Predictable Tuning of Absorption Properties in Modular Aromatic

Article pubs.acs.org/cm

Predictable Tuning of Absorption Properties in Modular Aromatic Donor−Acceptor Liquid Crystals Katherine R. Leight, Brooke E. Esarey, Alexandra E. Murray, and Joseph J. Reczek* Department of Chemistry and Biochemistry, Denison University, 500 West Loop, Granville, Ohio 43023, United States S Supporting Information *

ABSTRACT: This paper demonstrates a combinatorial design strategy in the generation of columnar liquid crystalline materials with tailored properties based on the molar (1:1) combination of complementary electron-rich and electron-poor aromatic components. Through the iterative study of relationships of individual component structure to combined material properties, a series of aromatic donor−acceptor columnar liquid crystal materials was developed whose charge-transfer absorption completely spans the visible spectrum. The red-onset of absorption in these materials is shown to correlate closely with straightforward orbital energy level calculations (density functional theory) of individual component molecules. This holds equally true regardless of the component or range of absorption characteristics exhibited by the molecules of this study. Charge-transfer band extinction coefficients are substantial in these materials, ranging from 3800−15500 M−1; the magnitude of which is shown to correlate to component identity. This ability to predictably design a range of functional material properties through preceding calculations, and achieving a tailored diversity of properties through combination of relatively simple component molecules, provides an impressive array of new materials and exemplifies this as a powerful strategy for efficient, targeted material design. KEYWORDS: columnar liquid crystals, donor−acceptor, charge-transfer, modular materials, tunable absorption

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INTRODUCTION Designing the self-assembly of relatively simple component molecules into higher-order systems to achieve desired properties is an attractive strategy for the development of new functional materials. Columnar liquid crystals (CLCs) are a class of materials formed from the self-assembly of disk-like aromatic molecules into π-stacked columns having thermotropic mesophase behavior. These systems have shown exciting potential in a variety of applications as molecular semiconducting materials,1 including organic light-emitting diodes, field-effect transistors, and in organic photovoltaics.1,2 Aromatic donor− acceptor columnar liquid crystals (DACLCs) are a subclass of CLCs fashioned from two dif ferent and complementary aromatic components. These materials, inspired in part by early work from Ringsdorf, Praefcke, and co-workers,3 consist of an electron-rich “donor” and an electron-poor “acceptor”, which self-assemble into alternating, face-centered columnar structures (Figure 1a).4 The electrostatic matching of mixed components is thought to provide a driving force for columnar assembly, stabilizing and sometimes inducing mesophase behavior.3,4 This often leads to enhanced material properties of DACLCs when compared to CLCs of the independent components, including broader mesophase temperature ranges, macrostucture stability, and charge mobility.3−5 In addition, the combinatorial nature of DACLC materials allows for independent tuning of several material properties, such as clearing and crystallization transition temperatures, simply through the mixing of relatively simple components.4b,d,6 This approach to materials design provides an © 2012 American Chemical Society

Figure 1. (a) Representation of donor−acceptor columnar liquid crystal assembly (DACLC). (b) Schematic of molecular and CT absorbance in DACLC materials.

opportunity for the facile study and optimization of many structure−property relationships in CLC materials, and overall offers a promising alternative to the synthesis-intensive requirements needed for tuning the properties of single-component organic molecules and materials.7 Of interest toward applications in organic optoelectronics and photovoltaics is the broad absorbance band usually exhibited in DACLC materials, unique to the mixture of components.4 This charge-transfer (CT) absorption frequently leads to highly colored materials, and is attributed to direct excitation of an electron from the highest occupied molecular orbital (HOMO) Received: March 11, 2012 Revised: July 25, 2012 Published: July 30, 2012 3318

dx.doi.org/10.1021/cm3007765 | Chem. Mater. 2012, 24, 3318−3328

Chemistry of Materials

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of the electron-rich to the lowest unoccupied molecular orbital (LUMO) of the electron-poor component (Figure 1b). Recently, aromatic donor−acceptor interactions have been extensively used to affect molecular assembly in solution, both intermolecular and intramolecular, in a variety of self-assembled structures including foldamers,8 catenanes,9 or rotaxanes,10 and have also played a role in molecular recognition11 and selective sensing applications,12 where the CT band offers an inherent signal of component or analyte interaction. These systems mostly exhibit interaromatic CT bands with relatively low extinction coefficients when compared to molecular absorptions, and the CT absorption ranges are challenging to accurately predict for different systems.9−13 This is largely due to dynamic equilibrium in solution and well-known effects of solvent on aromatic−aromatic association causing orbital perturbations that significantly influences CT transitions.14 However, upon exclusion of solvent from the noncovalent interaction, either through induced aggregation or bulk preparation as is the case in DACLCs, aromatic donor−acceptor materials exhibit a significantly high intensity of CT absorption, 10−100 times that of systems in solution.4,15 This has yet to be systematically investigated, and the ability to predictably tune the CT absorption and other optical properties of DACLC materials is of great importance toward significantly enhancing the understanding and optimization of these systems toward realizing their potential in organic electronic applications. Herein we report a systematic investigation of several new DACLC materials, assembled from the modular combination of series of aromatic components. Relationships of component identity and structure to bulk material properties are methodically investigated. Results reveal an intriguing variety of absorption characteristics, and offer the ability to rationally tune the CT absorption in DACLC columnar mesophases through exchange of relatively simple component molecules. All materials studied exhibit a highly colored columnar mesophase, and the available range of absorptivity completely spans the visible region of the electromagnetic spectrum. Additionally, density functional theory (DFT) calculations were performed on all component molecules to determine the shape, phasing, and relative energy of HOMO and LUMO molecular orbitals. These calculations were analyzed in the context of measured absorption data, revealing an excellent correlation between the measured red-onset of CT absorption in the mesophase with the calculated energy levels of independent HOMO and LUMO component orbitals. This correspondence between measured and calculated values holds equally well over the broad variety of absorption ranges achieved. In total, this study greatly expands the scope of known DACLC materials, and illustrates the ability to tune and predictably design important optical properties through the selfassembly of simple component molecules in this exciting class of materials.

