- Email: [email protected]
One Step Forward: A Novel “Step-Conjugated” Biphosphole
6. Stephan, D.W. (2015). Frustrated Lewis pairs. J. Am. Chem. Soc. 137, 10018–10032. 7. Stephan, D.W., and Erker, G. (2015). Frustrated Lewis pair chemistry: Development and perspectives. Angew. Chem. Int. Ed. 54, 6400–6441. 8. Niu, Z., Bhagya-Gunatilleke, W.D.C., Sun, Q., Lan, P.C., Perman, J., Ma, J.-G., Cheng, Y.,
Aguila, B., and Ma, S. (2018). Metal-organic framework anchored with Lewis pair as a new paradigm for catalysis. Chem 4, this issue, 2587–2599. 9. Bromberg, L., Diao, Y., Wu, H., Speakman, S.A., and Hatton, T.A. (2012). Chromium(III) terephthalate metal organic framework (MIL-101): HF-free synthesis, structure,
polyoxometalate composites, and catalytic properties. Chem. Meter. 24, 1664–1675. 10. Eisenberger, P., Bailey, A.M., and Crudden, C.M. (2012). Taking the F out of FLP: Simple Lewis acid-base pairs for mild reductions with neutral boranes via borenium ion catalysis. J. Am. Chem. Soc. 134, 17384– 17387.
Preview
One Step Forward: A Novel ‘‘Step-Conjugated’’ Biphosphole Jeffrey M. Lipshultz1 and Alexander T. Radosevich1,* New structural scaffolds for use in organic electronic materials are in high demand, especially if they possess unique optoelectronic or morphological properties. In this issue of Chem, Dobrovetsky, Baumgartner, and coworkers describe the synthesis of a new ‘‘step-conjugated’’ bi(dithienophosphole) and demonstrate its unique structural and electronic features. Fundamental discoveries in semiconducting p-conjugated organic molecular scaffolds are fueling an ongoing revolution in organic optoelectronics.1,2 New applications for organic lightemitting diodes, organic field-effect transistors, and organic photovoltaic cells all rely on innovations in the design and preparation of conjugated molecules, especially those containing heteroaromatic subunits. As structurefunction principles governing organic semiconductors have become better refined,3 synthetic chemists with a practical knowledge of the main-group elements have found an opening. The fundamental trends in atomic character impart ‘‘heavy-element’’ p-block compounds with interesting properties that might not be immediately apparent from their periodic relationship with venerable first-row organic congeners. The situation is well illustrated in group 15; phosphole—the heavier analog of pyrrole—exhibits low aroma-
ticity arising from the high s-character in the lone pair on phosphorus, diminishing delocalization into the surrounding p-system and resulting in a nonplanar geometry.4 Together, these effects impart differential electronic and photophysical properties to phospholes relative to other p-conjugated structures,5 thus offering new possibilities for organic electronics.6 In this issue of Chem, Dobrovetsky, Baumgartner, and coworkers report the discovery of a previously unknown bi(dithienophosphole) scaffold with an interesting molecular and electronic architecture arising from a novel P–P bond-forming reaction between P-aminophospholes.7 Baumgartner and coworkers had previously directed research toward P-aminophospholes as building blocks for the synthesis of varied phosphole derivatives,8 but in their current work, Fortuna’s hand weighed heavily on the rudder, given that attempted
purification of P-amino-dithienophosphole 1 by column chromatography on alumina unexpectedly led to the precipitation of crystalline material. Surprisingly, single-crystal X-ray crystallographic analysis showed the product to be bi(dithienophosphole) 2 (Scheme 1A). The unanticipated formation of a P–P bond under these otherwise innocuous conditions clearly warranted further investigation.9 Positing that the mildly acidic chromatographic conditions might be responsible for the unintended dimerization reaction, the authors were able to confirm that reaction of the Lewis acid BF3,OEt2 with P-aminophosphole 1 also yielded the biphosphole 2 (Scheme 1A). However, resolving the issue of why bi(dithienophosphole) 2 formed did not address the issue of how it was happening— what is the precise role of the acid in facilitating the dimerization? In situ spectroscopic investigations of the reaction of P-aminophosphole 1 with BF3,OEt2 provided a clue. 31P NMR spectra showed a set of doublet resonances with large, complementary coupling constants—telltale signs indicating the formation of an unsymmetrical P–P bond. Through correlated heteronuclear (1H and 31P) NMR spectra, it was further established that the unsymmetrical intermediate possessed a single pyrrolidinyl
1Department
of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2018.10.013
Chem 4, 2483–2498, November 8, 2018 ª 2018 Elsevier Inc. 2485
