Molecular Understanding of the Chemical Stability of Organic

Article pubs.acs.org/JPCC

Molecular Understanding of the Chemical Stability of Organic Materials for OLEDs: A Comparative Study on Sulfonyl, PhosphineOxide, and Carbonyl-Containing Host Materials Na Lin, Juan Qiao,* Lian Duan, Liduo Wang, and Yong Qiu Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Chemical stability of organic materials on service toward excitons and charge carriers is intrinsically associated with the operational stability and economics of state-of-the-art organic light-emitting devices. Here we conducted comprehensive experiments and theoretical calculations to comparatively investigate the intrinsic chemical stability of organic materials, which contain typical electron-accepting moieties of sulfonyl, phosphine-oxide, and carbonyl group. The materials with a diphenylsulfonyl moiety suffered a fatal chemical instability originating from the cleavage of C−S single bond whether under UV irradiation or in electrical-stressed devices. The material with a dibenzothiophene-S,S-dioxide moiety exhibited significantly improved chemical stability because of effective shielding of the weak C−S single bond in a ring. In contrast, the commercially used carbonyl-containing compound demonstrated the highest chemical stability with negligible degradation under the same condition. Quantum chemical calculations fully supported the experimental results and suggested that the bond strength of the weak chemical bonds of the molecules would determine the intrinsic chemical stability of the organic materials in their excited and charged states, which might be a plausible origin of the limited stability of high-energy blue-emitting materials and devices. Several implications have been drawn for the design of new blue-emitting materials.

1. INTRODUCTION Organic light-emitting diodes (OLEDs) have attracted tremendous attention over the past decades due to their unique features and great potential for flat-panel, flexible displays and solid-state lighting. With the realization of high efficiencies and high-quality colors throughout the visible spectrum, limited operational stability becomes instead the key impediment to wide commercialization of OLEDs, in particular for high-energy blue-emitting devices.1−3 Generally, operational stability is considered to be governed by the intrinsic degradation phenomena, which results in a progressive decrease of overall luminance and a rise of the operating voltage during prolonged device operation. From a historic perspective, several different intrinsic degradation modes were proposed, including thermal and morphological instability, formation of trap and luminescence quenchers, and both interface and anode degradation.4−6 However, most recent studies suggest that the intrinsic device degradation is mainly a consequence of the chemical deterioration of organic materials in OLEDs.5,6 Even though this chemical degradation is complex and not fully understood, the observed degradation processes suggest that in most cases the initial step would be a bond rupture induced either by excitons or charge carriers (radical anions or cations), accompanied by complex reactions with surrounding molecules. © 2014 American Chemical Society

The resulting products could assume functions of deep charge traps, nonradiative recombination centers, and luminescence quenchers.6 Therefore, the chemical stability of the employed materials toward excitons and charge carriers is crucial to the long-term stability of the devices. However, to date, the chemical stability and degradation mechanisms of only a few commonly used organic materials are known.5,6 To enable the development of novel materials with enhanced stabilities, it is of high importance to deepen the understanding of the relationship between molecular structure and chemical stability of the materials. More recently, considerable attention has been paid to develop ambipolar materials with both electron-donating and electron-accepting moieties, which may lead to more balanced recombination of hole and electron, thus improving the efficiency and lifetime of OLEDs.7,8 The chemical stability of widely used electrondonating moieties, such as carbazole and arylamine, have been well studied.9,10 In contrast, the chemical stability of electronaccepting moieties, such as phosphine oxides (PO), sulfone (SO), and nitrogen- and oxygen-containing heterocycles Received: December 25, 2013 Revised: March 19, 2014 Published: March 21, 2014 7569

dx.doi.org/10.1021/jp412614k | J. Phys. Chem. C 2014, 118, 7569−7578

The Journal of Physical Chemistry C

Article

Shimadzu AXIMA Performance MALDI-TOF instrument in both positive and negative detection modes with an applied voltage of 25 kV between the target and the aperture of the time-of-flight analyzer. The samples were excited at a wavelength of 337 nm, where the maximum pulsed laser beam intensity is 180 uJ/pulse (at 50 Hz), with a spot size of 0.01 mm2 and a pulse duration of 3−5 ns. The UV irradiation and photoluminescence (PL) spectra of thin films were performed inside a fluorospectrophotometer (JobinYvon, FluoroMax-3). X-ray photoelectron spectroscopy (XPS) analysis was carried out to detect the chemical changes of UV-aged thin films using Thermo Scientific ESCALAB 250Xi with a monochromatic Al KR source. High-performance liquid chromatography (HPLC) analysis and preparative separations on the UV-aged solutions and the extracts of stressed unipolar devices were carried out on an Agilent 1100 Series instrument equipped with an Agilent ZORBAX SB-C18 column. The mass spectra of the charge-induced degradation products were obtained using chemical ionization-mass spectroscopy (CIMS). 2.2. Preparation and Degradation of Thin Films, Solutions, and Devices. In a general procedure, ITO-coated glass substrates were carefully precleaned and treated by UV ozone for 8 min. The substrate was then transferred into an evaporation chamber, where the organic layers were deposited via vacuum thermal evaporation at a rate of 1 Å s−1 under a pressure of 1 × 10−4 Pa. The Cs2CO3/Al or MoO3/Ag bilayer cathode was evaporated under a pressure of 1 × 10−3 Pa, and the evaporation rates were 0.2, 5−10, 0.2−0.4, and 0.1−0.4 Å s−1 for Cs2CO3, Al, MoO3 and Ag, respectively. The electrononly devices were fabricated with the configuration of ITO/ CzSF, t-CzSF, CzDBTO, or SBFK (80 nm)/Cs2CO3 (2 nm)/ Al (100 nm), where Cs2CO3 served as electron-injecting layer. The hole-only devices have the configuration of ITO/MoO3 (2 nm)/CzSF, t-CzSF, or CzDBTO (80 nm)/MoO3 (5 nm)/Ag (100 nm), where MoO3 was used as hole-injecting layer and electron-blocking layer. To suppress degradation caused by water or oxygen from air, the devices were transferred to a nitrogen-filled glovebox and encapsulated right after preparation. The electron-only and hole-only devices were aged under a constant current density of 20 mA/cm2 for 48 h. Unaged fresh reference samples were stored for the same time under the same conditions. After aging, we analyzed the solutions extracted from the electrical stressed devices by HPLC and CIMS to identify the possible degradation products. Here CI-MS was chosen to avoid fragmentation of the molecules when ionized like LDI-TOF-MS. The metal-capped organic films for UV irradiation consist of ITO/CzSF, t-CzSF, CzDBTO, CzPO2, or SBFK (120 nm)/Ag (100 nm). The thin films were irradiated under 340 nm monochrome illumination through the side of ITO at a power density of ∼0.3mW/cm2, which were performed inside a fluorospectrophotometer (JobinYvon, FluoroMax-3). The anhydrous CH2Cl2 solutions (∼10−5 M) of these materials were also prepared and irradiated under an ultraviolet lamp (model: MUA-165) with the peak wavelength at 365 nm with a high power density of ∼24 mW/ cm2. Before UV irradiation, the solutions were degassed with argon for 20 min. Then, the possible degradation products were identified by HPLC and CI-MS. In addition, we also compared the retention times of the degradation products with authentic samples using similar HPLC analytical methods for the structure identification.

