Molecular Engineering of the Physical Properties of HighlyLuminescent π‑Conjugated PhospholesYi Ren, Felix Biegger, and Thomas Baumgartner*

Department of Chemistry and Centre for Advanced Solar Materials, University of Calgary, 2500 University Drive NW, Calgary ABT2N 1N4, Canada

*S Supporting Information

ABSTRACT: The syntheses as well as detailed studies on the thermal,photophysical, and self-organization properties of a new series ofphosphole-based ladder-type materials with exocyclic 5-alkylthienylsubstituents are reported. The studies also include DFT calculationsthat provide support for the experimentally determined photophysics. Incontrast to the related “phosphole-lipids” with fused conjugated head thatexhibit low luminescence in solution but high luminescence in the solidstate, the new system is highly luminescent in both solution and the solidstate. The extensive structure−property study reported herein revealedthat the physical properties of the new species can significantly be tunedvia the size of the conjugated system of the head group, thefunctionalization of the phosphorus center, as well as the length of the thiophene-appended alkyl chain that impacts theintermolecular interactions via π−π stacking, ionic, and van der Waals interactions, respectively. This molecular engineeringapproach allows to access materials with intriguing properties that range from highly emissive oils to self-organized one-dimensional fibers.

■ INTRODUCTION

Fused π-conjugated organic materials have gained considerableinterest in the past two decades due to their intriguing potentialfor practical application in organic electronics. Their com-paratively efficient luminescence and high charge mobilities,next to low production cost and/or applicability in flexible thinfilm devices, make these materials promising for use in organiclight-emitting diodes (OLEDs), organic field-effect transistors(OFETs), and organic photovoltaics (OPVs).1−7 Thiophene-based systems, in particular, have shown favorable optoelec-tronic properties and chemical reactivity for the targetedpurpose and are thus among the most popular building blocksto date.8−11 Phosphorus-based heterocycles, such as phosphole,on the other hand, had not been considered in this context untilrecently.12−16 However, the unique electronic nature and thechemical reactivity of the phosphorus center can lead topromising properties that are distinctly different from those ofrelated thiophene analogues. Notably, the versatile phosphoruschemistry allows to efficiently tailor the photophysical/redoxproperties and self-organization of the materials in the solidstate via simple chemical reactions that trigger noticeablechanges in the hyperconjugation between the pyramidalphosphorus center and the π-conjugated scaffold (Chart1).12−28

In the context of organic electronics, well-defined 1D or 2Dmicro/nanostructures, such as liquid crystals and organogels,have been found to favorably impact the charge-, energy-, andion-transport that are crucial features for the device perform-ance.3 A high degree of order can generally be achieved via

supramolecular interactions, such as π−π interactions, hydro-gen bonding, ionic interactions, and/or van der Waals forces, aswell as a combination thereof.29−36 Recently, our group hassuccessfully designed the bioinspired amphiphilic “phosphole-lipid” system (Chart 2, left), which can self-assemble into liquidcrystals and highly fluorescent organogels as a result ofbalanced intermolecular interactions that involve ionicinteractions, van der Waals forces, as well as π−π stacking.37−39

However, the steric bulk of the ionic phospholium head groupsin this system could potentially limit the π−π stackinginteractions of these conjugated groups and consequentlylower the energy- and charge-transfer capabilities of the system.In this contribution, we now report a new series of molecular

materials related to the original phosphole-lipids, in which alkylchains (for potential van der Waals interactions) were installed

Received: November 21, 2012Revised: February 2, 2013Published: February 27, 2013

Chart 1. P-Functionalization Possibilities of Phosphole (E,Left); n−π Orbital Coupling (A) and σ*−π*Hyperconjugation (B) (Right)

Article

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directly to the exocyclic P-substituent (here: thienyl) in orderto decrease the steric bulk at the phosphorus center andpotentially increase intermolecular π−π interactions (Chart 2,right). The altered molecular structure was expected toconsequently induce very different properties in the system.For this purpose, a series of different species with varying sizeof π-conjugated scaffold with three, four, and five fused rings,respectively, have been synthesized. In addition to neutralsystems that lack the ionic interaction component, less bulkymethylphosphonium species were also included in ourstructure−property studies to gain a better understanding ofthe effect of ionic interaction on the self-assembly features ofthese species. Finally, the length of the alkyl chains was alsotuned to provide deeper insight into their role forintermolecular organization. Our systematic studies of thenew system allowed us to evaluate the molecular structuraleffects on the photophysical and thermal properties, as well asself-organization features in the solid state.

■ EXPERIMENTAL SECTIONGeneral. All manipulations were carried out under a dry

nitrogen atmosphere employing standard Schlenk techniques.Solvents were dried using an MBraun Solvent PurificationSystem. Commercially available chemicals were purchased fromSigma-Aldrich, Alfa-Aesar, or Oakwood Chemicals and were, ifnot otherwise noted, used as received. NMR solvents werepurchased from Cambridge Isotope Laboratories. 3,3′-Dibro-mo-2,2′-bithiophene,21 3,3′-dibromo-2,2′-bis(benzo[b]-thiophene),40 and 2-(2′-bromophenyl)-3-bromobenzo[b]-thiophene26 were synthesized according to literature proce-dures. 31P{1H} NMR, 1H NMR, and 13C{1H} NMR wererecorded on Bruker Avance (-II, -III) 300 and 400spectrometers. Chemical shifts were referenced to external85% H3PO4 (

31P) and TMS (13C, 1H). Elemental analyses wereperformed at the Department of Chemistry at the University ofCalgary on a Perkin-Elmer Model 2400 series II instrument.MALDI/TOF, EI, and ESI mass spectra were run on a BrukerDaltonics AutoFlex III system, Finnigan SSQ 7000, and Agilent6520 Q-TOF instrument, respectively. Thermal analyses wereperformed using TA-Q200 DSC and TA-Q50 TGA instru-ments. All fluorescence and UV−vis experiments were recordedin dichloromethane solution on a Jasco FP-6600 spectrofluor-ometer and UV−vis−NIR Cary 5000 spectrophotometer.

