Circularly Polarized Luminescence in Chiral Aggregates

Letter pubs.acs.org/JPCL

Circularly Polarized Luminescence in Chiral Aggregates: Dependence of Morphology on Luminescence Dissymmetry Jatish Kumar, Takuya Nakashima,* Hiroyuki Tsumatori, and Tsuyoshi Kawai* Graduate School of Materials Science, Nara Institute of Science and Technology, NAIST, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan S Supporting Information *

ABSTRACT: The self-assembly of a chiral perylene bisimide bichromophoric derivative possessing a 1,1′-binaphthalene bridge was investigated by adopting two different methodologies, leading to the formation of aggregates with dissimilar morphologies. The chiral nature of the aggregated structures was optically probed with the help of circular dichroism (CD), vibrational circular dichroism (VCD), and circularly polarized luminescence (CPL). The one-dimensional aggregates formed in methylcyclohexane (MCH) exhibited twice the value of luminescence dissymmetry factor (glum) when compared with the spherical aggregates formed in chloroform at higher concentration. The summation of excitonic couplings between the individual chromophoric units in an aggregated system is responsible for the remarkably high luminescence dissymmetry exhibited by the chiral aggregates. The nanostructures could be successfully embedded into polymer films, leading to the fabrication of solid-state materials with high CPL dissymmetry that can find novel applications in chiroptical sensing, memory, and light-emitting devices based on organic nanoparticles. SECTION: Spectroscopy, Photochemistry, and Excited States high fluorescence efficiency, good solubility, and relatively high CD and CPL dissymmetry derived from the exciton coupling between PBI units arranged in a chiral fashion.27−30 The compounds were originally reported by Langhals et al., where basic optical properties including CD spectra were described.27 We then appended the CPL characteristic in a molecule28 and later observed a gradual increase in the CPL dissymmetry by increasing the concentration in a good solvent, chloroform.29 Further, Spano and co-workers have derived curious photophysical parameters of the system with the help of theoretical calculations.30 Recently, the molecule was used by Cohen and co-workers for the demonstration of enhanced enantioselectivity in the excitation of chiral molecules by superchiral light.31 Despite the recent advances in this field of research, no systematic exploration of the variation of CPL dissymmetry as a function of the morphology of aggregated structures has been carried out. Herein, we investigate the CPL dissymmetry of different morphological aggregates formed from the bichromophoric system, the molecular structures of the R and S isomers of which are shown in Chart 1. Different morphological features for the self-assembled molecular aggregates could be obtained by adopting two methodologies, (i) by varying the concentration and (ii) by varying the solvent composition. The morphological dependence on the CPL studies showed that the one-dimensional (1D) molecular wires exhibit higher chiral dissymmetry in luminescence in comparison with the crystalline spherical nanoparticles.

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nderstanding the complex nature of the organization of molecules in a self-assembled system is essential for the design and fabrication of organic-based photonic and electronic devices currently under development.1−4 One such class of molecules for understanding the mode of assembly in organic chromophoric systems is the perylenebisimdes (PBIs) that possess excellent photophysical and chiroptical properties.5−8 The mode of assembly and the nature of fluorescence in these classes of compounds can be modulated by introducing variations in parameters such as the nature of functional groups, solvent, and coexisting chemical substances.9−11 One of the interesting aspects with these systems is that they offer a high degree of helicity to the aggregates through the introduction of chiral centers on the ends of the molecules, and studies focused in this direction still remain a challenging area in the field of supramolecular chemistry.12,13 The groundstate and excited-state chirality in such molecular systems have been investigated with the help of circular dichroism (CD) and circularly polarized luminescence (CPL).14,15 However, similar studies on supramolecular systems still rely mostly on CD,14 and there have been few reports on the investigations of the CPL properties of aggregated systems.16−20 Intense CPL signals were observed from enantiomeric helicenes21,22 and aggregates of π-conjugated polymers with chiral side chains.23,24 Recently, the technique has been utilized for investigating the excited-state properties of helically self-assembled molecular aggregates.16−20 The excited-state properties of bichromophores possessing PBI units have gained much attention in recent years.14,25,26 Among these systems, binaphthalene compounds tethering two PBIs have been demonstrated as a chiral emitting molecule with © 2013 American Chemical Society

