J. Phys. Chem. B 2010, 114, 11975–11982
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Optical Properties of Light-Emitting Nematic Liquid Crystals: A Joint Experimental and Theoretical Study Alicia Liedtke,† Mary O’Neill,*,† Stephen M. Kelly,‡ Stuart P. Kitney,‡ Bernard Van Averbeke,§ Pol Boudard,§ David Beljonne,§ and Je´roˆme Cornil*,§ Departments of Physics and Chemistry, UniVersity of Hull, Cottingham Road, Hull, HU6 7RX, U.K., and Laboratory for Chemistry of NoVel Materials, UniVersity of Mons, Place du Parc 20, B-7000 Mons, Belgium ReceiVed: May 11, 2010; ReVised Manuscript ReceiVed: June 22, 2010
Semiempirical quantum-chemical calculations are used to simulate the optical properties of a series of green light-emitting nematic liquid crystals containing fluorene, thiophene, or thienothiophene groups with solidstate photoluminescence quantum efficiencies up to 0.36. We use a simple model of two parallel and closely spaced molecules in an anticofacial configuration to study intermolecular interactions in the solid state and slide one past the other to mimic the high orientational and low positional order of the nematic phase. We find that switching between H and J aggregates can be triggered by longitudinal displacements of the molecules with respect to one another by an extent that closely follows the chemical structure of the interacting chromophores. We discuss the implications of aggregate formation for efficient light emission in conjugated oligomers and polymers that show nematic or smectic order. 1. Introduction Nematic liquid crystals have orientational but no positional order and are ubiquitously used in liquid crystal televisions and other display devices. More recently, light-emitting nematic liquid crystals, including polymers and small molecules, have emerged as a new class of organic semiconductors with applications in light-emitting diodes (OLEDs), photovoltaics, and field-effect transistors.1-8 Red, blue, and green light-emitting nematic materials are available, whereas white electroluminescence has been obtained via energy transfer processes.9,10 Photoreactive nematic semiconductors can be cross-linked to form polymer networks, giving the added advantage that photolithography can be used to pixellate a full color display.1 The self-assembly properties of semiconducting liquid crystals yield many beneficial properties, such as high-charge carrier mobility and polarized emission. Well-oriented nematic thin films normally require annealing to the nematic phase and rapid quenching to form a nematic glass. Extremely high orientational order parameters (>0.9) have been found for some nematic semiconductors.11,12 The high orientational order of nematic polymers or extended oligomers in the nematic glassy state results in anisotropic charge carrier transport: the rate of intrachain transport of carriers is very much greater than the hopping rate between chains so that the mobility is substantially higher when the chains are parallel to the carrier transport direction.5,8 Polarized electroluminescence has been obtained from a wide range of liquid crystalline materials, including mainchain polymers, oligomers, reactive mesogens and small molecules, most of which are nematic, but some, smectic.13 It requires the uniform planar alignment of the light-emitting liquid crystal, normally obtained using an underlying alignment layer that can transport holes injected from the anode. * Corresponding authors. E-mails: (M.O.)
[email protected], (J.C.)
[email protected]. † Department of Physics, University of Hull. ‡ Department of Chemistry, University of Hull. § University of Mons.