Figure 2. (a) Series of electron-rich donor aromatics used in this study. (b) Electron-poor acceptors used in this study.

alkylation of the hexahydroxy terphenylene precursor.4d This compound was chosen as an electron-rich component with significantly different geometry compared to the naphthalene series (Figure 2, 13). Two aromatic imides were used as electronpoor components in this study, naphthalene diimide (NDI, A1) and melletic triimide (MTI, A2), synthesized according to literature procedures by condensation of 1,4,5,6-naphthalenetetracarboxylic dianhydride and 1,2,3,4,5,6-hexacarboxylic benzene with octyl amine, respectively. These acceptors were chosen on the basis of having completely different geometries with similar aromatic acceptor characteristics.4b,d Standard alkyl chains of length C6 were used uniformly for all electron-rich naphthalene series, and length C8 were used in the synthesis of HAT, NDI, and MTI for convenient mixing and solubility purposes, alternative side-chains of varying length and branching are currently being investigated for their effect on phase transition temperature and structure. Differential Scanning Calorimetry of Individual Components. The temperature and enthalpy of phase transitions upon cooling were measured using differential scanning calorimetry (DSC) for each individual compound 1−13, A1, and A2 (Table 1). This data provides a baseline for analyzing relationships between component and mixture phase-transition behavior. No mesophase behavior was observed for any of the pure compounds with the exception of 13; all single components gave a single phase transition from liquid to solid upon cooling with the exception of HAT8 (13), which is known to have a Colhex phase.16 In general, the measured crystallization temperature increased among derivatives with the same substituent positions as alkoxy functionality was substituted by amine. Comparing the 1,5-bis-substituted series (1−3), bisalkoxy 1 exhibits a crystallization temperature 11 °C lower than that of alkoxy-amine 2, and 2 in turn has a crystallization temperature 20 °C below that of bisamine 3. Bisthiane 4 showed the lowest crystallization temperature of the 1,5-series. Following this trend,

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RESULTS AND DISCUSSION Component Design and Synthesis. Several series of bissubstituted naphthalene derivatives were synthesized as electronrich components for this study. Commercially available bishydroxy, hydroxy-amino, bisamino, or bisthio starting materials, representing a variety of substitution positions, were alkylated with 1-bromohexane (Figure 2, 1−12). Reactions of compounds bearing arylamine functionality were carefully monitored for formation of the desired monobis-alkylated amines. The electron-rich hexylalkoxy triphenylene (HAT8) was synthesized according to literature procedures by complete 3319



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covering the complete visible spectrum from light yellow to black. DSC and Phase Changes of Donor−Acceptor Mixtures. The phase behavior of each donor−acceptor mixture was characterized on cooling via DSC to identify the presence and temperature range of mesophases and the enthalpy of phase transitions. All materials exhibited mesophase behavior distinct from the phase behavior of their components; none of the mixtures displayed characteristics of two separate compounds undergoing individual crystallization events.17 Representative DSC thermograms of 2, A1, and their 1:1 molar mixture 2:A1 are shown in Figure 4. The single, sharp, and intense transitions seen

Table 1. Phase Transition Temperatures and Enthalpy of Phase Transitions upon Cooling for the Independent Components in This Studya compound

T/ΔH(°C) (J/g)

compound

T/ΔH(°C) (J/g)

1 2 3 4 5 6 7 8

80/65.56 91/134.3 111/144.0 46/128.8 14/95.6 28/50.7 31/75.7 67/70.7

9 10 11 12 13c

29/165.3 −31b 50/122.8 21/102.6 54/0.6→33/54.4

A1 A2

180/40.3 88/35.2

a

The reported measurements were taken upon cooling at a rate of 5 °C/min. Each run was done in triplicate. bThe crystallization temperature of 10 approaches the lower limit of the DSC chiller range; a reproducible enthalpy of phase change could not be determined. cHAT8 (13) undergoes a transition to the Colhex phase upon cooling.