Received: December 3, 2013 Accepted: December 30, 2013 Published: December 30, 2013 316

dx.doi.org/10.1021/jz402615n | J. Phys. Chem. Lett. 2014, 5, 316−321

The Journal of Physical Chemistry Letters

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Chart 1. Chemical Structures of the S and R Isomers of the Compound 1 [R = CH(C6H13)2]

emission intensity at 540 nm can be attributed to the strong overlap between the emission and absorption spectra of the molecule (Figure S1c, Supporting Information), leading to reabsorption of the emitted photons by ground-state molecules. The nonlinear increase of the emission at 585 nm, where there is no considerable spectral overlap, is indicative of a selfquenching mechanism originating from intermolecular interactions at higher concentration. An increase in the intensity at 630 nm confirms the formation of molecular aggregates in solution at higher concentrations. It can be concluded that both reabsorption as well as self-quenching due to intermolecular interactions are responsible for the characteristic spectral features observed at higher concentrations. The nature of different emitting species in solution was investigated with the help of time-correlated single-photon counting (TCSPC) and the absolute quantum yield measurements. At lower concentration (1 × 10−5 M), the molecules in chloroform exhibited a monoexponential decay with a lifetime of 4.3 ns for emission monitored at 540 nm (Table S1, Supporting Information). A biexponential decay profile with lifetime values of 2.7 (97%) and 22.1 ns (3%) was observed at a higher concentration of 1 × 10−3 M. The long-lived species was prominent when the decay was monitored at 630 nm, and the contribution of this emission increased with increasing concentration (23.3 ns; 92% at 3 × 10−3 M). The dilute solution of molecules in chloroform exhibited a remarkably high quantum yield of 0.87, and a gradual quenching in fluorescence was observed with increasing concentration, resulting in a value of 0.42 for 3 × 10−3 M solution (Table S2, Supporting Information). The observed changes in the lifetime and fluorescence quantum yield values together with a considerable spectral shift in fluorescence are indicative of the specific intermolecular interactions operating at higher concentration leading to the formation of new emissive species in the ground state. The morphology of aggregated structures was directly probed by subjecting the sample to transmission electron microscopic (TEM) and atomic force microscopic (AFM) investigations. At higher concentrations, both R and S isomers of the compound formed spherical aggregates with an average diameter of 28 ± 3 nm (Figures S2 and S3, Supporting Information). Even though a higher fraction of the particles was spherical in nature, aggregated structures with other geometries were also observed. With an increase in concentration, a substantial increase in the number of aggregated structures was observed accompanied by a small increase in size. The lattice planes observed in the high-resolution TEM images and the diffraction pattern collected from the nanostructures suggest that the aggregated particles have ordered molecular stacking and/or possess crystalline nature to some extent (Figure 1a). At higher concentrations, the solvent does not dissolve the

The primary photophysical investigations of compound 1 were carried out in chloroform at different concentrations. A dilute solution of the molecule (1.0 × 10−5 M) in chloroform exhibited absorption bands at 528, 491, and 461 nm with a shoulder at 430 nm corresponding to the 0−0, 0−1, 0−2, and 0−3 vibrational transitions, respectively (Figure S1a, Supporting Information). These spectral features are intrinsically the same as those reported previously for similar molecules in their monomeric state.5−11 The fluorescence spectrum of the dilute solution of the molecule (1.0 × 10−5 M) displayed clearly separated peaks at 540 and 580 nm (Figure 1c). With an

