Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Synthesis and Spectral Properties of Push−Pull Dyes Based on Isobenzofuran Scaffolds Sean R. Norris,† Caroline C. Warner,† Bryan J. Lampkin, Paige Bouc, and Brett VanVeller* Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States
Org. Lett. Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 04/30/19. For personal use only.
S Supporting Information *
ABSTRACT: A new class of push−pull dyes based on the reactive isobenzofuran core have been synthesized. The new dyes have a smaller HOMO−LUMO gap than a related class of dyes based on benzofurazan and allow for isolation of structural factors that contribute to environmental sensitivity. Experimental and theoretical evidence implicate different photophysical processes are responsible for a reversal of emissive behavior that is observed between isobenzofuran and benzofurazan analogues.
E
nitrogen atoms can participate in hydrogen bonding with the solvent.8 To this end, our goal was to investigate how the nitrogen atoms at the 1,3-positions of 1 affect the properties of this class of small dyes. To achieve this aim, we synthesized the isobenzofuran analogue of 1effectively substituting the N atoms for CHto generate a new structural class of push−pull dyes (2, Scheme 1). The core of 2 presented its own challenge because isobenzofuran derivatives have traditionally been considered to be difficult to prepare and purify.11,12 Indeed, isobenzofurans are typically generated and subsequently reacted in situ to avoid decomposition via dimerization and other cycloaddition processes.13 While substitution at the 1,3-positions in isobenzofuran can lead to more stable structures that have been used in chromophore designs,14,15 our target in 2 leaves the 1,3-positions unsubstituted. However, a recent report of milligram-scale isolation of isobenzofuran16 encouraged us to pursue the synthesis of 2. Compound 2 was synthesized starting from commercially available 3 (Scheme 2). Compound 3 can be selectively monoaminated to give the regiochemistry shown in 4.17 The anthranilic acid moiety in 4 served as the precursor for in situ generation of benzyne18 that underwent cycloaddition with furan to provide 5.19 The isobenzofuran core (8) was established using the inverse-electron-demand Diels−Alder reactivity of 6 that generated pyridazine 7 as a byproduct.12,20 We found the solubility of 6 in the reaction mixture to be critical to realizing productive yields of 8 (Scheme 3). For example, 6 had a low solubility in CHCl3 which likely leads to a slow production of 8. Because 8 displayed cycloaddition reactivity of its own,21−23
nvironmentally sensitive dyes, which display changes in fluorescence wavelength or emission intensity, are useful tools to assess biomolecular interactions.1,2 In particular, small dyes have the greatest potential for fluorescence imaging while being minimally disruptive to native interactions.3 One class of dye that is well-suited to these requirements is the so-called “push−pull” benzoxadiazole (benzofurazan) family (1, Scheme 1).4−7 Compound 1 is a small fluorophore that absorbs visible light (∼450 nm) and emits a range of wavelengths and emission intensities depending on the polarity of the medium.4−7 Scheme 1. Novel Class of Environmentally Sensitive Dyes
There are several structural elements of the structure of 1 that have been proposed to be responsible for solvatofluorochromism. The identity of the group VI heteratom (O, S, or Se) at the 2-position in 1 and the electron-donating group at the 4-position have been studied and found to play a negligible role in the wavelength of absorption and emission.8,9 Alternatively, previous work was able to ascertain that the electron-withdrawing group in 1 plays a significant role in sensitivity of the dye toward the polarity and hydrogen bonding of the medium.8−10 However, the role that the nitrogen atoms in the five-membered ring might play in modifying the solvatofluorochromic behavior has not been fully investigated, despite previous work that suggested the © XXXX American Chemical Society
Received: April 10, 2019
A
DOI: 10.1021/acs.orglett.9b01260 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 2. Synthesis of Isobenzofuran Push−Pull Dyes
Scheme 4. Installation of Electron-Donating Groups
spectral properties. For example, while 1-NHMe displays strong emission intensity in a variety of solvents,26 1-OMe is frequently employed as a nonemissive quencher of fluorescence.27 Alternatively, when the emissive properties of 2NHMe and 2-OMe were studied, we observed a surprising reversal of behavior (Table 1 and Figure 1). Finally, derivative
Scheme 3. Competing Cycloaddition Processes
Figure 1. Comparison of the emissive behavior of electron-donating groups in within each class of dye. There is a reversal of behavior dependent on the core and electron-donating group.