bisalkoxy naphthalenes 5 and 7 gave crystallization temperatures 15 and 19 °C lower than the corresponding alkoxy-amines 9 and 11, respectively. This is likely the result of increased intermolecular hydrogen bonding as the hydrogen bond donor/acceptor arylamines are exchanged for the acceptor only alkoxy groups. Of notable exception to this trend is comparison of the 2,3-substituted naphthalenes 6 and 10. Bisalkoxy 6 displayed a dramatically higher crystallization point than alkoxyamine 10, which exhibited a melting point significantly lower than any other compound in this study. It is hypothesized that this is due to the ability of 10 to form very stable internal hydrogen bonds, significantly decreasing intermolecular interactions and lowering the crystallization temperature. Mixture Formation and Bulk Color. The DACLC materials were prepared by combining a 1:1 molar ratio of the desired electron-rich and electron-poor components as pure compounds in a small vial. The vial was then heated while continually mixing, and the two components were allowed to completely melt into an isotropic mixture. A dramatic color change was observed upon initial melting of every mixture, corresponding to the new CT absorbance band resulting from face-to-face interaction of the donor and acceptor aromatics. The isotropic melts were allowed to cool, and stored in the dark at room temperature. Acceptors A1 and A2 were independently combined with every electron-rich naphthalene in this study, 1− 13, in modular fashion. Mixtures 13:A1 and 13:A2 were prepared and studied to investigate a significantly different donor geometry. The wide variety in absorption properties between specific DACLCs is immediately apparent upon observation of the bulk color change for the series of materials (Figure 3). While all individual components are off-white to light-tan independently, the assembled mixtures ranged dramatically in color,

Figure 4. Representative DSC thermograms for donor and acceptor components individually and their mixture (2:A1) as a DACLC material.

for the independent 2 and A1 at 91 and 180 °C, respectively, are typical of liquid to solid single transitions. The 2:A1 mixture first undergoes a broad, low enthalpy liquid to mesophase transition at 159 °C, and a second crystallization event at 75 °C, typical of columnar mesophase behavior in DACLC materials. Data obtained for the phase transition temperatures and enthalpies is reported for all mixtures in this study in Table 2. All mixtures with acceptor A1 exhibited either 2, or 3 phase transitions, mixtures with acceptor A2 showed either 1, 2, or 3 transitions with the exception of mixtures 6:A2, 10:A2, and 12:A2 for which no phase transition was observed upon slow cooling down to −40 °C. Relationships in component-structure to material phase behavior were investigated. Considering first the naphthalene mixtures with acceptor A1, two distinguishable transitions were observed for mixtures of 1, 2, 5, 6, 7, and 12 with A1, while three transitions were observed with mixtures of 3, 4, 8, 9, 10, and 11 with A1, indicating an additional mesophase-mesophase transition. Of the twelve donor:A1 naphthalene-based mixtures, most exhibited typical liquid crystalline behavior with a lowenthalpy liquid-mesophase transition, and a significantly higher enthalpy crystallization event. Three exceptions to this were mixtures 7:A1, for which the two transitions are similar in enthalpy change, and 3:A1 and 10:A1, in which the first transition on cooling released more energy than the final crystallization event. No identifiable trends in the phase behavior

Figure 3. Representation of the range in bulk color achieved from simple mixing of components in this study, spanning the complete visible spectrum. Samples were melted and slowly cooled on TLC slides to enhance the color contrast between materials. 3320



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temperature clearing points were seen with mixtures 5:A2, 9:A2, and 12:A2 with no additional transitions. Previous studies concerning donor−acceptor mixtures of 1,5bisalkoxynaphthalenes with NDIs, in which only the alkyl substituent length was systematically varied, demonstrate a correlation between the clearing and the crystallization temperatures of these DACLC materials to the crystallization points of the acceptor and donor compounds, respectively. In general, the clearing point on cooling of mixtures in this study containing acceptor A1 are moderately below the crystallization temperature of A1 (180 °C), although they range from 123 to 167 °C. In a related trend, all of the DACLC mixtures consisting of a naphthalene donor and acceptor A2 have clearing points below the crystallization temperature of A2 (88 °C). The difference in crystallization temperature between the two acceptor molecules is 92 °C, and all materials made from the same donor component with acceptor A1 have a significantly lower clearing points, supporting a correlation between acceptor crystallization temperature and DACLC clearing point in mixtures with naphthalene-core donors. However, the same conclusion is not supported in the two mixtures with C3-symmetric donor 13, which as discussed above has a relatively low DACLC clearing point when combined with A1 (123 °C), and a higher clearing point than any other mixture, above the crystallization point of the acceptor component, when combined with A2 (138 °C). There is little consistent correlation between phase transition temperatures of the DACLC mixtures and the crystallization temperature of their donor components. While in some cases the final measured transition of a DACLC mixture on cooling matches reasonably with the crystallization temperature of the corresponding donor component (1:A1, 5:A1, and 8:A1), this is not observed for the majority of donor−acceptor materials. In particular, naphthalene donor 3, which has the highest overall crystallization temperature at 111 °C, yields a DACLC mixture with A1 exhibiting a significantly lower crystallization point at 22 °C. Other mixtures have transition temperatures significantly higher than their independent components (10:A1 and 12:A1). Mixtures with acceptor A2 exhibited no discernible trends in this respect, and showed no correlation to the corresponding A1 materials. This is perhaps not surprising given the mismatched geometry between the naphthalene donor and acceptor A2. Overall this leads to the conclusion that the phase transition temperatures of DACLCs do not consistently correspond to the structure of the donor component in an easily predictable way. Related to this, behavior of the CT band upon the mesophase to solid transition in the DACLC materials studied, easily observed by bulk color, was not consistent among mixtures. The CT induced color of crystalline materials was observed to remain the same, change slightly, or disappeared completely over times ranging from instantly to weeks. This appears to depend not only on the composition of components, but also on a variety of other factors including cooling rate and film thickness. A systematic investigation of the structure and property changes for materials undergoing this transition, in these and similar materials, is currently underway. Polarized Optical Microscopy. Polarized optical microscopy (POM) was performed on each mixture in correlation with the DSC results. For the purposes of this study, the f irst mesophase upon cooling of each donor−acceptor mixture was characterized by POM. As the liquid phase is not a focus of this study, the three mixtures that did not exhibit any phase behavior upon cooling (6:A2, 10:A2, and 11:A2) are not included in any following analysis. For each mixture, multiple images were taken