Fluorescence quantum yields and lifetimes were measuredusing an Edinburgh Instruments Ltd. FLS920P fluorescencespectrometer equipped with an integrating sphere. X-raycrystallographic data were collected on a Nonius Kappa CCDdiffractometer at 173 K. The structure was solved by directmethods and refined using SHELX. After full-matrix least-squares refinement of the non-hydrogen atoms with anisotropicthermal parameters, the hydrogen atoms were placed incalculated positions using a riding model. Theoreticalcalculations have been carried out at the B3LYP/6-31G(d)level using the Gaussian 03 suite of programs.41 This level oftheory has provided satisfactory results for the dithienophos-phole system in the past.21−26,40 Fluorescence confocalmicroscopy was performed on a Leica inverted confocal TCSSP2 microscope with water immersion lens.

General Experimental Procedure for Thienyldichlorophos-phanes. 2-Hexylthiophene (5.18 g; 30.7 mmol) was dissolvedin 100−150 mL of THF and cooled to −78 °C. Subsequently,n-BuLi (12.9 mL; 2.5 M solution, 1.05 equiv) was addeddropwise, and the solution was allowed to warm to roomtemperature and stirred for another 40 min. After cooling to−78 °C again, bis(diethylamino)chlorophosphane (7.76 g; 36.8mmol) was added slowly, and the reaction was stirred foranother 5 min at low temperature before warming up to roomtemperature and stirring for 5 h. The THF was subsequentlyreplaced with Et2O, which led to a white precipitate. Thesolution was then filtered before adding HCl (69.2 mL; 2 Msolution in Et2O, 4.5 equiv) at 0 °C. Another white precipitateformed instantaneously and was filtered off after stirringovernight at room temperature. After evaporation of thefiltrate, the product was obtained as a light yellow liquid.C6-species:

31P{1H} NMR (162 MHz, CDCl3): δ = 144.9ppm (s); 1H NMR (400 MHz, CDCl3): δ = 7.49 (dd, J = 7.2Hz, J = 3.6 Hz, 1H), 6.82 (m br, 1H), 2.90 (t, 3 JHH = 7.6 Hz,2H), 1.72 (m br, 2H), 1.35 (m br, 6H), 0.91 (t, 6.8 Hz, 3H)ppm.The same procedure was followed for the C12 analogue. 2-

Dodecylthiophene (5.08 g; 20.2 mmol), n-BuLi (8.5 mL; 2.5 Msolution in hexanes; 1.05 equiv), bis(diethylamino)-chlorophosphane (5.01 g; 24.3 mmol), and HCl (45.5 mL;2 M solution, 4.5 equiv) were used.C12-species:

31P{1H} NMR (162 MHz, CDCl3): δ = 145.9ppm (s); 1H NMR (400 MHz, CDCl3): δ = 7.48 (dd, J = 7.4Hz, J = 3.6 Hz, 1H), 6.81 (m br, 1H), 2.90 (t, 3 JHH = 7.8 Hz,2H), 1.73 (m br, 2H), 1.28 (m br, 18H), 0.90 (t, 6.8 Hz, 3H)ppm.The same procedure was followed for the nonsubstituted H

analogue: n-BuLi (8.0 mL, 20.0 mmol, 2.5 M in hexane), 1-bromothiophene (3.16 g, 20.0 mmol), bis(diethylamino)-chlorophosphane (4.21 g, 20.0 mmol), and HCl (40 mL, 80mmol, 2 M in diethyl ether) were used. The product wasdistilled to afford a clear oily liquid (yield: 2.78 g, 75%).31P{1H} NMR (162 MHz, CDCl3): δ = 145.5 ppm (s); 1HNMR (400 MHz, CDCl3): δ = 7.86 (dd, 3JHH = 4.8 Hz, 4JHH =1.1 Hz, 1H), 7.67 (ddd, 3JHP = 7.0 Hz, 3JHH = 3.8 Hz, 4JHH = 1.1Hz, 1H), 7.14 (ddd, 3JHH = 4.8 Hz, 3JHH = 3.8, 4JHP = 2.0 Hz,1H) ppm; 13C{1H} NMR (100 MHz, CDCl3): δ = 143.2 (d,JCP = 72.1 Hz), 136.1 (d, JCP = 6.7 Hz), 135.7 (d, JCP = 43.5Hz), 128.0 (d, JCP = 8.1 Hz) (all aromatics) ppm.

General Experimental Procedure for M, 1a/b, 2a/b, and3a/b. The corresponding dibromo species (1 mmol) weredissolved in Et2O (ca. 150 mL) and cooled to −78 °C.Subsequently, 2 equiv of n-BuLi was added dropwise at this

Chart 2. Original Phosphole-Lipid (Left) and New System(Right) with Their Potential for Intermolecular Interactions

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temperature. The reaction mixtures were stirred for a couple ofminutes at low temperature before stirring for 30 min at roomtemperature. After the reaction mixtures were cooled to −78°C once more, 1.1 equiv of dichloro(5-alkylthiophen-2-yl)phosphane was added before quickly warming up to roomtemperature. After stirring overnight, 2−3 mL of 30% H2O2was added to the reaction mixtures that were then stirred foranother 3 h. The compounds were purified using columnchromatography (silica, hexane/ethyl acetate).Compound M. Product was obtained by column chroma-

tography (silica:hexane/ethyl acetate: from 4:1). Yield: 72%.31P NMR (CDCl3, 162 MHz, δ): 11.1 ppm; 1H NMR (CDCl3,400 MHz, δ): 7.65−7.60 (m, 2H), 7.30 (dd, J = 5.2 Hz, J = 3.6Hz, 2H), 7.23 (dd, J = 4.8 Hz, J = 2.8 Hz, 2H), 7.15−7.13 (m,1H) ppm. 13C NMR (CDCl3, 100 MHz, δ): 145.5 (d, J = 26.3Hz), 138.8 (d, J = 118.5 Hz), 136.1 (d, J = 11.4 Hz,), 133.3 (d, J= 5.7 Hz), 128.4 (d, J = 12.8), 125.9 (d, J = 14.6) ppm.Compound 1a. Product was obtained by column

