Triplet energies and excimer formation in meta- and para-linked carbazolebiphenyl matrix materials

Abstract

We present a spectroscopic investigation on the effect of changing the position where carbazole is attached to biphenyl in carbazolebiphenyl (CBP) on the triplet state energies and the propensity to excimer formation. For this, two CBP derivatives have been prepared with the carbazole moieties attached at the (para) 4- and 4^′-positions (pCBP) and at the (meta) 3- and 3^′-positions (mCBP) of the biphenyls. These compounds are compared to analogous mCDBP and pCDBP, i.e. two highly twisted carbazoledimethylbiphenyls, which have a high triplet energy at about 3.0 eV and tend to form triplet excimers in a neat film. This torsion in the structure is associated with localization of the excited state onto the carbazole moieties. We find that in mCBP and pCBP, excimer formation is prevented by localization of the triplet excited state onto the central moiety. As conjugation can continue from the central biphenyls into the nitrogen of the carbazole in the para-connected pCBP, emission involves mainly the benzidine. By contrast, the meta-linkage in mCBP limits conjugation to the central biphenyl. The associated shorter conjugation length is the reason for the higher triplet energy of 2.8 eV in mCBP compared with the 2.65 eV in pCBP.

1. Introduction

Modern phosphorescent organic light-emitting diodes (OLEDs) comprise several functional layers such as injection layers, transport layers, blocking layers and the emissive layer. Achieving good performance requires confining the recombination of charges to the light-emitting chromophore in the emissive layer while avoiding quenching of the thus created triplet excited state by other triplets or charges. A common approach to obtain this is to embed a low percentage of the triplet emitter into an optically inert matrix material that keeps the triplet emitters sufficiently separated and that ensures an appropriate transport of the charge carriers and excitation to the emitter [1–4]. A key requirement for these kinds of compounds is a triplet excited state (T₁) that is energetically higher in the matrix material than in the emitter to avoid back energy transfer. While this requirement is relatively easy to fulfil for red and green emitters, where the triplet levels are in the range 1.7–2.6 eV, this becomes more difficult for blue phosphorescent emitters with triplet levels of 2.7 eV and higher.

A class of materials that is widely used for this purpose is based on the carbazole moiety, such as 4,4^′-bis(N-carbazolyl)-1,1^′-biphenyl (pCBP) and its derivatives [5–7]. While these compounds have been popular host materials for OLEDs for quite some time, the relation between their chemical structure, electronic structure and optoelectronic properties is being addressed only in current studies [8,9]. For example, Monkman and co-workers have addressed the propensity of carbazole-containing polymers (including poly(vinyl carbabzole)) to form dimers, excimers or exciplexes in the triplet state [9,10]. We have recently addressed the issue of triplet excimer formation in the low molecular weight compounds pCBP and 4,4^′-bis(N-carbazolyl)-2,2^′-dimethylbiphenyl (pCDBP) (figure 1) [8]. The comparatively low-lying triplet T₁ state of pCBP with a 0–0 energy at about 2.65 eV (468 nm) can be increased to a desirable high value of 3.0 eV (413 nm) in pCDBP by introducing torsion between the two central phenyl rings through attaching two sterically demanding CH₃ units [6,11]. However, this is accompanied by a higher tendency of pCDBP to form triplet excimers compared with pCBP. To understand this, we compared the absorption, fluorescence and phosphorescence spectra of pCBP and pCDBP with the spectra of N-phenylcarbazole (NPC) (i.e. just half of pCBP) and with the spectra of a pCBP derivative where the two central phenyls are replaced by a spiro-unit (i.e. a pCBP with two central phenyls locked in a planar position). It turns out that the torsion introduced by the methyl groups in pCDBP localizes the excited triplet state onto the NPC unit. Carbazole, however, due to its extended planar π-system, is rather susceptible to form sandwich-type triplet excimers [12]. By contrast, the triplet excited state in pCBP is localized onto the central moiety, with no electron density on the outer carbazole units. Thus, pCBP simply cannot form a carbazole-based triplet excimer. This insight is further confirmed by quantum chemical calculations and it also applies to related compounds based on the triphenylamine moiety [8].

Figure 1.

Chemical structures of the molecules used.

From this point of departure, we ask how the T₁ energy may be increased in this class of materials compared with pCBP without at the same time increasing the propensity to excimer formation. We show that when attaching the carbazole unit to the central biphenyl not in a para-position as in pCBP but instead in a meta-position to yield 3,3^′-bis(N-carbazolyl)-1,1^′-biphenyl (mCBP; (figure 1), a higher T₁ energy results while confinement of the triplet excited state onto the central biphenyl efficiently prevents excimer formation. By contrast, when torsion is introduced by a methyl group as in 3,3^′-bis(N-carbazolyl)-6,6^′-dimethylbiphenyl (mCDBP), excited state localization onto the outer carbazoles results and excimer formation is recovered.

2. Experimental

pCBP, polystyrene (PS), tetrahydrofuran (THF) and hexane were obtained from Sigma-Aldrich. pCDBP was synthesized as described by Schrögel et al. [11]. The synthesis of mCBP and mCDBP is described in [7]. The structures of all molecules are presented in figure 1.

