Polymorphism-Triggered Reversible Thermochromic Fluorescence of

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Polymorphism-Triggered Reversible Thermochromic Fluorescence of a Simple 1,8-Naphthyridine Antonio Fernández-Mato,† Marcos D. García,*,† Carlos Peinador,*,† José M. Quintela,† Manuel Sánchez-Andújar,† Breogán Pato-Doldán,† M. Antonia Señarís-Rodríguez,† Daniel Tordera,‡ and Henk J. Bolink‡ †

Departamento de Química Fundamental, Universidade da Coruña, Facultad de Ciencias, Rúa da Fraga 10, P.O. Box 15008, A Coruña, Spain ‡ Instituto de Ciencia Molecular, Universidad de Valencia, P.O. Box 22085, Valencia, Spain S Supporting Information *

ABSTRACT: The fluorescent behavior in the solid state of a naphthyridine-based donor−acceptor heterocycle is presented. Synthesized as a crystalline blue-emissive solid (Pbca), the compound can easily be transformed in its P21/c polymorphic form by heating. The latter material shows blue to cyan emission switching triggered by a reversible thermally induced phase transformation. This fact, the reversible acidochromism, and the strong anisotropic fluorescence of the compound in the solid state, account for the potential of 1,8-naphthyridines as simple and highly tunable organic compounds in materials science.

T

conveniently functionalized NAPY would result in an efficient solid-state fluorophore. Therefore, among the extensive library of NAPYs previously reported by our research group,14 compound 114d was chosen on account of its structural features (Figure 1): an electron-rich dimethoxyphenyl (DMP) antenna

he development of optoelectronic devices based on organic solids with efficient solid-state emission1 is an active and very promising area of research in the field of materials science due to its potential in many applicative fields (e.g., solid-state lasers,2 molecular-based logic,3 light-emitting diodes,4 light-emitting field-effect transistors,5 or sensors6). Therefore, the design and synthesis of easily tunable organic solids showing a dynamic fluorescence response upon external stimulation (by light, heat, pressure, etc.) has gained considerable attention.7 Nevertheless, the development of such materials displaying efficient and reversible switching between two luminescent statuses in the solid state still remains a challenge.8 The excellent fluorescent properties of 1,8-naphthyridine derivatives (NAPYs) in solution, as well as the appropriate arrangement of the nitrogen atoms on the heterocyclic core, have prompted a great deal of interest on their use as fluorescent probes for DNA and heavy transition-metal ions.9,10 Another important feature of these heterocycles, is that those containing electron-donating groups behave as intramolecular donor−acceptor fluorophores (ICTs).11 This accounts for their potential in many electronic systems (e.g., electroluminescent devices, field-effect transistors, or solid-state lasers), as the intramolecular charge transfer associated with this type of fluorophore helps to improve the solid-state quantum yield.12 Despite this evidence, the fluorescence properties of NAPYs in the solid state have been scarcely explored.13 By using simple and reliable principles of supramolecular chemistry and crystal engineering, we envisaged that a © 2013 American Chemical Society

Figure 1. 1,8-Naphthyridine derivative (NAPY) 1.

linked to the π-deficient NAPY core (as a requirement for the donor−acceptor nature of the fluorophore) and a phenyl antenna (PHE) at position 4 of the heterocycle, which was expected to modulate the aggregation of the compound in the solid state (vide inf ra). Prior to the study of the photochemical properties of compound 1 in the solid state and in order to check the ICT nature of 1, its fluorescent behavior was first studied in solution Received: November 12, 2012 Revised: December 19, 2012 Published: January 4, 2013 460

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Figure 2. HOMO (left) and LUMO (right) orbitals of 1 according to TD-DFT calculations.