(oxadiazoles, triazines, pyridines, phenantrolines, etc.), remains largely unknown. Of note, aryl PO and SO moieties have been widely used to develop high-efficiency blue-emitting, electrontransporting materials or host materials because of their high triplet energy and strong electron-accepting character.11−24 In our previous work, we found that a PO-containing material, 9(3,5-bis(diphenylphosphoryl)phenyl)-9H-carbazole (CzPO2) suffers a fatal chemical instability stemming from C−P single bond rupture.25 This finding raises the question about whether the SO moiety bears a similar problem and what may be underlying commonality behind the chemical stability of those organic materials. Herein, experimental investigation is combined with theoretic calculation to explore the chemical stability of three typical SO-containing materials, 4,4′-di(9H-carbazol-9-yl)diphenylsulfone (CzSF), 4,4′-di(3,6-di-tert-butyl-9H-carbazol9-yl)diphenylsulfone (t-CzSF), and 3,7-di(9H-carbazol-9-yl)dibenzothiophene-S,S-dioxide (CzDBTO), in comparison with PO-containing material CzPO2 and one commercially used carbonyl (CO)-containing material bis(9,9′-spirobifluorene-2yl)ketone (SBFK) (Figure 1). The introduction of two tert-

Figure 1. Chemical structures and monoisotopic masses in g/mol of the materials used.

butyl groups at the electrochemical active sites (3, 6-positions of carbazole) is generally considered to improve the electrochemical stability of carbazole-based materials, thus enhancing the device stability.26−28 It becomes intriguing to wonder if there are differences in the intrinsic chemical stability between carbazole unprotected CzSF and the protected t-CzSF. Also, a comparative study on CzDBTO and CzSF would be important for understanding the effects of protection of weak C−S single bond in a ring on the chemical stability of the corresponding materials. A comprehensive study including the investigation of molecular stability and photochemical behaviors, ultravioletand charge-induced degradation of thin films, identification of degradation products, and quantum chemical calculations is aimed at better understanding the relationship between molecular structure−chemical stability of these materials.

2. EXPERIMENTAL SECTION 2.1. Materials and Characterizations. CzSF, t-CzSF, CzDBTO, and CzPO2 were synthesized and purified as described in previous reports,21,24,29 and SBFK was purchased from Merck. The laser desorption ionization time-of flight mass spectrometry (LDI-TOF-MS) data were obtained using a 7570



Article

Figure 2. LDI-TOF-MS spectra taken at 20 μJ/pulse laser of CzSF at positive mode (a) and negative mode (b) as well as t-CzSF at positive mode (c) and negative mode (d).

Figure 3. LDI-TOF-MS spectra of CzDBTO taken at 20 μJ/pulse laser at the positive mode (a) and negative mode (b) and at 30 μJ/pulse laser at the positive mode (c) and negative mode (d). The LDI-TOF-MS spectra of SBFK taken at 30 μJ/pulse laser at the positive (e) and negative mode (f).

2.3. Quantum Chemical Calculations. The program Gaussian 0930 was used for all the calculations described in this paper. The geometry optimizations and vibration frequencies calculations were performed for each neutral molecule, radical, and charged species using density functional theory (DFT) with the B3LYP functional31,32 and a 6-31G (d) basis set. The unrestricted formalism was used for the geometry optimization and frequency calculations of neutral molecules, cation radicals and anion radicals. The bond dissociation energy (BDE) was calculated according to the enthalpy change in the corresponding reaction of homolytic cleavage of a single bond in the gas phase at 298 K and 1 atm.33 Calculations on the excited states energies of the molecules were carried out using timedependent DFT (TD-DFT) theory at the level of B3LYP/631G(d).

devices. To make assessment of the chemical stability of materials toward excitons or radical anions or cations, we investigated their photochemical behaviors using LDI-TOFMS. Leo and Scholz et al. employed this powerful technique to analyze the laser-induced photochemical reactions of organic layers as well as to detect the degradation products of long-term driven OLEDs.34−41 In our case, the sample was only pure powder of target material. The maximum pulsed laser beam intensity is 180 μJ/pulse. To avoid the formation of additional photochemical reactions during the analyzing process, the laser intensity was fixed at a low level. We set it to gradually increase to help track the possible degradation process. Similar to the PO-containing CzPO2,25 CzSF and t-CzSF proved to be highly reactive under laser irradiation. The LDITOF-MS spectra of CzSF and t-CzSF taken at low laser energy (20 μJ/pulse) at positive and negative mode, respectively, are shown in Figure 2. For CzSF, besides the expected signal of molecular ions mass/charge (i.e., m/z = 548.1), the spectrum for CzSF at the positive mode (a) showed additional signals at m/z = 242.9 and 258.9, which belong to 9-phenylcarbazole ion ([Ph + Cz]+) and [Ph + Cz + O]+ ions, respectively, and indicate a heterolytic cleavage at the C−S single bond either in

3. RESULTS AND DISCUSSION 3.1. Photochemical Stabilities of the Molecules under Laser Irradiation. Because materials in OLEDs usually work by providing excitons or radical anions or cations, the chemical stability of these highly energetic species would be intrinsically relevant to the operational stability of the corresponding 7571



