A Simple and Effective Method of Determining Lewis Acidity by Using Fluorescence

Abstract

•The fluorescence of dithienophospholes is used to visualize Lewis-acid strength•Lewis-acid strength can be precisely correlated with the chromaticity•Method is independent of the Lewis base•Method is applicable to main-group, cationic, and transition-metal species The theory of acids and bases has long been a key concept throughout the chemical sciences. Over the past century, Arrhenius, Brønsted, and Lowry, as well as Lewis, have developed several definitions of what constitutes an acid and a base. The Lewis acid-base theory has recently found rapidly increasing importance for chemistry of the 21st century, particularly in the areas of metal-free catalysis and materials science. Both fields have considerable societal impacts by enabling next-generation technologies and industrial processes. The strength of a Lewis acid is a critical factor in determining its practical utility—a certain threshold of Lewis acidity is required to induce catalytic chemical transformations. Here, we present a reliable method for quantifying this parameter with an easily observable response, which represents a de facto litmus test for Lewis acidity. The strength of a Lewis acid is an important parameter in establishing its utility for practical applications. Here, we report a universal fluorescence-based method for accurately determining the strength of a range of Lewis acids. Utilizing a fluorescent dithienophosphole oxide architecture, we are able to emulate the structural motif of the widely employed Gutmann-Beckett method. This allows us to precisely correlate the optical response to the strength of a Lewis acid. In comparison to commonly used methods that employ 31P NMR chemical shifts or quantum-chemical calculations, our method strongly suggests that using fluorescent Lewis acid-base adducts (FLAs) is more reliable. Moreover, our method provides a direct measure of the Lewis acidity of a given solution, independent of the Lewis base, with direct implications to the overall utility of a Lewis acid in practical applications. The strength of a Lewis acid is an important parameter in establishing its utility for practical applications. Here, we report a universal fluorescence-based method for accurately determining the strength of a range of Lewis acids. Utilizing a fluorescent dithienophosphole oxide architecture, we are able to emulate the structural motif of the widely employed Gutmann-Beckett method. This allows us to precisely correlate the optical response to the strength of a Lewis acid. In comparison to commonly used methods that employ 31P NMR chemical shifts or quantum-chemical calculations, our method strongly suggests that using fluorescent Lewis acid-base adducts (FLAs) is more reliable. Moreover, our method provides a direct measure of the Lewis acidity of a given solution, independent of the Lewis base, with direct implications to the overall utility of a Lewis acid in practical applications. It has been a hundred years since Gilbert Lewis first introduced the concept of Lewis acids (LAs) as electron-pair acceptors,1Lewis G.N. The atom and the molecule.J. Am. Chem. Soc. 1916; 38: 762-785Crossref Scopus (1139) Google Scholar and they have become one of the most important classes of compounds—today more than ever. LAs serve a vital role throughout the chemical sciences by facilitating chemical transformations,2Yamamoto H. Lewis Acids in Organic Synthesis. Wiley-VCH Verlag, 2008Google Scholar anion recognition,3Wade C.R. Broomsgrove A.E.J. Aldridge S. Gabbaï F.P. Fluoride ion complexation and sensing using organoboron compounds.Chem. Rev. 2010; 110: 3958-3984Crossref PubMed Scopus (906) Google