chromatography (silica:hexane/ethyl acetate: from 4:1). Yield:53%. 31P NMR (CDCl3, 162 MHz, δ): 10.6 ppm; 1H NMR(CDCl3, 400 MHz, δ): 7.47 (dd, J = 8.7, J = 3.5 Hz, 1H, Hc),7.31 (dd, J = 4.8 Hz, J = 3.3 Hz, 2H), 7.25 (dd, J = 4.9 Hz, J =2.6 Hz, 2H), 6.81−6.79 (m, 1H), 2.82 (t, J = 7.9 Hz, 2H), 1.66(m, J = 7.6 Hz, 2H), 1.26−1.39 (m, 6H), 0.89 (t, J = 6.9 Hz,3H) ppm. 13C NMR (CDCl3, 100 MHz, δ): 155.3 (d, J = 5.3Hz), 145.3 (d, J = 27.2 Hz), 139.6 (s), 138.4 (s), 136.5 (d, J =10.4 Hz,), 128.2 (d, J = 15.1 Hz), 125.9 (d, J = 14.2), 125.7 (d,J = 24.2), 31.4 (s), 30.2 (s), 28.7 (s), 22.5 (s), 14.0 (s) ppm;HRMS (EI-TOF): m/z = 378.0342 ([M], Calcd. 378.0336).Compound 1b. Product was obtained by column

chromatography (silica:hexane/ethyl acetate: from 4:1). Yield:66%. 31P NMR (CDCl3, 162 MHz, δ): 11.3 ppm; 1H NMR(CDCl3, 400 MHz, δ): 7.47 (dd, J = 8.6 Hz, J = 3.6 Hz, 2H),7.31 (dd, J = 4.8 Hz, J = 3.3 Hz, 2H), 7.25 (dd, J = 5.1 Hz, J =2.7 Hz, 2H), 6.81−6.79 (m, 1H), 2.81 (t, J = 7.7 Hz, 2H), 1.66(m, 2H,), 1.27−1.37 (m, 18H), 0.90 (t, J = 7.2 Hz, 3H) ppm;13C NMR (CDCl3, 100 MHz, δ): 165.3 (d, J = 5.5 Hz), 145.3(d, J = 25.9 Hz), 139.6 (s), 138.6 (s), 136.5 (d, J = 11.6 Hz)128.2 (d, J = 15.7 Hz), 125.9 (d, J = 14.6 Hz), 125.7 (d, J = 14.0Hz), 31.9 (s), 31.5 (s), 30.2 (s), 29.6 (s), 29.6 (s), 29.5 (s), 29.3(s), 29.2 (s), 29.1 (s), 22.7 (s), 14.1 (s) ppm; HRMS (EI-TOF): m/z = 462.1257 ([M], Calcd. 462.1275).Compound 2a. Product was obtained by column

chromatography (silica:hexane/ethyl acetate: from 4:1). Yield:61%. 31P {1H} NMR (162 MHz, CDCl3): 10.5 ppm; 1H NMR(400 MHz, CDCl3): δ = 8.02 (d br, J = 7.88 Hz, 2H), 7.87 (dbr, J = 7.88 Hz, 2H), 7.48−7.44 (m, 3H), 7.38−7.34 (m, 2H),7.43−7.32 (m, 4H), 6.78−6.77 (m, 1H), 2.77 (t, J = 7.64 Hz,2H), 1.65−1.57 (m, 2H), 1.33−1.22 (m, 6H), 0.84 (t, J = 7.56Hz, 3H) ppm; 13C{1H} NMR (100 MHz, CDCl3): δ = 155.6(d, J = 5.1 Hz), 146.1 (d, J = 26.0 Hz), 143.3 (d, J = 13.5 Hz),136.2 (d, J = 11.5 Hz), 136.0 (d, J = 13.3 Hz), 133.8 (d, J =116.3 Hz), 126.8 (d, J = 121.7 Hz), 126.3 (s), 125.8 (s), 125.7(s), 125.3 (s), 123.4 (d, J = 15.2 Hz), 31.3 (s), 31.3 (s), 28.6(s), 22.4 (s), 13.9 (s) ppm; HRMS (EI-TOF): m/z = 478.0632([M], Calcd. 478.0649).Compound 2b. Product was obtained by column

chromatography (silica:hexane/ethyl acetate: from 4:1). Yield:65%. 31P NMR (CDCl3, 162 MHz, δ): 10.6 ppm; 1H NMR(CDCl3, 400 MHz, δ): 8.05 (d br, J = 7.6 Hz, 2H), 7.9 (d br, J= 8.1 Hz, 2H), 7.49−7.45 (m, 3H), 7.40−7.36 (m, 2H), 6.70−6.77 (m, 1H), 2.77 (t, J = 8.0 Hz, 2H), 1.65−1.57 (m, 4H),1.31−1.23 (m, 16H), 0.87 (t br, J = 6.8 Hz, 3H) ppm; 13C

NMR (CDCl3, 100 MHz, δ): 155.7 (d, J = 5.2 Hz), 146.2 (d, J= 25.0 Hz), 143.4 (d, J = 13.5), 136,2 (d, J = 11.4 Hz), 136.1(d, J = 13.4 Hz), 134.5 (s), 133.31 (s), 126.4 (s), 125.8 (d, J =14.1 Hz), 125.4 (s), 123.5 (s), 123.4 (s), 31.92 (s), 31.4 (s),30.3 (s), 29.6 (d), 29.59 (s), 29.48 (s), 29.35 (s), 29.24 (s),29.09 (s), 22.69 (s), 13.14 (s) ppm; HRMS (MALDI-TOF):m/z = 562.1574 ([M], Calcd. 562.1588).

Compound 3a. Product was obtained by columnchromatography (silica:hexane/ethyl acetate: from 4:1). Yield:60%. 31P {1H} NMR (162 MHz, CDCl3): 18.0 ppm; 1H NMR(400 MHz, CDCl3): δ = 7.97−7.84 (d br, J = 7.84 Hz, 1H),7.85−7.82 (m, 1H), 7.81−7.76 (m, 1H), 7.52−7.44 (m, 2H),7.43−7.32 (m, 4H), 6.78−6.76 (m, 1H), 2.76 (t, JHH = 7.68 Hz,2H), 1.64−1.56 (m, 2H), 1.33−1.22 (m, 6H), 0.84 (t, JHH = 6.8Hz, 3H) ppm; 13C{1H} NMR (100 MHz, CDCl3): δ = 155.3(d, JCP = 7.6 Hz), 153.9 (d, JCP = 29.5 Hz), 143.0 (d, JCP = 12.5Hz), 136.9 (s), 136.6 (d, JCP = 20.1 Hz), 136.0 (d, JCP = 10.8Hz), 135.8 (d, JCP = 7.2 Hz), 133.0 (d, JCP = 2.0 Hz), 130.6 (d,JCP = 114.0 Hz), 129.3 (d, JCP = 5.7 Hz), 129.2 (d, JCP = 3.7Hz), 127.3 (d, JCP = 118.5 Hz), 125.9 (s), 125.7 (s), 125.5 (s),125.3 (s), 123.3 (d, JCP = 11.6 Hz), 121.8 (d, JCP = 9.3 Hz),31.2 (s), 31.2 (s), 28.5 (s), 22.3 (s), 13.8 (s) ppm; HRMS (EI-TOF): m/z = 422.0919 ([M], Calcd. 422.0928).

