Fluorescent Cyanine Dye J-Aggregates in the Fluorous Phase

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Fluorescent Cyanine Dye J‑Aggregates in the Fluorous Phase Wei Cao and Ellen M. Sletten* Department of Chemistry and Biochemistry, University of California, Los Angeles, 607 Charles E. Young Drive East, Los Angeles, California 90095, United States S Supporting Information *

ABSTRACT: We present a perfluorocarbon-hydrocarbon amphiphilic cyanine dye that J-aggregates in fluorous solvent. J-Aggregation is a special type of fluorophore aggregation, affording enhanced photophysical properties. Cyanine dyes are excellent J-aggregators in water but, until now, cyanine J-aggregates have not been translated to nonaqueous media. The fluorous phase J-aggregate displays enhanced photostability and processability compared to analogous aqueous aggregates. he striking visual properties of fluorophores have captured the attention of scientists for hundreds of years.1 The spectral properties of fluorescent molecules are dictated by their structure and environment. While thousands of fluorophores have been characterized as individual molecules, aggregates have distinct properties.2 Thus, controlling fluorophore self-assembly provides an avenue to access unique photophysical behaviors.3 J-Aggregates are an intriguing class of fluorophore aggregates.4 J-Aggregation occurs when chromophores are aligned such that constructive coupling of the transition dipole moments is achieved (Figure 1A).2a This coupling leads to narrow, bathochromically shifted absorption and emission bands, enhanced absorption coefficients (ε) and quantum yields (ΦF), as well as small Stokes shifts. J-Aggregates have found farreaching applications including photographic sensitizers,5 membrane potential probes,6 “superquenching” sensors,7 and materials for optoelectronic devices and organic semiconductors.8 Additionally, bacteriochlorophyll J-type aggregates are naturally found in green bacteria.9 Cyanine dyes, chromophores composed of two nitrogencontaining heterocycles linked via a polymethine chain,10 were the first J-aggregates discovered. Since the seminal reports in the 1930s,11 many cyanine dye scaffolds have been reported to Jaggregate in aqueous solution.4,12 A notable achievement was when Daehne and co-workers appended lipophilic and hydrophilic functionality onto the cyanine chromophores.13 This work produced amphiphilic 5,5′,6,6′-tetrachlorobenzimidacarbocyanine dyes which formed planar achiral or tubular chiral Jaggregates in water,14 with the premier dye being 3,3′-bis(3sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine (1, Figure 1B).15 Cyanine 1 has been studied as a mimic of bacteriochlorophyll J-aggregates,16 optimized for use on surfaces,17 and employed as antennas for energy transport.18 Despite this wide utility, 1 displays poor photostability18b,19 and is only applicable to devices in or fabricated from water.17,18 If cyanine J-aggregates can be created in nonaqueous solution, they can complement aggregates of 1.

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© XXXX American Chemical Society

Figure 1. (A) Generic cyanine dye and its J-aggregates. (B) 5,5′,6,6′Tetrachlorobenzimidacarbocyanine dyes: hydrophilic−lipophilic 1 (previous work) and fluorous−lipophilic 2 (this work).

Here, we report a cyanine dye that readily J-aggregates in nonaqueous media by employing hydrocarbon and perfluorocarbon moieties. Perfluorocarbons create an orthogonal phase to both aqueous and organic solution, deemed the fluorous phase.20 The nonpolarizability of perfluorocarbons make them attractive media for controlling aggregation.21 Furthermore, enhanced photophysical properties have been reported in fluorous solvent.22,23 With these promising attributes, we embarked on the synthesis and characterization of fluorous−lipophilic 5,5′,6,6′-tetrachlorobenzimidacarbocyanine dye 2 (Figure 1B) and demonstrated its orthogonality to 1. We prepared target 2 in three steps as shown in Figure 2A. Then, we explored its photophysical properties in varying solvents. To start, we dissolved 2 in the light fluorous solvent trifluorotoluene (PhCF3),20 which fully solubilized the amphiphilic cyanine and facilitated characterization of the monomeric fluorophore to have a λmax,abs = 533 nm and λmax,em = 549 nm (Figure 2B, black, Figure S1). With PhCF3 serving as a benchmark, a solvatochromism study with traditional organic solvents indicated that most solvents solubilized 2 in its monomeric form at 10−6 M (Figure S2/S3, Table S1). Notably, hexanes deviated from this trend and a distinct blue-shifted peak was observed, along with a shoulder where the monomer absorbs (Figure 2B, purple). In evaluating the emission of 2 in hexanes, we observed a peak with λmax,em = 564 nm. Excitation−emission correlation spectra confirmed that the emission was only from the shoulder corresponding to monomeric 2 and the blue-shifted aggregate was not emissive (Figure S4). Hypsochromically Received: November 9, 2017