Scheme 1. A ‘‘Step-Conjugated’’ Bi(dithienophosphole) Possesses Unique Whole-Molecule Structural and Electronic Properties (A) Serendipitous and optimized syntheses of the bi(dithienophosphole) scaffold. (B) X-ray crystal structure of bi(dithienophosphole) shows the head-to-head step-conjugated structure. (C) Calculated frontier molecule orbitals of 2 and its mono-oxidized derivative 3. (D) X-ray crystal structure of the bi(dithienophosphole) shows the herringbone packing arrangement. (E) Calculated frontier molecule orbitals of a three-molecule stack of the bi(dithienophosphole). Adapted from Wang et al. 7
unit attached to one of the phosphorus centers. The observation by positivemode high-resolution mass spectrometry (HRMS) of an ion corresponding to such a mono-pyrrolidinyl biphosphole cation
2486 Chem 4, 2483–2498, November 8, 2018
supported this inference. The ‘‘missing’’ pyrrolidine moiety could be accounted for by negative-mode HRMS analysis, which produced a peak corresponding to an anionic bis(trifluoroboron)pyrroli-
dine adduct. A plausible mechanistic pathway accounting for the formation of the observed intermediates, as well as their further conversion to the observed bi(dithienophosphole) product 2, was
satisfyingly supplied by density functional theory (DFT) calculations. In addition to the mechanistic interest associated with the formation of 2, structural and photophysical studies of this compound offered some unanticipated rewards. Single-crystal X-ray crystallographic analysis showed the out-of-plane nature of the two phosphole cores; the C–P–P bond angles were found to be 75 , indicative of strong p-character of the P–P s-bond. The structure further indicates that the biphosphole adopts a previously undescribed headto-head ‘‘step’’ arrangement of the two dithienophosphole subunits in the solid state (Scheme 1B). This conformation differs from prior reports of biphospholes from Re´au and colleagues,9 who found the subunits to be twisted about the P–P bond in a gauche conformation. A further crucial consequence of the outof-plane P–P s-bond linking the two phosphole moieties is that it permits extensive electronic delocalization through an uncommon orbital interaction. The computed electronic structure of 2 reveals the presence of low-lying unoccupied molecular orbitals that span the entire biphosphole core (Scheme 1C). But in contrast to the n-p orbital coupling that results in delocalization for first-row p-block heterocycles such as pyrroles, the conjugation in 2 does not involve delocalization of the lone-pair electronic orbital on P. Instead, the strong bending of the C–P–P angle positions the P–P s*orbital in such a stereoelectronic disposition as to permit mixing with the endocyclic p*-system of the butadiene moiety. This ‘‘s-p conjugation’’ is a distinctive feature of the heavier main-group heterocycles and serves as a bridge to allow a ‘‘step-conjugated’’ electronic structure across the bi(dithienophosphole) framework. Indeed, the electronic absorption features of bi(dithienophosphole) 2 are more than the simple sum of its monophosphole subunits. Relative to that of a P-phenyl monophosphole analog, the onset of absorption of 2 was red shifted
by approximately 50 nm. Consistent with the notion that the electron pair on phosphorus plays a minor role in the electronics of the biphosphole system, the local absorption maxima of both 2 and a derivative in which both P centers were converted to the corresponding P-sulfides were nearly identical, in contrast to previously described effects of P-oxidation on monomeric phospholes.10 This observation is again indicative of the great influence of the unique step-conjugated structure on the photophysical properties. However, in contrast to the similarities between 2 and its di-oxidized derivative, the mono-oxidized derivative 3 displays two pronounced absorption peaks roughly corresponding to p-p* transitions of previously studied dithienophosphole oxides and dithienophospholes, suggestive of localized transitions on each of the unsymmetrically functionalized subunits.10 This contrast is further illuminated by the computed frontier molecular orbitals of 2 and 3. As shown in Scheme 1C, in the case of 2, each of the highest occupied molecular orbital (HOMO), HOMO-1, and lowest unoccupied molecular orbital (LUMO) are evenly distributed across both dithienophosphole subunits. In contrast, the HOMO of 3 is entirely localized on the dithienophosphole subunit, whereas HOMO-1 is entirely localized on the dithienophosphole P-oxide subunit. However, the LUMO is primarily located on the P-oxide subunit, and there is minor orbital density on the phosphole moiety via s*-p* hyperconjugation. Perhaps more so than any other data, this contrast in the orbital architecture of 2 and its mono-oxidized derivative 3 clearly illustrates the profound impact of the ‘‘step conjugation’’ of this bi(dithienophosphole) architecture. It is also worth noting that time-dependent DFT calculations on 2 indicated that all three observed transitions involve both phosphole subunits, as expected for the step-conjugated architecture.
