Aggregation Dependent S1 and S2 Dual Emissions of Thiophene-Acrylonitrile-Carbazole Oligomer Xuanjun Zhang,† Kian Ping Loh,*,† Michael B. Sullivan,‡ Zhi-Kuan Chen,§ and Minghui Liu|
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2543–2546
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543, Singapore, Institute of High Performance Computing, I Science Park Road, #01-01 The Capricorn, Singapore 117528, Singapore, Institute of Material Research and Engineering, 3 Research Link, Singapore, and NUS Nanoscience and Nanotechnology InitiatiVe, Singapore ReceiVed February 20, 2008; ReVised Manuscript ReceiVed March 17, 2008
ABSTRACT: Dual emissions from S1 (blue region) and S2 (UV region) states of a chromophore with donor-acceptor structure were observed at room temperature. The photophysical properties of the chromophore depend critically on the packing of the molecules. The molecule exhibits weak S1 emission and strong S2 emission; however, when it undergoes solid aggregation, the S1 emission is enhanced while the S2 emission is quenched. The H-packing (face-to-face packing) of the molecules is revealed as the key factor for this interesting aggregation-dependent dual emission. Introduction Dual fluorescence is an anomalous emission behavior because it contradicts the well-established Kasha’s rule,1 which states that the fluorescence spectrum is dominated by a single S1 emission band arising from the first excited singlet state. Dual emission was first observed from the emission spectrum of 4-(dimethyl)aminobenzonitrile (DMABN) in 1959 by Lippert,2 and also discovered later in some less common molecules.3,4Dual fluorescent molecules can be sensitive probes of the molecular environment because the relative intensities of the two emission bands are influenced by parameters such as solvent polarity, viscosity and temperature.5,6 For example, it is possible for the molecules to interact with solvent to form a molecule-polar solvent complex which can exhibit two radiative pathways. The ability of a molecule to exhibit two-color emission is potentially useful in applications such as solvatochromic probe and biosensor. The different radiative pathways in such molecules for example can be either quenched or enhanced depending on coupling to the solvent bath or covalent bonding to a biomolecule.7 For organic molecules, aggregation is a common phenomenon that occurs in solution of high concentrations. Molecules exhibiting emission behavior which depends on its aggregated state are of particular interest because the aggregation-dependent luminescence can find applications in the fields of sensors or OLED. However, one deleterious side effect of aggregation is the problem of emission quenching due to strong electronic interactions, hydrogen bonding or π-π stacking.8 In this article, we report a novel D-A structured molecule 3-(9-ethyl-carbazol6-yl)-2-(thiophen-2-yl)acrylonitrile (1), in which carbazole and thiophene act as donors and -CN group as strong acceptor. The molecule exhibits dual emissions (normal S1 and anomalous S2) and the relative intensities of the two emission bands are affected by the aggregation behavior of the molecules in different solvents. Experimental Section Materials and Measurements. All chemicals were purchased from Sigma-Aldrich. 9-ethyl-3-carbazole and 9-ethyl-3-carba* E-mail:
[email protected]. † Department of Chemistry, National University of Singapore. ‡ Computional Materials Science and Engineering Programme, I Science Park Road, #01-01 The Capricorn Singapore Science Park II. § Institute of Material Research and Engineering, 3 Research Link. | NUS Nanoscience and Nanotechnology Initiative.