Polarized OLEDs with efficiencies up to 6.4 cdA-1 have been demonstrated.9 New 3-dimensional display configurations are possible with the polarization direction of the emitted light spatially patterned by means of photoalignment techniques.14-16 There has been very little work focusing on the impact of orientational order on the emission properties of light-emitting nematics. The transition dipole moment of a nematic liquid crystalline emitter is normally oriented along the molecular axis, which tends to lie in the plane of the thin film.17 Hence, the outcoupling efficiency of luminescence is higher than from thin films of similar refractive index but having isotropically oriented emitters.18 A transfer matrix approach has been used to predict that nematic order increases selfabsorption in thin films.19 In this context, we investigated the optical properties of nematics in solution and the solid state. The results are supplemented by corresponding quantum-chemical calculations to rationalize the evolution of the optical properties going from isolated molecules to the nematic phase. The molecules under study have extended aromatic cores with irregular alternating fluorene-thiophene or thieno-thiophene groups. They show nematic glassy phases at room temperature. They are green light-emitters with photoluminescence quantum efficiencies up to 0.36 in the solid state and have been processed in OLEDs with efficiencies up to 11 cdA-1.12 Here, we compare the calculated absorption and emission energies for isolated molecules to experimental spectra obtained in solution. In a next step, we consider simple dimers made of two parallel and closely spaced molecules in an anticofacial configuration. These calculations aim to study the impact on the optical properties of intermolecular interactions and, more specifically, the influence of sliding one chain past the other to mimic a nematic order. The theoretical results are analyzed in light of the experimental photoluminescence data. We discuss molecular design strategies that are expected to promote the formation of Jaggregates and highly luminescent nematic materials.
10.1021/jp104280w 2010 American Chemical Society Published on Web 08/31/2010
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TABLE 1: Compounds under Study and Their Liquid Crystal Transition Temperaturesa
a
Tg ) glass transition temperature; Cr ) crystalline; N ) nematic; I ) isotropic.
2. Methods 2.1. Experiment. Table 1 shows the chemical structure of the nematic liquid crystals used in this work and their liquid crystal transition temperatures. Compounds 1 and 3 are reactive mesogens, and compounds 2, 4, and 5 have nonpolymerizable (octyloxy) aliphatic terminal groups. The synthesis of compounds 1, 3, and 5 has been discussed previously,20,21 and that of 2 and 4 will be reported elsewhere. Absorption and photoluminescence (PL) spectra were recorded for the compounds in toluene solution and in thin films. The thin film samples were prepared by spin-casting on a quartz substrate typically from solutions of concentration 1 mg/0.1 mL. Materials 1, 2, 4, and 5 formed a glassy nematic state in thin films on evaporation of the solvent by annealing at 95 (1), 140 (2), 125 (4), and 100 °C (5). Thin films of 3 were annealed at 65 °C and retained the glassy state at room temperature for long periods, although the glass transition temperature is below room temperature. A Lambda40 (PerkinElmer) spectrophotometer was used for absorption measurements. PL was induced using a GaN laser at 405 nm and detected using a fiber coupled to an Ocean Optics spectrometer. The PL quantum efficiencies (PLQE) were measured using an integrating sphere.22 2.2. Theory. The semiempirical Hartree-Fock Austin model 1 (AM1) method was used to optimize the ground-state geometry of the various molecules in the gas phase.23 In the simulations, the fluorene side chains were replaced by (CH3)2, and for the isolated molecules, the terminal aliphatic chains were replaced by R ) OCH3 since the saturated parts of the molecules have a weak (direct) influence on the optical properties. The lowest-energy conformation was used to calculate the vertical electronic transitions from the ground state (GS) to the lowest excited states (ES) using the spectroscopic version of the semiempirical HartreesFock intermediate neglect of differential overlap method developed by Zerner and co-workers (ZINDO).24,25 ZINDO has been coupled to a single configuration interaction (SCI) scheme including all π-π* single excitations to account for electronic correlation effects in the excited states.26,27 The geometry of the lowest-energy excited state was optimized by
coupling the AM1 method to a SCI scheme scaling with the size of the system and ensuring the convergence of the results. The vertical emission energies were then simulated using the same approach from the excited-state geometry. The in-house ZOA software26 was used to visualize the outputs of the ZINDO calculations, such as the molecular orbital shapes, transition dipole densities, and the transition energies and oscillator strengths for the lowest electronic excitations. The influence of intermolecular interactions on the emission properties was analyzed by considering a model dimer with all torsion angles set to zero to assist the packing and mimic the solid-state behavior. The molecules were positioned in an anticofacial configuration to avoid interaction between neighboring fluorene units. We have also assessed the influence of the relative position of the two chains by displacing longitudinally one chain with respect to the other to mimic the relative positioning of the chains in the nematic phase. The impact of intermolecular interactions has been quantified by calculating the excitonic couplings, V12, between the lowest excited state of chains 1 and 2 in the framework of an excitonic model;27 the latter have been estimated here with an approach going beyond the traditional Fo¨rster theory by expanding the transition dipole moments into transition densities located at atomic sites:28 N
i µGSfES )
∑ qmGSfESrmi
(1)
qGSfES (m) qGSfES (n) 1 2 rmn
(2)
m
V12
1 ) 4πε0
∑∑ m
n
The atomic transition densities represent the overlap between the GS and ES wave functions and provide a local map for
Optics of Light-Emitting Nematic Liquid Crystals
Figure 1. Illustration of the splitting of the lowest excited state of an isolated molecule in H- and J-aggregates. The combination of the transition dipoles makes the emission symmetry-allowed in the Jaggregate and forbidden in the H-aggregate.