Properties of the bulk material are crucial for finding application in organic electronic devices. In the case of 2, the unique head-to-head solid-state orientation of the subunits results in a dense herringbone supramolecular arrangement (Scheme 1D). This conformation and packing of the biphosphole in the solid state bears positively upon the potential for high charge-carrier mobility. Computational models show substantial intermolecular overlap of the frontier molecular orbitals for a stack of three molecules (Scheme 1E), suggesting the likelihood of efficient charge transfer in the bulk, although experiments probing this notion are still needed. Through careful spectroscopic and computational study, Dobrovetsky, Baumgartner, and coworkers have demonstrated the synthesis of a new bi(dithienophosphole) scaffold possessing unique photophysical attributes and molecular orbital distribution arising from step conjugation via s*-p* hyperconjugation across the P–P s-bond. Notwithstanding the lack of observed emission from the bi(dithienophosphole) compounds, future work exploring the utility of this unique architecture in novel organic electronic applications seems warranted. To this end, the new synthetic protocol for the straightforward synthesis of symmetrical biphospholes and their derivatives should allow for ready access to a variety of new structures possessing similarly step-conjugated architectures. 1. Mazzio, K.A., and Luscombe, C.K. (2015). The future of organic photovoltaics. Chem. Soc. Rev. 44, 78–90. 2. Wang, C., Dong, H., Hu, W., Liu, Y., and Zhu, D. (2012). Semiconducting p-conjugated systems in field-effect transistors: A material odyssey of organic electronics. Chem. Rev. 112, 2208–2267. 3. Mishra, A., and Ba¨uerle, P. (2012). Small molecule organic semiconductors on the move: Promises for future solar energy technology. Angew. Chem. Int. Ed. 51, 2020– 2067. 4. Nyula´szi, L. (2001). Aromaticity of phosphorus heterocycles. Chem. Rev. 101, 1229–1246.
Chem 4, 2483–2498, November 8, 2018 2487
5. Baumgartner, T., and Re´au, R. (2006). Organophosphorus p-conjugated materials. Chem. Rev. 106, 4681–4727.
Baumgartner, T. (2018). An unexpected ‘‘step-conjugated’’ biphosphole via unique P–P bond formation. Chem 4, this issue, 2628–2643.
6. Duffy, M.P., Delaunay, W., Bouit, P.-A., and Hissler, M. (2016). p-Conjugated phospholes and their incorporation into devices: Components with a great deal of potential. Chem. Soc. Rev. 45, 5296– 5310.
8. Wang, Z., Gelfand, B.S., and Baumgartner, T. (2016). Dithienophosphole-based phosphinamides with intriguing selfassembly behavior. Angew. Chem. Int. Ed. 55, 3481–3485.
7. Wang, Z., Asok, N., Gaffen, J., Gottlieb, Y., Bi, W., Gendy, C., Dobrovetsky, R., and
9. Fave, C., Hissler, M., Ka´rpa´ti, T., RaultBerthelot, J., Deborde, V., Toupet, L.,
Nyula´szi, L., and Re´au, R. (2004). Connecting p-chromophores by s-P-P bonds: New type of assemblies exhibiting sp-conjugation. J. Am. Chem. Soc. 126, 6058– 6063. 10. Baumgartner, T., Neumann, T., and Wirges, B. (2004). The dithieno[3,2-b:20 ,30 d]phosphole system: A novel building block for highly luminescent p-conjugated materials. Angew. Chem. Int. Ed. 43, 6197–6201.