Table 2. Phase Transition Temperatures and Enthalpy of Phase Transitions for DACLC Materials Studieda

a

The reported measurements were taken upon cooling at a rate of 3 °C/min. Each run was done in triplicate.

of mixtures with acceptor A1 were observed relating to the position or identity of the naphthalene substituents on the donor component, although it was noted that the 2-phase materials are predominantly bisalkoxy substituted, while the 3-phase materials mostly contain one or more amine substituent. Perhaps surprisingly, mixtures 13:A1 and 13:A2 display similar phase behavior to each other, both having three recorded transitions of similar enthalpy. The phase behavior is again typical of liquid crystalline materials, with the first two transitions being very low enthalpy compared to the third. The only significant difference in thermal phase behavior noted was the temperature of the final crystallization event, which was 39 °C lower for 13:A1 than 13:A2, even though A1 has a signif icantly higher crystallization temperature than A2. The majority of mixtures of naphthalene donors with the MTI acceptor also exhibited mesophase behavior, although significantly less commonality was observed between materials. Mixtures with 1−5 substituted donors 1:A2, 3:A2, and 4:A2 underwent liquid to mesophase transitions at similar temperatures and relatively high enthalpies; however, 1:A2 displayed a second low-energy transition, and no additional phase change could be detected by DSC for the other two mixtures down to −40 °C, which is the cooling limit of the chiller used. Mixtures 2:A2 and 7:A2 each went through a very low enthalpy phase transition at relatively high temperature, the former then showed a second low-energy transition with no high-enthalpy crystallization event, and the latter underwent two additional transitions. Relatively low3321



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Figure 5. Representative polarized optical microscopy of the first mesophase upon cooling (3 °C/min) for several representative DACLC materials sandwiched between glass coverslips. All images were taken at a temperature 5 °C below mesophase onset and at 40× magnification. Mixtures were melted and recooled a minimum of three times while viewing different sample areas to ensure a representative texture. (a) 5:A1, (b) 8:A1, (c) 2:A1, (d) 9:A1, (e) 3:A1, (f) 4:A1, (g) 1:A2, (h) 2:A2, (i) 3:A2, (j) 13:A1, (k) 4:A2, (l) 5:A2, (m) 8:A2, (n) 9:A2, (o) 12:A2.

colored optical texture of randomly oriented rhomboids, possibly indicative of monoclinic columnar phase (Figure 5h), and 4:A2 shows a mosaic indicative of a plastic crystalline phase, consistent with the large enthalpy of phase change observed (Figure 5k). In total with the DSC results, this data confirms mesophase formation in the DACLCs studied and in all cases is highly suggestive of the columnar phase orientation anticipated. A comprehensive structural investigation of phase packing and changes upon phase transitions is currently underway. UV/vis Spectroscopy. The CT absorption of each DACLC material was investigated using thin-film UV/vis spectroscopy. Samples were prepared for analysis by sandwiching material between two quartz slides, then heating to an isotropic liquid and allowing the sample to cool slowly. Film thickness was measured with a digital micrometer upon liquid crystal formation, and the film massed to estimate density. The temperature was monitored using a thermocouple to ensure measurements were taken in the mesophase temperature range. An intense CT absorbance band was observed in the visible region of every donor: acceptor combination investigated, regardless of substituents or relative geometry of the components (Figure 6a−h). The UV−vis absorbance of each compound was also measured independently; none of the individual components displayed an absorption band in the visible region. Figure 6g is representative of all components and their mixtures, showing the thin-film spectrum of components 3 and A2 individually, overlaid with the spectra of the 3:A2 mixture. The dramatic difference in visiblenear-infrared absorbance between independent components and DACLC mixture is striking, highlighting the unique absorption properties of these materials beyond the sum of their components. Range and Onset of CT Absorption. The λmax‑CT of each mixture was determined, which, in conjunction with the measured film thickness, estimated density, and measurements of the % transmittance and reflectance, was used to estimate an extinction coefficient for each material (Table 3). The λonset for