For measurements in solution, pCBP, mCBP, pCDBP and mCDBP were dissolved in mTHF or hexane at a concentration of 10⁻⁵ M. Films comprising 2 wt% of the materials under investigation in PS were prepared by spin-coating from THF solution. Absorption and fluorescence spectra were recorded using a Varian Cary 5000 spectrophotometer and a Jasco FP-8600 spectrofluorimeter, respectively. Phosphorescence at 77 K was measured using the gated detection facility of the liquid nitrogen cooling unit PMU-830 of the Jasco spectrofluorimeter. The detector unit was opened at a delay of 5 ms after excitation and the signal was acquired for 50 ms. The excitation wavelength for both fluorescence and phosphorescence was 320 nm.

3. Results and discussion

(a). Spectral properties of solutions

The aim of this study is to assess how the electronic structure changes when the position at which the carbazole moieties are attached to the central biphenyl is altered from a para- to a meta-position. Figure 2 displays the absorption, fluorescence and phosphorescence spectra of the compounds under investigation obtained in mTHF solution. We shall first consider the two compounds where the methyl-substitution induces torsion between the two central phenyl rings, pCDBP and mCDBP. The spectra obtained for both, pCDBP and mCDBP, are identical to those of NPC [12,13], except that absorption and fluorescence are shifted slightly to lower energies by 10 meV. The small Stokes shift of 30 meV between absorption and emission and the well-resolved vibrational structure reflect the rigidity of the carbazole moiety. If we now consider the unsubstituted compounds, pCBP and mCBP, we observe the same resemblance to the NPC absorption and fluorescence for mCBP. By contrast, for pCBP, the 0–0 and 0–1 peaks of the absorption coincide with those of the other three compounds, yet the spectral weight of the 0–1 transition is increased and it merges into a shoulder at about 3.9 eV, previously associated with absorption involving the central biphenyl unit [7]. In this context of comparing pCBP and mCBP, it is necessary to be more specific on this point. Biphenyl itself does not absorb in this spectral range [14], yet the absorption of the biphenyl derivative benzidine (diaminobiphenyl) correlates well with this spectral feature [15]. In a similar way, the fluorescence of pCBP differs from that of the other three compounds under investigation. It is shifted to lower energies and has less structure. The phosphorescence spectra of pCBP and mCBP closely resemble each other in their spectral form, yet they differ strongly from those common to pCDBP, mCDBP and NPC. While the latter have the 0–0 peak of the T₁ state at 3.01 eV (412 nm), it is at 2.65 eV (486 nm) for pCBP, though at higher energy, namely at 2.81 eV (441 nm; 0–0 transition) for mCBP.

Figure 2.

Absorption (1), fluorescence (2) and phosphorescence (3) of pCBP, mCBP, pCDBP and mCDBP. Absorption and fluorescence were measured in mTHF (thick line) and hexane (thin line) at room temperature (concentration c=10⁻⁵ M). Phosphorescence spectra were obtained at 77 K in mTHF (thick line) and in PS matrix (thin line) (c=2 wt%). λ_exc= 320 nm.

What insight can be obtained from the comparison of these spectra? The torsion angles between the two central phenyl rings in pCDBP, mCDBP, pCBP and mCBP in the ground state geometry are about 74° for the two CDBPs and 33–38° for the two CBPs [8,11,16]. Evidently, the close similarity of the two CDBP spectra with the NPC spectra results from poor electronic coupling between the two almost orthogonal central phenyl rings, so that each half of the molecule forms an electronically nearly isolated chromophore.

In a similar way, the presence of both carbazole and benzidine features in the absorption spectrum of pCBP suggests the electron density in the ground state to be delocalized over the whole molecule, while the energetic close resemblance of its fluorescence and phosphorescence to fluorescence [15] and phosphorescence [17] of benzidine implies that the electron density is localized only on the central benzidine core in the excited singlet and triplets states. In this case, the luminescence of pCBP should bear some charge transfer nature that makes it sensitive to a polarity of the solvent. This well-known solvent effect on fluorescence emission is associated with the dipolar rearrangement of the solvent shell in the excited state of the molecule [18,19]. As is seen from figure 2, distinct changes in both fluorescence and phosphorescence of pCBP can be observed when changing from the more polar mTHF to the non-polar hexane and to PS, quite in contrast to pCDBP and mCDBP. This clearly confirms that the emission in pCBP involves some degree of charge transfer from the inner benzidine moiety towards the outer carbazole units.

The situation is more complex for mCBP. As the absorption and fluorescence spectra of mCBP fully coincide with those of pCDBP, mCDBP and NPC, one can conclude that, for all four compounds (mCBP, pCDBP, mCDBP and NPC), the electron density in the ground state and singlet excited state S₁ is localized on the same chromophore, namely, on the carbazole unit. This is not the case for the triplet excited state T₁ in mCBP. The shape of the phosphorescence of mCBP is much closer to the spectrum of pCBP than to that of pCDBP or mCDBP, while its energetic position and vibrational structure coincide with the phosphorescence of biphenyl [14]. We therefore consider that in the triplet state of mCBP, the electron density is localized exactly on the biphenyl core of this molecule. Thus, the higher T₁ energy in mCBP compared with pCBP is a result of confining the excited state onto the two central phenyl rings by attaching the carbazole at the meta-positions. By contrast, when attaching the carbazole at the para-position, the conjugation of the two phenyl ring extends into the nitrogen, thus lowering the T₁ energy.