Preview
Tandem Solar Flow Batteries for Conversion, Storage, and Utilization of Solar Energy Jian Luo1 and T. Leo Liu1,* In this issue of Chem, Jin and coworkers present the design principles and demonstration of a highly efficient integrated solar flow battery (SFB) device that can be configured to perform all the requisite functions from solar energy harvest to electricity redelivery with a record solar-to-output electricity efficiency of 14.1%. Worldwide economic growth is coupled with the increasing energy demand. Driven by the necessity to reduce the reliance on fossil fuels and carbon emissions, the development of renewable-energy sources, such as solar and wind power, has accelerated in recent years much faster than ever before. For example, the growth of electricity production from solar and wind energy is at an average rate of about 6.3% per year.1 The momentum toward renewable energy is irreversible. The practical utilization of solar energy requires both efficient, low-cost energy conversion and large-scale energy storage techniques because of the dispersion and intermittency of solar energy sources. Solar cells have been widely studied and implemented in the market. Meanwhile, several energy storage devices, such as secondary batteries
(e.g., lead-acid, Li-ion, and redox flow batteries), flywheels, and electrochemical super-capacitors, are available for electricity energy storage. Because of the technological merits of high power input and output, decoupled energy and power, scalability, safety features, and abundant redoxactive candidate materials, redox flow batteries (RFBs) are well suited for renewable-energy storage and electricity grid balancing.2 Simultaneous solar energy conversion and storage have received increasing interest for efficiently utilizing the abundant yet intermittent solar energy.3 Solar rechargeable batteries combine the advantages of photoelectrochemical (PEC) devices and batteries and have emerged as an attractive alternative to artificial photosynthesis for large-scale solar energy harvesting and storage.3
2488 Chem 4, 2483–2498, November 8, 2018 ª 2018 Elsevier Inc.
Under the irradiation of sunlight, the photo-generated electron-hole pair can drive the redox reactions, which makes the integration of a solar cell and RFB possible. In fact, recent studies have demonstrated the feasibility of integrating semiconductor solar cells and RFBs.4,5 In these solar flow batteries (SFBs), the anolyte or catholyte is directly reduced or oxidized by the photogenerated electron or hole on the surface of photoelectrodes. There are several apparent technological advantages to using SFBs for integrated solar energy conversion and storage. First, SFBs can directly utilize photo-generated current to charge a RFB while preserving the merits of RFBs, including decoupled energy and power storage, safety, and scalability. Second, integrated SFBs have a more compact design and smaller footprint than decoupled solar cells and batteries. Third, SFBs have a simpler design and are more practical than PEC solar fuel production and utilization, which requires a solar electrolyzer, a fuel storage system (e.g., a pressurized tank is required for hydrogen storage), and a second fuel cell to complete the whole process of solar energy conversion, storage, and utilization. Fourth and finally, SFBs can be used
1Department
of Chemistry and Biochemistry, Utah State University, Logan, UT 84322, USA *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2018.10.008
Aguila, B., and Ma, S. (2018). Metal-organic framework anchored with Lewis pair as a new paradigm for catalysis. Chem 4, this issue, 2587–2599. 9. Bromberg, L., Diao, Y., Wu, H., Speakman, S.A., and Hatton, T.A. (2012). Chromium(III) terephthalate metal organic framework (MIL-101): HF-free synthesis, structure,
polyoxometalate composites, and catalytic properties. Chem. Meter. 24, 1664–1675. 10. Eisenberger, P., Bailey, A.M., and Crudden, C.M. (2012). Taking the F out of FLP: Simple Lewis acid-base pairs for mild reductions with neutral boranes via borenium ion catalysis. J. Am. Chem. Soc. 134, 17384– 17387.
