Role of Molecular Packing in Determining Solid-State Optical

Mar 29, 2011 - (8, 9) Kiriy et al. and Garnier at al. have investigated the role of solid-state molecular organization in determining the field effect...
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PERSPECTIVE pubs.acs.org/JPCL

Role of Molecular Packing in Determining Solid-State Optical Properties of π-Conjugated Materials Shinto Varghese and Suresh Das* Photosciences and Photonics Section, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology, CSIR Trivandrum 695 019, Kerala, India ABSTRACT: The optical properties of π-conjugated organic molecules in their solid state are critically important in determining performance efficiencies of optoelectronic devices such as organic light-emitting diodes and organic thin-film transistors. This Perspective discusses some recent systematic explorations aimed toward arriving at an understanding of the role that molecular packing plays in determining these properties.

n the past few decades, π-conjugated organic materials have attracted considerable attention in view of their increasing use as active elements in electronic and optoelectronic devices such as light-emitting diodes, photovoltaic cells, and field effect transistors.1,2 Prototype systems of devices making use of organic materials are currently available, and the focus now is on improving device performances. The optical and electronic properties of π-conjugated materials in devices, which essentially deal with solid films, are defined not only by the chemical structure of the constituent molecules but also by the nature of their intermolecular electronic coupling. The nature of intermolecular interactions in solid films is, in turn, defined by the relative orientation of the nearest-neighbor molecules. In planar π-conjugated materials increased, overlap between π-orbitals of neighboring molecules results in delocalization of the polarons/excitons, leading to reduction in the activation barrier for charge transport, thereby increasing charge carrier mobilities, with the hopping mechanism playing a major role.3,4 Such interactions, on the other hand, open new pathways for the nonradiative decay of excitons which can compete with emissive routes, resulting in the quenching of solid-state luminescence, a phenomenon commonly referred to as concentration quenching.5,6 Thus, the two major issues that need to be addressed for the design of active elements in organoelectronic devices, namely, efficient charge transport and strong luminescence, are oppositely related. For tuning properties of materials in devices, it is therefore necessary to have a detailed understanding of the dependence of the charge transport and luminescence properties on the relative positions of adjacent molecules as well as on the factors that determine the nature of molecular packing in the solid state. The crystalline state of molecules can be considered as supramolecules par excellence because reliable and accurate information on the relative orientation and nature of intermolecular interactions of neighboring molecules can be obtained through crystallography.7 Much of our knowledge on the dependence of solid-state optical

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properties and intermolecular electronic interactions has been obtained from X-ray crystal structure analysis. This Perspective discusses recent advances made in understanding the role of molecular packing in controlling solid-state optical properties of π-conjugated molecular systems. Beginning with a brief explanation on the theoretical description of the role of molecular packing on solid-state optical properties, the Perspective extends to the description of illustrative examples of molecular systems in which the consequences of such packing on their solid-state optical properties have been explored with the help of single-crystal analysis. Stimuliresponsive systems in which changes in optical properties are brought about due to changes in the molecular packing are also

Information on the dependence of solid-state optical properties on molecular packing can reliably be obtained from X-ray and photophysical analysis of single crystals. briefly discussed. Molecular packing in π-conjugated materials also has a significant impact on their ability to transport charges and consequently on their optoelectronic properties. These aspects have been only briefly touched upon in the present Perspective due to restrictions of space and the availability of a few good reviews on this Received: January 21, 2011 Accepted: March 25, 2011 Published: March 29, 2011 863

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Figure 1. Schematic representation of exciton splitting W of the optically allowed transitions and the splitting energy between excited states both in H- and J-aggregates. Adapted from ref 12. Copyright (2001) Wiley-VCH.

Figure 3. Sketch of operations applied to a cofacial dimer formed by two stilbene molecules separated by 4 Å. The modifications are induced by (i) translation of one molecule along its long axis, (ii) translating one molecule along its short axis, (iii) rotating one molecule around its long axis, and (iv) rotating one molecule around the stacking axis while the planes of the molecular backbones are kept parallel. Adapted from ref 12. Copyright (2001) Wiley-VCH.

intermolecular packing distance varying from 20 down to 3.5 Å (Figure 2). It was observed that the lowest excited state of the cofacial dimer (H-aggregate) is not optically coupled to the ground state and that the oscillator strength is concentrated on the second excited state, regardless of the interchain distance. The excitonic splitting increases when the interchain distance is reduced, with the CT interactions becoming significant in the strong interacFigure 2. INDO/SCI-calculated transition energies of the two lowest optical transitions of a cofacial dimer formed by two stilbene molecules as a function of the interchain distance R (in Å). The horizontal line refers to the transition energy of the isolated molecule. Reproduced from ref 11. Copyright (1998) American Chemical Society.

Theoretical studies have contributed significantly toward understanding the effect of intermolecular coupling on photophysical properties in polymeric and molecular materials.

topic.8,9 Aggregation-induced enhancement in emission caused purely by restricted rotation of molecules in the solid state is an interesting phenomenon observed in few systems, and this aspect has also been well reviewed6 and is not covered in the present Perspective. Theoretical studies on π-conjugated materials have contributed significantly toward the understanding of effects of intermolecular electronic coupling on photophysical properties in polymeric and molecular materials, although an accurate prediction is still lacking. Optical properties of interacting conjugated systems have been explained by Kasha and co-workers in terms of exciton coupling theory in which the excited state of the aggregates splits (Davydov splitting) into two energy levels (Figure 1).10 Transition to the upper state is allowed in molecules with cofacial arrangement (H-aggregate), and transition to the lower state is allowed for molecules arranged in a head to tail fashion (J-aggregates). Bredas and co-workers have investigated the effect of intermolecular distance and relative orientations on exciton splitting energies in stilbene dimers.11,12 Their calculations on the cofacial dimer describe the evolution of the energy of the two excited states resulting from the interaction of the lowest excited state of isolated stilbene molecules as a function of the