Article

SBFK showed only the molecular ion signal. These results indicate that the weakest bond of SBFK molecule is the C(sp2)C(sp2) single bond between the aryl and CO unit, but it is much stronger than the C−S single bond of CzSF and t-CzSF molecules. 3.2. Photochemical Stability of Thin Films under UV Irradiation. Because organic materials serve as thin films in OLEDs, we further studied the UV-induced degradation behaviors of thin films of these materials. A group of samples consisting of ITO/CzSF, t-CzSF, CzDBTO, CzPO2, or SBFK (120 nm)/Ag (100 nm) were fabricated. It should be pointed out that the Ag layer was used to protect the organic materials from any possible photo-oxidation due to the presence of any trace amounts of oxygen in the environment.42 The samples were exposed to 340 nm illumination with the power density of ∼0.3 mW/cm2, which fits to the absorption peaks of all the materials (Figure S1 of Supporting Information (SI)). During continuous UV irradiation, in situ PL measurements were carried out to monitor changes in the PL spectra of the organic layers. The unaged reference samples were kept in the dark at room temperature. Figure 4 shows the changes in PL yields of these thin films versus the exposure time under UV irradiation. Details of the

the excited state or radical cation of CzSF under laser irradiation. At the negative mode (b), the spectrum gave only one signal at m/z = 306.2, corresponding to a negatively charged fragment [CzSF-(Ph + Cz)]−, which might come from a heterolytic cleavage of the C−S single bond in the excited state or a homolytic cleavage of the C−S single bond in the radical anion under laser irradiation. Similarly, a fragmentation by cleavage at the C−S single bond was also observed for the t-CzSF molecules (Figure 2c,d). The spectrum at the positive mode (c) showed no signal for tCzSF molecular ions but fragments at m/z = 354.1 and 370.1, which correspond to the 9-phenyl-3,6-di-tert-butylcarbazole ion ([Ph + t-Cz]+) and [Ph + t-Cz + O]+ ions, respectively. The spectrum at the negative mode (d), on the other hand, only exhibited one signal at m/z = 418.2, corresponding to a negatively charged fragment [CzSF-(Ph + t-Cz)]−. Note that both CzSF and t-CzSF have the hybrid structure of diphenylsulfonyl (DPSO) and carbazole units. However, the intensity of the signals at m/z = 166.8 and 278.3, representing the carbazole ([Cz]+) and 3,6-di-tert-butylcarbazole ([t-Cz]+) fragments, is much weaker than the others which are due to the dissociation of C−S bonds. These results indicate that the weakest bond of the two molecules is the C−S single bond from the DPSO moiety, not the C−N bond. Moreover, the introduction of two tert-butyl groups at the electrochemical active sites (3, 6-positions of carbazole) does not contribute to improving the photochemical stability as the electrochemical stability of carbazole-containing materials.26−28 From the strong intensity of the fragment signals due to the dissociation of C−S bonds, we can assume that the excited or charged CzSF and tCzSF molecules would undergo a severe fragmentation under the laser irradiation at 337 nm. In sharp contrast to the laser-induced fragmentation of the CzSF and t-CzSF molecules, the CzDBTO molecules showed no fragmentation at low laser energy and thus exhibited significantly higher photochemical stability. As shown in Figure 3a,b, at low laser energy (20 μJ/pulse), CzDBTO molecules gave only the molecular ion signal at m/z = 545.5 at the positive mode or 546.5 at the negative mode. When the laser intensity increased to 30 μJ/pulse, additional peaks came out from laser-induced dissociation reactions (Figure 3c,d). At the negative mode, the signal at m/z = 166.0 corresponds to the negatively charged carbazole. That suggests a heterolytic cleavage of the C−N single bond between carbazole anion and aryl free radical. At the positive mode, the relatively weaker signals at m/z = 241.8 and 481.6 correspond to [Ph + Cz]+ and [2(Ph + Cz)-H]+, respectively. That indicates a possible cleavage of C−S and C−C bond of the dibenzothiophene-S,Sdioxide (DBTO), which is followed by further free radical reactions at high laser intensity. These results would suggest that the weakest bond of the CzDBTO molecule is not the C− S bond of DBTO moiety but rather the C−N bond. Hence, we can conclude that the effective protection of weak C−S single bonds in a ring can significantly improve the photochemical stability of the involved molecules. Of particular concern is that the CO-containing SBFK in contrast exhibited the highest photochemical stability. At low laser intensity of 20 μJ/pulse, SBFK showed negligible fragmentation. When the laser intensity increased to 30 μJ/ pulse, besides the molecular ion signal at m/z = 658.3, a weak signal appeared at m/z = 343.0 at the positive mode (Figure 3e), which could be attributed to the loss of a spirobifluorene (SBF) free radical. However, at the negative mode (Figure 3f)

Figure 4. Changes in photoluminescence yields of samples with the structure of ITO/CzSF, t-CzSF, CzDBTO, CzPO2, or SBFK (120 nm)/Ag (100 nm) versus the irradiation time under 340 nm illumination with power density of ∼0.3 mW/cm2. The symbols show the experimental data, together with the fit using stretched exponential decay function (line).

corresponding PL spectra are given in Figure S2 of SI. For both CzSF and t-CzSF thin films, the PL yields decreased quickly at the early stage of the exposure to UV irradiation and a yield loss of ca. 65% was recorded when the exposure time reached 2 h, despite the fact that the electrochemical active sites (C3 and C6 of the carbazole) were blocked by tert-butyl units in the t-CzSF molecules. In contrast, a much slower decaying rate was observed for the CzDBTO thin film, showing that the DBTO moiety would be much more stable under UV irradiation in comparison with the DPSO moiety. The PO-containing CzPO2 undergo a remarkably slower degradation than CzSF and t-CzSF but much faster than CzDBTO and SBFK. It is noteworthy to point out that the CO-containing SBFK exhibited almost no detectable change in PL spectra after exposure to UV irradiation for a period up to 18 h. These results clearly suggest that the photochemical stability of these 7572



Article

Figure 5. XPS measurements of S 2p, N 1s, and C 1s electron binding energies for the thin films of CzSF, t-CzSF, CzDBTO, and SBFK, which were irradiated under 340 nm illumination at a power density of 0.3 mW/cm2 after 18 h. (a) S 2p1/2, 3/2 of CzSF; (b) S 2p1/2, 3/2 of t-CzSF; (c) N 1s of CzDBTO; (d) C 1s of SBFK.

