy; Mangini, A., Ed.; Pergamon: Oxford, 1962; pp 443−457. (23) Grabowski, Z. R.; Rotkiewicz, K.; Siemiarczuk, A.; Cowley, D. J.; Baumann, W. A New Class of Excited States with a Full Charge Separation. Nouv. J. Chim. 1979, 3, 443−454. (24) Rotkiewicz, K.; Grellmann, K. H.; Grabowski, Z. R. Reinterpretation of the ANOMALOUS Fluorescence of p-N,NDimethylamino-Benzonitrile. Chem. Phys. Lett. 1973, 19, 315−318. (25) Rettig, W. Charge Separation in Excited States of Decoupled SystemsTICT Compounds and Implications Regarding the Development of New Laser Dyes and the Primary Processes of Vision and Photosynthesis. Angew. Chem., Int. Ed. Engl. 1986, 25, 971−988.

(26) Leinhos, U.; Kühnle, W.; Zachariasse, K. A. Intramolecular Charge Transfer and Thermal Exciplex Dissociation with P-Aminobenzonitriles in Toluene. J. Phys. Chem. 1991, 95, 2013−2021. (27) Druzhinin, S. I.; Mayer, P.; Stalke, D.; von Bülow, R.; Noltemeyer, M.; Zachariasse, K. A. Intramolecular Charge Transfer with 1-tert-Butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) and Other Aminobenzonitriles. A Comparison of Experimental Vapor Phase Spectra and Crystal Structures with Calculations. J. Am. Chem. Soc. 2010, 132, 7730−7744 and references cited therein.. (28) Tian, H.; Yang, X.; Chen, R.; Pan, Y.; Li, L.; Hagfeldt, A.; Sun, L. Phenothiazine Derivatives for Efficient Organic Dye-Sensitized Solar Cells. Chem. Commun. 2007, 3741−3743. (29) Marszalek, M.; Nagane, S.; Ichake, A.; Humphry-Baker, R.; Paul, V.; Zakeeruddin, S. M.; Grätzell, M. Tuning Spectral Properties of Phenothiazine Based Donor-p-Acceptor Dyes for Efficient DyeSensitized Solar Cells. J. Mater. Chem. 2012, 22, 889−894. (30) Park, S. S.; Won, Y. S.; Choi, Y. C.; Kim, J. H. Molecular Design of Organic Dyes with Double Electron Acceptor for Dye-Sensitized Solar Cell. Energy Fuels 2009, 23, 3732−3736. (31) Meyer, T.; Ogermann, D.; Pankrath, A.; Kleinermanns, K.; Muller, T. J. J. Phenothiazinyl Rhodanylidene Merocyanines for DyeSensitized Solar Cells. J. Org. Chem. 2012, 77, 3704−3715. (32) Tsao, M.-H.; Wu, T.-Y.; Wang, H.-P.; Sun, I. W.; Su, S.-G.; Lin, Y.-C.; Chang, C.-W. An Efficient Metal-Free Sensitizer for DyeSensitized Solar Cells. Mater. Lett. 2011, 65, 583−586. (33) Tian, H.; Yang, X.; Chen, R.; Hagfeldt, A.; Sun, L. A Metal-Free “Black Dye” for Panchromatic Dye-Sensitized Solar Cells. Energy Environ. Sci. 2009, 2, 674−677. (34) Xie, Z.; Midya, A.; Loh, K. P.; Adams, S.; Blackwood, D. J.; Wang, J.; Zhang, X.; Chen, Z. Highly Efficient Dye-Sensitized Solar Cells Using Phenothiazine Derivative Organic Dyes. Prog. Photovoltaics 2010, 18, 573−581. (35) Kim, S. H.; Kim, H. W.; Sakong, C.; Namgoong, J.; Park, S. W.; Ko, M. J.; Lee, C. H.; Lee, W. I.; Kim, J. P. Effect of Five-Membered Heteroaromatic Linkers to the Performance of Phenothiazine-Based Dye-Sensitized Solar Cells. Org. Lett. 2011, 13, 5784−5787. (36) Wan, Z.; Jia, C.; Duan, Y.; Zhou, L.; Lin, Y.; Shi, Y. Phenothiazine−Triphenylamine Based Organic Dyes Containing Various Conjugated Linkers for Efficient Dye-Sensitized Solar Cells. J. Mater. Chem. 2012, 22, 25140−25147. (37) Agrawal, S.; Pastore, M.; Marotta, G.; Reddy, M. A.; Chandrasekharam, M.; Angelis, F. D. Optical Properties and Aggregation of Phenothiazine-Based Dye-Sensitizers for Solar Cells Applications: A Combined Experimental and Computational Investigation. J. Phys. Chem. C 2013, 117, 9613−9622. (38) Rodrigues, T.; dos Santos, C. G.; Riposati, A.; Barbosa, L. R. S.; Mascio, P. D.; Itri, R.; Baptista, M. S.; Nascimento, O. R.; Nantes, I. L. Photochemically Generated Stable Cation Radical of Phenothiazine Aggregates in Mildly Acid Buffered Solutions. J. Phys. Chem. B 2006, 110, 12257−12265. (39) Dance, Z. E. X.; Mi, Q.; McCamant, D. W.; Ahrens, M. J.; Ratner, M. A.; Wasielewski, M. R. Time-Resolved EPR Studies of Photogenerated Radical Ion Pairs Separated by p-Phenylene Oligomers and of Triplet States Resulting from Charge Recombination. J. Phys. Chem. B 2006, 110, 25163−25173. (40) Miura, T.; Carmieli, R.; Wasielewski, M. R. Time-Resolved EPR Studies of Charge Recombination and Triplet-Singlet Formation within Donor−Bridge−Acceptor Molecules Having Wire-Like Oligofluorene Bridges. J. Phys. Chem. A 2010, 114, 5769−5778. (41) Miura, T.; Wasielewski, M. R. Manipulating Photogenerated Radical Ion Pair Lifetimes in Wirelike Molecules Using Microwave Pulses: Molecular Spintronic Gates. J. Am. Chem. Soc. 2011, 133, 2844−2847. (42) Daub, J.; Engl, R.; Kurzawa, J.; Miller, S. E.; Schneider, S.; Stockmann, A.; Wasielewski, M. R. Competition Between Conformational Relaxation and Intramolecular Electron Transfer within Phenothiazine−Pyrene Dyads. J. Phys. Chem. A 2001, 105, 5655− 5665. 15993