Scholar, 4Jäkle F. Advances in the synthesis of organoborane polymers for optical, electronic, and sensory applications.Chem. Rev. 2010; 110: 3985-4022Crossref PubMed Scopus (906) Google Scholar and increasingly throughout materials science.5Baumgartner T. Jäkle F. Main Group Strategies Towards Functional Hybrid Materials. Wiley, 2018Google Scholar Recently, the discovery of frustrated Lewis pairs and the development of other organomain-group catalyst-driven reactions have reignited interest in LA development.6Stephan D.W. Erker G. Frustrated Lewis pair chemistry: development and perspectives.Angew. Chem. Int. Ed. Engl. 2015; 54: 6400-6441Crossref PubMed Scopus (1148) Google Scholar, 7Stephan D.W. The broadening reach of frustrated Lewis pair chemistry.Science. 2016; 354Crossref Scopus (547) Google Scholar, 8Fasano V. Ingleson M.J. Recent advances in water-tolerance in frustrated Lewis pair chemistry.Synthesis. 2018; 50: 1783-1795Crossref Scopus (24) Google Scholar The field is now bustling with activity, as evidenced by >10,000 articles on LAs published in the last decade alone.9Web of Science. With such vastly growing interest and practical value, assessment of the acceptor ability of LAs has become critical as this fundamental property provides the basis for the chemical and functional utility of newly developed species. Unlike traditional Brønsted acids, where acidity is a measure of a molecule’s propensity to dispatch a proton in solution, which can be quantified by the pKa scale,10Himmel D. Radtke V. Butschke B. Krossing I. Basic remarks on acidity.Angew. Chem. Int. Ed. Engl. 2018; 57: 4386-4411Crossref PubMed Scopus (35) Google Scholar no broadly accepted method has been developed for quantifying Lewis acidity to date. Historically, the Gutmann-Beckett (GB) method11Beckett M.A. Strickland G.C. Holland J.R. Sukumar Varma K. A convenient N.M.R. method for the measurement of Lewis acidity at boron centres: correlation of reaction rates of Lewis acid initiated epoxide polymerizations with Lewis acidity.Polymer. 1996; 37: 4629-4631Crossref Scopus (300) Google Scholar, 12Mayer U. Gutmann V. Gerger W. The acceptor number – a quantitative empirical parameter for the electrophilic properties of solvents.Monatsh. Chem. 1975; 106: 1235-1257Crossref Scopus (841) Google Scholar and fluoride ion affinity (FIA)13Müller L.O. Himmel D. Stauffer J. Steinfeld G. Slattery J. Santiso-Quiñones G. Brecht V. Krossing I. Simple access to the non-oxidizing Lewis superacid PhF→Al(ORF)3 (RF=C(CF3)3).Angew. Chem. Int. Ed. 2008; 47: 7659-7663Crossref PubMed Scopus (126) Google Scholar have been primarily used as methods for estimating Lewis acidity. The GB method utilizes triethylphosphine oxide as a probe, whereby the change in 31P NMR resonance from the free to the bound probe correlates to the relative LA strength. However, a number of factors have been found to affect 31P NMR chemical shifts, regularly leading to inconsistent results.14Sivaev I.B. Bregadze V.I. Lewis acidity of boron compounds.Coord. Chem. Rev. 2014; 270–271: 75-88Crossref Scopus (216) Google Scholar Other techniques that involve NMR spectroscopy have been reported by the Childs,15Childs R.F. Mulholland D.L. Nixon A. The Lewis acid complexes of α,β-unsaturated carbonyl and nitrile compounds. A nuclear magnetic resonance study.Can. J. Chem. 1982; 60: 801-808Crossref Google Scholar Piers,16Morgan M.M. Marwitz A.J.V. Piers W.E. Parvez M. Comparative Lewis acidity in fluoroarylboranes: B(o-HC6F4)3, B(p-HC6F4)3, and B(C6F5)3.Organometallics. 