after UV irradiation exhibited a reduced band with peak at ∼168.5 eV and ∼169.7 eV, corresponding to the S (2p1/2, 3/2) peak of the sulfonyl group, whereas an increased band with peak at 167.7 eV likewise corresponded to other chemical species. These results indicate that the UV-induced degradation of CzSF and t-CzSF thin films have analogous chemical modifications. In contrast to CzSF and t-CzSF films, CzDBTO film exhibited negligible change for S 2p electron binding energy (data not shown). As seen from the Figure 5c,d, UV irradiation otherwise resulted in a little decrease in the 400.6 and 285.4 eV, corresponding to N 1s electrons from carbazole47 of CzDBTO and C 1s electrons from carbonyl group48 of SBFK, respectively. To further identify the possible UV-induced degradation products, we have analyzed the possible degradation products of CzSF and t-CzSF in dilute solutions under UV irradiation. The anaerobic solutions were exposed to 365 nm excitation at a high power density of ∼24 mW/cm2 to accelerate the degradation. By employing HPLC and mass spectroscopy (MS) analysis, we found that the UV-irradiated solutions did yield some degradation products (Figure S3a,b of SI). In the HPLC chromatograms, besides the peak of CzSF and t-CzSF, several new peaks were detected. MS analysis performed on these degradation products gave the molecular weight m/z at 243.10 and 355.10, respectively, which can be tentatively assigned to [Ph + Cz] and [Ph + t-Cz]. These species are direct degradation products after UV irradiation, which are the cleavage products at the point of C−S single bonds. However, the UV-irradiated CzDBTO and SBFK solutions under the same irradiation did not yield detectable degradation products (Figure S3c,d of SI). The results here again demonstrate that the DPSO-containing materials CzSF and t-CzSF are much more vulnerable than the CO- and DBTO-containing materials in respect to UV irradiation. 3.3. Chemical Stability of Charged Species in Stressed Unipolar Devices. Considering the transport of holes and electrons in devices, chemical stability of organic materials

materials follows the order of CO > DBTO > PO > DPSOcontaining materials. Interestingly, we found that these UV-induced PL changes versus time (Figure 4) can be described by a stretched exponential decay (SED) function {L(t)/L(t0) = exp[−(t/ τ)β]},43 which is commonly used for degradation of electrically aged devices, where L(t0) is the initial luminance, L(t) is the luminance at time t, and τ and β are fitting parameters. Leo et al. also reported similar UV-induced degradation behavior of the 2,2′,7,7′-tetrakis(2,2-diphenylvinyl)spiro-9,9′-bifluorene.44 These results indicate that the UV-induced degradation mechanism of these thin films would be similar to that of their degradation under electrical stress. In this regard, the intrinsic photochemical stability of organic materials toward excitons in devices can be assessed by the UV-degradation behavior of their thin films. Clearly, this kind of assessment would have the advantage of avoiding any influence from other materials unavoidable in the measurements of device aging under electrical stress.41 To find out which chemical modifications are induced under UV irradiation, XPS measurements of the S, N, and C atoms were conducted to detect any changes in the electronic structures of these molecules in thin films. The thick silver layer was peeled off by adhesive tape right before the XPS measurements to expose these thin films. As seen from Figure 5a, the CzSF film showed a small spectral change in S 2p electron binding energies after UV irradiation. This change corresponds to a decrease in a band with a peak at ∼168.2 eV and ∼169.5 eV and an increase in a band with a peak at ∼167.5 eV. As the S (2p1/2, 3/2) peak of the SO moiety was found to be an unresolved 2:1 doublet at 168.2 and 169.5 eV,45,46 the band at 167.5 eV would correspond to other chemical species, suggesting UV irradiation causes a chemical change in the electronic structures of S atoms of the CzSF film. Notably, no obvious spectral change in N 1s binding energy was detected after UV irradiation. Similarly, as shown in Figure 5b, the XPS spectrum for S 2p electron binding energy in the t-CzSF film 7573



Article

Figure 6. HPLC chromatograms of the solutions extracted from electron-only devices after 48 h of operation at 20 mA/cm2. (a) CzSF; (b) t-CzSF; (c) CzDBTO; (d) SBFK.

Table 1. Excited State Energies and Bond Dissociation Energies (BDE) of the Weak Bonds Including C−S, C−N, C−P, and C(sp2)−C(sp2) in the Molecules compound CzSF

t-CzSF

energies of S1 and T1 (kcal/mol)

bond

initial state

bond dissociation reaction

average BDE(kcal/mol)

S1 = 78.6, T1= 70.2

C−S

neutral charged

C−N

neutral charged

C−S

neutral charged

C−N

neutral charged

CzSF→[CzSF-(Ph + Cz)]·+ (Ph + Cz)· CzSF+→[CzSF-(Ph + Cz)]·+ (Ph + Cz)+ CzSF−→[CzSF-(Ph + Cz)]− + (Ph + Cz)· CzSF→[CzSF-Cz]·+ Cz· CzSF+→[CzSF-Cz]·+ Cz+ CzSF−→[CzSF-Cz]·+ Cz− t-CzSF→[t-CzSF-(Ph + t-Cz)]·+ (Ph + t-Cz)· t-CzSF+→[t-CzSF-(Ph + t-Cz)]·+ (Ph+t-Cz)+ t-CzSF−→[t-CzSF-(Ph + t-Cz)]− + (Ph+t-Cz)· t-CzSF→[t-CzSF-(t-Cz)]·+ t-Cz· t-CzSF+→[t-CzSF-(t-Cz)]·+ t-Cz+ t-CzSF−→[t-CzSF-(t-Cz)]·+ t-Cz− CzDBTO→[CzDBTO-Cz]·+ Cz· CzDBTO+→[CzDBTO-Cz]·+ Cz+ CzDBTO−→[CzDBTO-Cz]·+ Cz− SBFK→[SBFK-SBF]·+ SBF· SBFK+→[SBFK-SBF]·+ SBF+ SBFK−→[SBFK-SBF]·+ SBF− CzPO2 →[CzPO2-Ph]·+ Ph· CzPO2 →[CzPO2-Ph2PO]· + Ph2PO· CzPO2+ →[CzPO2-Ph]+ + Ph· CzPO2+ →[CzPO2-Ph2PO]+ + Ph2PO· CzPO2− →[CzPO2-Ph]− + Ph· CzPO2− →[CzPO2-Ph2PO]− + Ph2PO· CzPO2 →[CzPO2-Cz]· + Cz· CzPO2+ →[CzPO2-Cz]+ + Cz· CzPO2− →[CzPO2-Cz]− + Cz·

66.7 91.6 29.7 81.4 98.2 53.3 67.4 93.4 29.9 80.5 91.4 52.1 81.1 95.3 61.3 90.5 110.2 93.1 81.2