Figure 1. (a) TEM image of the molecular aggregates of the R isomer of the compound in chloroform (3 × 10−3 M). The inset shows a highresolution TEM image of a single nanocrystal and the corresponding electron diffraction pattern. (b) VCD spectra of the R (solid lines) and S (broken lines) isomers of the molecule in chloroform (1 × 10−3 M). (c,d) Normalized fluorescence (R isomer) and the corresponding CPL spectra of the molecule in chloroform at different concentrations, 1 × 10−5 (black trace), 1 × 10−3 (red trace), and 3 × 10−3 M (blue trace).

increase in concentration (1.0 × 10−3 M), a reversal in the relative intensities between (0−0) and (0−1) peaks was observed. Interestingly, at still higher concentration (3.0 × 10−3 M), fluorescence quenching was observed at the main emission band, associated with a progressed weak band at 630 nm, which could be monitored from the normalized emission intensities. To get a deeper insight into the behavior of the molecule in solution, the concentration dependence of emission intensities at 540 and 580 nm was plotted (Figure S1d, Supporting Information). With increasing concentration, the emission intensity of the 540 nm band increases and then decreases with a slight red shift. However, the emission at 580 nm remains to be intensified and plateaus within the concentration range examined here. The decrease in the 317



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intensities of left and right circularly polarized light.38 The value of glum exhibited no variation at the monomeric region (at 540 nm); however, a gradual increase was observed in the aggregate region of the spectrum (at 630 nm) with increasing concentration (Figure S14a, Supporting Information). The glum value increased from ∼3 × 10−3 for dilute solution (1 × 10−5 M) to ∼8 × 10−3 for a highly concentrated solution (3 × 10−3 M). An increase in the glum value at higher wavelength is indicative of the contribution of the chiral aggregates to the enhanced fluorescence dissymmetry; however, the values were only slightly higher than those reported on similar organic chromophoric systems.27−30 Further, we extended the spectroscopic and microscopic investigations to different solvent systems. Systematic investigations on the absorption and fluorescence properties of dilute solutions of the compound revealed that the molecules remain stable in the monomeric state in solvents such as chloroform, toluene, and tetrahydrofuran at low concentration, whereas they undergo aggregation in nonpolar solvents like decane and methylcyclohexane (MCH) (Figure S7, Supporting Information). Remarkable spectral features corresponding to the monomeric and aggregated states were observed in chloroform and MCH, and hence, further studies were pursued in chloroform/MCH mixed solvent. Because the molecules were hardly dissolved in pure MCH, the aggregation behavior was monitored by diluting the chloroform dispersion with certain amounts of MCH. The spectral properties in varying chloroform/MCH solvent compositions revealed that the molecules remain stable in the monomeric state up to 80% of MCH and undergo aggregation for a composition above 90% of MCH, wherein the spectral properties corresponding to the aggregated state were observed, a red shift in the absorption spectrum with reversal in the peak intensities and quenching of fluorescence (Figure S8, Supporting Information). The fluorescence quantum yield decreased from 0.87 in chloroform to 0.44 in the MCH-rich solvent system. The fluorescence lifetime data showed the presence of a short-lived species with a lifetime of 3.9 ns (16%) and a relatively long-lived species with a lifetime of 13.6 ns (84%) for emission monitored at 630 nm. The aggregates showed good stability and solubility in solvents with higher MCH content. TEM and AFM images exhibited fibers that possessed a width of 4−6 nm, almost corresponding to a bimolecular thickness, and a length of a few micrometers (Figures S10 and S11 in the Supporting Information). The poor solubility of binaphthyl and PBI units compared to that of the alkyl part in MCH might result in the formation of very thin 1D assemblies. The scanning electron microscopic (SEM) images exhibited left- and right-handed fibers for the R and S isomers, respectively (Figures 2b and S9, Supporting Information). The XRD profile of a sample prepared from the MCH-rich solution showed a sharp peak at 2θ = 3.55° and a broad peak at 20.12° corresponding to d spacing of 2.48 and 0.44 nm, respectively (Figure S12b, Supporting Information). The peak at 2.48 nm corresponds to the molecular length, and that at 0.44 nm indicates a looser π−π stacking compared to spherical particles. CD can be employed as a powerful technique for investigating the chiral nature of supramolecular systems. The self-assembled structures of the compound exhibited mirror image CD with positive and negative first Cotton effects for S and R isomers, respectively (Figure 2c). The sign of the Cotton effect is well in agreement with the direction of twist of fibers observed in the SEM images. The CPL profile in MCH-rich