2013; 32: 317-322Crossref Scopus (37) Google Scholar and Hilt17Hilt G. Nödling A. The correlation of Lewis acidities of silyl triflates with reaction rates of catalyzed Diels-Alder reactions.Eur. J. Org. Chem. 2011; 2011: 7071-7075Crossref Scopus (32) Google Scholar groups to name a few. On the other hand, FIA that uses the equation LA + F− → LAF− (FIA: −ΔH) has been used as a powerful computational tool to estimate Lewis acidity, but it could be regarded more as a measure of fluoridophilicity rather than Lewis acidity.13Müller L.O. Himmel D. Stauffer J. Steinfeld G. Slattery J. Santiso-Quiñones G. Brecht V. Krossing I. Simple access to the non-oxidizing Lewis superacid PhF→Al(ORF)3 (RF=C(CF3)3).Angew. Chem. Int. Ed. 2008; 47: 7659-7663Crossref PubMed Scopus (126) Google Scholar Along similar lines, Krossing and co-workers have calculated Lewis acidity in terms of hydride and methyl anion affinities.18Böhrer H. Trapp N. Himmel D. Schleep M. Krossing I. From unsuccessful H2-activation with FLPs containing B(Ohfip)3 to a systematic evaluation of the Lewis acidity of 33 Lewis acids based on fluoride, chloride, hydride and methyl ion affinities.Dalton Trans. 2015; 44: 7489-7499Crossref PubMed Google Scholar Finally, the global electrophilicity index (GEI) has recently been applied to estimate LA strength in the absence of a Lewis base.19Jupp A.R. Johnstone T.C. Stephan D.W. The global electrophilicity index as a metric for Lewis acidity.Dalton Trans. 2018; 47: 7029-7035Crossref PubMed Google Scholar Web of Science. Although these methods are evidently suitable for the estimation of Lewis acidity, limitations based on inconclusive results, restrictive measurement parameters, or complex experimental setups clearly preclude their universal applicability. Moreover, these studies also suggest a strong dependence on the Lewis base that is used for the measurements. The latter can generally be attributed to (in)compatible bases giving to varied strengths of the resulting Lewis acid-base interaction.14Sivaev I.B. Bregadze V.I. Lewis acidity of boron compounds.Coord. Chem. Rev. 2014; 270–271: 75-88Crossref Scopus (216) Google Scholar It would thus be highly desirable to develop a general method that is straightforward, simple, and less prone to errors. We surmised that a fluorescence-based method would be able to fulfill these requirements. Fluorescent probes have found successful applications for the sensing of biological species,20Lakowicz J.R. Principles of Fluorescence Spectroscopy.Third edition. Springer, 2006Crossref Scopus (17808) Google Scholar small molecules, and anions (e.g., F− and CN−),3Wade C.R. Broomsgrove A.E.J. Aldridge S. Gabbaï F.P. Fluoride ion complexation and sensing using organoboron compounds.Chem. Rev. 2010; 110: 3958-3984Crossref PubMed Scopus (906) Google Scholar, 4Jäkle F. Advances in the synthesis of organoborane polymers for optical, electronic, and sensory applications.Chem. Rev. 2010; 110: 3985-4022Crossref PubMed Scopus (906) Google Scholar as well as in materials science.5Baumgartner T. Jäkle F. Main Group Strategies Towards Functional Hybrid Materials. Wiley, 2018Google Scholar The main benefits of using fluorescence in the signal-transduction process are derived from high sensitivity, allowing substantially low detection limits (ppm scale is often sufficient), which can exhibit a high degree of analyte specificity, and the ability to provide a distinct, often visible (i.e., easily detectable) change in the probe’s luminescence properties.20Lakowicz J.R. Principles of Fluorescence Spectroscopy.Third edition. Springer, 2006Crossref Scopus (17808) Google Scholar In addition, the experiments could also easily be performed under inert atmosphere to account for highly sensitive LAs. We introduced the dithieno[3,2-b:2′,3′-d]phosphole as stable, highly luminescent, and easily tunable fluorophore in 2004 (Figure 1).21Baumgartner T. Neumann T. Wirges B. The dithieno[3,2-b:2′,3′-d]phosphole system: a novel building block for highly luminescent π-conjugated materials.Angew. Chem. Int. Ed. Engl. 2004; 43: 6197-6201Crossref PubMed Scopus (182) Google Scholar In the context of our work, we were able to show that its optical properties can effectively be fine-tuned in various ways.22Romero-Nieto C. Baumgartner T. Dithieno[3,2-b:2′,3′-d]phospholes: a look back at the first decade.Synlett. 