at different sample positions, and each mixture was heated and cooled a minimum of three times to ensure reporting of a representative POM texture. Materials were melted between a glass slide and coverslip, and allowed to slowly cool (3 °C/min) below the first phase transition temperature. In all cases, onset of mesophase formation was observed by POM at a temperature consistent with that measured by DSC. For every DACLC material studied, sheet, needle, and/or mosaic type textures were observed, consistent with columnar mesophase textures (Figure 5).2,4,18 Shearing pressure was applied to each DACLC sample to perform a “slip test” within the mesophase temperature range. In each case the glass pieces slid past each other with medium to little resistance, confirming the presence of a partially ordered mesophase. Variations of relatively large, fan-type sheets were observed in all mixtures consisting of a bisalkoxy donor (1, 5−8) with acceptor A1 (Figure 5a,b). This was observed regardless of substituent position, and was also observed with the C3 symmetric HAT as donor in 13:A1 (Figure 5j). All DACLCs consisting of an alkoxy-amine compound (2, 9−12) with A1 also exhibited compact, fan structures, and consistently exhibited denser branching domains compared to bisalkoxy mixtures (Figure 5c,d). The bisamino mixture 3:A1 displayed the most condensed optical texture, adopting a mosaic structure with little observable isotropic domain (Figure 5d), while the bisthiol mixture 4:A1 exhibited the least dense texture (Figure 5f). All DACLC mixtures consisting of acceptor A1 exhibit domains of a unidirectional axis and relatively low birefringence, suggesting columnar rectangular (Colr) phase orientation, consistent with previous studies involving this component.4b,6 There is much greater variety in the optical textures of mixtures with acceptor A2, although all still exhibit textures in the first mesophase consistent with columnar liquid crystal or plastic crystal phases. Mixtures 1:A2, 3:A2, 8:A2, and 12:A2 yield thin radiating domains with circular origination patterns (Figure 5g,i,m,o), while 5:A2 and 9:A2 have fan-like circular domains, both suggesting a Colh phase (Figure 5l,n). 2:A2 displays a highly 3322



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Figure 6. UV/vis spectra of DACLC thin films. Mixtures were sandwiched between quartz slides and film thickness ranged from 10−15 μm; spectra are normalized for a film thickness of 12 μm. (a) 1,5-Bissubstituted donors 1, 2, 3, and 4 with A1. (b) Bis-alkoxy donors 5, 6, 7, and 8 with A1. (c) Alkoxyamine donors 9, 10, 11, and 12 with A1. (d) 1,5-Bissubstituted donors 1, 2, and 3 with A2. (e) Donors 4, 5, and 7 with A2. (f) Donors 8, 9, and 12 with A2. (g) Spectra of 3, A2, and 3:A2 highlighting the CT absorption unique to the mixture of components. (h) Donor 13 with acceptors A1 and A2.

Table 3. Measured CTmax Wavelength, Wavelength of CTonset, and CTmax Extinction Coefficient mixture

λmax‑CT (nm)

λonseta (nm)

εCTb (M−1 cm−1)

mixture

λmax‑CT (nm)

λonseta (nm)

εCTb (M−1 cm−1)

1:A1 2:A1 3:A1 4:A1 5:A1 6:A1 7:A1 8:A1

486 568 636 466 491 414c 450 465

656 759 812 594 705 550 613 631

6200 6000 6800 4200 3900 5100 6500 5500

1:A2 2:A2 3:A2 4:A2 5:A2 7:A2

458 516 573 454 490 445

595 691 761 570 656 565

15000 13000 13200 13300 7200 15200

9:A1 10:A1 11:A1 12:A1

606 551 572 562

826 730 728 725

3200 5200 6800 6400

8:A2 9:A2 12:A2

456 578 531

592 748 661

13800 11700 15500

13:A1 13:A2

512 474

712 676

6900 11000

a Determined by using the asymptote method on a minimum of three independent spectra of each mixture and averaging the values. bEstimated using the measured film thickness and the measured % transmittance - % reflection. cThis peak is a shoulder off the molecular absorbance.

naphthalene donors, each mixed with acceptor A1. There is a clear red-shift of the CT absorbance as the alkoxy groups of compound 1 are replaced by one (compound 2), and then two

DACLC materials was determined using the asymptote method by measuring the point of inflection at the red-end of each CT curve (Table 3). Figure 6a contrasts the four 1,5 substituted 3323



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Density Functional Theory Calculations. To investigate the relationship of individual component molecular orbitals to the measured CT absorption of the bulk mixtures, molecular orbital energy levels of the donor HOMO and HOMO-1, as well as the acceptor LUMO and LUMO+1, were calculated for each molecule in this study independently (Table 4). Orbital density for