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DOI: 10.1021/jacs.7b11925 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 2. (A) Synthesis of fluorous−lipophilic 5,5′,6,6′-tetrachlorobenzimidacarbocyanine dye 2. (B) Normalized absorption (Abs, solid line) and photoluminescence (PL, dashed line) spectra of 2 in hexanes (purple, 0.02 mM), trifluorotoluene (black, 0.01 mM), and fluorous solvent (red, 0.07 mM). (C/D) Change in absorption spectra of 0.07 mM 2 in fluorous solvent over time. (C) The arrows indicate spectral changes over time. (D) Changes in λmax.abs of the monomer (530 nm, black triangle) and aggregates (498 nm, purple circle; 564 nm, red square) vs time. The data were fit to a sigmoidal model (black line). (E/F) Change in absorption spectra with ratio of ROC4F9:CH2Cl2 measured 4 h after sample preparation (0.06 mM 2). (E) The arrows indicate spectral changes upon fluorous content increasing. (F) Absorbance of J-aggregate vs percent fluorous solvent with sigmoidal fit. Error bars represent the standard deviation of 3 replicate samples. (B−D) Fluorous = 99:1 v/v ROC4F9:CH2Cl2; R = 1:1 v/v ratio of Me and Et.

shifted spectral properties and decreased ΦF are the most common result of chromophore aggregation, giving rise to aggregation’s undesirable reputation.24 Next, we explored 2 in fluorous solvent. Compound 2 has limited solubility in perfluorocarbons; however, if first dissolved in trifluorotoluene or dichloromethane (CH2Cl2), 2 could be characterized in alkoxyperfluorobutane (ROC4F9). Under these conditions, we initially observed a hypsochromically shifted absorbance comparable to that of 2 in hexanes (Figure 2C).25 Over time this signal transformed into a red-shifted peak with λmax,abs = 564 nm and λmax,em = 584 nm (Figure 2B, red). A small shoulder corresponding to monomeric 2 was also present. Emission from the monomer and aggregate were confirmed by excitation−emission correlation spectroscopy (Figure S5). The full width at 2/3 maximum of the bathochromically shifted peak was 900 cm−1, which was narrower than the monomer (1658 cm−1). From these data, we can calculate that 2 forms Jaggregates with an effective coherent size of ∼3 at a slip angle of ∼40° in 99:1 ROC4F9/CH2Cl2.26 Collectively, these results show that the aggregation state of 2 can be controlled by organic and fluorous solvent to yield distinctive spectral properties (Figures 2B and S1, Table S2). Amphiphilic cyanine 2 is the first cyanine dye for which J-type aggregation can be readily obtained in nonaqueous solvent.27 We further investigated the solvent, temperature and time dependence of the aggregation of 2 in fluorous media. Dye 2 was dissolved in different ratios of CH2Cl2 and ROC4F9 (Figure 2E). With increasing fluorous solvent content, 2 changed from monomer to blue-shifted aggregate to J-aggregate. The solventand time-dependent data (Figure 2C,E) both show formation of the blue-shifted peak prior to J-aggregate formation, suggesting that the blue-shifted aggregate is transformed into the more thermodynamically stable J-aggregate. The sigmoidal kinetics suggested an autocatalytic process,28 which was confirmed by the addition of a seed aggregate (Figure S6). Finally, temperaturedependent studies on 2 in fluorous solvent revealed that the