2013; 24: 920-937Crossref Scopus (57) Google Scholar Although extension of the conjugated scaffold leads to a range of emission colors that span the full optical spectrum, the substitution pattern of the phosphorus center also has a noticeable impact on the optical properties of the system. The latter is the result of the phosphole-typical σ*-π* interaction between the exocyclic substituent and the conjugated system that alters the lowest unoccupied molecular orbital (LUMO) energy level as a function of the polarity of the exocyclic bond(s).23Baumgartner T. Insights on the design and electron-acceptor properties of conjugated organophosphorus materials.Acc. Chem. Res. 2014; 47: 1613-1622Crossref PubMed Scopus (296) Google Scholar In general, a more electronegative exocylic bonding partner with a larger contribution of E to the σ-orbital will concurrently lead to a larger contribution of the phosphorus atom to the σ*-orbital, which creates more overlap with the π*-system. This ultimately translates to a lowered LUMO that can be tuned by the electronegativity (EN) of the exocyclic substituent. Moreover, we were able to show that the system is quite robust, even in acidic media, showing some pronounced halochromism without degeneration of the molecular scaffold.24Stolar M. Baumgartner T. Synthesis and unexpected halochromism of carbazole-functionalized dithienophospholes.New. J. Chem. 2012; 36: 1153-1160Crossref Scopus (37) Google Scholar, 25Romero-Nieto C. Durben S. Kormos I.M. Baumgartner T. Simple and efficient generation of white light emission from organophosphorus building blocks.Adv. Funct. Mater. 2009; 19: 3625-3631Crossref Scopus (82) Google Scholar In the context of the present study, we thus deemed the pentavalent dithienophosphole oxide as particularly useful because it is structurally (and thus functionally) related to the triethylphosphine oxide used in the GB method. In a related study, Wolf and co-workers showed that the fluorescence of a conjugated bithiophene can effectively be altered via the establishment of an intramolecular P=O…B interaction.26Cao Y. Nagle J.K. Wolf M.O. Patrick B.O. Tunable luminescence of bithiophene-based flexible Lewis pairs.J. Am. Chem. Soc. 2015; 137: 4888-4891Crossref PubMed Scopus (68) Google Scholar Furthermore, in this context, Romero-Nieto and Hashmi have shown that the LA B(C6F5)3 can coordinate to the oxygen center of conjugated aldehydes to trigger strong luminescence, even in the solid state,27Hansmann M. López-Andarias A. Rettenmeier E. Egler-Lucas C. Rominger F. Hashmi A.S.K. Romero-Nieto C. B(C6F5)3: a Lewis acid that brings the light to the solid state.Angew. Chem. Int. Ed. 2015; 55: 1196-1199Crossref PubMed Scopus (41) Google Scholar whereas Melen and co-workers have used a similar concept in an attempt to establish such luminescent adducts as vapochromic sensors.28Soltani Y. Adams S.J. Börger J. Wilkins L.C. Newman P.D. Pope S.J.A. Melen R.L. Synthesis and photophysical properties of imine borane adducts towards vapochromic materials.Dalton Trans. 