Compound 3b. Product was obtained by columnchromatography (silica: hexane/ethyl acetate: from 4:1),Yield: 56%. 31P NMR (CDCl3, 162 MHz, δ): 18.1 ppm; 1HNMR (CDCl3, 400 MHz, δ): 7.99 (d br, J = 8.3 Hz, 1H), 7.90(d br, J = 7.7, 1H), 7.82−7.78 (m, 1H), 7.50−7.59 (m, 2H),7.37−7.47 (m, 4H), 6.79−6.77 (m, 1H), 2.79 (t, J = 7.7 Hz,2H), 1.64 (m, 2H), 1.21−1.38 (m, 18H), 0.90 (t, J = 7.1 Hz)ppm; 13C NMR (CDCl3, 100 MHz, δ): 155.6 (d, J = 4.7 Hz),154.1 (d, J = 129.9 Hz), 143.2 (d, J = 12.7 Hz), 137.1 (s), 136.8(d, J = 20.1), 136.2 (d, J = 20.1 Hz), 136.0 (d, J = 10.5 Hz),133.2 (d, J = 2.0 Hz), 131.4 (s), 130.2 (s), 129.5 (d, J = 1.7Hz), 129.4 (s), 128.0 Hz (s), 126.8 (s), 126.1 (s), 125.8 (d, J =14.0), 125.5 (s), 123.5 (d, J = 20.4 Hz), 122.0 (d, J = 9.8 Hz),53.4 (s), 31.9 (s), 31.4 (s), 30.3 (s), 29.63 (s), 29.60 (s), 29.48(s), 29.35 (s), 29.25 (s), 29.09 (s), 22.70 (s), 7.64 (s) ppm;HRMS (MALDI-TOF): m/z = 506.1851 ([M], Calcd.506.1867).The methylated species were obtained via a similar method,however, quenching with a slight excess of methyl triflateinstead of H2O2 in the final step.

Compound 2c. Product was obtained by column chroma-tography (silica:hexane/ethyl acetate: from 4:1). Yield: 58%.31P NMR (CDCl3, 162 MHz, δ): 4.5 ppm; 1H NMR (CDCl3,400 MHz, δ): 8.68 (dd, J = 9.2 H, J = 3.6 Hz, 1H), 8.04−7.99(m, 4H), 7.66−7.62 (m, 2H), 7.56−7.52 (m, 2H), 7.21−7.20(m, 1H,), 3.09 (d, J = 14.8 Hz, 3H), 2.82 (t, J = 8.0 Hz, 2H),1.64−1.56 (m, 2H), 1.32−1.21 (m, 6H), 0.82 (t br, J = 6.8 Hz,3H) ppm; 13C NMR (CDCl3, 100 MHz, δ): 162.1 (d, J = 5.4Hz), 150.3 (d, J = 22.2 Hz), 145.1 (d, J = 13.0 Hz), 143.4 (d, J= 14.5 Hz), 134.5 (d, J = 14.7 Hz), 129.6 (d, J = 15.4 Hz),128.3 (s), 127.2 (s), 124.2 (s), 122.8 (s), 122.9 (d, J = 101.6),106.4 (d, J = 106.1 Hz), 31.2 (s), 31.1 (s), 30.5 (s), 28.6 (s),22.4 (s), 13.9 (s), 8.32 (d, J = 54.7) ppm; HRMS (ESI): m/z =478.0632 ([M−H]+, Calcd. 478.0649).

Compound 2d. Product was obtained by columnchromatography (silica:hexane/ethyl acetate: from 4:1). Yield:60%. 31P NMR (CDCl3, 162 MHz, δ): 4.5 ppm; 1H NMR(CDCl3, 400 MHz, δ): 8.70 (dd, J = 9.4 Hz, J = 3.8 Hz, 1H),8.06 (d br, J = 8.5, 2H), 8.03 (d br, J = 8.3 Hz, 2H), 7.66 (m,2H), 7.56 (m, 2H), 7.23 (m, 1H,), 3.12 (d, J = 14.7 Hz, 3H),

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2.84 (t, J = 7.8 Hz, 2H), 1.66−1.59 (m, 2H), 1.18−1.35 (m,18H), 0.89 (t br, J = 7.2 Hz, 3H) ppm; 13C NMR (CDCl3, 100MHz, δ): 162.1 (d, J = 5.6 Hz), 150.4 (d, J = 22.3 Hz), 145.1(d, J = 13.0 Hz), 143.4 (d, J = 14.5 Hz), 134.5 (d, J = 14.52),129.6 (d, J = 14.0 Hz), 128.3 (s), 127.2 (s), 124.2 (s), 122.8 (s),107.0 (s), 105.8 (s), 31.9 (s), 31.2 (s), 30.9 (s), 29.55 (s), 29.34(s), 29.07 (s), 22.657 (s), 14.09 (s), 8.39 (d, J = 55.89) ppm;HRMS (EI-TOF): m/z = 561.1863 ([M]+, Calcd. 561.1868).Compound 3c. Product was obtained by column chroma-

tography (silica:hexane/ethyl acetate: from 4:1). Yield: 54%.31P {1H} NMR (162 MHz, CDCl3): 10.0 ppm; 1H NMR (400MHz, CDCl3): δ = 8.43 (m, 1H), 8.01 (m, 1H), 7.85−7.74 (m,3H), 7.6563 (m, 1H), 7.55 (m br, 2H), 7.15 (m, 1H), 2.96 (d, J= 14.8 Hz, 2H), 2.82 (d, J = 7.60 Hz, 2H), 1.64−1.57 (m, 2H),1.33−1.22 (m, 6H), 0.83 (t, JHH = 7.2 Hz, 3H) ppm; 13C{1H}NMR (100 MHz, CDCl3): δ = 161.7 (d, JCP = 5.3 Hz), 159.1(d, JCP = 25.6 Hz), 144.2 (d, JCP = 12.2 Hz), 143.1 (d, JCP =13.7 Hz), 137.8 (d, JCP = 17.6 Hz), 135.9 (d, JCP = 2.2 Hz),134.5 (d, JCP = 13.9 Hz), 133.1 (d, JCP = 11.3 Hz), 131.6 (d, JCP= 12.7 Hz), 129.2 (d, JCP = 3.7 Hz), 127.6 (s), 127.0 (s), 125.2(d, JCP = 551.7 Hz), 124.3 (s), 123.5 (d, JCP = 9.0 Hz), 122.6(s), 118.7 (d, JCP = 109.6 Hz), 117.2 (s), 108.9 (d, JCP = 104.7Hz), 31.3 (s), 31.1 (s), 30.4 (s), 29.7 (s), 28.6 (s), 22.4 (s), 13.9(s), 8.8 (d, JCP = 54.8 Hz) ppm.