the transition dipole, µGSfES. The sum in eq 1 runs over all atom-centered transition densities qmGSfESassociated with the excitation GS f ES and the spatial coordinates rmi (with i ) x, y, z). In eq 2, q1(m) and q2(n) refer to the atomic transition density on site m of molecule 1 and site n of molecule 2, respectively, and rmn to the separation between the two sites. A positive value of the excitonic coupling implies the formation of an H aggregate; in this case, the splitting of the lowest excited state leads in the dimer to (i) an optically forbidden lower-lying excited state resulting from the antisymmetric combination of the two transition dipoles and (ii) an optically allowed higher-lying excited state built from a symmetric combination of the dipoles. H aggregates are therefore only weakly emissive with radiative decay partly allowed as a result of localization of the excitation over one unit due to lattice relaxations in the excited state or energetic/ positional disorder.29 In contrast, a negative value of the exciton coupling corresponds to the formation of J-aggregates with an optically allowed low-energy state resulting from the symmetric combination of the dipoles and an optically forbidden high-energy state as the corresponding antisymmetric combination (see Figure 1). The formation of Jaggregates is highly desirable to optimize the emission properties of liquid crystals in the nematic phase and might be triggered by shifting longitudinally the chromophores, as described below. 3. Results 3.1. Isolated Chains. Figure 2 shows the experimental (in toluene at room temperature) absorption and PL spectra of all compounds, together with the calculated vertical transitions from the GS (absorption) and ES (emission) equilibrium geometries. The molecules adopt a twisted conformation in the GS, which explains the featureless absorption spectra; the experimental bands are, indeed, broadened by conformational disorder, primarily associated with a distribution of torsion angles between the rings along the chains (since the low-frequency torsional modes are easily populated by thermal excitations at room temperature).30 This disorder is reduced in the stiffer excited-state potential, where the molecules tend to adopt a more planar conformation (see the calculated torsion angles in the ground and lowest excited state of compound 3 in Table 2); similar trends also hold true for the other chains. The reduction of torsional disorder in the excited state is induced by a sharper torsion potential following the appear-
J. Phys. Chem. B, Vol. 114, No. 37, 2010 11977 ance of a relaxed semiquinoid geometry along the chain and rationalizes the appearance of a well-resolved vibronic structure in the emission spectra.31 The evolution of the optical properties among the different chains is nicely reproduced by the calculations, although the simulated wavelengths are systematically displaced to longer wavelengths by 0.05-0.19 eV for absorption and 0.02-0.13 eV for emission as compared with the experimental values. For the sake of illustration, Figure 3 plots the calculated atomic transition densities for the electronic transition involved in the lowest absorption and emission spectra of 3. The atomic transition densities are delocalized over the backbone, with a larger weight on the thiophene units. Note also that the weight on the outermost benzene rings is weaker, in line with the fact the torsion angles between the outer benzene and thiophene units (benzene-thiophene) change the least between GS and ES. Figure 3 also demonstrates that the distribution of the atomic transition densities is weakly affected by the geometry relaxation in the lowest excited state. Table 3 collects the theoretical and experimental results for the transition energies of the absorption and emission peaks for compounds 1-5. The emission peak corresponds here to the 0-1 vibronic transition for all compounds, which is expected to lie closer to the vertical transition energies provided by the calculations. The transition energies decrease with the number of thiophene rings, and the compounds incorporating bithiophene groups show lower transition energies than the corresponding thienothiophene-based compound (3 versus 2 or 5 versus 4). The transition dipole moment and, hence, the