tion regime. In the cofacial configuration, strong interchain interactions indicate a blue shift of the lowest optically allowed transition with respect to the isolated molecule, and the absence of optical coupling between the ground state and the lowest excited state of the cofacial dimer can lead to rapid interband relaxation, which is expected to reduce the luminescence quantum yields with respect to isolated monomers. Thus, their theoretical calculations predict considerable quenching in luminescence efficiency for a perfect cofacial dimer at short intermolecular distances (>4 Å). Bredas et al. have also analyzed the changes occurring in optical properties of the cofacial dimer of stilbene molecules separated by a distance of 4 Å upon applying translational and rotational operations, as shown in Figure 3. According to their 864

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Chart 1

Figure 4. Schematic illustrations of (a) “pitch” and b) “roll” angles/ distances (P/dP; R/dR) as defined by Curtis et al.4 The interplanar distances between the adjacent molecules (d) are also given. The inset shows the Cartesian coordinates in which the molecule is placed in the XY plane. Adapted from ref 4. Copyright (2004) American Chemical Society.

studies, translation and rotation along the long and short molecular axes (cases i, ii, and iii, Figure 3) are insufficient for recovery of high luminescence quantum yields because superposition and parallelism of the molecules is retained in such operations, whereas promoting a finite angle between the long molecular axes (case iv, Figure 3) leads to a decrease in the optical splitting, which becomes vanishingly small upon reaching the perpendicular stacking mode. Although the latter organization favors high luminescence quantum yields, reduced intermolecular π-overlap would result in a reduction of the charge-transport properties. A more preferred arrangement would be the operation shown in case i of Figure 3, in which the molecules are translated along the long molecular axis. Such an operation can lead to a head to tail arrangement of the molecules, resulting in conversion of the H-aggregate into a J-aggregate in which, as described earlier, the oscillator strength of the electronic transition is concentrated on the bottom of the exciton band (Figure 1). J-aggregates are the only molecular systems where electronic excitation is delocalized over several molecular units as a result of significant retention of π-overlap between neighboring molecules, giving rise to many cooperative and coherent phenomena, such as giant oscillator strength and superradiance, making such systems strongly emissive.13 π-Conjugated materials with such orientations can form ideal candidates for optoelectronic devices because they combine both high luminescence and charge-conducting properties of organic molecules. The charge-transport properties of π-conjugated materials are significantly dependent on molecular packing, and this aspect has been well reviewed.8,9 Kiriy et al. and Garnier at al. have investigated the role of solid-state molecular organization in determining the field effect mobility of some thiophene derivatives.14,15 The charge carrier mobility of β,β0 -DH6T was found to be ∑μmin = 3.9  103 cm2 V1 s1, which was an order of magnitude less than the PR-TRMC mobility found for R, ω-DH6T, which could be attributed to differences in the molecular packing. Single-crystal X-ray studies indicated a planar conformation of the β,β0 -DH6T backbone in the solid state and a herringbone molecular packing for unsubstituted and R,ω-substituted oligothiophenes. β,β0 -DH6T exhibited less dense crystalline packing than 6T and R,ω-DH6T. As a result, while the crystalline structure of 6T and R,ω-DH6T “allows” the current to flow through molecular stacks along two self-perpendicular directions, the crystalline structure of β,β0 -DH6T suppresses the charge transport in all directions. In order to understand the role of molecular packing in controlling charge-transport properties in the solid state, Curtis and co-

Figure 5. Fluorescence (left) and absorption spectra (right) of distyrylbenzene nanoparticles: (a) t-Bu4DSB, (b) DSB, (c) cocrystallized DSB/F12DSB, and (d) F12DSB. Spectra in hexane solution (dashed lines) are shown for comparison. A schematic representation of the respective condensed-phase structures is given on the right. Adapted from ref 16. Copyright (2005) American Institute of Physics.

workers have analyzed the solid-state packing motifs of a series of bithiazole and thiophene oligomers, as well as a series of substituted pentacenes, and discussed their effect on charge carrier mobility in such materials.3,4 The relative orientation of nearest-neighbor molecules was described in terms of “pitch and roll” inclinations from an “ideal” cofacial π-stack, with pitch inclinations describing translation of adjacent molecules relative to one another in the direction of the long molecular axis (Figure 4a) and roll inclinations describing translation of the molecules along the short molecular axis (Figure 4b). Thus, moderately large pitch distortions preserve ππ interactions between adjacent molecules, whereas roll translations greater than 2.5 Å, which is approximately equal to a width of a benzene ring, essentially destroy ππ overlap between adjacent molecules. Relatively large roll translations lead to the herringbone 865

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Figure 6. (a) Molecular structure of 2,5-diphenyl-1,4-distyrylbenzene (DPDSB), (b) cross-stacking conformation of the adjacent molecules in the crystal lattice, (c) cross stacking of molecules in a 1D molecular column along the b axis, and (d) emission spectra in dilute THF solution (blue) and in a thin film (red). The inset shows crystals of t-DPDSB under UV light (365 nm). Adapted from ref 17. Copyright (2005) American Chemical Society.