Preview
One Step Forward: A Novel ‘‘Step-Conjugated’’ Biphosphole Jeffrey M. Lipshultz1 and Alexander T. Radosevich1,* New structural scaffolds for use in organic electronic materials are in high demand, especially if they possess unique optoelectronic or morphological properties. In this issue of Chem, Dobrovetsky, Baumgartner, and coworkers describe the synthesis of a new ‘‘step-conjugated’’ bi(dithienophosphole) and demonstrate its unique structural and electronic features. Fundamental discoveries in semiconducting p-conjugated organic molecular scaffolds are fueling an ongoing revolution in organic optoelectronics.1,2 New applications for organic lightemitting diodes, organic field-effect transistors, and organic photovoltaic cells all rely on innovations in the design and preparation of conjugated molecules, especially those containing heteroaromatic subunits. As structurefunction principles governing organic semiconductors have become better refined,3 synthetic chemists with a practical knowledge of the main-group elements have found an opening. The fundamental trends in atomic character impart ‘‘heavy-element’’ p-block compounds with interesting properties that might not be immediately apparent from their periodic relationship with venerable first-row organic congeners. The situation is well illustrated in group 15; phosphole—the heavier analog of pyrrole—exhibits low aroma-
ticity arising from the high s-character in the lone pair on phosphorus, diminishing delocalization into the surrounding p-system and resulting in a nonplanar geometry.4 Together, these effects impart differential electronic and photophysical properties to phospholes relative to other p-conjugated structures,5 thus offering new possibilities for organic electronics.6 In this issue of Chem, Dobrovetsky, Baumgartner, and coworkers report the discovery of a previously unknown bi(dithienophosphole) scaffold with an interesting molecular and electronic architecture arising from a novel P–P bond-forming reaction between P-aminophospholes.7 Baumgartner and coworkers had previously directed research toward P-aminophospholes as building blocks for the synthesis of varied phosphole derivatives,8 but in their current work, Fortuna’s hand weighed heavily on the rudder, given that attempted
purification of P-amino-dithienophosphole 1 by column chromatography on alumina unexpectedly led to the precipitation of crystalline material. Surprisingly, single-crystal X-ray crystallographic analysis showed the product to be bi(dithienophosphole) 2 (Scheme 1A). The unanticipated formation of a P–P bond under these otherwise innocuous conditions clearly warranted further investigation.9 Positing that the mildly acidic chromatographic conditions might be responsible for the unintended dimerization reaction, the authors were able to confirm that reaction of the Lewis acid BF3,OEt2 with P-aminophosphole 1 also yielded the biphosphole 2 (Scheme 1A). However, resolving the issue of why bi(dithienophosphole) 2 formed did not address the issue of how it was happening— what is the precise role of the acid in facilitating the dimerization? In situ spectroscopic investigations of the reaction of P-aminophosphole 1 with BF3,OEt2 provided a clue. 31P NMR spectra showed a set of doublet resonances with large, complementary coupling constants—telltale signs indicating the formation of an unsymmetrical P–P bond. Through correlated heteronuclear (1H and 31P) NMR spectra, it was further established that the unsymmetrical intermediate possessed a single pyrrolidinyl
1Department
of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2018.10.013
Chem 4, 2483–2498, November 8, 2018 ª 2018 Elsevier Inc. 2485
Scheme 1. A ‘‘Step-Conjugated’’ Bi(dithienophosphole) Possesses Unique Whole-Molecule Structural and Electronic Properties (A) Serendipitous and optimized syntheses of the bi(dithienophosphole) scaffold. (B) X-ray crystal structure of bi(dithienophosphole) shows the head-to-head step-conjugated structure. (C) Calculated frontier molecule orbitals of 2 and its mono-oxidized derivative 3. (D) X-ray crystal structure of the bi(dithienophosphole) shows the herringbone packing arrangement. (E) Calculated frontier molecule orbitals of a three-molecule stack of the bi(dithienophosphole). Adapted from Wang et al. 7
unit attached to one of the phosphorus centers. The observation by positivemode high-resolution mass spectrometry (HRMS) of an ion corresponding to such a mono-pyrrolidinyl biphosphole cation
2486 Chem 4, 2483–2498, November 8, 2018
supported this inference. The ‘‘missing’’ pyrrolidine moiety could be accounted for by negative-mode HRMS analysis, which produced a peak corresponding to an anionic bis(trifluoroboron)pyrroli-
dine adduct. A plausible mechanistic pathway accounting for the formation of the observed intermediates, as well as their further conversion to the observed bi(dithienophosphole) product 2, was