oscillator strength of the transitions increases with the number of rings along the chains. The atomic transition densities in the longest chains are delocalized over the entire molecules, with yet larger weights on the central six rings (see Figure 4). There is no correlation between the calculated oscillator strength of the emission and the measured photoluminescence quantum efficiency (PLQE). This is expected because the PLQE is a function of both the radiative and nonradiative decay rates. The former varies with the oscillator strength; the nonradiative processes depend on a number of factors, including the nature of the aliphatic terminal groups (that are not the same for all compounds). The Stokes shift was estimated experimentally as the energy difference between the 0-1 vibronic transitions in the absorption and emission spectra to allow for a better comparison with the theoretical values obtained as the energy difference between the vertical transitions in absorption and emission. There is a good quantitative agreement between the two sets of values, although the experimental values are typically 10-15% larger than the calculated results. This small discrepancy can be explained by the uncertainty linked to not accounting for the vibronic structures in the simulations or to the values of the torsion angles, which could vary going from the gas phase to solution. The calculated and experimental Stokes shifts are found to be larger for bithiophenecontaining molecules than for those incorporating thienothiophene groups. There is a slight decrease in the Stokes shift with molecular size, since the geometric changes over the molecular backbone have a smaller amplitude in larger chains.30 3.2. Interacting Chains. We investigated the way intermolecular interactions in thin films modify the optical properties of the investigated compounds. Representative spectra are shown in Figure 5 for compound 5 when going from solution to the solid state. The prominent feature is a red shift of both the absorption and emission bands in thin
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Figure 2. Experimental absorption (a) and emission (b) spectra of the compounds; the corresponding calculated transition energies are represented by vertical lines.
TABLE 2: AM1-Calculated Torsional Angles between the Aromatic Rings of Compound 3
3
state
benzenethiophene
thiophenethiophene
thiophenefluorene
fluorenethiophene
thiophenethiophene
thiophenebenzene
GS ES
27.53° 14.31°
25.63° 0.80°
26.40° 0.07°
26.11° 0.07°
25.40° 0.43°
27.37° 11.03°
films due to the planarization of the chains induced by solidstate packing.30 The red shift of 0.12 eV for absorption is fully consistent with the calculated value of 0.15 eV going from a twisted to planar geometry. To further shed light on the emission properties of interacting chains, we considered model dimers made of molecules 4 and 5 with an interchain distance fixed at 4.5 Å according to recent X-ray diffraction experiments.17 Two distinct intermolecular separations of 0.45 and 1.5 nm are actually identified by X-ray analysis for 1, 3, and 5, indicating that the molecules organize into lamellae.17,32 The larger spacing of 1.5 nm results from microphase separation of the aromatic core of the molecule and
their alkyl side chains. We analyzed the influence of the relative orientation of the two molecules by translating longitudinally one chain with respect to the other by up to 60 Å (i.e., the length of the molecule) by steps of 0.1 Å, starting from an anticofacial geometry to avoid steric interactions between the fluorene units. The imposed parallelism between the chains is a reasonable assumption, given the extremely high-order parameters, >0.9, obtained for the materials;12 on the other hand, the anticofacial orientation is not guaranteed in the films, since there is no evidence for biaxial ordering in our materials.17 Systems with different orientations or larger numbers of contributing molecules would lead to inhomogeneous broadening of the spectral
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Figure 3. Atomic transition densities associated with the lowest absorption band (a) and emission (b) for compound 3. The color and size of the spheres represent the sign and amplitude of the atomic transition densities.