zolecarboxaldehyde were synthesized by a method similar to those reported for carbazole or phenothiazine derivatives. 9–11 The UV-vis and PL were measured on UV-3101 PC UV-vis-NIR scanning spectrophotometer and RF-5301 PC spectrofluorophotometer, respectively. All of the measurements were carried out using freshly prepared solutions. The quantum efficiencies (Φfl) of the molecules in dilute THF solution and in the nanoaggregate form were measured using quinine sulfate in 0.1 mol L-1 sulfuric acid as standard. The quantum efficiency of the solid sample was measured in a HORIBA Jobin Yvon FluoroLog-3 spectrofluorometer equipped with a 450 W Xe lamp as the excitation source, and an iHR320-FSS spectrometer equipped with a Hamamatsu R-928 PMT detector and an F-3018 integrating sphere. Crystal Structure Determination. Single crystals suitable for X-ray diffraction were obtained by slow evaporation of acetone solution at room temperature. Data collections were performed using a Bruker-AXS SMART CCD area detector diffractometer with Mo KR radiation with an ω-scan mode (λ ) 0.71073Å). The structure was solved with direct methods using the SHELXTL program12 and refined anisotropically with SHELXTL using full-matrix least-squares procedures. Crystal data are summarized in Table 1. Selected bond lengths and angles are illustrated in Table 2. Calculations. Geometry of the isolated molecule was optimized using the B3LYP hybrid density functional with the 6-31G(d) basis set in Gaussian03.13 Based on the geometry, TD-DFT14 was used to calculate the HOMO and LUMO orbital geometry and energies, as well as the absorption spectrum.15 The energy gaps of 1 are illustrated in Table 3. Synthesis of 1. 9-Ethyl-3-carbazolecarboxaldehyde (5 mmol, 1.12 g) and 2-thiopheneacetonitrile (5 mmol, 0.615 g) were stirred in 60 mL of ethanol, t-BuOK (5 mmol, 0.56 g) was added and then the mixture was refluxed for 3 h. After cooling to room temperature, a yellow crystalline product was obtained, and this was washed with cold ethanol thoroughly and dried in vacuum. Yield: 83%. Anal. Calcd for C21H16N2S: C, 76.73; H, 4.87; N, 8.53. Found: C, 76.46; H, 4.82; N, 8.45%. 1H NMR (400 MHz, 300K, CDCl3, TMS, ppm): δ ) 1.47 (t, J ) 7.2 Hz, 3 H), 4.40 (q, J ) 7.2 Hz, 2 H), 7.08 (s, 1 H), 7.2-7.53 (m, 6 H), 7.58 (s, 1 H), 8.10(d, J ) 8.4 Hz, 1 H), 8.15 (d, J ) 7.6 Hz, 1 H), 8.59 (s, 1 H).
10.1021/cg800190w CCC: $40.75 2008 American Chemical Society Published on Web 05/28/2008
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Table 1. Crystallographic Data for 1 empirical formula
C21H16N2S
formula weight crystal system space group T (K) a (Å) b (Å) c (Å) β (°) V (Å3) Z Dc (g cm-3) F (000) µ (mm-1) λ (Å) crystal size (mm) 2θmax (°) N No (I > 2.0σ(I)) R1 wR2 goodness-of-fit largest diff peak and hole (e Å-3)
328.42 monoclinic P2(1)/c 295(2) 5.139(9) 16.668(3) 19.676(3) 91.53(4) 1684.7(5) 4 1.295 688 0.195 0.71073 0.40 × 0.08 × 0.06 50.0 9533 2951 0.0698 0.1677 1.083 0.263 and -0.321
Figure 1. UV-vis absorption (black curve), excitation and emission spectra of 1 in THF (1 × 10-5 M). The excitation spectrum was taken using the S2 emission peak at 316 nm, and the emission spectrum was obtained by 289 nm excitation.