satisfyingly supplied by density functional theory (DFT) calculations. In addition to the mechanistic interest associated with the formation of 2, structural and photophysical studies of this compound offered some unanticipated rewards. Single-crystal X-ray crystallographic analysis showed the out-of-plane nature of the two phosphole cores; the C–P–P bond angles were found to be 75 , indicative of strong p-character of the P–P s-bond. The structure further indicates that the biphosphole adopts a previously undescribed headto-head ‘‘step’’ arrangement of the two dithienophosphole subunits in the solid state (Scheme 1B). This conformation differs from prior reports of biphospholes from Re´au and colleagues,9 who found the subunits to be twisted about the P–P bond in a gauche conformation. A further crucial consequence of the outof-plane P–P s-bond linking the two phosphole moieties is that it permits extensive electronic delocalization through an uncommon orbital interaction. The computed electronic structure of 2 reveals the presence of low-lying unoccupied molecular orbitals that span the entire biphosphole core (Scheme 1C). But in contrast to the n-p orbital coupling that results in delocalization for first-row p-block heterocycles such as pyrroles, the conjugation in 2 does not involve delocalization of the lone-pair electronic orbital on P. Instead, the strong bending of the C–P–P angle positions the P–P s*orbital in such a stereoelectronic disposition as to permit mixing with the endocyclic p*-system of the butadiene moiety. This ‘‘s-p conjugation’’ is a distinctive feature of the heavier main-group heterocycles and serves as a bridge to allow a ‘‘step-conjugated’’ electronic structure across the bi(dithienophosphole) framework. Indeed, the electronic absorption features of bi(dithienophosphole) 2 are more than the simple sum of its monophosphole subunits. Relative to that of a P-phenyl monophosphole analog, the onset of absorption of 2 was red shifted
by approximately 50 nm. Consistent with the notion that the electron pair on phosphorus plays a minor role in the electronics of the biphosphole system, the local absorption maxima of both 2 and a derivative in which both P centers were converted to the corresponding P-sulfides were nearly identical, in contrast to previously described effects of P-oxidation on monomeric phospholes.10 This observation is again indicative of the great influence of the unique step-conjugated structure on the photophysical properties. However, in contrast to the similarities between 2 and its di-oxidized derivative, the mono-oxidized derivative 3 displays two pronounced absorption peaks roughly corresponding to p-p* transitions of previously studied dithienophosphole oxides and dithienophospholes, suggestive of localized transitions on each of the unsymmetrically functionalized subunits.10 This contrast is further illuminated by the computed frontier molecular orbitals of 2 and 3. As shown in Scheme 1C, in the case of 2, each of the highest occupied molecular orbital (HOMO), HOMO-1, and lowest unoccupied molecular orbital (LUMO) are evenly distributed across both dithienophosphole subunits. In contrast, the HOMO of 3 is entirely localized on the dithienophosphole subunit, whereas HOMO-1 is entirely localized on the dithienophosphole P-oxide subunit. However, the LUMO is primarily located on the P-oxide subunit, and there is minor orbital density on the phosphole moiety via s*-p* hyperconjugation. Perhaps more so than any other data, this contrast in the orbital architecture of 2 and its mono-oxidized derivative 3 clearly illustrates the profound impact of the ‘‘step conjugation’’ of this bi(dithienophosphole) architecture. It is also worth noting that time-dependent DFT calculations on 2 indicated that all three observed transitions involve both phosphole subunits, as expected for the step-conjugated architecture.
Properties of the bulk material are crucial for finding application in organic electronic devices. In the case of 2, the unique head-to-head solid-state orientation of the subunits results in a dense herringbone supramolecular arrangement (Scheme 1D). This conformation and packing of the biphosphole in the solid state bears positively upon the potential for high charge-carrier mobility. Computational models show substantial intermolecular overlap of the frontier molecular orbitals for a stack of three molecules (Scheme 1E), suggesting the likelihood of efficient charge transfer in the bulk, although experiments probing this notion are still needed. Through careful spectroscopic and computational study, Dobrovetsky, Baumgartner, and coworkers have demonstrated the synthesis of a new bi(dithienophosphole) scaffold possessing unique photophysical attributes and molecular orbital distribution arising from step conjugation via s*-p* hyperconjugation across the P–P s-bond. Notwithstanding the lack of observed emission from the bi(dithienophosphole) compounds, future work exploring the utility of this unique architecture in novel organic electronic applications seems warranted. To this end, the new synthetic protocol for the straightforward synthesis of symmetrical biphospholes and their derivatives should allow for ready access to a variety of new structures possessing similarly step-conjugated architectures. 1. Mazzio, K.A., and Luscombe, C.K. (2015). The future of organic photovoltaics. Chem. Soc. Rev. 44, 78–90. 2. Wang, C., Dong, H., Hu, W., Liu, Y., and Zhu, D. (2012). Semiconducting p-conjugated systems in field-effect transistors: A material odyssey of organic electronics. Chem. Rev. 112, 2208–2267. 3. Mishra, A., and Ba¨uerle, P. (2012). Small molecule organic semiconductors on the move: Promises for future solar energy technology. Angew. Chem. Int. Ed. 51, 2020– 2067. 4. Nyula´szi, L. (2001). Aromaticity of phosphorus heterocycles. Chem. Rev. 101, 1229–1246.