■ RESULTS AND DISCUSSION

Synthesis. The necessary dibrominated starting materialsfor the synthesis of the various π-conjugated phospholes wereobtained via the reported procedures.21,26,40 Ring-closingreactions for the installation of the central phosphole ring

involved lithiation of the respective dibromo precursors andsubsequent quenching with the corresponding dichlorophos-phanes, in analogy to our established procedures,21−26 andafforded the target phosphole systems in moderate yields(Scheme 1).Further functionalization of the P-center involved oxidation

with H2O2, or methylation with methyl triflate, both followingestablished protocols as well.21−28 For comparison we have alsoincluded the nonalkylated model compound M withdithienophosphole scaffold in this study, to “isolate” the effectsof the thiophene group on the photophysical properties and theorganization in the solid state. All compounds werecharacterized by 1H, 31P, and 13C NMR spectroscopy, as wellas high-resolution mass spectrometry. The 31P{1H} NMRspectrum of model compound M (δ = 11.2 ppm) shows apronounced high-field-shifted resonance compared to P-phenyl-substituted dithienophosphole P (Scheme 1; δ = 18.8ppm),21 due to the electron-donor character of the 2-thienylgroup at phosphorus. Compared to the P-oxide species (1a, δ =10.6 ppm; 1b, δ = 11.3 ppm; 2a, δ = 10.5 ppm; 2b, δ = 10.6ppm; 3a, δ = 18.0 ppm; 3b, δ = 18.1 ppm), themethylphosphonium derivatives show high-field-shifted 31PNMR resonances (2c, δ = 4.5 ppm; 2d, δ = 4.5 ppm; 3c, δ= 10.0 ppm), which are consistent with our previous results andthe literature. However, all shifts are also high-field-shiftedcompared to the related P-phenyl congeners.21,26,40,42

Thermal Properties. The thermal properties of thecompounds were analyzed by differential scanning calorimetry(DSC) with heat−cool−heat scans with rates of 5 °C/min andare summarized in Table 1. It was observed that the length of

Scheme 1. Synthesis of the Modified, New Phosphole-Lipids

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the alkyl chain, the size of the π-conjugated head, as well as thenature of the P-center play an important role in the thermalproperties of the system (Figure 1).Oxide species 1a (Tm = −25 °C) and 3a (Tg = −23 °C) with

relatively small conjugated heads actually are oils at roomtemperature (Figure 1b, c) with very low melting and glasstransition temperatures, respectively. Upon increasing the sizeof the conjugated head, the melting temperature was alsosignificantly increased (2a, Tm = 167 °C). Compound 1b withthe longer dodecyl chain is a solid at room temperature andshows a glass transition at Tg = 35 °C, a crystallization processat TCr = 33 °C, and a small endothermic transition at Ten = 77°C. Compared to 1a with hexyl chain, 1b exhibits a muchhigher melting point (Tm = 91 °C), which is probably due to amore organized structure induced by phase segregation of thelong apolar dodecyl chain and the somewhat polar conjugatedhead. The asymmetric 3b with dodecyl chain, on the otherhand, exhibits a broad and lower endothermic transition (Tm =−20 °C), probably due to the limited degree of order in theracemic mixtures (the P-atom is a stereocenter in thissystem).26

As for series 2 with a larger conjugated backbone, thephospholium species 2c (Tm = 146 °C) and 2d (Tm = 107 °C)generally exhibit relatively low melting points compared to theoxide species 2a (Tm = 167 °C) and 2b (Tm = 132 °C). A verysmall glass transition (Tg = 22 °C) and two recrystallizationprocesses (TCr = 68 and 117 °C) are observed for 2a, indicatingthat this compound is in a thermal transition between anamorphous (metastable) and crystalline (stable) state during

the second heating process. For 2b with the longer C12 alkylchain, a significant exothermic transition was observed at 86 °Cupon cooling from the isotropic liquid. The melting point isdecreasing upon elongation of the alkyl chain (2a, Tm = 167°C; 2b, Tm = 132 °C). Compared to oxide species 2a,b, methylphosphonium species 2c,d exhibit very different thermaltransition features. Compounds 2c and 2d show meltingtransitions in the first heating (Tm = 146 °C and Tm = 107 °C,respectively) and glass transition in the second heating process(2c, Tg = 42 °C; 2d, Tg = 24 °C), which can be attributed tothe different history of the samples. These observations indicatethat a metastable amorphous state can be accessed due to theslow kinetics of crystallization during the rather fast coolingprocess. Based on the current studies, compounds with smallerconjugated heads (i.e., series 1) show a “rodlike” shape due tothe small size of the heads and large size of alkyl chains.Therefore, the transition temperatures (melting and glasstransition) generally increase upon elongation of the alkylchain, which is due to the increased ordered molecularorganization resulting from the enhanced rodlike shape ofmolecules. As for molecules with bigger conjugated heads (i.e.,series 2), the size of the head seems to play a more importantrole for the molecular organization, particularly for thecompounds with shorter hexyl chain. Furthermore, it wasfound that lower transition temperatures (melting and glasstransition) are generally observed upon elongation of the alkylchain, which can be attributed to an increase in disorderresulting from the high flexibility of the alkyl chain. Moreover,the methylated systems also show lower melting transitiontemperatures when compared to the oxide counterparts, whichcan be attributed to the presence of the large counterions thatlimit strong interactions between the cationic phospholes.