energy barrier between the red- and blue-shifted aggregates can be readily traversed (Figure S7). The chromophore scaffolds of 1 and 2 are identical. Differences between the stability and photophysical behavior of monomeric and aggregated 1 and 2 are primarily a result of contrasting media and/or assembly of the chromophores. Whereas 1 and 2 have similar properties when characterized in alcohols (Tables 1A and S3),29 the aggregates have distinct features. Aggregates of 1 have been extensively characterized in aqueous solution and are known to form chiral tubes with two main J-bands.14−18 Fluorous phase aggregates of 2 do not display as red-shifted absorption and emission or as high ε and ΦF as aqueous phase aggregates of 1. These inequivalencies could arise from differences in solvation or coupling of the transition dipoles. Further exploration of the aggregates via circular dichroism (CD) spectroscopy indicated that aggregates of 2 are achiral. These results are in contrast to the chiral tubes formed by 1 in water14−18 and suggest that 2 forms planar aggregates in the fluorous phase (Figure S8). The exceptional light harvesting and exciton transport properties of J-aggregates poise them for use in devices, providing the aggregates are stable and processable.30 Poor photostability is a major limitation of J-aggregates of 1.18b,19 Table 1. Photophysical Characterization of Monomer (A) and J-Aggregates (B) of 1 and 2

A. B.

Compound (solvent)

λmax,abs (nm)

ε (M−1 cm−1)

λmax,em (nm)

ΦF (%)

kphotobleach (relative)

1 (MeOH) 2 (MeOH) 1 (water) 2 (fluorous)

520 518 589 564

147,000 126,000 219,000c 153,000d

540 538 601 584

3.3a 6.9a 6.9 1.2

1b 1.09b 1e 0.065e

a

In EtOH. bRelative to kphotobleach of 1 in MeOH. cFrom ref 16a. Apparent ε per monomeric unit at 564 nm. eRelative to kphotobleach of 1 in water. d

B


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Figure 3. J-Aggregates of 1 and 2 on surfaces. (A−C) Solution of 2 dropcast onto glass (A/B) or mica (C). (D−F) Solution of 1 dropcast onto glass (D/ E) or mica (F). (G−I) Solution of 2 dropcast onto glass (G,H) or mica (I) followed by dropcast of solution of 1. (J−L) Solution of 1 dropcast onto glass (A,B) or mica (C) followed by dropcast of solution of 2. (A,D,G,L) The absorption (Abs, solid line) and photoluminescence (PL, dashed line) spectra of the glass. The spectra represent the smoothed average of 10 (Abs) or 4 (PL) points on the film. (B,E,H,K) Contact angle of the films. (C,F,I,L) Tappingmode height atomic force microscopy images and height profile through indicated line. Arrows denote aggregates of 2 (red) and 1 (blue). (A−L) Solution of 2 = 0.034 mM 2 in 99:1 v/v ROC4F9:CH2Cl2; R = 1:1 v/v ratio of Me and Et. Solution of 1 = 0.34 mM 1 in water.

transfer of the aggregates of 2 to mica. Three nanometers is the expected height of one layer of 2 and consistent with the CD data suggesting planar aggregates. The orthogonal nature of the fluorous phase is advantageous for the fabrication of advanced materials on surfaces and in solution.34 We showcase the flexibility in fabrication that fluorous aggregate 2 provides by preparing films and solutions containing distinct J-aggregates of 1 and 2. If true orthogonality is achieved, surfaces prepared by dropcasting 1 followed by 2 or 2 followed by 1 should be identical. To test this, we first prepared films of Jaggregates of 2 and 1 alone and measured their absorption, emission, contact angle, and surface profiles (Figure 3A−F). Films of 1 show an imperfect mixture of tubes and bundles, demonstrating that the transfer of 1 to surfaces is less straightforward than it is for 2 (Figure S12). Next, we dropcast an aqueous solution of aggregates of 1 onto surfaces containing 2 or a fluorous solution of aggregates of 2 onto surfaces containing 1. The absorption and emission of these two films were similar and had features of each aggregate (Figures 3G,J and S13), whereas the contact angles of the surfaces changed to intermediate values (Figure 3H,K). AFM images convincingly show isolated 3 nm tall discs and long bundles indicative of Jaggregates of 2 and 1, respectively (Figure 3I,L). These results demonstrate the compatibility of fluorous and aqueous Jaggregates on surfaces. The orthogonality of aggregates of 1 and 2 is also apparent in solution (Figures S14 and S15). In summary, we have prepared the first cyanine dye that readily J-aggregates in fluorous media. We accomplished this by leveraging the orthogonality of perfluorocarbons and hydro-