2-R3 was of interest because azetidine (R3 = N(CH2)3) donating groups have been shown to increase the emissive properties of push−pull chromophores.26,28,29 Unfortunately, we were unable to isolate 2-R3 in pure form for study. The known compound, 1-NHMe, displayed high quantum yields in nonpolar solvents (Figure 1, black). In contrast, the isobenzofuran variant (2-NHMe, green) showed far lower quantum yields across all solvents studied. Intriguingly, this behavior was reversed for OMe variants, in which isobenzofuran 2-OMe (red) displayed higher emission intensity compared with 1-OMe (blue). In general, we observed both isobenzofuran derivatives (2NHMe and 2-OMe) to exhibit red-shifted absorbance compared to their benzofurazan (1) analogues (Figure 2, top), indicating the isobenzofuran core leads to a smaller HOMO−LUMO gap. We observed that the absorbance maxima for 1-NHMe and 2-NHMe were similar (only ∼20 nm apart). Alternatively, a larger difference between the absorbance maxima (∼60 nm) for 1-OMe and 2-OMe was observed. As anticipated, the more strongly electron-donating NHMe derivatives display more red-shifted absorbance maxima compared with the OMe derivatives. This observation is congruent with the strategy behind using stronger electrondonating groups to red-shift the absorbance wavelength in push−pull dyes. The trend in the position of the absorbance maxima is mirrored in the position of the emission maxima (Figure 2,
the major product (9) isolated from the CHCl3 mixture was the product of the cycloaddition of 8 with unreacted starting material 5. The undesired product 9 was unambiguously characterized by X-ray crystallography (Scheme 3). Alternatively, the reaction of 5 with 6 was accelerated in DMF, likely due to the greater solubility of 6 in DMF contributing to a higher rate of reaction with 5, and an improved yield of 8 was possible. We initially viewed 8 as a convenient branch point to install electron-donating groups to provide derivatives of 2 for study (see 8 → 2, Scheme 2). Unfortunately, the reactivity of the isobenzofuran core in 8 prevented direct installation of electron-donating groups in useful yields. We therefore sought to unveil the sensitive isobenzofuran moiety in the final step by installing the electron-donating group earlier in the synthesis via nucleophilic aromatic substitution (SNAr) with 5 (Scheme 4). Two common electron donating-groups (OMe and NHMe) found in dyes related to 1 were installed in moderate yield to provide precursor 10. Compound 10 was then reacted with tetrazine 6 to furnish the final target molecule class (2) after HPLC purification. We selected OMe and NHMe as archetypal electrondonating groups because they have been previously studied24,25 in benzofurazan scaffolds (1) and show dramatically different B
DOI: 10.1021/acs.orglett.9b01260 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 1. Comparison of Spectral Properties of 1 and 2 solvent
abs λmax (nm)
em λmax (nm)
log ε
ΦF
1-OMe
Toluene Dioxane THF CH2Cl2 MeCN MeOH H2O
368 359 366 370 369 357 381
405 406 405 406 400 402 ND
2.86 2.89 2.97 2.93 2.84 2.91 2.76
0.019b 0.027b 0.008b 0.023b 0.010b 0.005b ND
2-OMe
Toluene Dioxane THF CH2Cl2 MeCN MeOH H2O
423 425 423 433 431 429 451
505 519 515 529 541 569 607
4.06 4.09 4.12 4.07 4.02 4.04 4.03
0.22a 0.31a 0.26a 0.12a 0.004a 0.004a 0.001a
1-NHMe
Toluene Dioxane THF CH2Cl2 MeCN MeOH H2O
442 448 454 449 458 461 478
508 516 520 514 527 532 552
4.05 4.07 4.00 4.05 4.10 4.11 4.23
0.44a 0.71a 0.71a 0.55a 0.48a 0.23a 0.013a
2-NHMe
Toluene Dioxane THF CH2Cl2 MeCN MeOH H2O
469 471 477 475 482 485 512
495 501 502 518 520 536 548
3.53 3.73 3.77 3.74 3.83 3.81 3.95
0.18a 0.128b 0.047b 0.030b 0.017b 0.008a 0.001a
Figure 2. Comparison of wavelength dependence on polarity of absorbance (top) and emission (bottom) for previously known benzofurazan (1-NHMe, 1-OMe) derivatives and isobenzofuran (2NHMe, 2-OMe) derivatives. Trend lines for the absorbance data do not include the data point for water.
a
Average of two measurements. QY determined relative to coumarin 153 in ethanol. bAverage of two measurements. QY determined relative to Ru(bipy)3Cl2 in water.