S1 = 76.1, T1 = 68.3

CzDBTO

S1 = 70.9, T1 = 62.3

C−N

neutral charged

SBFK

S1 = 79.3, T1 = 61.3

C−C

neutral charged

CzPO225

S1 = 82.6, T1 = 73.2

C−P

neutral charged

C−N

neutral charged

70.5 88.5 42.9 59.8 80.7 93.4 61.3

nm)/CsCO3 (2 nm)/Al (100 nm) were employed to study the chemical stability of materials toward electrons. Figure 6a showed the chromatogram of the CzSF solutions extracted from the electron-only devices operated at 20 mA/cm2 after 48 h. Besides the peak of CzSF, an additional peak labeled with an arrow arose as the result of degradation. The HPLC and MS results both showed that the degradation product can be

toward charge carriers equally cannot be ignored. Then we explored the electrical degradation behaviors of these materials in the unipolar devices. In the previous work, we found that the PO-containing material CzPO2 in unipolar devices suffers similar chemical degradation like that under UV irradiation.25 The electron-only devices of the other four materials with the configuration of ITO/CzSF, t-CzSF, CzDBTO, or SBFK (80 7574



Article

Figure 7. Isocontour plots of HOMO and LUMO for the neutral molecules obtained at the B3LYP/6-31G(d) level. All the MO surfaces correspond to an isocontour value of |Ψ| = 0.03.

bond at both excited singlet and triplet states. Importantly, the BDE of C−N single bond in CzSF and t-CzSF is about 81.4 and 80.5 kcal/mol, which is much higher than their excited S1 and T1 energies. Likewise, the BDE of C−N bond connecting the phenyl moiety and carbazole moiety in CzDBTO molecule is comparable to that of CzSF and t-CzSF, also much higher than its excited state energies of S1 (3.1 eV, 70.9 kcal/mol) and T1 (2.7 eV, 62.3 kcal/mol). In comparison, the BDE of the C(sp2)−C(sp2) single bond of SBFK is the highest (90.5 kcal/ mol), over 11 kcal/mol higher than its singlet state energies. Thus, it can be ruled out that CzDBTO and SBFK molecules would dissociate in excited states, which could be the origin of the high chemical stability of these two materials, in particular for SBFK. Furthermore, we investigated the electron density distributions of frontier orbitals involved in the S1 and T1 states of these molecules, which would be additional factors affecting the bond cleavage in these excited states.49,50 The TD-DFT calculations showed that the HOMO→LUMO transition significantly contributes to the S1 (100% for CzSF, 97% for tCzSF, 100% for CzDBTO, and 69% for SBFK) and T1 (71% for CzSF, 74% for t-CzSF and 67% for CzDBTO) states of these molecules (Table S1 of SI). For both CzSF and t-CzSF, the HOMOs are delocalized over the whole molecule, whereas the LUMOs are dominantly localized on the carbazole moieties (Figure 7). Therefore, upon excitation to the S1 or T1, both CzSF and t-CzSF would have the electron density transferring from DPSO to carbazole moieties, which is expected to weaken the C−S bonds and strengthen the C−N bonds. Similarly, the C−N bonds of CzDBTO would be likewise strengthened when the molecule is excited to the S1 or T1 states. In comparison, for SBFK, the HOMO spreads widely in the two spirobifluorene side groups, and the LUMO highly resides in the center of the molecule including the carbonyl group and the two neighboring phenyl rings. Thus, the C−C bonds would be reinforced when the molecule is excited to the S1 state. These results again suggested that upon excitation to S1 or T1 state, the C−S bonds of CzSF and t-CzSF molecules would be undermined, and the C−N bonds of CzDBTO and C−C bonds of SBFK would be consolidated instead, which agrees well with the above discussion from the BDE of these molecules. Considering the moving of electron or hole through the materials, we also calculated the dissociation energy of C−S and C−N single bond in their charged species. The dissociation energy of the C−S single bond of CzSF and t-CzSF molecules in negatively charged states (as radical anion) is markedly reduced by half (29.7 kcal/mol for CzSF and 29.9 kcal/mol for t-CzSF). So it is reasonable to conclude that the vulnerable C− S single bond of CzSF and t-CzSF molecules would dissociate not only in their excited states but also more easily in the

assigned to [Ph + Cz] corresponding to the cleavage of C−S single bond. As shown in Figure 6b, t-CzSF exhibited a very similar scenario of electrical degradation behavior. The degradation product labeled with an arrow can be identified as [Ph + t-Cz]. Although the underlying chemical mechanisms are too complex to be completely elucidated, this identified degradation product would correspond to the dissociation of relatively weak C−S bonds. It means that CzSF and t-CzSF have undergone comparable chemical degradation in similar electrical aged electron-only devices under UV irradiation. In contrast to the CzSF and t-CzSF, the electron-only devices of CzDBTO and SBFK showed negligible chemical change under the same condition (Figure 6c,d). These results indicate that the introduction of a DBTO or CO moiety can significantly improve the chemical stability of organic materials in negatively charged species (radical anions). Except for SBFK only with electron-transporting capability, CzSF, t-CzSF, CzDBTO, and CzPO2 have the typical D−A structures and thus exhibit ambipolar transporting character. The hole-only devices with the structure of ITO/MoO3 (2 nm)/CzSF, t-CzSF, or CzDBTO (80 nm)/MoO3 (5 nm)/Ag (100 nm) were employed to study the chemical stability of these materials toward holes. Surprisingly, chemical analysis on the solutions extracted from the hole-only devices did not give detectable degradation products after 48 h of operation at 20 mA/cm2 (Figure S4 of SI), even for CzSF and t-CzSF devices. That is quite different from those of the electron-only devices, thus indicating that CzSF, t-CzSF, and CzDBTO have much better chemical stability in positively charged species (radical cations) than in their negative counterparts. Overall, these results strongly suggest that the chemically unstable C−S single bonds of DPSO derivatives would undermine the stability of the corresponding OLEDs, especially when they serve as electron-transporting materials. 3.4. Quantum Chemical Calculations. To gain further insight into the role of the C−S, C−N, and C(sp2)-C(sp2) single bonds in the chemical degradation of the involved molecules, quantum chemical calculations were carried out to calculate the corresponding BDE. The first excited singlet (S1) and triplet (T1) states of these molecules including the excited state energies and the molecular orbitals involved in these excited states were also calculated to investigate whether molecules in these excited states would undergo bond cleavage. The results are listed in Table 1. The calculated average BDE of C−S single bonds in CzSF and t-CzSF molecules are about 66.7 and 67.4 kcal/mol, which are markedly lower than the excited state energies of both S1 (78.6 and 76.1 kcal/mol) and T1 (70.2 and 68.3 kcal/mol), respectively. It means that CzSF and t-CzSF molecules may suffer a unimolecular homolytic dissociation of the C−S single 7575