Figure 7. (a) Molecular structure of (2Z,20 Z)-2,20 -(1,4-phenylene) bis(3-(4-butoxyphenyl) acrylonitrile (DBDCS). (b) Photo of a single crystal before annealing, under ordinary light (i) and UV light (ii), and after annealing, under ordinary light (iii) and UV light (iv) (scale bar: 0.2 mm). (c) Photo of the pristine powder under ordinary light (left) and UV light (right) (scale bar: 5 mm). (d) Photo of the ground powder under ordinary light (left) and UV light (right). (scale bar: 5 mm). (e) Normalized PL spectra of DBDCS under different conditions. (f) Schematic illustration of two different modes of slip-stacking in DBDCS molecular sheets, dictated by different ways of antiparallel/head to tail coupling of local dipoles. Adapted from ref 20. Copyright (2010) American Chemical Society.

packing in which the π-overlap is minimum. They have shown that thiophenes tend to exhibit large roll translations, whereas thiazoles have small roll but large pitch translations, and substituted pentacenes tend to have both moderate pitch and roll distances.4 Herringbone is the generally observed packing pattern of long conjugated molecules without substituents, such as in oligophenylenes, oligophenylenevinylenes, and acenes. Molecules adopt this packing pattern in order to avoid energetically unfavorable repulsive electronic interactions as well as to afford increased packing efficiency. It is recognized that charge carrier mobility increases upon generating large valence or conduction bandwidths in crystalline π-conjugated materials. Because the latter is proportional to the extent of orbital overlap of adjacent molecules, crystals with π-stacked molecules would be expected to be

better than those with herringbone packing with respect to charge-transport properties. Modification of stacking modes is therefore essential for fine-tuning the optical and electronic properties of π-conjugated organic materials, and one way of bringing this about is by introducing minor changes in the nature of substituents on the π-conjugated backbone. Solid-state luminescence of trans-distyryl benzene (DSB) derivatives has been extensively investigated because they can serve as model compounds for studying intermolecular interactions in para-phenylenevinylene (PPV) polymers. Gierschner and co-workers have investigated the consequences of the different packing motifs in single crystals and nanoparticle suspensions of a series of distyrylbenzene derivatives (DSB, t-Bu4DSB, F12DSB, and a 1:1 cocrystal of DSB/F12DSB; 866

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Chart 2

Figure 8. Optical properties of monoalkoxy-substituted diphenylbutadienes, MBC1(i), MBC4 (ii), MBC8 (iii), and MBC12 (iv); (a) thin films under UV illumination, (b) normalized solid-state fluorescence spectra and (c) diffuse reflectance spectra. Adapted from ref 22. Copyright (2005) The Royal Society of Chemistry.

steric hindrance of the bulky t-butyl group preventing electronic interactions between neighboring molecules (Figure 5a). Theoretical predictions that luminescence efficiencies could be maximized by arranging the long axes of adjacent molecules

Chart 1) on their photophysical properties using absorption and fluorescence spectroscopy.16 Whereas the absorption and fluorescence spectra of all molecules in solution were quite similar, large differences were observed in their solid-state optical properties (Figure 5). They observed that DSB organized in a herringbone manner, with the long axes of the molecules oriented in parallel but the short axes almost perpendicular to each other, whereas the fluorinated distyrylbenzene, F12DSB, as well as the DSB/F12DSB cocrystals preferred cofacial stacking in the solid state. For all structures, the consequence of the parallel alignment of the transition moments was a strongly blue-shifted H-type absorption spectrum and a low radiative rate constant. The perpendicular arrangement of the short axes in DSB crystals led to only very weak intermolecular vibronic coupling, as a result of which its emission spectrum is well structured, very similar to the one in solution (Figure 5b). For F12DSB and DSB/F12DSB, the cofacial arrangement of the adjacent molecules enabled strong intermolecular vibronic coupling of adjacent molecules, resulting in an unstructured and strongly red-shifted excimer-like emission spectrum (Figure 5c and d). The solid-state photophysical properties of t-Bu4DSB however were very similar to those in its solution, and this was attributed to a lack of long-range order of the constituting molecules due to the

Awareness of the role of molecular packing in controlling solid-state optical properties of π-conjugated materials can be utilized for developing stimuli-responsive materials.

perpendicular to each other were confirmed by Ma and coworkers, who reported that 2,5-diphenyl-1,4-distyrylbenzene (DPDSB, Figure 6a) adopted a stable cross-stacking mode in its crystalline state (Figure 6b).17 As a result, a high fluorescence quantum yield was observed for this molecule in its crystalline state, with the emission range very close to that of the monomer 867

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(Figure 6d). Additionally, the cross stacking of these molecules with overlap in their central phenyl rings resulted in the onedimensional molecular columns (b axis), which can act as a channel for electron transport, producing high carrier mobility (Figure 6c). The authors also observed formation of a “brick wall” packing arrangement in crystals of t-DPDSB grown by the physical vapor transport method, in which the quantum yield was considerably reduced (∼48%.) compared to that of the crossstacked polymorph (80%).18 The combined properties of strong blue emission and good electron-transporting property of DPDSB crystals made this material a promising candidate for OLED and lasing applications. Knowledge of the role of molecular packing in controlling solidstate optical properties of DSB derivatives has been utilized for developing stimuli-sensitive materials, where an external stimulus can be utilized to fine-tune the molecular packing. Reversible piezochromism in cyano-distyrylbenzene was reported by Weder and co-workers.19 Recently, Park, Gierschner, and co-workers have undertaken a detailed investigation of the changes in fluorescence properties of the cyano-distyrylbenzene derivative DBDCS when subjected to external stimuli (Figure 7).20 DBDCS was observed to form highly fluorescent “molecular sheets” assisted by the multiple CH 3 3 3 N and CH 3 3 3 O hydrogen bonds with stacking and shear-sliding capabilities upon exposure to external stimuli such as temperature, pressure, and solvent vapor. On the basis of structural, optical, photophysical, and computational studies, two different phases, that is, a metastable green-emitting G-phase and a thermodynamically stable blue-emitting B-phase, were identified. The metastable G-phase was attributed to a kinetically trapped structure that was stabilized by antiparallel coupling of the local dipoles. This structure was observed to have a moderate excitonic coupling. The structure however exhibited efficient emission, which was attributed to excimer formation. Upon annealing, a smooth slip of the molecular sheets with a low activation barrier formed the B-phase with a head to tail arrangement of the local dipoles. Here, the excimer formation is diminished, while excitonic interaction substantially increases. It was