Article

(43) Stockmann, A.; Kurzawa, J.; Fritz, N.; Acar, N.; Schneidre, S.; Daub, J.; Engl, R.; Clark, T. Conformational Control of Photoinduced Charge Separation within Phenothiazine−Pyrene Dyads. J. Phys. Chem. A 2002, 106, 7958−7970. (44) Acar, N.; Kurzawa, J.; Fritz, N.; Stockmann, A.; Roman, C.; Schneider, S.; Clark, T. Phenothiazine−Pyrene Dyads: Photoinduced Charge Separation and Structural Relaxation in the CT State. J. Phys. Chem. A 2003, 107, 9530−9541. (45) Frisch, M. J. et al. Gaussian 09, Revision C.01; Gaussian Inc.: Wallingford CT, 2009. (46) Bauernschmitt, R.; Ahlrichs, R. Treatment of Electronic Excitations within the Adiabatic Approximation of Time Dependent Density Functional Theory. Chem. Phys. Lett. 1996, 256, 454−464. (47) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. Molecular Excitation Energies to High-Lying Bound States from TimeDependent Density-Functional Response Theory: Characterization and Correction of the Time-Dependent Local Density Approximation Ionization threshold. J. Chem. Phys. 1998, 108, 4439−4449. (48) Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid Exchange− Correlation Functional using the Coulomb-Attenuating Method (CAM-B3LYP). Chem. Phys. Lett. 2004, 393, 51−57. (49) Dunning, J. T. H. Gaussian Basis Sets for use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (50) Improta, R.; Barone, V.; Scalmani, G.; Frisch, M. J. A StateSpecific Polarizable Continuum Model Time Dependent Density Functional Theory Method for Excited State Calculations in Solution. J. Chem. Phys. 2006, 125, 054103. (51) Improta, R.; Scalmani, G.; Frisch, M. J.; Barone, V. Toward Effective and Reliable Fluorescence Energies in Solution by a New State Specific Polarizable Continuum Model Time Dependent Density Functional Theory Approach. J. Chem. Phys. 2007, 127, 074504. (52) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models. Chem. Rev. 2005, 105, 2999−3094. (53) Gong, S.; He, X.; Chen, Y.; Jiang, Z.; Zhong, C.; Ma, D.; Qin, J.; Yang, C. Simple CBP Isomers with High Triplet Energies for Highly Efficient Blue Electroluminescence. J. Mater. Chem. 2012, 22, 2894− 2899. (54) Schrögel, P.; Langer, N.; Schildknecht, C.; Wagenblast, G.; Lennartz, C.; Strohriegl, P. Meta-Linked CBP-Derivatives as Host Materials for a Blue Iridium Carbine Complex. Org. Electron. 2011, 12, 2047−2055. (55) The experimental HOMO energy level of the neat film was determined to be 5.5 eV using a Riken-Keiki AC-2 photoelectron spectrometer; the LUMO energy level was estimated to be 3.0 eV by subtracting the optical energy gap (2.5 eV) estimated from the UV−vis absorption spectrum from the measured HOMO energy level. (56) Hasegawa, M.; Sonobe, Y.; Shindo, Y.; Sugimura, T.; Karatsu, T.; Kitamura, A. Photophysical Processes in Aromatic Polyimides. 2. Photoreduction of Benzophenone-Containing Polyimide Model Compounds. J. Phys. Chem. 1994, 98, 10771−10778. (57) Spies, C.; Lorenc, A.; Gehrke, R.; Kricheldorf, H. R. Charge Transfer Interactions of N-(4-Carboxyphenyl)trimellitimide Dibutyl Ester. J. Phys. Chem. A 2003, 107, 456−463. (58) Fö rster, T. Zwischenmolekulare Energiewanderung und Fluoreszenz. Ann. Phys. 1948, 2, 55−75. (59) Cerullo, G.; Nisoli, M.; Stagira, S.; De Silvestri, S.; Lanzani, G.; Graupner, W.; List, E.; Leising, G. Ultrafast Energy-Transfer Dynamics in a Blend of Electroluminescent Conjugated Polymers. Chem. Phys. Lett. 1998, 288, 561−566.

15994


Dual Intramolecular Charge-Transfer Fluorescence Derived from a

Recommend Documents