J. Phys. Chem. C 2009, 113, 11927–11935
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Correlation between Solid-State Photophysical Properties and Molecular Packing in a Series of Indane-1,3-dione Containing Butadiene Derivatives† N. S. Saleesh Kumar,‡ Shinto Varghese,‡ C. H. Suresh,§ Nigam P. Rath,| and Suresh Das*,‡ Photosciences and Photonics Section, Chemical Sciences and Technology DiVision, National Institute for Interdisciplinary Science and Technology, TriVandrum-695 019, Kerala, India, Computational Modeling and Simulation Section, National Institute for Interdisciplinary Science and Technology, CSIR, TriVandrum-695 019, Kerala, India, and Department of Chemistry and Biochemistry and Center for Nanoscience, UniVersity of Missouri-St. Louis, St. Louis, Missouri 63121 ReceiVed: March 19, 2009; ReVised Manuscript ReceiVed: May 4, 2009
The solid-state photophysical and photochromic properties and the molecular packing in single crystals of a series of donor-acceptor-substituted butadiene derivatives with alkoxy groups as donor and indane-1,3dione as acceptor are reported. These materials showed significant enhancement and red-shift in fluorescence in the solid state compared to that in solution. The single crystal analysis of these derivatives indicated that these effects could be attributed to both improved intramolecular charge transfer due to planarization of the molecules and to intermolecular exciton coupling between adjacent molecules leading to aggregate fluorescence. The character of the aggregate formed (H- or J-type) and extent of aggregation were strongly dependent on the length of the alkyl substituent, and these differences could be correlated to variations in the molecular packing observed in their single crystals. Some of the derivatives could be super cooled to a metastable glassy state with significantly different optical properties. Transformation from crystalline to a highly stable glassy form could also be induced by light, making these materials useful for recording optical images. Introduction In recent years the synthesis and study of photophysical properties of organic π-conjugated materials has been attracting increasing attention, in view of their potential application in electronic and optoelectronic devices.1,2 To achieve successful incorporation of such conjugated materials into functional devices it is necessary to understand and to be able to manipulate their solid-state optical properties,3-6 for which structure-property relationships need to be established. The absorption spectra of π-conjugated systems are known to undergo a red-shift in the solid state compared to that in the solution phase, resulting in reduction of luminescence efficiency.7 These effects are generally attributed to formation of molecular aggregates. Several recent studies have, however, shown that molecular aggregation can lead to significant enhancement in luminescence efficiency.8 Such aggregate-induced emission (AIE) has been attributed to various factors including restricted molecular motion in the aggregates and planarization of the molecules.8c-g Aggregation of molecules can also lead to exciton coupling. In this context formation of J aggregates is especially interesting, since this can lead to strongly emissive systems.8h,i The nature of the aggregates formed in the bulk state and their optical properties are determined by the relative positions of adjacent molecules and their dipole moments.9 The molecular packing in the bulk is in turn controlled by a delicate balance of various weak intermolecular forces involving both the
CHART 1
π-conjugated units as well as remote functional groups which are not in conjugation.10 Variations in the structure of remote functional groups can therefore be utilized as an effective strategy to fine-tune the solid-state photophysical properties of a particular chromophore.11 Alternatively, the molecular packing of chromophores in the bulk can be altered by external stimuli such as heat, light, pressure, and chemicals.12 Materials which can respond to external stimuli in such a manner have significant potential for application in imaging13 and sensing14 devices. Here we report on a detailed study of the role of molecular packing in determining the solid-state luminescence properties of a series of donor-acceptor-substituted butadiene molecules (Chart 1). Variations in the molecular packing induced by external stimuli such as heat and light resulted in significant changes in the luminescence properties of two of the derivatives, BINC8 and BINC12, and the use of these processes in imaging applications was explored.
†
Part of the “Hiroshi Masuhara Festschrift”. * To whom correspondence should be addressed. E-mail: sureshdas55@ gmail.com. ‡ Photosciences and Photonics Section, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology. § Computational Modeling and Simulation Section, National Institute for Interdisciplinary Science and Technology. | Department of Chemistry and Biochemistry and Center for Nanoscience, University of Missouri-St. Louis.
Experimental Section Instrumentation. The reagents and materials for synthesis were used as obtained from Aldrich and S. D. Fine chemical suppliers. The butadiene derivatives were synthesized in a multistep process using reported procedures15 and their structures were established by FT-IR, 1H, and 13C NMR analysis.16 Purity
10.1021/jp902482r CCC: $40.75 2009 American Chemical Society Published on Web 05/19/2009
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J. Phys. Chem. C, Vol. 113, No. 27, 2009
Kumar et al.
TABLE 1: Summary of Crystallographic Data for the Butadiene Derivatives empirical formula mol wt crystal system space group Z a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 dcalc, Mg/cm3 η, mm-1 total no. of reflns no. of unique reflns Rint final R indices R1, wR2 R indices (all data) R1, wR2
BINC1
BINC4
BINC8
BINC12
C19 H14 O3 290.30 monoclinic P21/n 4 3.85440(10) 25.3459(7) 14.3517(4) 90 92.410(1) 90 1400.82(7) 1.376 0.093 15896 2756 0.032 0.0589, 0.1211 0.0662, 0.1248
C22 H20 O3 332.38 triclinic P1j 6 10.2989(8) 14.2552(11) 18.2675(12) 101.355(5) 92.229(5) 101.836(5) 2564.7(3) 1.291 0.085 58395 9082 0.14 0.0749, 0.1570 0.1781, 0.2099
C26 H28 O3 388.48 triclinic P1j 2 9.6787(3) 11.4861(4) 11.5181(4) 105.191(2) 105.812(2) 111.716(2) 1046.03(6) 1.233 0.079 17785 5599 0.017 0.0428, 0.1190 0.0508, 0.1247
C30H36 O3 444.59 triclinic P1j 2 5.1374(3) 15.7168(9) 16.2649(9) 108.964(3) 90.663(3) 97.203(3) 1230.28(12) 1.200 0.076 32017 32017 0.0320 0.0430, 0.1131 0.0623, 0.1272
of the material was ascertained by crystallizing the samples from 10% ethyl acetate:hexane solvent mixtures. Fluorescence quantum yields, with an estimated reproducibility of around 10%, were determined by using 10-methylacridinium trifluoromethane sulfonate in water (ΦF ) 0.99)17 as standard. Measurements of solid-state photoluminescence (PL) were carried out by using the front face emission scan mode on a SPEX Fluorolog F112X spectrofluorimeter. The PL quantum yield in thin films (Φfilm) was determined by using a calibrated integrating sphere system. Steady-state photolysis was carried out with a 200 W highpressure mercury lamp in combination with a 445 nm bandpass filter with a half bandwidth (HBW) of 100 nm. The measured intensity at the irradiated region was 8 mW/cm2. For measuring the fluorescence of the crystalline forms the materials were placed between two quartz slides. Thin films were prepared by heating the crystalline powders above their respective melting points followed by cooling to room temperature. The quantum yield was determined by comparing the spectral intensities of the lamp and the sample emission as reported in the literature.18 By using this experimental setup and the integrating sphere system, the solid-state fluorescence quantum yield of a thin film of the standard green OLED material tris-8-hydroxyquinolinolato aluminum (Alq3) was determined to be 19((2)%, which is consistent with previously reported values.19 Fluorescence lifetimes were measured with an IBH (FluoroCube) TimeCorrelated Picosecond Single Photon Counting (TCSPC) system. The solutions were excited with a pulsed diode laser (