found that upon application of pressure, the B-phase crystal could be restored to the original G-phase, and this is schematically represented in Figure 7f. Using this materials rewritable fluorescent optical recording media with fast and reversible response to multiple stimuli could be demonstrated. Similar type of mechanochromic effects on the luminescence behavior of bifluoroboron avobenzone was recently reported.21 Donoracceptor-substituted diphenyl butadienes (DPBs) are structurally closely related to DSB derivatives, and the correlation between molecular packing and solid-state photophysical properties as well as the sensitivity of these properties to external stimuli of this class of molecules have been well studied.2225 The photophysical properties of a series of alkoxy-substituted diphenylbutadienes (Chart 2) in the solid state were observed to be very sensitive to the molecular packing. The role of the number of alkoxy substituents and the chain length in

Figure 11. Schematic representation of change in the molecular arrangement within the thermodynamically stable and kinetically trapped forms of MBC8. Adapted from ref 23. Copyright (2008) American Chemical Society.

Chart 3

Figure 9. Solid-state fluorescence of MBC8 in the two fluorescing states. Reproduced from ref 22. Copyright (2005) The Royal Society of Chemistry.

Figure 10. (left) Time-dependent changes in the fluorescence spectrum of a metastable MBC8 film: (a) 0 h, (m) 6 h, in steps of 30 min. (right) Timedependent changes in the diffuse reflectance spectrum of a metastable MBC8 film: (a) 0 h, (m) 6 h, in steps of 1 h. Adapted from ref 22. Copyright (2005) The Royal Society of Chemistry. 868

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Figure 12. (a) Molecular orientation of the nearest neighbors of (a) BINC1 and (b) BINC8, viewed along short molecular axis, and (c) BINC12 viewed along short and long molecular axes. (d) Fluorescence spectra of crystals (i) BINC12, (ii) BINC4, (iii) BINC8 and (iv) BINC1. The dashed curve shows an emission spectrum of BINC12 dissolved in acetonitrile (λex = 465 nm). Adapted from ref 24. Copyright (2009) American Chemical Society.

controlling the nature of the molecular packing and, consequently, their fluorescence properties hve been elucidated for this class of molecules. Whereas in the di- and trialkoxy-substituted derivatives the solidstate fluorescence was independent of the length of the alkyl chains, the optical properties of monoalkoxy-substituted diphenylbutadienes were observed to be highly sensitive to the molecular structure and environmental factors such as temperature.22,23 The derivatives with longer alkoxy chains (MBC8, MBC12) exhibited monomer-like fluorescence (λmax ≈ 450 nm) in the solid state, whereas those of the short alkoxy chain derivatives (MBC1, MBC4) were significantly broader and red-shifted (λmax ≈ 500 nm) (Figure 8). The fluorescence decay profiles of all of the derivatives showed biexponential fits, indicating the existence of two distinct emitting states. Evidence for two species could also be obtained from their ground-state absorption measured using reflectance spectroscopy (Figure 8). Whereas the diffuse reflectance absorption spectrum of MBC12 shows only a broad band with an absorption maximum centered at 385 nm, MBC8 possesses an additional band in the long-wavelength region with an absorption maximum centered at 430 nm. With a further decrease in the alkoxy chain length, a relative increase in absorption in the long-wavelength region is observed with MBC1 and MBC4 possessing intense longwavelength absorption bands (Figure 8c). The red-shifted absorption and emission observed in the solid films of these butadiene derivatives, which are absent in solution, were assigned to those of the J-aggregates, whereas the short-wavelength absorption and emission bands, which correspond closely to the solution spectra, were assigned to those of the monomer. MBC8 was observed to possess two thermally interconvertible polymorphic states with drastically different fluorescence properties, exhibiting blue fluorescence in its stable state and green fluorescence when obtained as its freshly solidified melt. This form was found to be metastable and reverted to the stable blue fluorescent state over a period of 6 h at 27 °C (Figure 9), and the corresponding changes in emission and absorption during this transformation were monitored at regular intervals (Figure 10). The differences in the fluorescence behavior of the two polymorphic forms of MBC8 were explained on the basis of the crystal packing of the diphenylbutadiene derivatives, which shows that in the green fluorescent derivatives, MBC1and MBC4, the nearest-neighbor molecules are packed through an edge-to-face interaction between molecules of adjacent stacks. The similarity of the optical properties (absorption, emission, and emission lifetimes) strongly suggests that the metastable form of MBC8 possesses a similar