(compound 3) amine substituents. Interestingly, bisthiol 4 displayed the shortest wavelength CT band, appearing as a shoulder on the molecular absorbance of the 4:A1 spectrum. Spectra of the bisalkoxy donors 5−8 and alkoxy-amine donors 9−12, each combined with acceptor A1, are shown in Figures 6b and 6c, respectively. It is interesting to note that there is generally little variation in the CT absorption range when comparing between mixtures of these constitutional isomers, regardless of substitution position. One notable exception is the absorption range for mixtures 5:A1 and 9:A1, in each case with a 1,4substitution pattern on the naphthalene donor. These mixtures display a significantly broadened CT peak with increased λonset and a decreased λmax compared to other corresponding mixtures of identical atom composition. As discussed above for the 1,5substituted donor series, comparing absorption in DACLCs with donors of the same connectivity pattern shows a λonset of absorption comes at a shorter wavelength for all bisalkoxynaphthalenes compared to their alkoxy amino counterparts. The 1,5-substituted naphthalene series with C3 symmetric acceptor A2 is shown in Figure 6d. It is somewhat remarkable that an identical trend in the red-shift of λmax‑CT and λonset for the A2 materials, as compared to the respective A1 materials, is observed as the donor component is altered from compound 1 to 2 to 3. Similar to the A1 mixtures, comparing absorption in DACLCs with donors of the same connectivity with acceptor A2, a shorter wavelength for λonset of absorption is observed for the bisalkoxy-naphthalenes compared to their alkoxy amino counterparts (Figure 6e−f). As is the case with the A1 mixtures, the A2 mixtures with 1,4-substituted donors 5 and 9 each have a redshifted absorbance and decreased extinction coefficient compared to their constitutional isomers. Interestingly, each A2 material exhibits a higher energy CT transition when compared to the respective A1 material; for example 1:A2, 2:A2, and 3:A2 each have a blue-shifted λmax‑CT and λonset when compared to the corresponding 1:A1, 2:A1, and 3:A1 materials. This trend holds true for every mixture studied, including with C3 donor 13; the 13:A2 mixture is blue-shifted by a consistent amount compared to 13:A1 (Figure 6h). Variations in CT Absorptivity. In contrast to variations in the wavelength of CT absorption discussed above, comparing DACLCs with donor components that have the same substituent positions reveals a relative consistency in εCT regardless of substituent identity. The 1,5-substituted donors 1, 2, and 3 with acceptor A1 all exhibit values of ε ∼6300 M−1, as do the three 1,6substituted donors 7, 11, and 12, irrespective of the difference in alkoxy versus amine substituents. This trend continues with other same-position substituted donors; 1,4-substituted 5 and 9 exhibit the two lowest εCT with A1 at ∼3500 M−1, and the two materials with 2,3-substituted donors, 6:A1 and 10:A1, have nearly identical εCT at ∼5200 M−1. While subtle, the intensity of CT absorption in DACLC materials is affected by the identity of the donor compound, and appears to be correlated to the structure rather than composition of the donor component. The mixtures with A2 also exhibit relatively consistent εCT regardless of the donor component, with the exception of the 1,4substituted donors 5 and 9, which again show the lowest εCT of the series. A significantly more pronounced difference in the magnitude of εCT between DACLCs with acceptor A1 versus those with acceptor A2 was observed. In each case, when the donor molecule is conserved, the observed εCT approximately doubles in materials with acceptor A2 compared to those with A1. Again, this trend remains consistent for the C3 donor 13; 13:A2 having almost double the observed εCT of 13:A1.

Table 4. Energy Levels Determined by DFT Calculations for Donor Molecule HOMO and HOMO-1, and Acceptor Molecule LUMO and LUMO+1a D

HOMO (eV)

HOMO-1 (eV)

D

HOMO (eV)

1 2 3 4

−5.34 −5.14 −5.00 −5.55

−6.62 −6.19 −5.83 −6.00

9 10 11 12 13

−5.01 −5.17 −5.18 −5.22 −5.21

HOMO-1 (eV) −6.57 −5.61 −5.71 −5.99 −5.21*

5 6 7 8

−5.22 −5.69 −5.49 −5.42

−6.63 −5.74 −6.08 −6.25

A A1 A2

LUMO −3.48 −3.34

LUMO+1 −1.76 −3.34*

a

All calculations were performed in triplicate from randomized starting geometries. *Denotes the presence of degenerate orbitals.

donor-HOMOs and acceptor-LUMOs were also calculated and visually rendered (Figure 7). Calculations were performed at the DFT level using the B3LYP functional with a 6-311G* basis set for each molecule in vacuo, implemented by Spartan 08 Mac. Slight differences in calculated orbital energies were observed upon comparing methyl vs ethyl groups as alkyl substituents; however, alkyl-chain lengths beyond ethyl were found to have no influence on overall HOMO/LUMO calculations, and so alkylethyl chains were used for all derivatives to minimize calculation time without affecting orbital energy levels. Comparing molecules with the same substituent connectivity within the donor set, as oxygen atoms are substituted with nitrogen atoms the calculated energy level of the HOMO orbital increases as expected. For example, in the 1,5-substituted series, diamine 3E‑HOMO > alkoxy-amine 2E‑HOMO > bisalkoxy 1E‑HOMO. The magnitude of the increase in HOMO energy upon substituting a single nitrogen is fairly consistent, approximately 0.20 eV. One exception to this trend in energy difference is seen with the 2,3-substituted derivatives, where there is a larger increase of 0.52 eV when comparing bisalkoxy 6E‑HOMO to alkoxyamine 10E‑HOMO. Within sets of constitutional isomers there is a fair amount of consistency in the calculated energies of donor molecule HOMO orbitals. Energies of the alkoxy-amine HOMOs 2, 10−12 all fall within a range of 0.09 eV. A single exception in this set was observed in the HOMO energy of 1,4-alkoxy-amine 9, which has a calculated value slightly lower (by 0.12 eV) than the average of the other alkoxy-amine isomers. Similarly, bisalkoxy naphthalenes 1, 7, 8 all exhibit similar HOMO energies within a range of 0.09 eV, the 1,4-bisalkoxy isomer 5 again having a lower energy HOMO consistent with this substitution pattern in the alkoxyamine series. Interestingly, 2,3-bisalkoxy naphthalene 6 has an increased E-HOMO when compared to its constitutional isomers, yielding the overall highest HOMO energy calculated for a donor molecule in this study. There is a much greater difference in the calculated energy of the donor HOMO-1 orbitals, for which no obvious trend among components was observed. Donor molecule 13 has a HOMO energy level that falls within the range of the naphthalene series, making it an excellent option for 3324