2018; 47: 12656-12660Crossref PubMed Google Scholar Herein, we introduce a simple and powerful, naked-eye litmus test for determining the strength of LAs via the use of fluorescent phosphole oxides, by generating fluorescent Lewis acid-base adducts (FLAs) with distinctly altered fluorescence and coloration properties. In this proof-of-concept study, we report the detailed fluorescence and chromaticity responses of the dithienophosphole system in the presence of a series of important LAs to showcase the available Lewis-acidity range for our method. Moreover, we utilize the chromaticity coordinates of a series of representative probes and adducts to generate an experimental Lewis-acidity scale that is independent of the Lewis-basic probe. The underlying hypothesis of our study is based on coordination of a LA that will change the polarity of the P=O bond by reducing the negative hyperconjugation that constitutes the π-bond (Scheme 1).23Baumgartner T. Insights on the design and electron-acceptor properties of conjugated organophosphorus materials.Acc. Chem. Res. 2014; 47: 1613-1622Crossref PubMed Scopus (296) Google Scholar This interaction should directly reflect the LA strength. In other words, a stronger LA will considerably increase the polarity of the P=O bond, effectively increasing the formal EN of the oxo-group and thereby strengthening the σ*-π* interaction in the dithienophosphole scaffold. This lowers the LUMO further, which ultimately translates to a red-shifted emission. To set the foundation for the FLA method, we employed three dithienophosphole oxide probes with different emission wavelengths (Figure 2). Notably, the different π-conjugated backbones should also translate to different Lewis basicities of the phosphoryl groups. The simplest dithienophosphole oxide, 1 (Figure 2), which has a pronounced blue luminescent nature (λem = 446 nm, ϕPL = 0.74), was the first probe investigated.21Baumgartner T. Neumann T. Wirges B. The dithieno[3,2-b:2′,3′-d]phosphole system: a novel building block for highly luminescent π-conjugated materials.Angew. Chem. Int. Ed. Engl. 2004; 43: 6197-6201Crossref PubMed Scopus (182) Google Scholar To initiate our study, we selected B(C6F5)3 as a representative LA to test. This LA is currently one of the most versatile and commonly used main-group catalysts.29Rao B. Kinjo R. Boron-based catalysts for C-C bond-formation reactions.Chem. Asian J. 2018; 13: 1279-1292Crossref PubMed Scopus (53) Google Scholar To our satisfaction, the reaction of 1 with one equiv of B(C6F5)3 in CH2Cl2 at room temperature resulted in an immediate shift in the emission color of the reaction mixture from sky blue to aquamarine (under UV-lamp). Moreover, multinuclear NMR analysis supported an adduct formation between the phosphoryl group and the borane.24Stolar M. Baumgartner T. Synthesis and unexpected halochromism of carbazole-functionalized dithienophospholes.New. J. Chem. 2012; 36: 1153-1160Crossref Scopus (37) Google Scholar, 30Gordon T.J. Szabo L.D. Linder T. Berlinguette C.P. Baumgartner T. Structure–property relationships of acylated asymmetric dithienophospholes.C. R. Chim. 2010; 13: 971-979Crossref Scopus (11) Google Scholar Most distinctively, the 31P{1H} NMR resonance shifted downfield to 30.2 from 18.2 ppm. The 11B NMR spectrum showed a resonance at −1.31 ppm, and the 19F NMR spectrum showed three resonances for the ortho-, para-, and meta-fluorine atoms of the pentafluorophenyl rings at −133.2, −158.0, and −164.3 ppm, respectively, fully supporting a strong adduct formation.31Hogben M.G. Graham W.A.G. Chemical shifts and coupling constants in pentafluorophenyl derivatives. I. Correlations of chemical shifts, coupling constants, and pi-electronic interactions.J. Am. Chem. Soc. 1969; 91: 283-291Crossref Scopus (120) Google Scholar The coordination of the borane to 1 was definitively proven via single-crystal X-ray crystallography (Figure 3). The structure of 1-B(C6F5)3 shows coordination through the P–O bond