structure. The crystal structure of the thermodynamically stable form of MBC8 indicated that the main overlap between neighboring molecules occurs between the butadiene chromophore of one molecule and the insulating oligomethylene chain of the other molecule. Upon heating, the weak interactions present in MBC8 could break, followed by translation of the molecules along the long axis and twist of the aromatic group, resulting in an arrangement in which the aromatic units lie close to each other, stabilized by edge-toface interactions, as observed in MBC1 and MBC4, to give rise to the metastable green fluorescent state upon rapid cooling. The proposed changes in the alignment are schematically represented in Figure 11. This could then slowly evolve over a period of time back into the thermodynamically stable arrangement in which stacks are stabilized by CH 3 3 3 O hydrogen bonds.23 Emission arising from planar monomers, as well as J- and H-aggregates, was observed in a series of indanedione-substituted butadiene derivatives (Chart 3). Analysis of the crystal structures indicated that their crystal packing could be classified into three types depending on the relative orientation of their nearest neighbors (Figure 12 ac), and the extent of their π-overlap was quantified in terms of pitch and roll distances.24 In type 1 packing, which was observed for BINC1, the molecules are oriented virtually one on top of the other to form a near-perfect H-dimer (Figure 12a). In type 2 packing, which was observed for the dimers of BINC4 and BINC8, the molecules show substantial displacement along the long axis of the molecule while still retaining a significant degree of π-overlap, resulting in a head to tail arrangement of the neighboring molecules characteristic of J-type dimers (Figure 12b). In type 3 packing, which was observed for BINC12, the molecules are displaced substantially along the short axis of the molecules, resulting in virtually no π-overlap between the neighboring molecules (Figure 12c). As observed with the molecular packing in the crystals of the BINC derivatives, their photophysical properties could also be classified into three distinct types, and a clear correlation between the photophysical properties and crystal packing could be observed. All three crystalline forms showed significantly higher fluorescence compared to that observed for the monomers in solution. Although the emission spectra of all four derivatives were identical in solution, their emission bands were significantly broader and red-shifted in the solid state (Figure 12d). The maximum red shift in the emission band was observed for BINC1 (λmax ≈ 656 nm). The emission maxima were observed to undergo a blue shift with an increase in the length of the alkoxy substituent, with BINC1 > BINC4 ≈ BINC8 > BINC12 (Figure 12d). 869

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Chart 4

Figure 14. (a) Molecular structure of O-alkylated derivatives of hydroxylbenzylideneimidazolinone (ROBDI). (b) Solid-state fluorescence spectra of ROBDI crystals. Adapted from ref 29. Copyright (2009) American Chemical Society.

Figure 13. Schematic representation of the molecular alignment of BP4 in stable blue fluorescent and in the kinetically trapped yellow fluorescent forms. Reproduced from ref 25. Copyright (2008) American Chemical Society.

The enhanced emission observed in BINC4 and BINC8 crystals could be attributed to the formation of J-type dimers, which are known to be highly fluorescent, while the enhanced fluorescence of the isolated monomers observed in BINC12 compared to their emission in solution can be attributed to rigidization of the molecules in the solid state. In BINC1, molecular packing in the crystals had indicated the formation of H-type dimers between the nearest neighbors, and H-aggregates are generally known to be nonfluorescent because transition to the top of the exciton band in these systems, which is optically allowed, is followed by rapid interband relaxation, usually resulting in fluorescence quenching. In the cofacial arrangements of some π-conjugated organic chromophores (H-dimers) however, excitation is known to lead to a decrease of the interchromophoric distance, and in such cases, a long-lived and strongly red-shifted broad fluorescence band arising from a low-energy state assigned to an excimer like state has been reported.26,27 The strongly red-shifted unstructured emission band from BINC1 could therefore be attributed to excimer-like emission resulting from excitation of its H-type dimers. The role of molecular packing in controlling the solid-state fluorescence of a series of alkoxyphenylpyridyl butadiene derivatives (Chart 4) was investigated by studying the X-ray crystal structure of these molecules.25 One of the derivatives, BP4, exhibited polymorphism, with the different polymorphs exhibiting visually distinguishable fluorescence. In the natural state, it existed as a polymorph exhibiting blue fluorescence, while its cooled melt emitted yellow light. The difference could be attributed to a transformation in the molecular packing of the material from a herringbone to a brickstone arrangement, resulting in a change from monomer to J-type aggregate fluorescence (Figure 13). The polymorph exhibiting yellow fluorescence was fairly stable (>6 months) but could be converted back to the original form by keeping the film at 100 °C for a short period of time (∼810 min) before slowly cooling to room temperature. The thermally induced changes in fluorescence behavior were clearly reproducible

Figure 15. (a) Molecular structures of phenyleneethynylene derivatives, (b) the slip-stacked pairs of interacting molecules and the photographs of the thin films of these derivatives under UV illumination, (c) fluorescence emission spectra of the derivatives, and (d) variation of the emission maximum with the spacing between the interacting molecules in crystals of the derivatives. Adapted from ref 31. Copyright (2009) The Royal Society of Chemistry.

over several cycles, indicating the utility of this material for thermal imaging application. Formation of polymorphs with distinct optical properties has also been reported for tetraphenylbutadienes.28 A very similar tuning of solid-state optical properties of O-alkylated derivatives of hydroxybenzylideneimidazolinone (ROBDI), the chromophoric unit of the green fluorescent protein (GFP) (Figure 14a) in which the molecular packing was altered by varying the length of the alkyl substituent, was reported by Tolbert and co-workers.29 Whereas the modified chromophoric unit of the GFP derivatives possessed similar absorption and emission spectra in solution, the size of the O-alkyl substituents of these derivatives played a significant role in determining their solid-state luminescence properties. With increasing length of the alkyl group, the interaction between the aromatic molecules in the lattice became weaker, resulting in a significant hypsochromic shift in the emission spectra of their crystals (Figure 14b). Phenyleneethynylenes exhibit interesting spectroscopic properties in solution and have been investigated extensively for understanding optical properties of poly(p-phenyleneethynylene) 870