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Figure 7. Representations for donor HOMO and acceptor LUMO orbitals from DFT calculations (B3LYP functional with a 6-311G*; Spartan 08 Mac, in vacuo). Compounds 13 and A2 each have a degenerate HOMO (132) and LUMO (A22) orbital respectively.

The energy gap (Eg) determined from the empirically measured λonset of the CT band (Table 2), and the corresponding Eg of the acceptorLUMO−donorHOMO as determined through DFT calculations on each independent component (Table 1) were compared (Figure 8). There is remarkable agreement between the empirical and the calculated values for the Eg (and therefore λonset) in DACLC materials, equally over a broad range of values. Linear regression analysis of the two data sets shows that all calculated values have a difference within 3% of those determined by UV/vis spectroscopy (Figure 8b), and only a single mixture, 1:A2, displayed a difference of greater than 2% between values (Figure 8a). This holds true in all 23 DACLC materials studied, with Eg values ranging across the visible spectrum and into the near-infrared from 1.5−2.1 eV (800−600 nm), regardless of component structure or composition. The precision and consistency of this correlation is extraordinary, and suggest the possibility for targeted and efficient design of optical absorption characteristics in a variety of DACLC materials. Orbital Calculations and CT Extinction Coefficients. The CT band of DACLC materials is the result of a spin-allowed πD−πA* transition. In the solvent-less mesophase, assembled through favorable alternating face-centered electrostatic association, this transition is relatively highly conserved, leading to the intensely colored films with significant extinction coefficients observed. It is important to note that the magnitude of εCT in these materials is significantly greater than extinction coefficients observed for most solvent based aromatic donor−acceptor systems in which values are typically an order of magnitude less in intensity.

exploring the effects of molecular geometry irrespective of orbital energy. Similarly, the respective LUMOs of the two acceptor molecules are similar in energy, A2 being slightly higher in energy (0.06 eV) than A1. It is noted that donor 13 and acceptor A2 each have a pair of degenerate orbitals at the respective HOMO and LUMO energy level. DFT Calculations Correlating to Onset of CT Absorbance. The red λonset of absorption in semiconducting materials most often corresponds to the lowest energy transition from the “top” of the valence band to the “bottom” of the conduction band, essentially a HOMO to LUMO transition. Bulk DACLC systems consist of extended stacks, which can transport charge via overlapping π orbitals. However, unlike covalent donor− acceptor conducting polymer systems, so-called “push−pull” polymers, association of components through noncovalent π−π interactions usually results in minimal perturbation of the overall energy level of the molecular HOMO and LUMO orbitals.13b,19 This is particularly relevant in solvent-less bulk systems, such as those in this study, where CT absorption is not affected by solvent and depends only on the interaction of component molecules. The lowest energy CT transition for DACLC materials likely results from the excitation of a relatively isolated and poorly stacked donor/acceptor pairing with little orbital mixing, and therefore almost no orbital perturbation. In this case, the absorption energy should resemble the energy difference between the free-standing donor-HOMO and acceptor-LUMO orbitals, thus enabling prediction of the red λonset for CT absorption through relatively simple DFT calculations of orbital energies for the independent components. 3325