to the B center. The B–O bond length was found to be 1.549(3) Å, whereas the P–O bond was 1.5296(15) Å (cf.: 1.4859(17) and 1.4883(17) Å for the uncomplexed P–O bond; see Figure S101) and had a P–O–B angle of 130.59(13)°. Another interesting structural feature is the π-stacking observed between one of the pentafluorophenyl rings and the conjugated scaffold of the dithienophosphole (d = 3.5 Å). The X-ray structure helps us to gauge the relative steric demand of our probe. Given that the GB method is based on Et3PO, we chose the corresponding B(C6F5)3 adduct as the most suitable comparator to 1-B(C6F5)3.32Beckett M.A. Brassington D.S. Coles S.J. Hursthouse M.B. Lewis acidity of tris(pentafluorophenyl)borane: crystal and molecular structure of B(C6F5)3・OPEt3.Inorg. Chem. Commun. 2000; 3: 530-533Crossref Scopus (159) Google Scholar Judging by the space-filling diagrams for each species (Figure S105) it is inherently evident that our probe is sterically more accessible, which should give us at least the same range as for the GB method in terms of steric limitation. Density functional theory (DFT) calculations at the B3LYP/6-31G+(d) (polarizable continuum model [PCM], solvent: toluene)33Frisch M.J. Trucks G.W. Schlegel H.B. Scuseria G.E. Robb M.A. Cheeseman J.R. Scalmani G. Barone V. Mennucci B. Petersson G.A. et al.Gaussian 09, Revision E.01. Gaussian, Inc., 2013Google Scholar provided a deeper understanding of the photophysical properties of 1 upon LA binding. The highest occupied molecular orbital (HOMO) of 1 has an energy level of −6.01 eV, and the LUMO has an energy of −2.14 eV. The orbitals are predominately delocalized over the dithienophosphole scaffold (see Figure S106). Upon coordination of B(C6F5)3, both orbital energies decrease to −6.56 and −2.83 eV, respectively. This larger impact on the LUMO energy level confirms an increased acceptor character of the phosphoryl group in the adduct.23Baumgartner T. Insights on the design and electron-acceptor properties of conjugated organophosphorus materials.Acc. Chem. Res. 2014; 47: 1613-1622Crossref PubMed Scopus (296) Google Scholar To illustrate the value of a fluorescence-based determination of Lewis acidity, we subjected 1 to a broad and representative scope of commonly employed main-group and transition-metal LAs with different sizes, steric bulk, and charges (Table 1). To maintain consistency and compatibility with the largest number of tested LAs, toluene was used as a solvent in all cases. To our satisfaction, all resulting FLAs were visibly luminescent, many of which exhibited appreciable quantum yields (ϕ = 0.41–0.99) with only a few anomalous exceptions. Although the emission spectra show an apparent correlation between LA strength and magnitude of the observed bathochromic shifts for the FLAs, the ultraviolet visible (UV-vis) absorption spectra do not (Table 1). The corresponding emission spectra are shown in Figure 4. On the basis of the observed luminescence, BPh3 is the weakest LA, whereas [Ph3C][B(C6F5)4)] is the strongest of the series. It should be noted that complete adduct formation between 1 and BPh3, Al(OTf)3, or Zn(OTf)2 could only be achieved with a large excess of the LA by using 2,000, 1,000, or 1,000 equiv, respectively. In the case of the other acids, a ratio of 1:5 with the dithienophosphole oxide is generally sufficient to generate a nearly saturated solution of the probe. Differences in emission between the adducts of weak and strong LAs can easily be seen even by the naked eye. The weak LA BPh3 generates an FLA that has a red-shifted emission of 6 nm, whereas the FLA with [Et3Si][B(C6F5)4)] exhibits a bathochromic shift of 69 nm. According to the compiled fluorescence data, the LA strength increases