Chem. Mater. 2008, 20, 6595–6596
Novel Anthracene Diimide Fluorescent Sensor
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Scheme 1. Synthesis of Dianhydride 4 and Diimide 1b
Daniel S. Tyson,† Ashley D. Carbaugh,§ Faysal Ilhan,† Javier Santos-Pe´rez,† and Michael A. Meador§,* Polymers Branch, Structures and Materials DiVision, NASA Glenn Research Center, CleVeland, Ohio 44135, and Ohio Aerospace Institute, 22800 Cedar Point Road, CleVeland, Ohio 44142 ReceiVed September 11, 2008 ReVised Manuscript ReceiVed October 8, 2008
Aromatic imides and diimides have attracted considerable attention in molecular electronics,1 as fluorescent probes in biological systems,2 and as sensors3 for the detection of chemical species. With a few exceptions, these reports have been confined to perylene and naphthalene imides and diimides, in part because of the ready availability of the corresponding anhydride and dianhydride starting materials. We have reported the synthesis of an aniline capped anthryl diimide, 1a, and its ability to function as an “on-off” sensor for pH, acid halides, and chemical surrogates for Sarin.4 In the “off state”, fluorescence from 1a is completely quenched by intramolecular excited state charge transfer (CT) from the terminal amines. Reaction of these groups inhibits CT quenching and activates the fluorescence. This approach to chemical species detection is highly sensitive, because it is easy to recognize the “on” and “off” states of the dye. Synthesis of 1a involves the generation of o-xylylenols by photolysis of 2,5-dibenzoyl-p-xylene, 2, and their trapping in situ with 4-nitrophenylmaleimide. Although this approach is fairly straightforward, it is limited in scope because only maleimides, which are photochemically stable, can be used. To address this, we have recently prepared the corresponding anthracene dianhydride, 4, and used it to synthesize triaryl amine-terminated diimide 1b (Scheme 1). Herein, we present the synthesis and photophysics of 1 and its potential use as a fluorescent sensor for pH and nitroaromatics and as an indicator in thermochromic materials. Dianhydride 4 was prepared in four steps from 2,5dibenzoyl-p-xylene and dimethyl acetylenedicarboxylate as outlined in Scheme 1. Reaction of 3 and N,N-diphenylaniline in acetic acid at 120 °C afforded diimide 1b as a red solid in 30% yield. Spectral data and elemental analysis were consistent with the formation of 1b. X-ray crystallographic analysis of single crystals of 1b grown by vapor diffusion of diethyl ether into dichloromethane revealed that the anthracene core is planar and
undistorted and the pendant 1,5-Ph groups are nearly orthogonal (83.9°) to this core. The imide N-Ph substitutents are also rotated from planarity by ca. 38°. The unit cell of 1b consists of columns in which the triphenyl amine termini are located above and below the anthracene core of adjacent diimide molecules (Figure 1) and suggests that CT within 1b in the solid state is a facile process accounting for its red color. Representative absorption and emission spectra of 1b in toluene and dichloromethane at room temperature are shown in Figure 2. Fluorescence from 1b is from a polar CT excited state. The broad emission from 1b is solvatochromic, shifting from 595 nm in toluene to 685 in dichloroethane. The fluorescence quantum yield for 1b in toluene is 0.035 and decreases with increasing solvent polarity. Excited state CT in 1b can be exploited as a means of detecting certain chemical species. For example, formation of this CT state can be supressed, and fluorescence intensity
Figure 1. Unit cell of 1b.
* Corresponding author. E-mail:
[email protected]. † Ohio Aerospace Institute. § NASA Glenn Research Center.
(1) Zang, L.; Che, Y.; Moore, J. S. Acc. Chem. Res. 2008, ASAP. (2) Zhu, L.; Wu, W.; Zhu, M-Q; Han, J. J.; Hurst, J. K.; Li, A. D. Q. J. Am. Chem. Soc. 2007, 129, 3524–3526. (3) (a) Che, Y.; Yang, X.; Loser, S.; Zhang, L. Nano Lett. 2008, 8, 2219– 2223. (b) Cao, H.; Diaz, D. I.; DiCesare, N.; Lakowicz, J. R.; Heagy, M. D. Org. Lett. 2002, 4, 1503–1505. (4) Ilhan, F.; Tyson, D. S.; Meador, M. A. Chem. Mater. 2004, 16, 2978– 80.