Photophysical Properties. In contrast to the originalphosphole-lipids with low luminescence in solution,37,38 all newcompounds are highly luminescent in solution and the solidstate (Figure 2). Table 2 summarizes the obtained data fromUV−vis and fluorescence spectroscopy of 10−5 M solutions inCH2Cl2. Compared to the P-phenyl-substituted phosphole P(λabs = 361 nm and λem = 446 nm), the model compound M(λabs = 367 nm, λem = 452 nm) exhibits a slightly red-shiftedabsorption and emission, which is in line with the strongerdonor nature of the thienyl group.21,42 These observationssupport the impact of the changes in the phosphole-typicalσ*−π* hyperconjugation on the photophysical proper-ties.17,37−39,43,44 Red-shifted absorption and emission features

Table 1. Thermal Transitions for the Phosphole-Lipids,Obtained via DSCa

compd Tendoa (°C) Tg

b (°C) TCrc (°C)

1a n.o.d −25 n.o.1b 77/91 −35 332a 25/152/167 22 68/1172b 108/132 n.o. n.o.2c 116e/146e 42 n.o.2d 107e 24 n.o.3a n.o. −23 n.o.3b −20 n.o. n.o.

aMeasured in the second heating process, melting point in bold.bGlass-transition temperature in the second heating. cCrystallizationtemperature. dn.o.not observed. eMelting point in the first heatingprocess.

Figure 1. (a) DSC traces of compounds, second heating, 5 °C/min; Tg, glass transition temperature; TCr, crystallization temperature; Ten,endothermal transition temperature; Tm, melting temperature, * only observed in the first heating scan. (b) Photograph of liquid 3a. (c) Photographof 3a under UV light (λex = 365 nm).

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can be observed from series 3, over series 1, to series 2 due toincreasing conjugation length and/or donor strength of thefused peripheral rings that is consistent with theoretical studies(vide infra). Since the photophysical properties of thesecompounds in solution mainly depend on the electronic natureof the π-conjugated backbones, the length of the alkyl chain (C6vs C12) does not show any effect on the absorption or emissionmaxima. In addition, compared to the phosphole oxide species2a and 2b, the methylated phosphonium species 2c and 2dgenerally display a red-shifted absorption of ca. Δλabs = 15 nmand emission of about Δλem = 26 nm (Table 2), which is due tothe increased acceptor character of the positive phospholiumcenter and again in line with our earlier observations.26

Compared to the high quantum yield of ϕPL = 0.81 for themodel compound M, the values for compounds 1a (ϕPL =0.53) and 1b (ϕPL = 0.52) are noticeably lower. This can beattributed to the high flexibility of the alkyl chain leading tolarger nonradiative decay constants (M, Knr = 0.73 × 107 s−1;1a, Knr =5.2 × 107 s−1; 1b, Knr = 5.3 × 107 s−1).45 Series 2 withits extended conjugated backbone shows significantly higherquantum yields, even reaching unity (2a, ϕPL = 0.95; 2b, ϕPL =0.84; 2c, ϕPL = 0.99; 2d, ϕPL = 1.00), which can be attributed toa significantly lowered nonradiative and concomitantlyincreased radiative decay rates (Table 2). Furthermore, thequantum yield is larger for the methyl phosphonium species 2c

and 2d than for the corresponding oxides 2a and 2b. The highvalues are the result of significantly lowered nonradiative decayrates (2a, Knr = 0.4 × 107 s−1; 2b, Knr = 1.3 × 107 s−1; 2c, Knr =0.06 × 107 s−1). Surprisingly, the series of asymmetricphospholes 3 shows rather low quantum yields of ϕPL = 0.19,0.18, and 0.38 for 3a, 3b, and 3c, respectively, which are theconsequence of substantially higher nonradiative decay rateconstants (3a, Knr = 32.4 × 107 s−1; 3b, Knr = 32.8 × 107 s−1;3c, Knr = 8.49 × 107 s−1).Table 3 shows the photophysical data for all compounds in

the solid state, and Figure 2b shows the emission of

representative dropcast film samples in the solid state. In thesolid state, M shows a significantly red-shifted emission (λem =494 nm) compared to its solution (λem = 452 nm), which couldbe attributed to π−π stacking in a dimerlike structuralarrangement (vide infra). Remarkably, even in the liquid oilystate, 1a (ϕPL = 0.18), 3a (ϕPL = 0.21), and 3b (ϕPL = 0.18)show relatively strong fluorescence at room temperature(Figure 1c). For series 3, the absorption of the alkyl-substitutedspecies does not show significant changes in the solid statecompared to their solutions indicating the absence ofintermolecular interactions in the ground state (Table 3). Bycontrast, oxide series 1 (1a, Δλabs = 7 nm; 1b, Δλabs = 5 nm)and 2 (2a, Δλabs = 8 nm; 2b, Δλabs = 19 nm) with the largerconjugated head and longer alkyl chain show a more red-shiftedabsorption (Figure 3); particularly series 2 (2a, Δλonset = 22nm; 2b, Δλonset = 35 nm) exhibits a significantly red-shiftedshoulder compared to series 1 (1a, Δλonset = 14 nm; 1b, Δλonset= 15 nm) and 3 (3a, Δλonset = 10 nm; 3b, Δλonset = 11 nm),which can be attributed to the stronger π−π interactionsbetween molecules with bigger conjugated heads in series2.26,37,38 Furthermore, 2a (Δλem = 6 nm) and 2b (Δλem = 19nm) also show a relatively strongly red-shifted emissioncompared to the other species (1a, Δλem = 10 nm; 1b, Δλem= 6 nm; 2c, Δλem = 1 nm; 2d, Δλem = 1 nm), which is likelydue to stronger π−π interactions via the pronounced phasesegregation in 2 with the longer dodecyl chain.

Figure 2. Luminescence of all new compounds in CH2Cl2 solution(top) and of selected species in the solid state (bottom), excited with ahandheld UV lamp (λex = 365 nm).