molecules completely but should assist them to crystallize into stable nanoparticles in solution. Formation of similar crystalline nanostructures during the self-assembly of molecules has already been reported on PBI systems.32,33 Dynamic light scattering (DLS) measurements showed a gradual increase in the size of the particle with an increase in concentration (Figure S4, Supporting Information). Particles with an average hydrodynamic diameter of 19 and 26 nm could be observed in solutions at concentrations of 2 × 10−3 and 3 × 10−3 M, respectively. The sizes were consistent with those observed from the microscopic measurements, suggesting that the molecular aggregates were already formed in solution and not upon evaporation of solvent. The XRD pattern of the sample displayed a weak band at 2θ = 3.48° and two broad bands at approximately 11.4 and 22.4° corresponding to d spacing of 2.54, 0.78, and 0.39 nm, respectively (Figure S12a, Supporting Information). These features are indicative of lamellar packing with a π−π stacking distance of 0.39 nm in a spherical aggregate.34 To understand the nature of chirality in the monomeric and aggregated states, CD, vibrational CD (VCD), and CPL measurements were carried out at different concentrations. CD measurements of R and S isomers of the molecule in chloroform (1 × 10−5 M) indicate clear circularly polarized dissymmetry in the absorption band corresponding to the perylene units between 450 and 550 nm (Figure S5, Supporting Information). The CD response of the dilute solution of molecules in chloroform originates from the chiral center present on the binaphthalene unit. Better insight into the nature of chiral aggregates at higher concentration was obtained from the VCD spectroscopy, and this method has been recently utilized for understanding the chirality in the supramolecular architectures of fibrils and aggregates.35−37 The two isomers of the molecule exhibited mirror image VCD spectra (Figure 1b), and the peak wavenumbers matched well with the corresponding infrared spectra (Figure S6, Supporting Information). Positive and negative couplets corresponding to ring stretching at 1320 and 1595 cm−1 were observed for S and R isomers, respectively. This suggested the chiral arrangement of PBI units in clockwise and anticlockwise manners, respectively. In addition, peak patterns observed between 1640 and 1740 cm−1 corresponding to imide CO stretching were indicative of the left and right helical twisting arrangement of the CO moieties within the aggregated structures for the R and S isomers, respectively.35,36 The CPL spectra revealed more direct evidence for chiral dissymmetry in fluorescence of the aggregates. CPL at the respective emission wavelength originates from the chiral centers existing in a dissymmetric environment in the excited state. At varying concentrations, the S and R isomers of the molecule exhibited mirror image CPL profiles at the corresponding fluorescence wavelengths with positive and negative signs, respectively (Figure 1d). The sign of CPL matches with the corresponding CD spectra at the longest wavelength, suggesting that the bands arise solely from the same transitions. At low concentration (1 × 10−5 M), peaks were observed at 540 and 580 nm with a higher ratio between the A0−0/A1−0 peak intensities. With increasing concentration, the CPL profile exhibited spectral variations in accordance with the fluorescence changes, a gradual increase in intensity of the peak at 630 nm finally leading to dominance of this peak at a concentration of 3 × 10−3 M. The degree of CPL is given by the luminescence dissymmetry factor, which is defined as glum = 2(IL − IR)/(IL + IR), where IL and IR are the luminescence 318



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Figure 3. Plot of glum versus concentration for R (solid lines) and S (broken lines) isomers of the molecule in chloroform (red traces) and a mixture (1:19) of chloroform/MCH (blue traces).