bottom). Three of the derivatives display emission maxima clustered between 500 and 600 nm, whereas the emission maxima for 1-OMe are situated apart from the other derivatives at ∼400 nm. Notably, 1-OMe displays almost no solvatofluorochromism, in contrast to 2-OMe which displays the greatest solvatofluorochromism of the group. A plot of the absorbance and emission spectra of 2-OMe is shown in Figure 3 and is representative of the molecules reported in Table 1. Based on these results, we propose that the nitrogen atoms at the 1,3-positions in 1 do not play a substantive role in the environmentally sensitive spectral properties of this dye. This result may not be surprising based on the fact that the furazan moiety displays a pKaH = −8.3.30 The furazan unit does not appear to be basic enough to participate in significant hydrogen bonding effects. The nitrogen atoms, however, do appear to modify the emissive intensity of the dye, but we propose that there are different effects between NHMe and OMe as outlined below. In the case of nitrogen donating groups, it has been proposed that pyramidalization of the −NR2 group in the excited state plays a role in determining the fluorescence efficiency in 1.26 We note that the NHMe group in the computed optimized excited-state geometries is planar for 1NHMe and pyramidal for 2-NHMe (see Figures S26 and S27). Thus, we propose that pyramidalization of the NHMe in 2-
Figure 3. Absorbance and emission spectra for 2-OMe showing the solvent polarity and quantum yield effects in various solvents.
NHMe contributes to a nonradiative relaxation process of the excited state. The structural relaxation associated with this pyramidalization is small (barrier to inversion is ∼0.5 kcal/mol, Figure S26) and likely explains why similar Stokes shifts are observed between 1-NHMe and 2-NHMe, despite changes in excited-state structure. In the case of oxygen donating groups, pyramidalization at oxygen is more difficult to define and we are unable to draw similar conclusions for 1-OMe and 2-OMe. However, we have observed pronounced differences in the quantum yield of fluorescence for 2-OMe in the presence and absence of oxygen (Table 2), which is in contrast to the behavior of the NHMe derivatives. This behavior indicates that oxygen is operating as a fluorescence quencher, although further study is necessary to C
DOI: 10.1021/acs.orglett.9b01260 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 2. Comparison of Emission Intensitya ΦF, aerated toluene
ΦF, degassed toluene
0.49 0.17 0.085
0.44 0.18 0.22
1-NHMe 2-NHMe 2-OMeb
Ellern and the Molecular Structure Laboratory of Iowa State University.
■
(1) Klymchenko, A. S.; Mely, Y. Fluorescent Environment-Sensitive Dyes as Reporters of Biomolecular Interactions. Prog. Mol. Biol. Transl. Sci. 2013, 113, 35−58. (2) Klymchenko, A. S. Solvatochromic and Fluorogenic Dyes as Environment-Sensitive Probes: Design and Biological Applications. Acc. Chem. Res. 2017, 50, 366−375. (3) Lavis, L. D.; Raines, R. T. Bright Ideas for Chemical Biology. ACS Chem. Biol. 2008, 3, 142−55. (4) Zhuang, Y. D.; Chiang, P. Y.; Wang, C. W.; Tan, K. T. Environment-Sensitive Fluorescent Turn-on Probes Targeting Hydrophobic Ligand-Binding