Perfluorocarbons have been reported to enhance the photostability of fluorophores22 and we investigated whether this phenomenon would apply to aggregates. We irradiated Jaggregates of 1 in D2O or 2 in 99:1 ROC4F9/CH2Cl2 with a 530 nm LED (1.78 mW/cm2) and measured the loss of absorption over time. After 20 min, aggregates of 1 were significantly degraded, whereas 90% of aggregates of 2 remained (Figure S9, Tables S4 and S5). The photobleaching rate constants reveal aggregates of 2 in fluorous solvent are 15 times more photostable than aggregates of 1 in aqueous media. Interestingly, we did not observe enhanced photostability with monomeric 2 in increasing ratios of ROC4F9:CH2Cl2 (Figure S10, Table S6). Processability and transfer of aggregates to surfaces and solid devices has been a challenge for cyanine J-aggregates.17 When transferring aggregates from aqueous solution to a thin film, a dramatic change in the dielectric constant (κ) of the surrounding environment occurs (κ: water 80, air ∼1), which can destabilize self-assembled structures.28 Perfluorocarbons are the most gaslike phase having very low κ (perfluorohexane: 1.57); therefore, we expect aggregates of 2 to easily transfer to surfaces.31,32 Indeed, a simple dropcasting procedure resulted in the transfer of aggregates of 2 to glass with similar absorption and emission profiles to that observed in fluorous solvent (Figures 3A and S11).33 Contact angle measurements indicated that the surface was hydrophobic after dropcasting 2 (Figure 3B). We further explored surfaces containing 2 by performing atomic force microscopy (AFM). AFM revealed discs uniformly 3 nm in height (Figure 3C, noted by red triangles), indicating robust C