Further, temperature-dependent studies were carried out on the self-assembled structures to understand the stability of the aggregates at higher temperatures. Upon increasing the temperature, a gradual decrease in the intensity of the aggregated bands with the regaining of the monomeric absorption spectral features was observed (Figure 4a). An

Figure 2. (a,b) SEM images at different magnifications of the R isomer of the molecule in a mixture (1:19) of chloroform/MCH. (c,d) CD and the corresponding absorption (R isomer) spectra (red lines) and CPL and the corresponding fluorescence (R isomer) spectra (blue lines) of R (solid lines) and S (broken lines) isomers of the molecule in a mixture (1:19) of chloroform/MCH (1 × 10−5 M).

solvents exhibited peaks at the aggregate region of the spectrum (Figure 2c). Interestingly, the glum value showed a remarkable increase from ∼3 × 10−3 for the monomeric state in chloroform (at 540 nm) to ∼1.8 × 10−2 for the chiral aggregates in a mixture (1:19) of chloroform/MCH (at 630 nm). The sign of the CD and CPL signals of the stereoisomers remained the same in the monomeric as well as aggregated states, suggesting that the peaks arises from the same transitions, (i) from the intramolecular excitations of the monomeric state of the chiral molecule in chloroform and (ii) from the exciton coupled state of the helical aggregates in MCH-rich solvent. The intramolecular exciton coupling dominates in the monomeric state, whereas the excitons are delocalized along the aggregated structures, leading to predominance of the intermolecular coupling in the self-assembled state. The summation of interand intramolecular couplings may contribute to the significantly enhanced CPL activity in the helical nanowires. The concentration-dependent CPL investigations in the MCH-rich system showed a steady increase in the CPL intensity with increasing concentration, consistent with the fluorescence spectral changes (Figure S13, Supporting Information). Unlike the case with pure chloroform, the glum value remained constant (∼1.8 × 102) for varying concentrations of the molecule in MCH-rich solvent, suggesting that there is no further induction of chiral dissymmetry to the aggregated structures with an increase in concentration (Figure S14b, Supporting Information). A plot of glum against the concentration of the molecule exhibited a gradual increase in the luminescence dissymmetry with increasing concentration in pure chloroform, whereas similar studies in a mixture (1:19) of chloroform/MCH exhibited no considerable variations in the glum values (Figure 3). A comparison of the CPL responses of the aggregates suggests the induction of higher-order chirality to the 1D nanowires formed in MCH-rich solvent rather than the crystalline nanoparticles in pure chloroform. The stronger π−π stacking of PBI units in crystalline nanoparticles as observed in the XRD study might cost the chiral arrangement of PBI units.25 The glum value of 0.018 observed for the helical nanowires is among the highest values reported on such chromophoric aggregates in the solution phase.16−20

Figure 4. Temperature-dependent (a) absorption, (b) fluorescence, (c) CD, and (d) CPL spectral changes of the R (red lines) and S (blue lines) isomers of the molecule in a mixture (1:19) of chloroform/ MCH. The R isomer was used for absorption and fluorescence measurements.

enhancement in fluorescence with the regaining of the structured emission bands was observed at elevated temperatures (Figure 4b). These spectral features are the characteristic of the monomeric form of the molecule, indicating that the aggregated structures dissociate to monomers at higher temperatures. The CD responses of the R and S stereoisomers of the molecule displayed similar spectral features at elevated temperatures; a gradual suppression in the peaks corresponding to the transition in the aggregated structures and progression of new peaks in the monomeric bands were observed (Figure 4c). The broad CPL band at 630 nm vanished with the formation of structured CPL peaks at 540 and 580 nm at higher temperature (Figure 4d). The glum value showed a sudden decrease from ∼0.018 (at 630 nm) to ∼0.005 (at 540 nm). Further, upon cooling the solution to room temperature, the molecules recovered to chiral aggregated structures, which were evident from the specific spectral responses, and this cycle could be repeated a number of times without the degradation of the molecules. To the best of our knowledge, this is the first 319