Figure 3. Mercury projections of 1 (Pbca) with arbitrary numbering schemes, illustrating the following: minimum asymmetrical unit within the crystal structure with 50% ellipsoids (top) and the unit cell viewed across the crystallographic a axis with the PHE antennas involved in the isolated π-stacking interactions highlighted in red (bottom). Hydrogen atoms are omitted for clarity.

nature of 1 can also explain the observed solvatochromic effect, as solvents with relatively large electrical dipole moments, such as acetonitrile, can stabilize the charge-transfer excited state. Recrystallization of 1 from CH3CN afforded crystals suitable for single-crystal X-ray diffraction. In these conditions, compound 1 crystallized in the orthorhombic crystal system (Pbca).17 As shown in Figure 3, the molecular conformation of 1 differs only slightly from that calculated in solution (∠C4C3C20C21 = 118.7°, ∠C6C7C9C10 = 28.4°). In the solid state, the fluorescence of 1 (Pbca) was shown to be very similar to that of the compound in solution, resulting in a highly emissive material (λexc = 323 nm, λmax = 467 nm; Φ = 0.47, (CIE) = 0.193 and 0.220, Figure 6). As a probable consequence of the nonplanar configuration of the aromatic substituents around the heterocyclic core promoted mostly by the PHE antenna, π-stacking interactions are nearly avoided on the crystal packing. Therefore, only isolated pairs of molecules interconnected by the PHE antennas by an OFF π-stacking motif (parallel planes, dcentroids = 4.33 Å, dplanes = 3.42 Å) can be found in the crystal (Figure 3). As anticipated, this lack of effective π-stacking along one or more of the directions of the

and correlated with TD-DFT [at the B3LYP/6-31G (d) level]15 calculations. The UV−vis spectrum of CH3CN showed an absorption maximum centered at λmax = 378 nm, with a shoulder at λmax = 270 nm.16 On the other hand, the emission spectrum showed a solvent-dependent broad peak (λmax = 465 nm, CH2Cl2; λmax = 500 nm, CH3CN), as well as a large Stokes shift in acetonitrile (122 nm).16 The main absorption band of 1 in CH2Cl2 is in good agreement with that predicted in this solvent using TD-DFT calculations and was assigned to the intramolecular charge-transfer transition from the HOMO delocalized over the DMP antenna to the LUMO, mainly localized on the NAPY core (Figure 2). The optimized molecular geometry in solution, obtained by the above-mentioned quantum mechanical calculations, shows the DMP being almost planar relative to the NAPY core (∠C6C7C9C10 = 2.3°), facilitating therefore the intramolecular charge transfer. On the other hand, and crucially in order to avoid extensive π-stacking in the crystalline state, the phenylene antenna adopts a twisted conformation, regarding the heterocyclic core, in order to minimize steric effects (∠C4C3C20C21 = 121.4°). In addition, the donor−acceptor 461

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Figure 4. Fluorescence microscopy images of 1 (Pbca). Bright field image (left), emission detected through a 435−485 nm filter upon 325−375 nm excitation (center), and emission detected through a 600−660 nm emission filter upon 540−580 nm excitation (right).

crystal would be the reason for the strong fluorescence of the material.18 Compound 1 (Pbca), which crystallizes in the form of transparent plates, displayed strong fluorescence anisotropy. This effect can be explained by the waveguide effect of this type of single crystal, with only part of the fluorescent light waves escaping in the middle of the plate and the rest being reabsorbed or emitted at the edges (Figure 4). Differential scanning calorimetric curves (Figure S7 of the Supporting Information) of a bulk sample of compound 1 (synthesized as the Pbca polymorph, Figure S9 of the Supporting Information) showed two endothermic peaks: a small peak at 151 °C and a large peak at 199 °C.16 As the lower temperature peak could be related with a structural transition, the sample was heated above 170 °C for 6 h and quenched to room temperature. We established with LeBail refinement of the XRPD pattern of the quenched sample that the obtained compound was single-phased, showing good agreement between the experimental data and the proposed model for the fitting (Figure 5), consisting in a monoclinic cell, a =

temperature polymorph is very similar to the high-temperature polymorphic form, with both forms showing a blue emission but with a decreased quantum yield for the P21/c form (λexc = 338 nm, λmax = 467 nm; Φ = 0.29, CIE = 0.193 and 0.245, Figure 6), a fact that can be attributed to a similar crystal packing in both polymorphs.19 Remarkably, the low-temperature polymorph triggers the activation of a reversible thermochromic behavior because when heated to melt (199 °C) and quickly cooled down to room temperature, the blue emission is transformed into an intense cyan emission (λexc = 323 nm, λmax = 495 nm, Φ = 0.3, CIE = 0.221 and 0.400), with the resulting material in good agreement with an amorphous phase.16 The above-mentioned cycle is reversible since if the melted solid (amorphous phase) is annelated to 155 °C, the compound reverts to the P21/c polymorph, restoring the blue emission (Figure 6). In this case, the red-shift less-intense emission of the amorphous phase can be correlated with the increased probabilities of π-stacking interactions.