Table 2. Photophysical Data for the Phosphole-Lipids inSolution (c = 10−5 M in CH2Cl2)

compdλabs(nm) log εa

λem(nm) ϕPL

b τd (ns) Krc Knr

c

M 367 3.30 452 0.81 13.8 6.5 0.731a 365 3.85 450 0.53 9.0 5.8 5.21b 365 3.52 451 0.52 9.1 5.7 5.32a 405 3.88 496 0.95 12.6 7.5 0.42b 405 3.94 497 0.84 12.6 6.7 1.32c 420 3.98 522 0.99 16.2 6.1 0.062d 420 4.06 521 1.00 16.1 6.2 0.03a 330 3.95 428 0.19 2.4 (96.7%) 7.1 32.4

5.8 (3.3%)3b 330 3.94 430 0.18 2.4 (98.3%) 7.2 32.8

9.8 (1.58%)3c 340 3.93 446 0.38 7.3 5.2 8.49

aMolar absorption coefficient (M−1 cm−1). bFluorescence quantumyield determined by a calibrated integrating sphere system. cRateconstants (107 s−1) calculated according to the formulas Kr = Φpl/τand Knr = (1 − Φpl)/τ.

dLifetime.

Table 3. Photophysical Data for the Phosphole-Lipids in theSolid Statea

compdλabs(nm) λem

b (nm) ϕPL τ (ns)

M n.a.c 494 0.25 8.21a 372 460 0.18 0.5 (41.0%), 1.5 (52.4%), 3.4

(6.6%)1b 370 457 0.15 0.1 (12.5%), 0.9 (46.4%), 3.4

(41.1%)2a 406 576,

506 (502)0.40 2.8 (61.7%), 4.9 (38.3%)

2b 424 564,519 (512)

0.53 2.8 (42.1%), 7.3 (57.9%)

2c 422 523 0.61 0.4 (36.0%), 1.6 (50.0%), 6.5(14.0%)

2d 421 522 0.78 1.0 (18.6%), 3.2 (58.2%), 8.7(23.2%)

3a 332 437 0.21 0.2 (15.4%), 1.4 (56.5%), 5.3(28.0%)

3b 332 440 0.18 1.8 (67.4%), 4.8 (32.6%)3c 339 448 0.25 1.2 (15.2%), 3.4 (66.8%), 10.5

(18.0%)aAll measurements were conducted with thin films obtained by dropcasting of 10−3 M solutions in CH2Cl2 onto cleaned glass slides.bValues in parentheses show maxima for samples that were heatedupon drop casting. cn.a.not available

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As for the methylphosphonium species, 2c (λem = 523 nm)and 2d (λem = 522 nm) display a red-shifted emission (andabsorption, see Table 3) compared to the corresponding oxidecompounds (2a, λem = 502 nm; 2b, λem = 514 nm), which issimilar to the observations in solution. However, themethylphosphonium species 2c,d show almost the sameabsorption and emission in both solution and the solid state(Table 3). Based on these observations, it seems that thestructural features, such as the less bulky phosphorus oxidecenter, more extended conjugated head, and longer alkyl chain,are very important for strong π−π interactions that lead tophase segregation in the solid state.A peculiar observation for 2a,b in the solid state is the fact

that two emission maxima can be observed for freshly dropcastfilms of the oxides 2a and 2b (Figure 4), which is in agreementwith the two observed morphologies for 2a and 2b (yellow andorange appearance) as well as the DSC studies (vide supra).While the thin films of both compounds for UV−vis andfluorescence spectroscopy were prepared via dropcasting fromsolution, the glass slides were usually gently heated to acceleratesolvent evaporation. The metastable morphology, with red-shifted emission, could thus be frozen within the bulk matrixupon fast evaporation of the solvent. If the glass slides were notheated upon evaporation, the low-energy peak only appears as ashoulder and the emission maxima are slightly red-shifted.Notably, annealing overnight at 80 °C led to a continueddecrease of the intensity of the shoulder further supporting themetastable phase slowly converting into the stable (yellow)phase, in line with the DSC results.Despite the fact that the fluorescence quantum yields for

compound series 1 (ϕPL ≈ 0.15) and 2 (ϕPL ≈ 0.36) are lowerin the solid state when compared with the corresponding values

in solution (1, ϕPL ≈ 0.50; 2, ϕPL ≈ 0.90), as a consequencefrom intermolecular quenching, they are still an appreciablesolid-state emitter. The asymmetric compounds 3a, 3b, and 3con the other hand show almost the same quantum yield valuesfor both solid state and in solution, which is consistent with theobservation of similar absorption between the solution andsolid state (see Figure 3b,c) and suggest that the racemicmixtures prevent a more organized molecular organization inthe solid state.

Theoretical Calculations. DFT calculations were carriedout at the B3LYP/6-31G(d) level of theory (polarizedcontinuum model, PCM; solvent = CH2Cl2)

41 to furtherunderstand the photophysical properties of this new phosphole-

Figure 3. Absorption spectra of selected species in CHCl3 solution and the solid state: (a) 2a, (b) 2b, (c) 3a, (d) 3b.

Figure 4. Emission spectra of 2a and 2b in the solid state after fastevaporation and thermal annealing overnight.

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lipid system. In order to reduce computation time, the alkylchain was replaced by a methyl substituent for the calculations,and these compounds are marked 1a/b′, 2a/b′, 2c/d′, 3a/b′,and 3c′. The HOMO (highest occupied molecular orbital) andLUMO (lowest unoccupied molecular orbital) orbitals aredepicted in Figure 5 and show the expected distributions withthe HOMO being the π-system excluding the P-center, and theLUMO being the quinoidal π*-system that exhibits thephosphole-typical σ*−π* hyperconjugation involving theexocyclic substituents.21−26

According to the DFT calculations, the S0−S1 transition ofthese phosphole-lipids can be assigned to occur betweenHOMO and LUMO. Consistent with the experimental data(Table 4), extending the effective π-conjugation from 3a/b′

(EH−L = 3.44 eV), via 1a/b′ (EH−L = 3.29 eV) to 2a/b′ (EH−L =2.92 eV), leads to a decrease in the HOMO−LUMO gap, whilemethyl derivatives 2c/d′ (EH−L = 2.77 eV) and 3c′ (EH−L =3.22 eV) exhibit a smaller HOMO−LUMO gap than thecorresponding oxide derivatives. The relatively higher oscillatorstrength of 2a/b′ and 3a/b′ also supports the higherfluorescence quantum yields of these compounds in solution(see Table 4).