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assembled state. An increase in concentration resulted in spherical aggregates in chloroform, whereas varying the solvent system led to the formation of 1D fibrous structures. Both of the aggregated structures exhibited high luminescence dissymmetry in solution as well as in the solid state when compared with the monomeric state of the molecule. The molecules undergo self-assembly through the π−π interaction of the perylene units, and the sum of excitonic couplings between the individual chromophoric units contributes to the high CPL dissymmetry of the nanostructures. The higher-order excitonic coupling in the 1D nanowires relative to that in the spherical particles makes them superior in terms of the luminescence dissymmetry factor. The value of glum exhibited by the fibrous structures is among the highest values reported for organic self-assembled systems in solution to date. The morphological dependence on CPL dissymmetry of fluorescent nanoparticles can lead to controlled modulation of chiroptical properties, making these materials potential candidates for applications in the field of chiroptical sensors and as sources for circularly polarized light.

attempt toward temperature-dependent switching of CPL dissymmetry in self-organized multichromophoric structures. As is well-known, the utilization of any material for practical application demands the demonstration of its properties on solid surfaces. The nanostructures in chloroform and MCHrich solvents were successfully embedded into polymethyl(methacrylate) (PMMA) polymer matrixes for the fabrication of flexible and self-standing uniform films. The polymer films of the molecular aggregates were found to be highly fluorescent and CPL active (Figure 5a). The films with aggregates in

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Absorption, fluorescence, CD, and CPL spectra at different experimental conditions, TEM, AFM, and SEM images of the aggregates, glum mapping plots, and lifetime and quantum yield data. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors

*E-mail: [email protected] (T.N.). *E-mail: [email protected] (T.K.).

Figure 5. (a) Photographs of fluorescent polymer films under 365 nm UV illumination; films were formed from (i−iv) chloroform and (v) a mixture (1:19) of chloroform/MCH. Fluorescence and the corresponding CPL spectra from polymer films of R (solid lines) and S (broken lines) isomers of the compound formed from (b) chloroform at a concentration of 1 × 10−3 (red lines) and 3 × 10−3 M (blue lines) and (c) a mixture (1:19) of chloroform/MCH (1 × 10−4 M). The R isomer was used for photographs, absorption, and fluorescence measurements.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Ms. S. Fujita for the measurement of highresolution TEM. We also acknowledge Hitachi HighTechnolgies and Jasco Corporations for the measurement of SEM with SU9000 and VCD with FVS-6000, respectively. This work was partly supported by Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan under the Green Photonics Project at NAIST and the Grand-in-Aid for Scientific Research A (25248019).

chloroform at a concentration of 1 × 10−3 M showed peaks at 540 and 590 nm whereas a single peak at 630 nm was observed at concentrations of 3 × 10−3 M (Figure 5b). The polymer films with 1D aggregates exhibited a fluorescence peak at 630 nm, and the CPL at the corresponding wavelength exhibited negative and positive signs for the R and S stereoisomers, respectively (Figure 5c). CPL mapping analysis is a new technique that confirms the consistency of luminescence dissymmetry at different domains of the aggregated structures (see Figure S15, Supporting Information for details). The mapping analysis carried out at different points on the polymeric surface showed an average glum value of ∼0.01 for CPL collected at 630 nm from the polymer films in chloroform (3 × 10−3 M). Similar analysis on the 1D aggregate-entrapped polymer films showed remarkably high luminescence dissymmetry (glum = ∼0.02). The sign of CPL in all of the cases was consistent with that of corresponding solution-phase spectra, suggesting that the self-assembled structures maintain their chiroptical properties even when trapped inside of polymer matrixes. In summary, we have investigated the chiroptical properties of a bichromophoric system in the monomeric as well as self-

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REFERENCES

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Circularly Polarized Luminescence in Chiral Aggregates

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