Article

4. CONCLUSIONS In conclusion, we have comprehensively explored the intrinsic chemical stability of a series of organic materials with typical electron-accepting moieties of SO, PO, and CO. The same as the strong electron acceptor bearing desirable electrontransporting capability, CO moiety has the best chemical stability, PO follows, and the DPSO is the worst. It was found that the materials with a DPSO and PO moiety would undergo a fatal chemical instability originating from the weak C−S and C−P single bonds. The cleavage of the C−S single bond in the DPSO moiety would occur not only in excited singlet and triplet states but also more easily in negatively charged states. It would be detrimental to operation stability of OLEDs when these materials serve as host, emitter, or electron-transporting materials. However, the materials with a DBTO or CO moiety showed significantly improved chemical stability via shielding of weak C−S bonds in a ring or employing a strong C(sp2)− C(sp2) bond. Several implications have been drawn for the development of new materials, in particular for high-energy blue-emitting materials. These findings would provide a facile approach for rational design of new organic materials with desirable chemical stability through theoretic screening of the weak bond strength and the excited state energies of the corresponding molecules. Although this study focused on organic materials for OLEDs, we anticipate that these findings and methodologies would help to understand the fundamental degradation mechanism of organic materials for other organic optoelectronic devices.

negatively charged states, which is similar to the behavior of the C−P single bond in CzPO2.25 The calculated result is in good agreement with the above identified degradation products of the materials under UV irradiation and electrical stress. However, in positively charged states (as radical cations), the C−S and C−P bond behave quite differently. The dissociation energy of the C−P single bond in the CzPO2 molecule decreases about 10 kcal/mol in positively charged states, thus indicating that it would suffer rupture. But the dissociation energy of the C−S single bonds in CzSF and t-CzSF molecules instead increase about 25 and 18 kcal/mol in positively charged states, thus suggesting it would be much more robust. These results agree well with negligible electrical degradation of holeonly devices based on CzSF and t-CzSF. Then we discuss the C−N single bonds between carbazole and the aryl groups in these compounds except for SBFK. In all neutral molecules, the average BDE of C−N single bond is about 80.9 ± 0.5 kcal/mol, which is comparable to the S1 energy of the CzPO2 molecule but significantly higher than the excited S1 and T1 of CzSF, t-CzSF and CzDBTO molecules. So in CzPO2, the C−N single bond would dissociate likewise in the excited singlet states. However, in CzSF, t-CzSF, and CzDBTO, the C−N single bonds would be robust in excited singlet and triplet states. Furthermore, in all four molecules, the dissociation energy of C−N single bond in positively charged states has a big increase, ranging from 11 to 17 kcal/mol, than that in neutral molecules, whereas that in negatively charged states has a big decrease ranging from 20 to 28 kcal/mol. These results quantitatively suggest that the C−N single bond between carbazole and the aryl group would be more robust in radical cations but more vulnerable in radical anions. 3.5. Implications for the Design of New Materials. First, considering the good accordance with the experimental results, quantum chemical calculation is highly suggested as a prediction tools for evaluating the intrinsic chemical stabilities of organic materials. It has been found that the relationship between the magnitude of the weakest BDE and the excited singlet/triplet energy of the molecules would determine the chemical stability of organic materials in their excited and/or charged states. That might be a plausible origin of the limited stability of blue-emitting materials and devices. Second, toward rational design of organic materials with high chemical stability, several implications could be drawn as follows: (i) Because of the easy cleavage of weak C−S and C−P single bonds, the DPSO and PO moieties would be detrimental to operational stability of the devices, in particular when the corresponding materials serve as electron-transport and/or emitting materials with high singlet/triplet energy. (ii) The CO moiety in contrast exhibits high chemical stability due to a strong C(sp2)−C(sp2) bond, thus having great potential as a desirable electronaccepting group for electron-transporting materials or ambipolar host materials, in particular for high-energy blue-emitting materials. (iii) Carbazole is a robust electron-donating moiety for design of hole-transporting materials, but it may encounter risk as the hole-transporting subunit for ambipolar host materials, especially when the host material is in the negatively charged state. (iv) By a comparison study on the chemical stability of DPSO and DBTO moieties, we confirmed that the effective protection of molecular weak bonds in a ring is an efficient strategy to improve chemical stability of organic materials.

■

ASSOCIATED CONTENT

S Supporting Information *

The absorption spectra of CzSF, t-CzSF, CzDBTO, SBFK, and CzPO2 thin films. The normalized PL spectra of Ag-capped thin films obtained under 340 nm monochromatic illumination. The HPLC chromatograms of the UV-irradiated solutions and the HPLC chromatograms of the solutions extracted from electrical-stressed hole-only devices. The transitions and orbital contributions of S1 and T1 states of CzSF, t-CzSF, CzDBTO, and SBFK obtained from TD-DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Fax: +86-10-62795137. Tel.: +86-10-62773109. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the National Nature Science Foundation of China (under grants 91233118 and 51073089) and the National Key Basic Research and Development Program of China (under grant 2011CB808403). The computation in this research was performed on the “Explorer 100” cluster system of Tsinghua National Laboratory for Information Science and Technology. J.Q. is grateful to Prof. Bo-Qing Xu for his valuable suggestions on the preparation of the manuscript.

■

REFERENCES

(1) Xiao, L.; Chen, Z.; Qu, B.; Luo, J.; Kong, S.; Gong, Q.; Kido, J. Recent Progresses on Materials for Electrophosphorescent Organic Light-Emitting Devices. Adv. Mater. 2011, 23, 926−952.