Table 2. Bond Lengths and Angles for 1 C(8)-C(9) C(13)-C(14 N(2)-C(21) S(1)-C(15) C(14)-C(13)-C(9) C(13)-C(14)-C(21)
1.395(4) 1.340(4) 1.142(5) 1.704(3) 133.3(3) 122.3(3)
C(9)-C(13) C(14)-C(21) C(14)-C(15) S(1)-C(18) C(13)-C(14)-C(15) N(2)-C(21)-C(14)
1.453(4) 1.435(5) 1.469(4) 1.688(4) 124.0(3) 176.9(4)
Table 3. Simulated Energy Gaps of S0, S1, and S2 for 1 and Experimental (Denoted with *) Absorption
S0-S1 S0-S2 S1-S2
eV
nm
cm-1
Abs.*/nm
ε*/M-1cm-1
3.388 4.320 0.932
365.94 286.98 1329.9
27327 34846 7519
365 289
35000 41000
Nanoparticle Preparation. With vigorous stirring, 0.3 mL of a THF solution of 1 (1 mg · mL-1) was injected into water (14.7 mL). The turbid mixture was further stirred at room temperature for 3 min and then stood for 4 h. Relatively uniform nanoparticles of 1 with diameters of 80-100 nm formed. Results and Discussion Molecule 1 exhibits three absorption bands with two main peaks at 289 and 365 nm in THF (10-5 M), which agrees well with computer simulation results (Table 3) for S0 f S2 (4.32 eV, 287 nm) and S0 f S1 (3.39 eV, 365.9 nm) absorptions, respectively. The molecule shows dual emissions (excited at 289 nm) in dilute THF solution with one peak at 316 nm and another peak at about 440-460 nm (Figure 1). The high-energy emission at 316 nm is assigned as a S2 peak because its wavelength is shorter than the lowest absorbance of 365 nm. In addition, the dual emissions were observed in different solvents such as toluene, THF, and ethanol, indicating that is not due to the formation of a specific solvent-molecule complex. Furthermore, the 0-0 band of high energy emission overlaps with S2 absorption and excitation and displays the expected mirror image relationship (Figure 1). Consequently, the emission can be assigned to fluorescence from S2.3 The high energy S2 emission is most likely related to local emission from the carbazole moiety in the molecule, because modification of the carbazole with phenothiazine, a closely related structure with one extra S atom, resulted in only S1 emission.16 Similar to
Figure 2. UV-vis spectra of 1: (a) the THF solution; (b) nanoparticles. Inset: molecular structure of 1.
other rare molecules exhibiting anomalous emission, the S2 f S0 emission is from fluorescence localized to specific functional moieties in the molecule, while the S1f S0 emission arises from delocalized molecular states.4c The S1 emission at the blue region (about 440-460 nm) is very weak in solution but greatly enhanced in the solid (aggregated) state, whereas the S2 emission shows the opposite behavior. To the best of our knowledge, this is the first example of a molecule exhibiting S1 and S2 dual emissions that depend on its aggregation behavior. To understand the structure-property relationships further, we investigated the optical properties of the molecule in different aggregation states, ranging from dilute solution, nanoparticles, to single crystal. Nanoparticles of 1 can be fabricated by injecting 0.3 mL of the THF solution containing the molecules (1 mg · mL-1) into water (14.7 mL). UV-vis spectra of THF solution and colloid particles dispersed in THF/H2O are shown in Figure 2. One noteworthy difference of UV absorption spectra between THF solution and nanoparticles is the direction of spectral shift. The absorption maxima are blue-shifted in the case of the nanoparticles. The blue shift is explained by H-aggregate (face to face) formation resulting from the strong π-stacking interactions.17 The long tail in the visible region is attributed to Mie scattering caused by nanosized particles.18 The formation of nanoparticles was revealed by SEM analysis. As shown in Figure 3a and Figure 3b, the as-prepared colloid particles were relatively uniform with diameters of 80-100 nm, which can easily form film by simple drop casting on a Cu plate. The photoluminescence spectra of the molecules in water/ THF mixtures revealed that the S2 emission is reduced while S1 emission is enhanced when the water fraction in the water/ THF mixture increases, which is correlated to the aggregation of the molecule to form nanoparticles. Figure 3c showed the emission photograph of THF solution and nanoparticles, from
S1 and S2 Dual Emissions of Oligomer
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Figure 3. (a) SEM image of the colloid particles of 1 by injection of THF solution into water. (b) Microspheres assembled on a Cu plate. (c) Nonemission and visible blue emission of 1 in THF (left) and water (right) under UV illumination. (d) Confocal fluorescence microscopy image of single crystals of 1.