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polymers.30 An interesting correlation of the interplanar distance of the interacting molecules (nearest neighbors) with the fluorescence properties in the solid state was demonstrated by Kulkarni and coworkers in a series of alkoxy-substituted phenyleneethynelenes (C1C6) possessing varying alkyl chain lengths (Figure 15a).31 Analysis of the molecular packing of the crystals indicated the nearest neighbors to be arranged in a nearly head to tail fashion, akin to a J-aggregate (Figure 15b). A linear correlation was observed between the solid-state emission maxima and the spacing between the interacting molecules, with a blue shift being observed with increasing spacing, and this was attributed to varying strengths in the dipolar coupling between the interacting molecules (Figure 15 c and d). Acenes are another class of molecules which have been extensively explored as elements in optoelectronic devices due to their large planar π-surface. The role of molecular structure and molecular packing on their charge-transport properties in the solid state has been extensively investigated.8 The role of molecular packing in

controlling the solid-state optical packing of anthracenes has been studies by Tohnai and co-workers in organic salts of anthracene-2, 6-disulfonic acid (ADS) possessing linear alkyl amines as counterions, wherein the solid-state florescence could be modulated by varying the nature of the amines (Chart 5).32,33 The solid-state fluorescence emission of these salts was found to significantly depend on the change in the alkyl chain lengths of the amines. The salts with amines possessing long alkyl chains exhibited a weak fluorescence (ADS4, ADS5), whereas those having short alkyl chains exhibited more than 10-fold higher fluorescence quantum yields (ADS1, ADS2), and these differences could be attributed to differences in the relative orientation of the anthracene moieties. On the basis of crystal structure analysis, the molecular arrangements of anthracene moieties were divided into two types, namely, form I for ADS1 and ADS2, which exhibited a 2D arrangement of anthracene moieties, and form II exhibited by ADS4 and ADS5, which had a 1D arrangement of anthracene moieties (Figure 16). The differences in the nature of packing were attributed to two different driving forces for crystallization, namely, alkyl chain packing of amines and ππ stacking of ADS. Whereas ππ stacking acts as the principal driving force in form I, both alkyl chain packing and ππ stacking contribute in form II. On the basis of these studies, they conclude that factors essential for high fluorescence quantum yield in the solid state for this class of molecules are prevention of contact between π-planes of anthracene moieties and immobilization of the anthracene rings. Enhancement in the solidstate photoluminescence quantum yield of achiral 2-anthracenecarboxylic acid upon complexation with achiral benzylamine to yield chiral helical columnar supramolecular organization and its solidstate circularly polarized emission has been reported.34 Wang and co-workers have demonstrated a clear link between molecular packing and solid-state luminescence for a series of dimeric 9-anthryl pyrazole (ANP) derivatives (Chart 6).35 By slow diffusion of petroleum ether into solutions of these compounds in selected solvents, they were able to grow crystalline polymorphs of each of these derivatives. The polymorphs classified as a- and b-types differed from each other mainly in the extent of π-overlap between the neighboring anthracene molecules, and this was found to have a significant influence on the emission properties of these crystals. Except for ANP2a and ANP2b, whose emission spectra were nearly identical, all other a-type crystals exhibited significantly red-shifted emission compared to their b-type polymorphs (Table 1). For example, about 2/3 of an anthracene overlap was observed for crystals of ANP1a, which exhibited green emission, while no

Chart 5

Figure 16. (a) Crystal structure of ADS1 form I and (b) the 2D arrangement of anthracene moieties. (c) Crystal structure of ADS5 form II and (d) the 1D arrangement of anthracene moieties. Reproduced from ref 32. Copyright (2005) The Royal Society of Chemistry.

Chart 6

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type of arrangement also involves substantial π-overlap between the neighboring molecules, the charge-transport properties of these materials will not be significantly compromised. Another factor that needs significant attention is the selfassembling properties of such materials on various surfaces where competition between moleculemolecule interactions

Table 1. Anthracene Packing Modes and Emission Data of ANP Crystals (Adapted from ref 35. Copyright (2009) American Chemical Society)

Highly fluorescent materials in which charge-transport properties are not compromised can be obtained when molecules are arranged in a slip-stacked J-aggregated manner.

overlap was found in crystals of ANP1b, which showed deep blue emission. The variation in the degree of π-overlap of the neighboring anthracene moieties and the emission properties of the ANP derivatives are summarized in Table 1. It may be noted that in the green-light-emitting crystals, the neighboring anthracene moieties are significantly pitched with respect to each other, with very little roll displacement, indicating formation of J-type aggregates. The fluorescence quantum yields of the blueemitting crystals were significantly high, making them potentially useful as blue emitters in optoelectronics. Future Directions and Outlook. A critical issue to be addressed with regards to the study of π-conjugated materials is that these materials, by their nature, tend to aggregate very strongly to yield π-stacked structures in their solid state, resulting in molecular arrangements, which, although beneficial for charge transport, are highly detrimental for generating luminescent materials. Successful incorporation of these materials into electroptic devices requires optimization of both of these properties. This requires a detailed understanding of how molecular packing affects these properties as well as an understanding of the various weak intermolecular forces which control the relative orientation of these molecules in their solid state. This Perspective provides an overview of the recent progress made in understanding the role of molecular packing in controlling the functional properties, in particular, the optical properties of different classes of π-conjugated organic materials. Even though substantial progress has been made toward understanding the relationship between molecular arrangement and optical and electronic properties of different classes of π-conjugated organic materials and their supramolecular self-assembling properties, the challenge of controlled design and construction of novel materials with tailormade properties for specific applications remains unmet. Earlier studies suggest that one good way of optimizing charge transport and luminescence in such materials is to construct molecules which can arrange themselves in a brick stone or slipstacked J-type aggregated structures. Such structures are known to be highly fluorescent, with their fluorescence properties sometimes far surpassing those of the monomer. Because this

and moleculesubstrate interactions will define the relative orientation and thereby the functional properties of the materials. It is also essential to understand the sensitivity of molecular arrangement toward external stimuli such as temperature because conditions used for device fabrication will also define the nature of molecular packing in devices. On the other hand, the sensitivity of molecular packing and, consequently, their optical properties to external stimuli such as temperature, pressure, and chemicals can be utilized for the construction of stimuli-specific sensing devices.36,37