7576



Article

Phosphorescence Hybrid White Organic Light-Emitting Device. Adv. Mater. 2012, 24, 3410−3414. (21) Zhang, Q.; Li, J.; Shizu, K.; Huang, S.; Hirata, S.; Miyazaki, H.; Adachi, C. Design of Efficient Thermally Activated Delayed Fluorescence Materials for Pure Blue Organic Light Emitting Diodes. J. Am. Chem. Soc. 2012, 134, 14706−14709. (22) Christensen, P. R.; Nagle, J. K.; Bhatti, A.; Wolf, M. O. Enhanced Photoluminescence of Sulfur-Bridged Organic Chromophores. J. Am. Chem. Soc. 2013, 135, 8109−8112. (23) Zheng, C.-J.; Wang, J.; Ye, J.; Lo, M.-F.; Liu, X.-K.; Fung, M.-K.; Zhang, X.-H.; Lee, C.-S. Novel Efficient Blue Fluorophors with Small Singlet-Triplet Splitting: Hosts for Highly Efficient Fluorescence and Phosphorescence Hybrid WOLEDs with Simplified Structure. Adv. Mater. 2013, 25, 2205−2211. (24) Moss, K. C.; Bourdakos, K. N.; Bhalla, V.; Kamtekar, K. T.; Bryce, M. R.; Fox, M. A.; Vaughan, H. L.; Dias, F. B.; Monkman, A. P. Tuning the Intramolecular Charge Transfer Emission from Deep Blue to Green in Ambipolar Systems Based on Dibenzothiophene S,SDioxide by Manipulation of Conjugation and Strength of the Electron Donor Units. J. Org. Chem. 2010, 75, 6771−6781. (25) Lin, N.; Qiao, J.; Duan, L.; Li, H.; Wang, L.; Qiu, Y. Achilles Heels of Phosphine Oxide Materials for OLEDs: Chemical Stability and Degradation Mechanism of a Bipolar Phosphine Oxide/Carbazole Hybrid Host Material. J. Phys. Chem. C 2012, 116, 19451−19457. (26) Brunner, K.; van Dijken, A.; Börner, H.; Bastiaansen, J. J. A. M.; Kiggen, N. M. M.; Langeveld, B. M. W. Carbazole Compounds as Host Materials for Triplet Emitters in Organic Light-Emitting Diodes: Tuning the HOMO Level without Influencing the Triplet Energy in Small Molecules. J. Am. Chem. Soc. 2004, 126, 6035−6042. (27) Lin, N.; Sun, Y.; Qiao, J.; Duan, L.; Qiu, Y. The Electrochemical Stability of Organic Materials and Their Influence on the Operational Stability of Organic Light-Emitting Devices. Sci. China: Chem. 2013, 43, 407−417. (28) Gu, C.; Zhang, Z.; Sun, S.; Pan, Y.; Zhong, C.; Lv, Y.; Li, M.; Ariga, K.; Huang, F.; Ma, Y. In Situ Electrochemical Deposition and Doping of C60 Films Applied to High-Performance Inverted Organic Photovoltaics. Adv. Mater. 2012, 24, 5727−5731. (29) Jiang, W.; Duan, L.; Qiao, J.; Dong, G.; Wang, L.; Qiu, Y. Tuning of Charge Balance in Bipolar Host Materials for Highly Efficient Solution-Processed Phosphorescent Devices. Org. Lett. 2011, 13, 3146−3149. (30) Frisch, M. J.; et al. Gaussian 09, revision C.1; Gaussian, Inc.: Wallingford CT, 2010. (31) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle− Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (32) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652. (33) DiLabio, G. A.; Pratt, D. A.; LoFaro, A. D.; Wright, J. S. Theoretical Study of X−H Bond Energetics (X = C, N, O, S): Application to Substituent Effects, Gas Phase Acidities, and Redox Potentials. J. Phys. Chem. A 1999, 103, 1653−1661. (34) Scholz, S.; Corten, C.; Walzer, K.; Kuckling, D.; Leo, K. Photochemical Reactions in Organic Semiconductor Thin Films. Org. Electron. 2007, 8, 709−717. (35) Scholz, S.; Walzer, K.; Leo, K. Analysis of Complete Organic Semiconductor Devices by Laser Desorption/Ionization Time-ofFlight Mass Spectrometry. Adv. Funct. Mater. 2008, 18, 2541−2547. (36) Scholz, S.; Meerheim, R.; Walzer, K.; Leo, K. Chemical Degradation Mechanisms of Organic Semiconductor Devices. Proc. SPIE 2008, 6999, 69991B(1)−69991B(10). (37) Scholz, S.; Meerheim, R.; Lüssem, B.; Leo, K. Laser Desorption/ Ionization Time-of-Flight Mass Spectrometry: A Predictive Tool for the Lifetime of Organic Light Emitting Devices. Appl. Phys. Lett. 2009, 94, 043314(1)−043314(3). (38) de Moraes, I. R.; Scholz, S.; Lüssem, B.; Leo, K. Role of OxygenBonds in the Degradation Process of Phosphorescent Organic Light Emitting Diodes. Appl. Phys. Lett. 2011, 99, 053302(1)−053302(3).