Figure 2. Room-temperature absorption and emission spectra of 1b in toluene and dichloroethane.
10.1021/cm8024704 CCC: $40.75 2008 American Chemical Society Published on Web 10/22/2008
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Chem. Mater., Vol. 20, No. 21, 2008
Communications
Figure 3. Stern-Volmer quenching of the fluorescence from 1b with 2,4DNT.
Figure 5. Comparison of emssion spectra of 1 in LLDPE at 153 °C, PS at 25 °C, and MeTHF glass at 77 °K.
Figure 4. Effect of temperature on emission spectra of film of 0.1 wt % 1b in LLDPE.
tion occurs as the temperature of the film is increased, and the emission shifts from the red (excimer) to green (single molecule). Although it is likely that room-temperature emission from the 1b doped LLDPE is due to the fomation of molecular aggregates, this mechanism cannot fully explain its thermochromic behavior in these films, because the emission at 153 °C is blue-shifted by ca. 50 nm from the single molecule emission from 1b in toluene. Some insight into the origin of this green emission is given by comparing the fluorescence spectra of 1b in LLDPE at 25 and 153 °C with that of 1b in MeTHF glass at 77 °K (Figure 5). In MeTHF glass, 1b assumes a conformation in which the N-phenyl substituent is not coplanar with the anthryl diimide core. This inhibits excited-state CT by disrupting conjugation between the triphenylamine termini and the anthryl imide leading to the observed green emission. Heating the LLDPE film to 153 °C does lead to disruption of 1b aggregates, as was observed by Weder for cyano-OPVs in LLDPE. However, the individual molecules of 1b are still in a highly viscous medium which also restricts rotation of the N-phenyl group, inhibiting excited-state CT and leading to the same green emission. It is also worth noting that the same concentration of 1b in a polystyrene (PS) film has a similar emision spectrum. The better solubility of 1b in PS than in LLDPE which enables a molecular level dispersion of 1b rather than aggregation. Molecules of 1b are immobilized by the PS matrix and have the same conformational restriction as observed in MeTHF glass. The synthesis of other anthryl diimides and their use as sensors and in chromogenic materials is underway.
reduced, by protonating the terminal amines (see the Supporting Information), making 1b a potential pH sensor. Fluorescence from 1b was also quenched by addition of strong electron acceptors, such as 2,4-dinitrotoluene, 2,4DNT (Figure 3). This quenching in toluene follows typical Stern-Volmer behavior with a kq of 7.67 × 109 L mol-1 s-1, slightly less than the rate of diffusion (1.1 × 1010 L mol-1 s-1).5 We are currently exploring the use of 1b in the sensing of other nitroaromatics such as TNT. Linear low-density polyethylene (LLDPE) films doped with 1b (0.01 to 0.1 wt %) were prepared by melt mixing in a twin-screw mini-extruder. Emission from these films at RT had a λmax at ca. 623 nm, red-shifted by ca. 30 nm from that in toluene. As the films were heated, the emission spectrum shifted to the blue with λmax reaching a limit of 543 nm at 153 °C (Figure 4). Cooling the film back to room temperature or lower restored the red emission. Thermochromism has been reported by Weder6 for cyano substituted oligo(p-phenylene vinylene), cyano-OPV, doped LLDPE and has been ascribed to the formation of excimers via cyano-OPV aggregation within the LLDPE. Deaggrega(5) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry, 2nd ed.; Marcel Dekker: New York, 1993. (6) Crenshaw, B. R.; Weder, C. Chem. Mater. 2003, 15, 4717–4724.
Acknowledgment. F.I., J.S.-P., and D.S.T. were supported under NASA Cooperative Agreement NCC06ZA47A. A.D.C. was a 2007 NASA Undergraduate Research Program participant. This work was funded by the Fundamental Aeronautics Research Program. Supporting Information Available: Synthesis of 1b and intermediates, photophysical data on 1b, effects of pH on fluorescence of 1b (PDF).This material is available free of charge via the Internet at http://pubs.acs.org. CM8024704