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Mentioned above, this variation is present comparing mixtures with donors having different substitution patterns, regardless of the substituent identities. Again, we consider data obtained from the DFT calculations comparing orbital geometries. In almost all cases, the geometry of the HOMO orbital for donor molecules is most consistent between molecules with the same substitution pattern, regardless of the overall energy level of the orbitals. This could explain similarities in εCT between mixtures with these donor components, irrespective of overall orbital energies and λonset, based on the structural efficiency of photon absorption as related to matching geometry and phase of the donorHOMO and acceptorLUMO. Modular Tuning of Absorption Range, Bulk Color, and Overall Energy. The variability in optical properties achievable through the straightforward mixing and matching of simple complementary components is immediately evident in the wide range of colors achieved for the DACLC materials presented. It is interesting to note that the extent of the visible spectrum absorbed can also be tuned; different DACLC materials absorb in either a single region or multiple regions of the visible spectrum. If only one CT band significantly contributes to absorption in the visible region, seen in the UV/vis spectrum as a smooth CT curve until reaching the point of intramolecular absorbance, then the material exhibits a discrete color such as orange (6:A1), green (2:A1), or blue (9:A1) (Figure 6). Alternatively, some DACLC materials absorb more broadly in the visible because of a second CT absorption band seen as an additional perturbation between the intramolecular and primary CT absorption bands (Figure 6b, c, e). This is present in systems with a relatively high energy HOMO-1 orbital on the donor component such as 3:A2, 3:A1, and 10:A1 (Table 1), resulting in browns (3:A2) and blacks (3:A1) respectively (Figure 3). An additional advantage of this approach, practically important for optoelectronic material design, is the potential to tune the overall energy of electrons in a material in conjunction with or independently of, the absorbance. This is well illustrated through the comparison of mixtures 2:A1 and 3:A2. In DFT calculations, these mixtures have a near identical value for the acceptorLUMO− donorHOMO gap of 1.66 eV. Correspondingly, when their absorption spectra are self-normalized, the measured CT band for these two mixtures is remarkably similar, especially with respect to λonset (Figure 10a). However, the overall energy of the HOMO and LUMO orbitals in 3:A2 is calculated to be 0.14 eV greater than that of 2:A1 (Figure 10b). Therefore, while the Eg

Figure 8. Correlation of the CT λonset measured from UV/vis/NIR spectra to that of the predicted value from calculated LUMOacceptor− HOMOdonor Eg. (a) Values overlaid for each DACLC mixture. (b) Linear regression analysis of the two CT λonset values of each mixture.

As discussed above, the εCT of DACLC films in this study varies most significantly with a change in the identity of the electron-poor component. Materials with acceptor A1 gave an average εCT value of 5500 M−1, while DACLC films consisting of a donor with acceptor A2 gave an average value of more than twice that at 13,000 M−1. Although the underlying reasons for this significant difference are not completely understood at this time, we hypothesize it is related to two factors: (1) A2 has two degenerate LUMO orbitals while A1 has only a single orbital at the energy of the LUMO.20 An additional degenerate LUMO should increase the probability of an allowed transition occurring upon interaction with a photon, increasing the efficiency of absorption and therefore εCT for materials with A2 compared to A1. (2) Phasing of the donorHOMO orbitals is better matched with that of the LUMO of acceptor A2 compared to the LUMO of A1 (Figure 9).21 Again, this will increase the efficiency of absorption

Figure 9. HOMO orbitals for naphthalene donors in this study have inversion-like symmetry phase-matching well with one LUMO orbital of acceptor A2, but not with the LUMO of A1.

transitions between the donorHOMO and acceptorLUMO, increasing εCT for the A2 mixtures. Either or both of these reasons may contribute to the general increase in εCT for the A2 mixtures, and further studies to better determine the contributing factors of the acceptor component to this important optical property are currently underway. Although less pronounced, there is also significant variation in the εCT of materials with different donor components.

Figure 10. (a) Overlay of the UV/vis spectra for mixtures 2:A1 and 3:A2 for which the absorption scales have been adjusted to directly overlap each λmax‑CT. (b) Energy band diagram illustrating the different overall energy levels of mixtures 2:A1 and 3:A2 while having an identical Eg of 1.66 eV. 3326



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and therefore λonset of these DACLC materials is almost identical, the overall energy of electrons in these systems is not. The ability to tailor overall energy levels of DACLC materials independently of the Eg, with minimal synthetic effort, offers exciting and rare potential for the optimization of both optical and electrochemical transitions toward a variety of applications.

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CONCLUSION This study illustrates the remarkable tunability of optical properties in a series of new aromatic donor−acceptor aromatic liquid crystal materials. Through the mixing and matching of simple components, the band gap energy and corresponding absorption range can be specifically tailored over the complete visible spectrum and into the near-infrared. In addition, the onset of CT absorption for a given donor−acceptor pair has been shown to tightly correlate with calculations of the acceptorLUMO− donorHOMO energy gap, obtained from straightforward DFT studies on the individual components. This allows not only for tuning of the absorption range, but also juxtaposed adjustment of the overall energy of the valence electrons in the CT system, important for many electronic and electrochemical applications. The extinction coefficient for thin films of these DACLCs is in a respectable range for absorption-related applications such as photoconductivity, and is also able to be affected in these materials, in particular having a significant correlation to the identity of the acceptor component. In all, this study significantly enhances the scope and understanding of structure−property relationships in aromatic donor−acceptor liquid crystalline materials and will greatly aid in the design of next-generation materials for use in optoelectronic applications, where components can be efficiently designed to achieve tailored band-gaps through preliminary DFT calculations. Current studies are directed toward determining specific mesophase structural parameters with characterization of the mesophase to solid transition, investigation of charge mobility’s and photoconductivity of these DACLCs, as well as optimizing phase transition temperatures through side-chain modifications on components.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental details including synthetic details and 1 H NMR spectra of components. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mitchell Legg and David Porter for aid with synthesis, and Dr. Jordan Fantini for helpful discussions. This work was supported in part by funds from Research Corporation (CC10792) and the Lindbergh Foundation.

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REFERENCES

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Predictable Tuning of Absorption Properties in Modular Aromatic

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