Figure 6. Normalized emission spectra of crystalline samples of 1 (Pbca, land P21/c) and amorphous phase upon melting.

Figure 5. LeBail refinement of the XRPD pattern of 1 (P21/c) at room temperature. The observed data (+) and calculated profile (solid line) are provided; the difference plot is drawn below the profile. Tick marks indicate peak positions of the sample.

Another expected, but interesting, feature found for compound 1 was its acidochromism,20 displaying a solid− solid acid and base stimuli-responsive change in its solid-state fluorescence. When compound 1 (in its bulk form) was ground with p-toluenesulphonic acid (PTSA), it turned into an orange material with a yellow emission (λexc = 341 nm, λmax = 570 nm, Φ = 0.16, CIE = 0.514 and 0.484, Figure S3 of the Supporting Information),21 that was restored to the original emission by grinding the mixture with K2CO3 (CIE = 0.214 and 0.289, Figure S3 of the Supporting Information). Protonation of the NAPY can be responsible for a substantial alteration of the HOMO−LUMO gap, prompting therefore a red-shift emission.22

13.731(1) Å, b = 15.848(1) Å, c = 9.694(1) Å, β = 94.33 (1)°, and space group P21/c.16 Therefore, the high temperature peak on the DSC thermogram can be assigned to the melting point corresponding to the monoclinic crystal structure. Compound 1 was therefore crystallized in two polymorphic forms: one corresponding to an orthorhombic cell [a = 15.922(1) Å, b = 9.802(1) Å, c = 27.129(1) Å, space group Pbca] and the above-mentioned monoclinic polymorph, P21/c. These two polymorphic forms display relatively similar cell parameters: aorth ≈ bmono., borth ≈ cmono, but with quite a different c axis, corth ≈ 2amono. The fluorescence of the low 462