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(13) De Rossi, U.; Moll, J.; Spieles, M.; Bach, G.; Dahne, S.; Kriwanek, J.; Lisk, M. J. Prakt. Chem./Chem.-Ztg. 1995, 337, 203−208. (14) Kirstein, S.; Daehne, S. Int. J. Photoenergy 2006, 2006, 1−21. (15) Pawlik, A.; Ouart, A.; Kirstein, S.; Abraham, H.-W.; Daehne, S. Eur. J. Org. Chem. 2003, 2003, 3065−3080. (16) (a) von Berlepsch, H.; Kirstein, S.; Hania, R.; Pugžlys, A.; Böttcher, C. J. Phys. Chem. B 2007, 111, 1701−1711. (b) Eisele, D. M.; Cone, C. W.; Bloemsma, E. A.; Vlaming, S. M.; van der Kwaak, C. G. F.; Silbey, R. J.; Bawendi, M. G.; Knoester, J.; Rabe, J. P.; Vanden Bout, D. A. Nat. Chem. 2012, 4, 655−662. (17) Eisele, D. M.; Knoester, J.; Kirstein, S.; Rabe, J. P.; Vanden Bout, D. A. Nat. Nanotechnol. 2009, 4, 658−663. (18) (a) Eisele, D. M.; Arias, D. H.; Fu, X.; Bloemsma, E. A.; Steiner, C. P.; Jensen, R. A.; Rebentrost, P.; Eisele, H.; Tokmakoff, A.; Lloyd, S.; Nelson, K. A.; Nicastro, D.; Knoester, J.; Bawendi, M. G. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, E3367−E3375. (b) Caram, J. R.; Doria, S.; Eisele, D. M.; Freyria, F. S.; Sinclair, T. S.; Rebentrost, P.; Lloyd, S.; Bawendi, M. G. Nano Lett. 2016, 16, 6808−6815. (19) Qiao, Y.; Polzer, F.; Kirmse, H.; Kirstein, S.; Rabe, J. P. Chem. Commun. 2015, 51, 11980−11982. (20) Gladysz, J. A.; Curran, D. P.; Horvath, I. T. Handbook of Fluorous Chemistry; Wiley-VHC: Weinheim, 2004. (21) Jochyms, Q.; Mignard, E.; Vincent, J. M. J. Fluorine Chem. 2015, 177, 11−18. (22) (a) Sun, H. R.; Putta, A.; Kloster, J. P.; Tottempudi, U. K. Chem. Commun. 2012, 48, 12085−12087. (b) DiMagno, S. G.; Dussault, P. H.; Schultz, J. A. J. Am. Chem. Soc. 1996, 118, 5312−5313. (23) Lim, J.; Swager, T. M. Angew. Chem., Int. Ed. 2010, 49, 7486− 7488. (24) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: New York, 1970. (25) The Abs values above 1 are in the linear range of the spectrometer, see Figure S16. (26) (a) Busse, G.; Frederichs, B.; Petrov, N. K.; Techert, S. Phys. Chem. Chem. Phys. 2004, 6, 3309−3314. (b) Knapp, E. W. Chem. Phys. 1984, 85, 73−82. (c) Rousseau, E.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 2000, 16, 8865−8870. See SI for equations and detailed calculations. (27) An absorbed cyanine dye has been shown to J-aggregates in hexane. See: Heier, J.; Steiger, R.; Nüesch, F.; Hany, R. Langmuir 2010, 26, 3955−3961. (28) (a) Fukui, T.; Kawai, S.; Fujinuma, S.; Matsushita, Y.; Yasuda, T.; Sakurai, T.; Seki, S.; Takeuchi, M.; Sugiyasu, K. Nat. Chem. 2017, 9, 493−499. (b) Kaiser, T. E.; Stepanenko, V.; Würthner, F. J. Am. Chem. Soc. 2009, 131, 6719−6732. (29) The QY of 2 is higher than 1. See SI for further discussion. (30) (a) Katz, H. E.; Bao, Z. N.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359−369. (b) Allard, S.; Forster, M.; Souharce, B.; Thiem, H.; Scherf, U. Angew. Chem., Int. Ed. 2008, 47, 4070−4098. (31) Kim, H.; Jo, S. H.; Jee, J.-H.; Han, W.; Kim, Y.; Park, H.-H.; Jin, H.J.; Yoo, B.; Lee, J.-K. New J. Chem. 2015, 39, 836−842. (32) Laurence, C.; Nicolet, P.; Dalati, M. T.; Abboud, J.-L. M.; Notario, R. J. Phys. Chem. 1994, 98, 5807−5816. (33) Emission was collected at 340°. (34) Yoder, N. C.; Yüksel, D.; Dafik, L.; Kumar, K. Curr. Opin. Chem. Biol. 2006, 10, 576−583. (35) Chen, Z.; Liu, Y.; Wagner, W.; Stepanenko, V.; Ren, X.; Ogi, S.; Wuerthner, F. Angew. Chem., Int. Ed. 2017, 56, 5729−5733. (36) Zarzar, L. D.; Sresht, V.; Sletten, E. M.; Kalow, J. A.; Blankschtein, D.; Swager, T. M. Nature 2015, 518, 520−524.

carbons to create nonpolar cyanine amphiphiles. We expect this strategy will be applicable to other chromophore classes with amphiphilicity-induced J-aggregation.35 The aggregation of fluorous−lipophilic 5,5′,6,6′-tetrachlorobenzimidacarbocyanine dye 2 can be controlled via changes in solvent and temperature. The fluorous phase J-aggregates of 2 display enhanced photostability and are readily transferred to surfaces. The processability advantages that the fluorous phase provides will offer opportunities for printing and patterning of aggregates. Solution phase assemblies of complex emulsions36 containing Jaggregates are also of interest. Taken together, fluorous cyanine J-aggregate 2 offers enhanced photostability and processability: two important advances for transforming J-aggregates from basic science curiosities into integral components of emerging technologies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b11925. Synthesis and characterization of 4, 5, and 2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Ellen M. Sletten: 0000-0002-0049-7278 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by UCLA and ACS-PRF 57379-DNI4. We thank Prof. Justin R. Caram for helpful discussions. REFERENCES

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Fluorescent Cyanine Dye J-Aggregates in the Fluorous Phase

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