(2) Yook, K. S.; Lee, J. Y. Organic Materials for Deep Blue Phosphorescent Organic Light-Emitting Diodes. Adv. Mater. 2012, 24, 3169−3190. (3) Sasabe, H.; Kido, J. Development of High Performance OLEDs for General Lighting. J. Mater. Chem. C 2013, 1, 1699−1707. (4) Aziz, H.; Popovic, Z. D. Degradation Phenomena in SmallMolecule Organic Light-Emitting Devices. Chem. Mater. 2004, 16, 4522−4532. (5) So, F.; Kondakov, D. Degradation Mechanisms in SmallMolecule and Polymer Organic Light-Emitting Diodes. Adv. Mater. 2010, 22, 3762−3777. (6) Schmidbauer, S.; Hohenleutner, A.; Kö nig, B. Chemical Degradation in Organic Light-Emitting Devices: Mechanisms and Implications for the Design of New Materials. Adv. Mater. 2013, 25, 2114−2129. (7) Chaskar, A.; Chen, H.-F.; Wong, K.-T. Bipolar Host Materials: A Chemical Approach for Highly Efficient Electrophosphorescent Devices. Adv. Mater. 2011, 23, 3876−3895. (8) Duan, L.; Qiao, J.; Sun, Y.; Qiu, Y. Strategies to Design Bipolar Small Molecules for OLEDs: Donor−Acceptor Structure and NonDonor−Acceptor Structure. Adv. Mater. 2011, 23, 1137−1144. (9) Kondakov, D. Y.; Lenhart, W. C.; Nichols, W. F. Operational Degradation of Organic Light-Emitting Diodes: Mechanism and Identification of Chemical Products. J. Appl. Phys. 2007, 101, 024512(1)−024512(7). (10) Kondakov, D. Y. Role of Chemical Reactions of Arylamine Hole Transport Materials in Operational Degradation of Organic LightEmitting Diodes. J. Appl. Phys. 2008, 104, 084520(1)−084520(9). (11) Jeon, S. O.; Lee, J. Y. Phosphine Oxide Derivatives for Organic Light Emitting Diodes. J. Mater. Chem. 2012, 22, 4233−4243. (12) Huang, T. H.; Lin, J. T.; Chen, L. Y.; Lin, Y. T.; Wu, C. C. Dipolar Dibenzothiophene S,S-Dioxide Derivatives Containing Diarylamine: Materials for Single-Layer Organic Light-Emitting Devices. Adv. Mater. 2006, 18, 602−606. (13) Huang, T. H.; Whang, W. T.; Shen, J. Y.; Wen, Y. S.; Lin, J. T.; Ke, T. H.; Chen, L. Y.; Wu, C. C. Dibenzothiophene/Oxide and Quinoxaline/Pyrazine Derivatives Serving as Electron-Transport Materials. Adv. Funct. Mater. 2006, 16, 1449−1456. (14) Liu, J.; Zou, J.; Yang, W.; Wu, H.; Li, C.; Zhang, B.; Peng, J.; Cao, Y. Highly Efficient and Spectrally Stable Blue-Light-Emitting Polyfluorenes Containing a Dibenzothiophene-S,S-dioxide Unit. Chem. Mater. 2008, 20, 4499−4506. (15) Hsu, F.-M.; Chien, C.-H.; Hsieh, Y.-J.; Wu, C.-H.; Shu, C.-F.; Liu, S.-W.; Chen, C.-T. Highly Efficient Red Electrophosphorescent Device Incorporating a Bipolar Triphenylamine/BisphenylsulfonylSubstituted Fluorene Hybrid as the Host. J. Mater. Chem. 2009, 19, 8002−8008. (16) Li, K.; Qu, J.; Xu, B.; Zhou, Y.; Liu, L.; Peng, P.; Tian, W. Synthesis and Photovoltaic Properties of Novel Solution-Processable Triphenylamine-Based Dendrimers with Sulfonyldibenzene Cores. New J. Chem. 2009, 33, 2120−2127. (17) Grisorio, R.; Melcarne, G.; Suranna, G. P.; Mastrorilli, P.; Nobile, C. F.; Cosma, P.; Fini, P.; Colella, S.; Fabiano, E.; Piacenza, M.; et al. First Disubstituted Dibenzothiophene-5,5-Dioxide Monodispersed Molecular Materials for Efficient Blue-Electroluminescence. J. Mater. Chem. 2010, 20, 1012−1018. (18) Zhou, G.; Wang, Q.; Wang, X.; Ho, C.-L.; Wong, W.-Y.; Ma, D.; Wang, L.; Lin, Z. Metallophosphors of Platinum with Distinct MainGroup Elements: A Versatile Approach Towards Color Tuning and White-Light Emission with Superior Efficiency/Color Quality/Brightness Trade-Offs. J. Mater. Chem. 2010, 20, 7472−7484. (19) Kim, S.-J.; Leroy, J.; Zuniga, C.; Zhang, Y.; Zhu, L.; Sears, J. S.; Barlow, S.; Brédas, J.-L.; Marder, S. R.; Kippelen, B. High-Efficiency Blue-Green Electrophosphorescent Light-Emitting Devices Using a Bis-Sulfone As Host in the Emitting Layer. Org. Electron. 2011, 12, 1314−1318. (20) Ye, J.; Zheng, C.-J.; Ou, X.-M.; Zhang, X.-H.; Fung, M.-K.; Lee, C.-S. Management of Singlet and Triplet Excitons in a Single Emission Layer: A Simple Approach for a High-Efficiency Fluorescence/ 7577



Article

(39) de Moraes, I. R.; Scholz, S.; Lüssem, B.; Leo, K. Analysis of Chemical Degradation Mechanism within Sky Blue Phosphorescent Organic Light Emitting Diodes by Laser-Desorption/Ionization Timeof-Flight Mass Spectrometry. Org. Electron. 2011, 12, 341−347. (40) de Moraes, I. R.; Scholz, S.; Lüssem, B.; Leo, K. Chemical Degradation Processes of Highly Stable Red Phosphorescent Organic Light Emitting Diodes. Org. Electron. 2012, 13, 1900−1907. (41) Seifert, R.; de Moraes, I. R.; Scholz, S.; Gather, M. C.; Lüssem, B.; Leo, K. Chemical Degradation Mechanisms of Highly Efficient Blue Phosphorescent Emitters Used for Organic Light Emitting Diodes. Org. Electron. 2013, 14, 115−123. (42) Wang, Q.; Williams, G.; Tsui, T.; Aziz, H. Photochemical Deterioration of the Organic/Metal Contacts in Organic Optoelectronic Devices. J. Appl. Phys. 2012, 112, 064502(1)−064502(7). (43) Fery, C.; Racine, B.; Vaufrey, D.; Doyeux, H.; Cina, S. Physical Mechanism Responsible for the Stretched Exponential Decay Behavior of Aging Organic Light-Emitting Diodes. Appl. Phys. Lett. 2005, 87, 213502(1)−213502(3). (44) Seifert, R.; Scholz, S.; Lussem, B.; Leo, K. Comparison of Ultraviolet- and Charge-Induced Degradation Phenomena in Blue Fluorescent Organic Light Emitting Diodes. Appl. Phys. Lett. 2010, 97, 013308(1)−013308(3). (45) Hopkins, J.; Badyal, J. P. S. CF4 Plasma Treatment of Asymmetric Polysulfone Membranes. Langmuir 1996, 12, 3666−3670. (46) Gardella, J. A.; Ferguson, S. A.; Chin, R. L. Shakeup Satellites for the Analysis of Structure and Bonding in Aromatic Polymers by X-Ray Photoelectron Spectroscopy. Appl. Spectrosc. 1986, 40, 224−232. (47) Kessel, R.; Schultze, J. W. Surface Analytical and Photoelectrochemical Investigations of Conducting Polymers. Surf. Interface Anal. 1990, 16, 401−406. (48) Briggs, D.; Beamson, G. Primary and Secondary OxygenInduced C1s Binding Energy Shifts in X-ray Photoelectron Spectroscopy of Polymers. Anal. Chem. 1992, 64, 1729−1736. (49) Budyka, M. F.; Biktimirova, N. V.; Gavrishova, T. N.; Laukhina, O. D.; Zemtsov, D. B. Photodissociation of 4-Azidopyridine and 4Azidoquinolinein Neutral and Cationic Forms. J. Photochem. Photobiol., A 2005, 173, 70−74. (50) Albani, B. A.; Durr, C. B.; Turro, C. Selective Photoinduced Ligand Exchange in a New Tris-Heteroleptic Ru(II) Complex. J. Phys. Chem. A 2013, 117, 13885−13892.

■

NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on April 1, 2014, with the molecular structure of SBFK incorrectly defined throughout the paper. The corrected version was reposted on April 10, 2014.

7578


Molecular Understanding of the Chemical Stability of Organic

Recommend Documents