Solid-State Organization. We were able to obtain suitablesingle crystals of model compound M for an X-ray diffractionstudy (Figure 6), confirming that M exhibits a very similarsolid-state structure to its P-phenyl substituted rela-tives.21−26,38,40 Similar to other phosphole relatives, a dimericstructure is observed in M with an intermolecular π−πinteraction ca. 3.6 Å (Figure 6b) that correlates well with itsred-shifted emission in the solid state.Based on the stronger π−π interactions suggested by the

optical properties in the solid state, we chose compound 2bwith the more extended conjugated head for further studies oftheir potential self-assembly behavior. As a proof of concept,modified dip-coating experiments were carried out with 2b.46,47

Instead of pulling the glass slide out of the solution containingthe respective compounds, the slide was left in the samplesolution (∼10−3 M) of different organic solvents (Et2O,acetone, CHCl3, and CH2Cl2) followed by slow evaporationof the solvent. After complete evaporation of the solvent,different microstructures were observed. In Et2O, bundlelike,highly crystalline fiber structures of 2b were observed whosesize appears to depend on their relative position within thesolid sample. Generally, the size of the bundles is larger at thebottom of glass slide (Figure 7b) compared to the top part(Figure 7a), which can be attributed to the increasingconcentration of the solution upon slow evaporation of thesolvents. Interestingly, well-dispersed, highly crystalline fiberstructures of 2b could be observed on the whole slide duringthe slow evaporation process from both CHCl3 (Figure 7c) andCH2Cl2 (Figure 7d). In the more polar solvent acetone, 2bforms very different microstructures. On the top of thesubstrate, larger and more densely packed crystalline structures(Figure 7e) were observed, while long and straight 1Dcrystalline fibers (Figure 7f) were observed on the bottompart of the substrate. The 1D fiberlike structures of 2b couldpotentially provide efficient channels for charge and excitontransfer.3

Although the optical studies of 2c and 2d with the ionic π-conjugated backbone in the solid state suggested less efficientπ−π interactions, these species were able to form highlybranched fiberlike structures in heptane via solvophobicinteractions between the ionic π-conjugated heads and thenonpolar hydrocarbon solvent. In order to access the fibers, afew drops of a concentrated solution of 2c and 2d in CH2Cl2(10−3∼10−2 M) were added to 15 mL of heptane. Heavilybranched fiber structures could be obtained after sitting

Figure 5. HOMO and LUMO orbital geometries and energy values: S = yellow, P = orange, O = red, C = gray, H = white.

Table 4. Theoretical Data for the Phosphole-Lipid ModelCompoundsa

compdHOMO(eV)

LUMO(eV)

transition(weight) ΔE (eV) ( fb) EOP (eV)

c

1a/b′ −5.73 −1.90 HOMO ⇒LUMO

3.29 (0.18) 3.11 (1a)

(83%) 3.18 (1b)2a/b′ −5.63 −2.20 HOMO ⇒

LUMO2.92 (0.33) 2.77 (2a)

(85%) 2.76 (2b)2c/d′ −6.35 −3.05 HOMO ⇒

LUMO2.77 (0.27) 2.63 (2c)

(86%) 2.62 (2d)3a/b′ −5.87 −1.87 HOMO ⇒

LUMO3.44 (0.20) 3.21 (3a)

(86%) 3.22 (3b)3c′ −6.58 −2.72 HOMO ⇒

LUMO3.23 (0.160) 3.11 (3c)

(87%)aTD-B3LYP/6-31G(d) (PCM; solvent = CH2Cl2).

bOscillatorstrength. cOptical band gap from UV/vis absorption onset.

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overnight at room temperature. Polarized optical microscopy(POM) studies clearly revealed the branched fiberlikestructures of 2c and 2d, and confocal fluorescence microscopyconfirmed their microscale.(Figure 8).

■ CONCLUSIONSIn conclusion, we have designed a series of new highlyfluorescent phosphole-lipid materials that exhibit less steric bulkat the phosphorus center for potentially improved intermo-lecular π−π interactions, next to their potential for van derWaals and ionic interactions. All of the obtained molecularmaterials are highly luminescent in solution and the solid state.Moreover, our detailed structure−property study has revealedthat molecular engineering of the conjugated head groups viavarying their size, as well as the functionalization of the P-center, but also the length of the thienyl-appended alkyl groups,can be used to effectively manipulate their thermal, optical, andself-organization properties. We could show that intermolecularionic interactions are not an essential feature for creating one-

dimensional structures, as such organized 1D fibers could beobtained by simply balancing π−π stacking and van der Waalsinteractions between molecules as representatively shown forcompound 2b; however, the addition of intermolecular ionicinteractions does not preclude the formation of fibrousstructures either, leading to branched 1D-structures for thephospholium species 2c. Remarkably, by modifying thestrengths/contributions of the various π−π stacking, van derWaals, and ionic interactions between the molecules, we wereable to create luminescent oils as well as 1D solid fibers, while

Figure 6. (a) Molecular structure ofM in the solid state (50% probability level). (b) Molecular packing ofM. The graphics were generated using theMercury 3.0 software package. Selected bond lengths (Å) and angles (deg): P1−O1, 1.486(2); P1−C3, 1.812(3); P1−C6, 1.803(3); P1−C11,1.784(3); C4−C5, 1.453(4); C3−C4, 1.380(4); C5−C6, 1.386(4); C3−P1−C6, 92.37(14); C3−P1−C11, 107.14(13); C6−P1−C11, 106.08(13).

Figure 7. Polarized optical micrographs of the fibers of 2b from Et2O(a, ×20; b, ×4), CHCl3 (c, ×20), CH2Cl2 (d, ×20), and acetone (e,×20; f, ×20).

Figure 8. Polarized optical micrographs of branched fibers of 2c (a,20× magnification), 2d (b, 10× magnification), and confocalfluorescence micrograph of the branched fiber of 2d (c).

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at the same time maintaining the highly emissive nature of thenew species.

■ ASSOCIATED CONTENT

*S Supporting InformationAtomic coordinates for DFT-minimized structures; crystal data,structure refinement, and CIF file of compound M. The CIFfile was also deposited with the Cambridge CrystallographicData center (CCDC), and CCDC code 911793 was allocated.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: thomas.baumgartner@ucalgary.ca. Phone: +1 (403)220-3039.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

Financial support by NSERC of Canada and the CanadaFoundation for Innovation (CFI) is gratefully acknowledged.Y.R. thanks Alberta Ingenuity, now part of Alberta InnovatesTechnology Futures, as well as Talisman Energy for a graduatescholarship, respectively. The authors also thank Prof. C. P.Berlinguette for access to the fluorescence instrument.

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