in the following order: BPh3 < Zn(OTf)2 < Al(OTf)3 < AlMe3 < BF3 < In(OTf)3 ≈ Sc(OTf)3 < GaCl3 < AlCl3 < B(C6F5)3 ≈ B(p-C6F4H)3 < BCl3 < [Et3Si][B(C6F5)4)] ≈ Me3SiOTf < [Ph3C][B(C6F5)4)].Table 1Spectroscopic Properties for the FLAs of 1 in TolueneLewis Acidλabs (nm)λem (nm)ɛ (M−1cm−1) (ϕ)aAbsolute quantum yield; determined from solution using an integrating sphere.31P δ (ppm)Stokes Shift (cm−1)Δλem (nm)None3664465,600 (0.74)14.24,900–BPh3b2,000 equiv of BPh3.35545215,000 (0.90)21.56,0456BF33804946,500 (0.67)31.96,07348BCl33875144,900 (0.41)30.86,38568B(C6F5)33895095,100 (0.89)30.26,06163B(p-C6F4H)33965095,100 (0.62)29.85,60663AlCl33855035,200 (0.58)NDcAdduct not clearly detectable by NMR.6,09357[Ph3C][B(C6F5)4)]429523110,800 (0.04)dValues due to large absorption contribution from Ph3C+ chromophore.28.54,19077[Et3Si][B(C6F5)4)]3875154,900 (0.44)27.06,42269AlMe33734895,900 (0.07)14.76,36043GaCl33845005,000 (0.99)NDcAdduct not clearly detectable by NMR.6,04254In(OTf)33734975,300 (0.77)27.26,68951Sc(OTf)33784974,800 (0.83)NDcAdduct not clearly detectable by NMR.6,33451Me3SiOTf3765155,800 (0.21)32.67,17869Al(OTf)3e1,000 equiv of Al(OTf)3.300fNot feasible because of sample scattering.488N/AfNot feasible because of sample scattering.24.3N/AfNot feasible because of sample scattering.42Zn(OTf)2g1,000 equiv of Zn(OTf)2.310fNot feasible because of sample scattering.470N/AfNot feasible because of sample scattering.27.7N/AfNot feasible because of sample scattering.24a Absolute quantum yield; determined from solution using an integrating sphere.b 2,000 equiv of BPh3.c Adduct not clearly detectable by NMR.d Values due to large absorption contribution from Ph3C+ chromophore.e 1,000 equiv of Al(OTf)3.f Not feasible because of sample scattering.g 1,000 equiv of Zn(OTf)2. Open table in a new tab By and large, this series represents a very good match for LA strength when considering fundamental parameters, such as bonding, orbital interactions, and ENs. However, the 31P NMR spectra of compound 1 with BCl3, AlCl3, and GaCl3, respectively, suggested not only unexpected LA strengths (Table 1) but also indicated the presence of a mixture of species (see Figures S88, S91, and S95); even with a 1:1 ratio of acid and base. This can be explained by the polarization of a chloride substituent by another ECl3 unit, which was generally employed in excess to achieve a fully saturated solution of the FLA.34Sgro M.J. Dömer J. Stephan D.W. Stoichiometric CO2 reductions using a bis-borane-based frustrated Lewis pair.Chem. Commun. 2012; 48: 7253-7255Crossref PubMed Scopus (79) Google Scholar Moreover, a few of the FLAs have identical or nearly identical emission maxima: those of the two metal triflates Sc(OTf)3 and In(OTf)3, as well as the boranes B(C6F5)3 and B(p-C6F4H)3. However, literature data suggest that B(C6F5)3 exhibits a slightly stronger LA character.16Morgan M.M. Marwitz A.J.V. Piers W.E. Parvez M. Comparative Lewis acidity in fluoroarylboranes: B(o-HC6F4)3, B(p-HC6F4)3, and B(C6F5)3.Organometallics. 2013; 32: 317-322Crossref Scopus (37) Google Scholar Finally, the trityl cation showed a larger bathochromic shift than either of the silylium cations, which is atypical on the basis of the available literature data as well.35Gusev D.G. Ozerov O.V. Calculated hydride and fluoride affinities of a series of carbenium and silylium cations in the gas phase and in C6H5Cl solution.Chem. Eur. J. 2011; 17: 634-640Crossref PubMed Scopus (31) Google Scholar As mentioned above, it is known that these measurements can be biased by the nature of the employed Lewis basic probe.14Sivaev I.B. Bregadze V.I. Lewis acidity of boron compounds.Coord. Chem. Rev. 2014; 270–271: 75-88Crossref Scopus (216) Google Scholar, 36Ashley