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ BIOGRAPHIES Shinto Varghese obtained his M.Sc. (Chemistry) degree in 2005 from St. Thomas College, Thrissur of Calicut University, Kerala. He has been conducting his doctoral research under the supervision of Dr. Suresh Das since 2005. His research interests are in the study of the photophysics and photochemistry of functional organic materials. Dr. Suresh Das, Director of the National Institute for Interdisciplinary Science and Technology, Trivandrum, obtained his Ph. D. in Chemistry in 1981 from the University of Newcastle upon Tyne, U.K, under the supervision of Professor G. Alistair Johnson. He is a Fellow of the Indian National Science Academy, New Delhi, and the Indian Academy of Sciences, Bangalore. His research interests are in the areas of photoresponsive soft materials, squaraine-dye-based sensitizers, organic solid-state luminescent materials, and photoinduced electron transfer. For more details, visit: http:// www.niist.res.in/english/scientists/suresh-das/personal.html ’ ACKNOWLEDGMENT A research grant from the Council of Scientific and Industrial Research (CSIR), India, under the network Project NWP 023 and student fellowships from CSIR, India, are gratefully acknowledged. 872

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’ REFERENCES

(22) Davis, R.; Rath, N. P.; Das, S. Thermally Reversible Fluorescent Polymorphs of Alkoxy-Cyano-Substituted Diphenylbutadienes: Role of Crystal Packing in Solid State Fluorescence. Chem. Commun. 2004, 74–75. (23) Davis, R.; Kumar, N. S. S.; Abraham, S.; Suresh, C. H.; Rath, N. P.; Tamaoki, N.; Das, S. Molecular Packing and Solid-State Fluorescence of Alkoxy-Cyano Substituted Diphenylbutadienes: Structure of the Luminescent Aggregates. J. Phy. Chem. C 2008, 112, 2137–2146. (24) Kumar, N. S. S.; Varghese, S.; Suresh, C. H.; Rath, N. P.; Das, S. Correlation Between Solid-State Photophysical Properties and Molecular Packing in a Series of Indane-1,3-dione Containing Butadiene Derivatives. J. Phys. Chem. C 2009, 113, 11927–11935. (25) Kumar, N. S. S.; Varghese, S.; Rath, N. P.; Das, S. Solid State Optical Properties of 4-Alkoxy-Pyridine Butadiene Derivatives: Reversible Thermal Switching of Luminescence. J. Phys. Chem. C 2008, 112, 8429. (26) Van Hutten, P. F.; Krasnikov, V. V.; Brouwer, H. -J.; Hadziioannou, G. Excimer Luminescence from Single Crystals and Films of a CyanoSubstituted Phenylene-Vinylene Model Compound. Chem. Phys. 1999, 241, 139–154. (27) Liu, Y.; Tao, X.; Wang, F.; Shi, J.; Sun, J.; Yu, W.; Ren, Zou, D.; Jiang, M. Intermolecular Hydrogen Bonds Induce Highly Emissive Excimers: Enhancement of Solid State Luminescence. J. Phys. Chem. C 2007, 111, 6544–6549. (28) Girlando, A.; Ianelli, S.; Bilotti, I.; Brillante, A.; Valle, R. G. D.; Venuti, E.; Campione, M.; Mora, S.; Silvestri, L.; Spearman, P.; Tavazzi, S. Spectroscopic and Structural Characterization of Two Polymorphs of 1,1,4,4-Tetraphenyl-1,3-butadiene. Cryst. Growth Des. 2010, 10, 2752–2758. (29) Dong, J.; Solntsev, K. M.; Tolbert, L. M. Activation and Tuning of Green Fluorescent Protein Chromophore Emission by Alkyl Substituent-Mediated Crystal Packing. J. Am. Chem. Soc. 2009, 131, 662–670. (30) Kim, J.; Swager, T. M. Control of Conformational and Interpolymer Effects in Conjugated Polymers. Nature 2004, 411, 1030–1034. (31) Thomas, R.; Varghese, S.; Kulkarni, G. U. The Influence of Crystal Packing on the Solid State Fluorescence Behavior of Alkyloxy Substituted Phenyleneethynylenes. J. Mater. Chem. 2009, 19, 4401–4406. (32) Mizobe, Y.; Tohnai, N.; Miyata, M.; Hasegawa, Y. A Tunable Solid-State Fluorescence System Consisting of Organic Salts of Anthracene-2,6-disulfonic acid with Primary Amines. Chem. Commun. 2005, 1839–1841. (33) Mizobe, Y.; Hinoue, T.; Yamamoto, A.; Hisaki, I.; Miyata, M.; Hasegawa, Y.; Tohnai, N. Systematic Investigation of Molecular Arrangements and Solid-State Fluorescence Properties on Salts of Anthracene-2,6-disulfonic Acid with Aliphatic Primary Amines. Chem.—Eur. J. 2009, 15, 8175–8184. (34) Imai, Y.; Murata, K.; Asano, N.; Nakano, Y.; Kawaguchi, K.; Harada, T.; Sato, T.; Fujiki, M.; Kuroda, R.; Matsubara, Y. Selective Formation and Optical Property of a 21-Helical Columnar Fluorophore Composed of Achiral 2-Anthracenecarboxylic Acid and Benzylamine. Cryst. Growth Des. 2008, 8, 3376–3379. (35) Zhang, Z.; Zhang, Y.; Yao, D.; Bi, H.; Javed, I.; Fan, Y.; Zhang, H.; Wang, Y. Anthracene-Arrangement-Dependent Emissions of Crystals of 9-Anthrylpyrazole Derivatives. Cryst. Growth Des. 2009, 9, 5069–5076. (36) Davis, R; Das, S. Fluorescent Aggregates of 1-(p-Butyloxyphenyl)4-(p-cyanophenyl) Buta-1E,3E-diene: Temperature Sensing and Photoimaging Applications. J. Fluoresc. 2005, 15, 749–53. (37) Mutai, T.; Satou., H.; Araki, K. Reproducible OnOff Switching of Solid-State Luminescence by Controlling Molecular Packing through Heat-Mode Interconversion. Nat. Mater. 2005, 4, 685.