TABLE 3: Calculated and Experimental Transition Energies for the Absorption (abs) and Emission (em) of the Isolated Compounds 1-5a theory compound 1 2 3 4 5 a
Abs Em Abs Em Abs Em Abs Em Abs Em
experiment
energy (eV)
oscillator strength
Stokes shift (eV)
3.10 2.65 2.80 2.42 2.77 2.34 2.70 2.33 2.64 2.23
2.46 2.63 2.78 3.00 3.07 3.30 4.02 4.01 4.01 3.90
0.447 0.383 0.431 0.371 0.408
energy (eV) 3.15 2.67 3.03 2.58 2.92 2.44 2.87 2.45 2.82 2.36
PLQE (soln) (%)
Stokes shift (eV)
78b
0.475 0.455
33
0.481
67
0.424
48
0.467
PLQE ) photoluminescence quantum yield. b Measured on a compound with the same core as 1 but with polymerizable terminal chains.
Figure 4. Atomic transition densities associated with the lowest absorption band of compound 5. The color and size of the spheres represent the sign and amplitude of the atomic transition densities.
Figure 5. Absorption and PL spectra of a solution and thin film of 5.
peaks without affecting the global picture. For the sake of comparison, similar results were generated for an oligothiophene of the same size (with 15 units) and for two molecule chains in which the fluorene units have been replaced by alkyl chains of the same length (CH2)7 and by twisted biphenyl units, respectively. Figure 6a shows the evolution of the exciton coupling as a function of the degree of translation for molecules 4 and 5, the 15-ring oligothiophene, and derivatives of the chains,
including biphenyl moieties. In the case of the oligothiophene, the atomic transition densities are delocalized over all the rings, with a dominant weight on the central units. The excitonic coupling smoothly decreases with the degree of translation and changes sign around 25 Å, which corresponds to half the molecular length (28 Å); this shift is thus the threshold value to promote the formation of J-aggregates and, hence, improved luminescence properties. For longer shifts, the exciton coupling passes by a maximum and finally decays down to zero when there is no more spatial overlap between the molecules. Similar trends are observed for molecules 4 and 5, except that a fine structure appears at intermediate longitudinal shifts. The fine structure is enhanced when introducing twisted biphenyl units (with the appearance of a clear minimum around 10 Å), thus suggesting that the degree of conjugation in the bridge connecting the bithiophene units plays a key role. This hypothesis is further reinforced by Figure 6b, showing the evolution of the exciton coupling for molecules 4 and 5 and for derivatives including alkyl chains. In the latter case, the exciton coupling switches sign back and forth from positive to negative as one molecule is translated with
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Figure 6. Evolution of the exciton coupling as a function of the degree of translation (a) for dimers of molecules 4 and 5 as well as for the oligothiophene and the chains incorporating biphenyl units and (b) for dimers of molecules 4 and 5 and for the derivatives including alky chains.
respect to the other. In particular, the Coulomb interaction is negative for shifts as small as 3-4 Å, suggesting J-like aggregates; it then converges toward zero for longitudinal displacements in excess of the molecular size. The atomic transition densities reported for the different compounds in Figure 7 reveal that the conjugation breaking induced by the spacer units (going from fluorene to twisted byphenyl and alkyl chains) progressively splits the total transition dipole moment into aligned local dipoles centered over the bithiophene units, as schematized in Figure 7. Hence, in the fully unconjugated case, small shifts bring the local dipoles in a J-like configuration with respect to one another so that the overall coupling is negative. Note that the differences in the excitonic coupling of molecules 4 and 5 incorporating alkyl chains for zero translation originate from
the fact that there are significant atomic transition densities on the terminal thiophene-phenyl groups for 4. 4. Conclusions Many conjugated polymers or oligomers have nematic or other liquid crystalline phases4,33-35 where H aggregation can affect their PLQE. For example, the fluorene copolymer poly(9,9-dioctylfluorene-alt-thieno[3,2-b]-thiophene) forms a nematic liquid crystal glass by rapid quenching from its hightemperature nematic phase. Its solid-state PLQE is 0.12, less than half the value in solution. Smectic liquid crystals have positional order, and the resulting formation of H aggregates may explain in general their relatively low efficiency in OLEDs.13 The molecules studied here are nematic with high
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Figure 7. Atomic transition densities associated with the lowest absorption band for (a) the oligothiophene, (b) molecule 5, and its derivatives, including (c) biphenyl groups and (d) alkyl chains. The color and size of the spheres represent the sign and amplitude of the atomic transition densities. We also introduce a schematic view of the total transition dipole moment in (a) fully delocalized structures; (b, c) structures where the atomic transition densities are localized on weakly connected fragments (such as molecules 4 and 5 incorporating biphenyl units); and (d) structures with isolated local transition dipole moments for H-aggregates (left) and J-aggregates (right), such as molecules 4 and 5 with alkyl chains.