Domains for Selective Protein Detection. Angew. Chem., Int. Ed. 2013, 52, 8124−8128. (5) Liu, T. K.; Hsieh, P. Y.; Zhuang, Y. D.; Hsia, C. Y.; Huang, C. L.; Lai, H. P.; Lin, H. S.; Chen, I. C.; Hsu, H. Y.; Tan, K. T. A Rapid Snap-Tag Fluorogenic Probe Based on an Environment-Sensitive Fluorophore for No-Wash Live Cell Imaging. ACS Chem. Biol. 2014, 9, 2359−2365. (6) Appelqvist, H.; Stranius, K.; Borjesson, K.; Nilsson, K. P. R.; Dyrager, C. Specific Imaging of Intracellular Lipid Droplets Using a Benzothiadiazole Derivative with Solvatochromic Properties. Bioconjugate Chem. 2017, 28, 1363−1370. (7) Uchiyama, S.; Kimura, K.; Gota, C.; Okabe, K.; Kawamoto, K.; Inada, N.; Yoshihara, T.; Tobita, S. Environment-Sensitive Fluorophores with Benzothiadiazole and Benzoselenadiazole Structures as Candidate Components of a Fluorescent Polymeric Thermometer. Chem. - Eur. J. 2012, 18, 9552−9563. (8) Gota, C.; Uchiyama, S.; Yoshihara, T.; Tobita, S.; Ohwada, T. Temperature-Dependent Fluorescence Lifetime of a Fluorescent Polymeric Thermometer, Poly(N-Isopropylacrylamide), Labeled by Polarity and Hydrogen Bonding Sensitive 4-Sulfamoyl-7-Aminobenzofurazan. J. Phys. Chem. B 2008, 112, 2829−2836. (9) Thooft, A.; Cassaidy, K.; VanVeller, B. A Small Push-Pull Fluorophore for Turn-on Fluorescence. J. Org. Chem. 2017, 82, 8842−8847. (10) Uchiyama, S.; Santa, T.; Fukushima, T.; Homma, H.; Imai, K. Effects of the Substituent Groups at the 4- and 7-Positions on the Fluorescence Characteristics of Benzofurazan Compounds. J. Chem. Soc., Perkin Trans. 2 1998, 2, 2165−2174. (11) Tobia, D.; Rickborn, B. Substituent Effects on Rates of Interand Intramolecular Cycloaddition Reactions of Isobenzofurans. J. Org. Chem. 1987, 52, 2611−2615. (12) Warrener, R. N. Isolation of Isobenzofuran, a Stable but Highly Reactive Molecule. J. Am. Chem. Soc. 1971, 93, 2346−2438. (13) Rodrigo, R. Progress in the Chemistry of Isobenzofurans: Applications to the Synthesis of Natural Products and Polyaromatic Hydrocarbons. Tetrahedron 1988, 44, 2093−2135. (14) Meek, S. T.; Nesterov, E. E.; Swager, T. M. Near-Infrared Fluorophores Containing Benzo[C]Heterocycle Subunits. Org. Lett. 2008, 10, 2991−2993. (15) Xiang, Z.; Nesterov, E. E.; Skoch, J.; Lin, T.; Hyman, B. T.; Swager, T. M.; Bacskai, B. J.; Reeves, S. A. Detection of Myelination Using a Novel Histological Probe. J. Histochem. Cytochem. 2005, 53, 1511−1516. (16) Peters, M. K.; Herges, R. Preparation and Isolation of Isobenzofuran. Beilstein J. Org. Chem. 2017, 13, 2659−2662. (17) Adnan, M. M.; David, J.; Gohimmukkula, D. R.; Huang, G.; Zhu, J.; Rao, M.; Andrews, R. C.; Ren, T. Benzazole Derivatives and Their Preparation, Compositions, and Methods of Use as Β-Secretase Inhibitors. Wo2006099379a2. 2006. (18) Stiles, M.; Miller, R. G.; Burckhardt, U. Reactions of Benzyne Intermediates in Non-Basic Media. J. Am. Chem. Soc. 1963, 85, 1792− 1797. (19) Best, W. M.; Wege, D. Intramolecular Diels-Alder Additions of Benzynes to Furans - Application to the Total Synthesis of Biflorin,
a
Average of two measurements. Quantum yields determined relative to coumarin 153 in ethanol. bEmission intensity of 1-OMe was too low to draw reasonable conclusions as to differences.