(1) Forrest, S. R. The Path to Ubiquitous and Low-Cost Organic Electronic Appliances on Plastic. Nature 2004, 428, 911–918. (2) Meijer, E. W.; Schenning, A. P. H. J. Chemistry: Material Marriage in Electronics. Nature 2002, 419, 353–354. (3) Koren, A. B.; Curtis, M. D.; Francis, A. H.; Kampf, J. W. Intermolecular Interactions in π-Stacked Conjugated Molecules. Synthesis, Structure and Spectral Characterization of Alkyl Bithiazole Oligomers. J. Am. Chem. Soc. 2003, 125, 5040–5050. (4) Curtis, M. D.; Cao, J.; Kampf, J. W. Solid-State Packing of Conjugated Oligomers: From π-Stacks to the Herringbone Structure. J. Am. Chem. Soc. 2004, 126, 4318–4328. (5) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London, 1970. (6) Hong, Y.; Lama, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission: Phenomenon, Mechanism and Applications. Chem. Commun. 2009, 4332–4353. (7) Desiraju, G. R. Designer Crystals: Intermolecular Interactions, Network Structures and Supramolecular Synthons. Chem. Commun. 1997, 1475–1482. (8) Anthony, J. E. Functionalized Acenes and Heteroacenes for Organic Electronics. Chem. Rev. 2006, 106, 5028–5048. (9) Murphy, A. R.; Frehet, J. M. J. Organic Semiconducting Oligomers for Use in Thin Film Transistors. Chem. Rev. 2007, 107, 1066–1096. (10) Kasha, M. Energy Transfer Mechanism and the Molecular Exciton Model for Molecular Aggregate. Radiat. Res. 1963, 20, 55–71. (11) Cornil, J.; dos Santos, D. A.; Crispin, X.; Silbey, R.; Bredas, J. L. Influence of Interchain Interactions on the Absorption and Luminescence of Conjugated Oligomers and Polymers: A Quantum-Chemical Characterization. J. Am. Chem. Soc. 1998, 120, 1289–1299. (12) Cornil, J.; Beljonne, D.; Calbert, J.-P.; Bredas, J.-L. Interchain Interactions in Organic π-Conjugated Materials: Impact on Electronic Structure, Optical Response and Charge Transport. Adv. Mater. 2001, 13, 1053–1067. (13) Da Como, E.; Loi, M. A.; Murgia, M.; Zamboni, R.; Muccini, M. J-Aggregation in R-Sexithiophene Submonolayer Films on Silicon Dioxide. J. Am. Chem. Soc. 2006, 128, 4277–4281. (14) Kiriy, N.; Kiriy, A.; Bocharova, V.; Stamm, M.; Richter, S.; Pl€otner, M.; Fischer, W.-J.; Krebs, F. C.; Senkovska, I.; Adler, H.-J. Conformation, Molecular Packing, and Field Effect Mobility of Regioregular β, β0 -Dihexylsexithiophene. Chem. Mater. 2004, 16, 4757–4764. (15) Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.; Servet, B.; Ries, S.; Alnot, P. Molecular Engineering of Organic Semiconductors: Design of Self-assembly Properties in Conjugated Thiophene Oligomers. J. Am. Chem. Soc. 1993, 115, 8716. (16) Gierschner, J.; Ehni, M.; Egelhaaf, H.-J.; Medina, B. M.; Beljonne, D.; Benmansour, H.; Bazan, G. C. Solid-State Optical Properties of Linear Polyconjugated Molecules: π-Stack Contra Herringbone. J. Chem. Phys. 2005, 123, 144914–144919. (17) Xie, Z.; Yang, B.; Li, F.; Cheng, G.; Liu, L.; Yang, G.; Xu, H.; Ye, L.; Hanif, M.; Liu, S.; Ma, D.; Ma, Y. Cross Dipole Stacking in the Crystal of Distyrylbenzene Derivative: The Approach Toward High Solid-State Luminescence Efficiency. J. Am. Chem. Soc. 2005, 127, 14152–14154. (18) Xie, Z.; Wang, H.; Li, F.; Xie, W.; Liu, L.; Yang, B.; Ye, L.; Ma, Y. Crystal Structure of a Highly Luminescent Slice Crystal Grown in the Vapor Phase: A New Polymorph of 2,5-Diphenyl-1,4-distyrylbenzene. Cryst. Growth Des. 2007, 7, 2512–2516. (19) Kunzelman, J.; Kinami, M.; Crenshaw, B. R.; Protasiewicz, J. D.; Weder, C. Oligo(p-Phenylene Vinylene)s as a “New” Class of Piezochromic Fluorophores. Adv. Mater. 2008, 20, 119. (20) Yoon, S.-J.; Chung, J. W.; Gierschner, J.; Kim, K. S.; Choi, M.-G.; Kim, D.; Park, S. Y. Multistimuli Two-Color Luminescence Switching via Different Slip-Stacking of Highly Fluorescent Molecular Sheets. J. Am. Chem. Soc. 2010, 132, 13675–13683. (21) Zhang, G.; Lu, J.; Sabat, M.; Fraser, C. L. Polymorphism and Reversible Mechanochromic Luminescence for Solid-State Difluoroboron Avobenzone. J. Am. Chem. Soc. 2010, 132, 2160–2162. 873

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