orientational order and no positional order, and we find that H- or J-aggregates are formed depending on the relative longitudinal displacement of the molecules of the dimer. The PLQE of thin films of 5 and 4 (36% and 31%, respectively), though appreciable, is smaller than the values reported in solution (see Table 3). A strategy to improve PLQE is to increase the intermolecular separation; for example, by using bulky side chains, although this might prove detrimental for charge transport properties. Such a strategy may explain the high PLQE of 70% for a thin film of a nematic oligofluorene with branched side chains.36 Alternatively, the present calculations suggest that nematic materials should be designed with local transition dipole moments along the backbone so that the transition from H- to J-aggregate occurs at small displacements. One possible strategy to achieve this goal would be to design donor-acceptor molecules in which the electronic structure of the donor and acceptor moieties and their relative lengths would be tuned so that (i) the electronic excitations would spread mostly over the donor domains and (ii) the chains would pack in a dense lamellae architecture with the energy donors and acceptors facing each other, thereby providing the required longitudinal shift for the formation of strongly emissive J-like aggregates.
Acknowledgment. The work in Mons is supported in part by the Interuniversity Attraction Pole IAP 6/27 of the Belgian Federal Government and the Belgian National Fund for Scientific Research (FNRS/FRFC). J.C. and D.B. are FNRS Research Fellows. References and Notes (1) Aldred, M. P.; Contoret, A. E. A.; Farrar, S. R.; Kelly, S. M.; Mathieson, D.; O’Neill, M.; Tsoi, W. C.; Vlachos, P. AdV. Mater. 2005, 17, 1368–1372. (2) Carrasco-Orozco, M.; Tsoi, W. C.; O’Neill, M.; Aldred, M. P.; Vlachos, P.; Kelly, S. M. AdV. Mater. 2006, 18, 1754–1758. (3) O’Neill, M.; Kelly, S. M. AdV. Mater. 2003, 15, 1135–1146. (4) Geng, Y.; Chen, A. C. A.; Ou, J. J.; Chen, S. H.; Klubek, K.; Vaeth, K. M.; Tang, C. W. Chem. Mater. 2003, 15, 4352–4360. (5) Yasuda, T.; Fujita, K.; Tsutsui, T.; Geng, Y.; Culligan, S. W.; Chen, S. H. Chem. Mater. 2005, 17, 264–268. (6) Neher, D. Macromol. Rapid Commun. 2001, 22, 1365–1385. (7) Levermore, P. A.; Jin, R.; Wang, X.; De Mello, J. C.; Bradley, D. D. C. AdV. Funct. Mater. 2009, 19, 950–957. (8) Sirringhaus, H.; Wilson, R. J.; Friend, R. H.; Inbasekaran, M.; Wu, W.; Woo, E. P.; Grell, M.; Bradley, D. D. C. Appl. Phys. Lett. 2000, 77, 406–408. (9) Chen, A. C. A.; Culligan, S. W.; Geng, Y.; Chen, S. H.; Klubek, K. P.; Vaeth, K. M.; Tang, C. W. AdV. Mater. 2004, 16, 783–788. (10) Liedtke, A.; O’Neill, M.; Wertmoller, A.; Kitney, S. P.; Kelly, S. M. Chem. Mater. 2008, 20, 3579–3586.
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