for emission from the solid film (prepared by spin-coating). Figure 4b shows the change in relative intensities of the S2/S1 emission as a function of the water/THF fractions. In principle, the S2 f S0 fluorescence may appear if the S2 f S1 radiationless internal conversion process is sufficiently slow. This depends on the interplay of two factors, namely, the energy gap between the S2 and S1 levels (∆E) and the density of states in these two electronic states. One criterion is the existence of a large energy gap (∆E > 3000 cm-1) between the S2 and S1 states which reduces the vibronic coupling between these states and slows down the rate of S2 f S1 internal conversion process.19 Our calculation (Table 3) shows that the energy gap of 7519 cm-1 in molecule 1 is sufficiently large for the potential observation of anomalous S2 emission.
Figure 4. (a) Comparison of S2 and S1 emission intensities: solution (curve a), nanoparticles (curve b), solid film (curve c). (b) Relative intensity of S2/S1 in THF/water systems with different water fractions.
which we can see that the nanoparticles exhibit enhancement of blue emission under UV illumination. The quantum yield of S1 emission in THF solution is about 0.24%, while that of nanoparticles is about 5.2%. It is noted that the low energy S1 emission of the nanoparticles further enhanced after the solution was allowed to stand for 2 days at room temperature, which can be explained by the slow crystallization of the particles. Single crystals of 1 exhibit bright emission under UV illumination with emission quantum yield of 13%. The confocal fluorescence image of the microcrystals in Figure 3d exhibits good brightness and contrast. The change in the intensities of the dual emission intensities in different aggregation states of the molecules is illustrated in Figure 4a. The anomalous S2 emission is dominant in dilute solution. In the aggregated state (nanoparticle), the S2 and S1 dual emissions are peaked at 365 and 490 nm, with some redshifts. However, the relative peak intensities of S2 compared to S1 are now reduced greatly. This trend is more pronounced
To further elucidate the structure-property relationship, the molecular packing in the solid state was analyzed using XRD. As shown in Figure 5, the main skeleton of 1 exhibits planar geometry. Along the a cell axis, the molecular packing is similar to H aggregation (Figure 5b). The perpendicular distance of the centroid of one ring to the other ring plane is 3.38 Å, indicating very strong π-π interactions. This H packing mode agrees well with the blue shift of the UV-vis absorption in the nanoparticles. The close packing of the molecular units should enhance the electronic interaction between the π-electrons and reduce the delocalized excited energy level, leading to a faster S2 f S1 interconversion process.20 Therefore, in the aggregates, the localized S2 anomalous emission is suppressed and the S1 emission from the whole conjugated molecule is enhanced. As the dual emission can be switched by intermolecular interactions, it should be influenced by different packing modes like H or J aggregation, or by H-bonding or electrostatic interaction with the carbazole moiety. It is noted that some of the derivatives of 1 also exhibit these intriguing S1 and S2 dual emissions. We found that modification of the alkyl chain at the 9-position of carbazole group does not influence the dual emission character; this provides a handle for us to modify the molecule with different functional groups. Further work to
2546 Crystal Growth & Design, Vol. 8, No. 7, 2008
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References
Figure 5. (a) Crystal structure of 1. (b) Molecular packing along the a-axis. (c) Very strong π-π packing between two molecules.
modify this series of molecules in terms of its conjugation and hydrophilicity is now in progress. Conclusions In summary, an interesting molecule exhibiting aggregationdependent dual emissions from S1 and anomalous S2 states has been reported. The S2 state is quenched when the molecule undergoes aggregation, while the S1 state is enhanced. The switching between these two radiative pathways may be related to the H aggregation (face-to-face packing) in the molecules. Acknowledgment. We are grateful to SERC, Grant No. 052 117 00029, “Molecular and Material Engineering Approaches to Organic and Polymer Electronics Devices”, for financial support. Supporting Information Available: X-ray crystallographic file of the structure in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.
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CG800190W