The statement that the lowest excited singlet and triplet states belong to different electronic configurations is supported by their different reaction to the solvent polarity. One can see from figure 2 that upon changing the polarity of the solvent, the fluorescence shows the same minor hypsochromic shift at the onset of the fluorescence as pCDBP and mCDBP while the phosphorescence displays the same larger bathochromic shift as the pCBP phosphorescence.

(b). Spectral properties of neat films

By comparison with the reference compound NPC, we were able to assess the spectral effects of increasing torsion between the two central phenyl rings and para- versus meta-linkage on the phosphorescence, fluorescence and absorption for the carbazole derivatives CBP/CDBP in liquid and solid solutions. The investigation of the luminescence properties of neat films allows us to find out how these structural changes influence the formation of excimers in these typical OLED host materials. Figure 3 presents absorption, fluorescence and phosphorescence spectra of the neat films.

Figure 3.

Absorption (1), fluorescence (2) and phosphorescence (3) of pCBP, mCBP, pCDBP, and mCDBP neat films (thick line). Absorption was measured at room temperature; fluorescence and phosphorescence at 77 K. Thin lines represent the phosphorescence spectra of 2% solid solution in PS. λ_exc= 320 nm.

When comparing the data presented in figures 2 and 3, we see that changing the environment from the non-polar hexane solution to the neat film does not influence the absorption and fluorescence spectra except for a shift of the latter to lower energies on 190 meV for pCBP and on 140 meV for other molecules. This polarization effect is similar to solvatochromism and is well understood. The phosphorescence spectra of pCBP and mCBP evolve similarly. By contrast, the well-structured phosphorescence spectra of the strongly twisted derivatives pCDBP and mCDBP, with the 0–0 peak near 3.0 eV in PS, change strongly when films are made instead of solutions. They lose most of their vibrational structure and shift significantly to the red. For pCDBP, the broad emission centred around 2.5 eV had previously been shown to arise from triplet excimer emission [6]. Comparison with model carbazolophanes has shown that the triplet excimer can be attributed to a sandwich-type arrangement between two carbazole moieties [12]. Obviously, the broad phosphorescence of mCDBP film can be attributed to the same mechanism.

4. Conclusion

On the basis of the spectroscopic data, we have obtained the following information. In the strongly twisted pCDBP and mCDBP, the fluorescence and phosphorescence are dominated by the individual properties of the NPC moiety. There is little coupling between the two halves of the molecule. While these twisted compounds have a high triplet T₁ energy around 3.0 eV associated with NPC, they also share the tendency of carbazole to form excimers in neat films. In contrast to this, the fluorescence and phosphorescence of pCBP are dominated by the properties of the central part of the molecule, that is, the benzidine moiety. The lack of electron density on the outer carbazole units prevents the formation of excimers in a neat film, yet the T₁ energy is as low as that in benzidine, i.e. 2.65 eV (468 nm). The spectroscopic findings on pCBP and pCDBP are further supported by quantum chemical calculations published elsewhere [8]. A way to increase the T₁ energy over that in pCBP while at the same time preventing spin triplet excimer formation is evident in mCBP. In the case of mCBP, fluorescence involves predominantly the carbazole moieties. By contrast, the phosphorescence originates only from the central core. Importantly, the usage of a meta-linkage in place of a para-connection to attach the carbazole largely prevents conjugation between the central biphenyl and the carbazole units. Thus, whereas in pCBP the phosphorescence is localized onto the benzidine unit, it is confined to merely the biphenyl in mCBP. The shorter conjugated system in biphenyl compared with benzidine results in the higher T₁ energy of about 2.8 eV (443 nm) in mCBP. Analogous to pCBP, the localization of the triplet state onto the centre of the molecule and the concominant lack of electron density on the outer carbazoles prevent the formation of triplet excimers in thin films.

Funding statement

A.K. and S.B. acknowledge support by the Federal Ministry of Education and Research (BMBF project OLYMP). A.R. and P.S. are grateful for funding by the German Science Foundation (DFG) through the Research Training Group GRK 1640 ‘Photophysics of Synthetic and Biological Multichromophoric Systems’.

Author contributions

The idea to the synthesis of the compounds and to the study was conceived by P.S. P.S., A.K., S.B. and A.R. participated in the design of the study. P.S. synthesized the compounds. S.B. and A.R. carried out the fluorescence and phosphorescence measurements and analysed the data. The data were interpreted by S.B. and A.K. The manuscript was drafted by S.B. with input from A.R., P.S. and A.K. The work was coordinated and the manuscript was finally revised by A.K. All authors gave final approval for publication.

Conflict of interests

We do not have competing interests.