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(6) (a) Thomas, S. W., III; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107, 1339. (b) Basabe-Desmonts, L.; Reinhoudt, D. N.; CregoCalama, M. Chem. Soc. Rev. 2007, 36, 993. (c) Wolfbeis, O. S. J. Mater. Chem. 2005, 15, 2657. (d) Martinez-Máñez, R.; Sancenon, F. Chem. Rev. 2003, 103, 4419. (7) For selected examples: (a) Anthony, S. P.; Draper, S. M. J. Phys. Chem. C 2010, 114, 11708. (b) Yoon, S. J.; Chung, J. W.; Gierschner, J.; Kin, K. S.; Choi, M. G.; Kim, D.; Park, S. Y. J. Am. Chem. Soc. 2010, 132, 13675. (c) Bell, T. D. M.; Bhosale, S. V.; Forsyth, C. M.; Hayne, D.; Ghiggino, K. P.; Hutchison, J. A.; Jani, C. H.; Langford, S. J.; Lee, M. A.-P.; Woodward, C. P. Chem. Commun. (Cambridge, U.K.) 2010, 46, 4881. (d) Sagara, Y.; Kato, T. Nat. Chem. 2009, 1, 605. (e) Mutai, T.; Tomoda, H.; Ohkawa, T.; Yabe, Y.; Araki, K. Angew. Chem., Int. Ed. 2008, 47, 9522. (f) Mutai, T.; Satou, H.; Araki, K. Nat. Mater. 2005, 4, 685. (g) Davis, R.; Rath, N. P.; Das, S. Chem. Commun. (Cambridge, U.K.) 2004, 74. (8) (a) Zhao, Y. F.; Gao, H. Z.; Fan, Y.; Zhou, T. L.; Su, Z. M.; Liu, Y.; Wang, Y. Adv. Mater. (Weinheim, Ger.) 2009, 21, 3165. (b) Hadjoudis, E.; Mavridis, I. M. Chem. Soc. Rev. 2004, 33, 579. (c) Al-kaysi, R. O.; Bardeen, C. J. Adv. Mater. (Weinheim, Ger.) 2007, 19, 1276. (9) (a) Sato, Y.; Honjo, A.; Ishikawa, D.; Nishizawa, S.; Teramae, N. Chem. Commun. 2011, 47, 5885. (b) Nakatani, K.; Horie, S.; Saito, I. J. Am. Chem. Soc. 2003, 125, 8972. (10) (a) Li, H.-J.; Fu, W.-F.; Li, L.; Gan, X.; Mu, W.-H.; Chen, W.-Q.; Duan, X.-M.; Song, H.-B. Org. Lett. 2010, 12, 2924. (b) Yu, M.-M.; Li, Z.-X.; Wei, L.-H.; Wei, D.-H.; Tang, M.-S. Org. Lett. 2008, 10, 5115. (11) (a) Shelar, D. P.; Birari, D. R.; Rote, R. V.; Patil, S. R.; Toche, R. B.; Jachak, M. N. J. Phys. Org. Chem. 2011, 24, 203. (b) Shelar, D. P.; Patil, S. R.; Rote, R. V.; Toche, R. B.; Jachak, M. N. J. Fluoresc. 2011, 21, 1033. (12) Zhang, X.; Zhang, X.; Wang, B.; Zhang, C.; Chang, J. C.; Lee, C. S.; Lee, S.-T. J. Phys. Chem. C 2008, 112, 16264. (13) (a) Fernández-Mato, A.; Quintela, J. M.; Peinador, C. New J. Chem. 2012, 36, 1634. (b) Yun-Ying, W.; Chen, Y.; Gou, G. Z.; Mu, W. H.; Lv, X. J.; Du, M. L.; Fu, W. F. Org. Lett. 2012, 14, 5226. (c) Quan, L.; Chen, Y.; Lv, X. J.; Fu, W. F. Chem.−Eur. J. 2012, 18, 14599. (d) Li, H. F. J.; Fu, W. F.; Li, L.; Gan, X.; Mu, W. H.; Chen, W. Q.; Duan, X. M.; Song, H. B. Org. Lett. 2010, 12, 2924. (e) Harada, N.; Abe, Y.; Karasawa, S.; Koga, N. Org. Lett. 2012, 14, 6282−6285. (14) (a) Fernández-Mato, A.; Blanco, G.; Quintela, J. M.; Peinador, C. Tetrahedron 2008, 64, 3446. (b) Veiga, C.; Quintela, J. M.; Peinador, C.; Chas, M.; Fernández, A. Heterocycles 2006, 68, 295. (c) Vilar, J.; Peinador, C.; Quintela, J. M. Heterocycles 2005, 67, 329. (d) Quintela, J. M.; Peinador, C.; González, L.; Iglesias, R.; Paramá, A.; Á lvarez, F.; Sanmartín, M. L.; Riguera, R. Eur. J. Med. Chem. 2003, 38, 265. (e) Vilar, J.; Peinador, C.; Veiga, M. C.; Ojea, V.; Quintela, J. M. Heterocycles 1995, 41, 111. (15) Frisch, M. J. et al. Gaussian 03; Gaussian Inc.: Wallingford, CT, 2004. (16) See Supporting Information for further details. (17) Crystal data for 1: C25H20N3O3, Mr = 410.44, Orthorhombic (Pbca), a = 15.9397(6) Å, b = 9.6503(4) Å, c = 26.8957(12) Å, V = 4137.2(3) Å3, T = 100(2) K, space group Pbca, Z = 8, μ(Mo Kα) = 0.088 mm−1, 38540 reflections measured, 3669 independent (Rint = 0.1075). The final R1 values were 0.0504 [I > 2σ(I)]. The final wR(F2) values were 0.1261 [I > 2σ(I)]. The final R1 values were 0.0922 (all data). The final wR(F2) values were 0.1664 (all data). The goodness of the fit on F2 was 1.089. (18) Langhals, H.; Potrawa, T.; Nöth, H.; Linti, G. Angew. Chem., Int. Ed. 1989, 28, 478. (19) Unfortunately, attempts to achieve a single crystal to single crystal transformation for the Pbca → P21/c were unsuccessful, due to a significant loss of crystallinity upon heating single crystals of the orthorhombic form. (20) In solution, protonation of the nitrogen atoms within the 1,8naphtyridine core has been postulated to disrupt the conjugated electronic system of the ICT moiety in similar systems, see for instance: Zhou, Y.; Xiao, Y.; Qian, X. Tetrahedron Lett. 2008, 49, 3380.