identify the precise mechanism by which this may be occurring.31,32 This work details the design and synthesis of a new class of dyes based on isobenzofuran (2). The isobenzofuran moiety has traditionally been considered to be highly reactive, often requiring substitution at the 1,3-positions to ensure isolation. Our design leaves the 1,3-positions unsubstituted, yet leads to isolable push−pull dyes which display an ∼100 nm red shift in photophysical features compared with analogous scaffolds based on benzofurazan (1). These new derivatives allowed us to evaluate the role that the nitrogen atoms in furazan might play in the environmental sensitivity of 1. The results of this study indicate that the electron-withdrawing partner in the push−pull system is solely responsible for environmentally dependent changes in the wavelength of absorption and emission. The emission intensity of both classes of dyes, however, is highly dependent on the identity of the electrondonating group, which contributes to different mechanisms of nonradiative relaxation in 1 and 2.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01260. Synthetic procedures, compound characterization, 1H and 13C NMR, HRMS, computational details (PDF) Accession Codes
CCDC 1908797 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
■
REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Brett VanVeller: 0000-0002-3792-0308 Author Contributions †
S.R.N. and C.C.W. contributed equally.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Acknowledgement is made to the Donors of the American Chemical Society Petroleum Research Fund (57219DNI4) and Iowa State University of Science and Technology for support of this research. We also acknowledge Dr. Arkady D
DOI: 10.1021/acs.orglett.9b01260 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters and the Mansonone-E, Mansonone-I, and Mansonone-F. Aust. J. Chem. 1986, 39, 647−666. (20) Boger, D. L. Diels-Alder Reactions of Heterocyclic Aza Dienes. Scope and Applications. Chem. Rev. 1986, 86, 781−793. (21) Man, Y. M.; Mak, T. C. W.; Wong, H. N. C. Arene Synthesis by Extrusion Reaction. Part 14. Synthesis of Benzo-Fused Tetraphenylenes and Crystal Structure of a 4:1 Clathrate Inclusion Compound of Dibenzo[B,H]Tetraphenylene with P-Xylene. J. Org. Chem. 1990, 55, 3214−3221. (22) Wong, H. N. C.; Man, Y.-M.; Mak, T. C. W. Synthesis of Tetrabenzo[B,H,N,T]Tetraphenylene and Crystal Structure of Its 1:1 Clathrate Inclusion Compound with P-Xylene. Tetrahedron Lett. 1987, 28, 6359−6362. (23) Takeshita, H.; Wada, Y.; Mori, A.; Hatsui, T. The Cycloaddition Reaction of Isobenzofuran with Some Tropones. Chem. Lett. 1973, 2, 335−336. (24) Song, F. B.; Li, Z. F.; Li, J. Y.; Wu, S.; Qiu, X. B.; Xi, Z.; Yi, L. Investigation of Thiolysis of Nbd Amines for the Development of H2s Probes and Evaluating the Stability of Nbd Dyes. Org. Biomol. Chem. 2016, 14, 11117−11124. (25) Fery-Forgues, S.; Vidal, C.; Lavabre, D. Unusual Solvatochromic Behaviour of the 4-Hydroxy-7-Nitrobenzofurazan Conjugated Anion in Protic Solvents. J. Chem. Soc., Perkin Trans. 2 1996, 2, 73− 77. (26) Saha, S.; Samanta, A. Photophysical and Dynamic Nmr Studies on 4-Amino-7-Nitrobenz-2-Oxa-1,3-Diazole Derivatives: Elucidation of the Nonradiative Deactivation Pathway. J. Phys. Chem. A 1998, 102, 7903−7912. (27) Yamaguchi, T.; Asanuma, M.; Nakanishi, S.; Saito, Y.; Okazaki, M.; Dodo, K.; Sodeoka, M. Turn-on Fluorescent Affinity Labeling Using a Small Bifunctional O-Nitrobenzoxadiazole Unit. Chem. Sci. 2014, 5, 1021−1029. (28) Grimm, J. B.; English, B. P.; Chen, J.; Slaughter, J. P.; Zhang, Z.; Revyakin, A.; Patel, R.; Macklin, J. J.; Normanno, D.; Singer, R. H.; Lionnet, T.; Lavis, L. D. A General Method to Improve Fluorophores for Live-Cell and Single-Molecule Microscopy. Nat. Methods 2015, 12, 244−250. (29) Liu, X.; Qiao, Q.; Tian, W.; Liu, W.; Chen, J.; Lang, M. J.; Xu, Z. Aziridinyl Fluorophores Demonstrate Bright Fluorescence and Superior Photostability by Effectively Inhibiting Twisted Intramolecular Charge Transfer. J. Am. Chem. Soc. 2016, 138, 6960−6963. (30) Boulton, A. J.; Ghosh, P. B. Benzofuroxans. In Advances in Heterocyclic Chemistry; Katritzky, A. R., Boulton, A. J., Eds.; Academic Press: 1969; Vol. 10, pp 1−41. (31) Uchiyama, S.; Santa, T.; Imai, K. Fluorescence Characteristics of Six 4,7-Disubstituted Benzofurazan Compounds: An Experimental and Semi-Empirical Mo Study. J. Chem. Soc., Perkin Trans. 2 1999, 2, 2525−2532. (32) Uchiyama, S.; Takehira, K.; Kohtani, S.; Imai, K.; Nakagaki, R.; Tobita, S.; Santa, T. Fluorescence on-Off Switching Mechanism of Benzofurazans. Org. Biomol. Chem. 2003, 1, 1067−1072.
E
DOI: 10.1021/acs.orglett.9b01260 Org. Lett. XXXX, XXX, XXX−XXX