A.E. Herrington T.J. Wildgoose G.G. Zaher H. Thompson A.L. Rees N.H. Krämer T. O’Hare D. Separating electrophilicity and Lewis acidity: the synthesis, characterization, and electrochemistry of the electron deficient Tris(aryl)boranes B(C6F5)3−n(C6Cl5)n (n=1–3).J. Am. Chem. Soc. 2011; 133: 14727-14740Crossref PubMed Scopus (115) Google Scholar To investigate this feature in more detail, we decided to expand our method to other fluorescent dithienophosphole oxides, so as to maintain a consistent probe architecture and character. Conveniently, the emission color of the scaffold is easily tuned by extension of the conjugated backbone via the 2,6-positions (Figure 2).22Romero-Nieto C. Baumgartner T. Dithieno[3,2-b:2′,3′-d]phospholes: a look back at the first decade.Synlett. 2013; 24: 920-937Crossref Scopus (57) Google Scholar Importantly, the donor-acceptor-donor (D-A-D) architecture would likely enable a more pronounced bathochromic response upon coordination to a LA,24Stolar M. Baumgartner T. Synthesis and unexpected halochromism of carbazole-functionalized dithienophospholes.New. J. Chem. 2012; 36: 1153-1160Crossref Scopus (37) Google Scholar as supported by DFT calculations. Compound 2 (Figure 2), which has a 31P{1H} NMR chemical shift of 15.3 ppm,37Dienes Y. Durben S. Kárpáti T. Neumann T. Englert U. Nyulászi L. Baumgartner T. Selective tuning of the band gap of π-conjugated dithieno[3,2-b:2′,3′-d]phospholes toward different emission colors.Chem. Eur. J. 2007; 13: 7487-7500Crossref PubMed Scopus (169) Google Scholar was investigated as the second LA probe. The extended conjugation of the bithiophene backbone affords it a bathochromic shift with an emission maximum at 520 nm (ϕPL = 0.31) in the green region of the optical spectrum. We investigated the same series of 15 benchmark LAs (Table 2), which provided similar trends in the optical spectra as observed with probe 1. Again, the weakest LA was found to be BPh3. In contrast to the results found with probe 1, this probe now implies that B(C6F5)3 is stronger than B(p-C6F4H)3 on the basis of the emission properties of the corresponding FLAs (Table 2). In addition, the strongest LA is [SiEt3][B(C6F5)4] and not [Ph3C][B(C6F5)4], although the emission curve of 2-[Ph3C]+ is rather broad. The emission spectra are shown in Figure 5 and a clear trend can be observed resulting in the Lewis acidic order BPh3 < Zn(OTf)3 < AlMe3 < Al(OTf)3 ≈ BF3 < In(OTf)3 < Sc(OTf)3 < GaCl3 < AlCl3 < Me3SiOTf < BCl3 ≈ B(p-C6F4H)3 < B(C6F5)3 < [Ph3C][B(C6F5)4] < [Et3Si][B(C6F5)4].Table 2Spectroscopic Properties for the FLAs of 2 in TolueneLewis Acidλabs (nm)λem (nm)ɛ (M−1cm−1) (ϕ)aAbsolute quantum yield; determined from solution using an integrating sphere.Stokes Shift (cm−1)Δλem (nm)None42252019,300 (0.81)4,466–BPh3b2,000 equiv of BPh3.42552326,100 (0.99)4,4093BF344357612,200 (0.85)5,21256BCl345059612,500 (0.86)5,44376B(C6F5)345459712,700 (0.65)5,27677B(p-C6F4H)344959613,300 (0.80)5,49376AlCl345258913,700 (0.73)5,14669[Ph3C][B(C6F5)4)]432599126,600 (0.04)cValues due to large absorption contribution from Ph3C+ chromophore.6,45479[Et3Si][B(C6F5)4)]45360012,100 (0.90)5,40880AlMe343656316,000 (0.50)5,17443GaCl344958715,500 (0.89)5,23667In(OTf)343858114,300 (0.63)5,61961Sc(OTf)343858213,700 (0.57)5,64962Me3SiOTf44359514,200 (0.82)5,76775Al(OTf)3d1,000 equiv of Al(OTf)3.300eNot feasible due to sample scattering.576N/AeNot feasible due to sample scattering.N/AeNot feasible due to sample scattering.56Zn(OTf)2f1,000 equiv of Zn(OTf)2.406eNot feasible due to sample scattering.557N/AeNot feasible due to sample scattering.N/AeNot feasible due to sample scattering.37a Absolute quantum yield; determined from solution using an integrating sphere.b 2,000 equiv of BPh3.c Valu