In summary, compound 1 (owning an intramolecular donor−acceptor nature) was crystallized in two similar polymorphic forms: while the Pbca form shows a strong anisotropic fluorescence (blue), the less luminescent polymorph (P21/c, blue), easily prepared from the Pbca form, triggers a reversible thermally induced transformation to a highly emissive amorphous phase (cyan). Furthermore, 1 displays reversible acidochromism upon treatment with PTSA and restoration of the initial blue emission with K2CO3. These results exemplify how the careful election of the pattern of substitution in NAPYs, a well-known class of luminescent compounds in solution, resulted in the discovery of the efficient and highly versatile stimuli-responsive solid-state fluorophore 1. This novel organic luminescence material, which can be efficiently reversibly switched between different luminescent phases based on external stimulation, has the potential for application in the development of display, sensing, or memory devices.

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ASSOCIATED CONTENT

S Supporting Information *

General methods, crystallographic data for 1 (Pbca), spectroscopic characterization of 1, extra figures, and computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M.D.G.: e-mail, [email protected]; fax, +34 981 167065; tel, +34 981 167000 ext: 2247. C.P.: e-mail, carlos.peinador@ udc.es; fax, +34 981 167065; tel, +34 981 167000 ext: 2172. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Ministerio de Ciencia e Innovación and FEDER (CTQ2010-16484/BQU, MAT201124594, and MAT2010-21342-C02-01). The authors are indebted to Centro de Supercomputación de Galicia (CESGA) for providing the computer facilities. D.T. acknowledges the Spanish Ministry of Education, Culture, and Sport for a FPU grant.

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REFERENCES

(1) See: Shimizu, M.; Hiyama, T. Chem.Asian J. 2010, 5, 1516 and references cited therein. (2) (a) Samuel, I. D. W.; Turnbull, G. A. Chem. Rev. 2007, 107, 1272. (b) Samuel, I. D. W.; Turnbull, G. A. Mater. Today (Oxford, U.K.) 2004, 7, 28. (c) Scherf, U.; Riechel, S.; Lemmer, U.; Mahrt, R. F. Curr. Opin. Solid State Mater. Sci. 2001, 5, 143. (d) McGehee, M. D.; Heeger, A. J. Adv. Mater. (Weinheim, Ger.) 2000, 12, 1655. (3) (a) de Silva, A. P.; James, M. R.; McKinney, B. O. F.; Pears, D. A.; Weir, S. M. Nat. Mater. 2006, 5, 787. (b) Raymo, F. M.; Giordani, S. J. Am. Chem. Soc. 2002, 124, 2004. (c) Raymo, F. M.; Giordani, S. Org. Lett. 2001, 3, 1833. (d) Irie, M.; Ed. Chem. Rev. 2000, 100, 1683, special issue on “Photochromism: Memories and Switches”. (4) (a) Highly Efficient OLEDs with Phosphorescent Materials; Yersin, H., Ed.; Wiley-VCH: Weinheim, Germany, 2008. (b) Organic LightEmitting Materials and Devices (Eds.: Li, Z.; Meng; H.), Taylor & Francis: Boca Raton, FL, 2007. (c) Organic Light-Emitting Devices. Synthesis, Properties and Applications; Müllen, K., Scherf, U., Eds.; Wiley-VCH: Weinheim, Germany, 2006. (5) (a) Zaumseil, J.; Sirringhaus, H. Chem. Rev. 2007, 107, 1296. (b) Cicoira, F.; Santato, C. Adv. Funct. Mater. 2007, 17, 3421. 463

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(21) Further characterization of this material, as well as others prepared by grinding of 1 with organic acids, is currently under investigation in our laboratory. (22) This observation is supported by the behavior of compound 1 in solution (CH2Cl2), where few drops of trifluoroacetic acid turn the original blue emission into yellow, with a significant quenching of the fluorescence intensity.16

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