Chem 4, 2483–2498, November 8, 2018 2487
5. Baumgartner, T., and Re´au, R. (2006). Organophosphorus p-conjugated materials. Chem. Rev. 106, 4681–4727.
Baumgartner, T. (2018). An unexpected ‘‘step-conjugated’’ biphosphole via unique P–P bond formation. Chem 4, this issue, 2628–2643.
6. Duffy, M.P., Delaunay, W., Bouit, P.-A., and Hissler, M. (2016). p-Conjugated phospholes and their incorporation into devices: Components with a great deal of potential. Chem. Soc. Rev. 45, 5296– 5310.
8. Wang, Z., Gelfand, B.S., and Baumgartner, T. (2016). Dithienophosphole-based phosphinamides with intriguing selfassembly behavior. Angew. Chem. Int. Ed. 55, 3481–3485.
7. Wang, Z., Asok, N., Gaffen, J., Gottlieb, Y., Bi, W., Gendy, C., Dobrovetsky, R., and
9. Fave, C., Hissler, M., Ka´rpa´ti, T., RaultBerthelot, J., Deborde, V., Toupet, L.,
Nyula´szi, L., and Re´au, R. (2004). Connecting p-chromophores by s-P-P bonds: New type of assemblies exhibiting sp-conjugation. J. Am. Chem. Soc. 126, 6058– 6063. 10. Baumgartner, T., Neumann, T., and Wirges, B. (2004). The dithieno[3,2-b:20 ,30 d]phosphole system: A novel building block for highly luminescent p-conjugated materials. Angew. Chem. Int. Ed. 43, 6197–6201.
Preview
Tandem Solar Flow Batteries for Conversion, Storage, and Utilization of Solar Energy Jian Luo1 and T. Leo Liu1,* In this issue of Chem, Jin and coworkers present the design principles and demonstration of a highly efficient integrated solar flow battery (SFB) device that can be configured to perform all the requisite functions from solar energy harvest to electricity redelivery with a record solar-to-output electricity efficiency of 14.1%. Worldwide economic growth is coupled with the increasing energy demand. Driven by the necessity to reduce the reliance on fossil fuels and carbon emissions, the development of renewable-energy sources, such as solar and wind power, has accelerated in recent years much faster than ever before. For example, the growth of electricity production from solar and wind energy is at an average rate of about 6.3% per year.1 The momentum toward renewable energy is irreversible. The practical utilization of solar energy requires both efficient, low-cost energy conversion and large-scale energy storage techniques because of the dispersion and intermittency of solar energy sources. Solar cells have been widely studied and implemented in the market. Meanwhile, several energy storage devices, such as secondary batteries
(e.g., lead-acid, Li-ion, and redox flow batteries), flywheels, and electrochemical super-capacitors, are available for electricity energy storage. Because of the technological merits of high power input and output, decoupled energy and power, scalability, safety features, and abundant redoxactive candidate materials, redox flow batteries (RFBs) are well suited for renewable-energy storage and electricity grid balancing.2 Simultaneous solar energy conversion and storage have received increasing interest for efficiently utilizing the abundant yet intermittent solar energy.3 Solar rechargeable batteries combine the advantages of photoelectrochemical (PEC) devices and batteries and have emerged as an attractive alternative to artificial photosynthesis for large-scale solar energy harvesting and storage.3
2488 Chem 4, 2483–2498, November 8, 2018 ª 2018 Elsevier Inc.
Under the irradiation of sunlight, the photo-generated electron-hole pair can drive the redox reactions, which makes the integration of a solar cell and RFB possible. In fact, recent studies have demonstrated the feasibility of integrating semiconductor solar cells and RFBs.4,5 In these solar flow batteries (SFBs), the anolyte or catholyte is directly reduced or oxidized by the photogenerated electron or hole on the surface of photoelectrodes. There are several apparent technological advantages to using SFBs for integrated solar energy conversion and storage. First, SFBs can directly utilize photo-generated current to charge a RFB while preserving the merits of RFBs, including decoupled energy and power storage, safety, and scalability. Second, integrated SFBs have a more compact design and smaller footprint than decoupled solar cells and batteries. Third, SFBs have a simpler design and are more practical than PEC solar fuel production and utilization, which requires a solar electrolyzer, a fuel storage system (e.g., a pressurized tank is required for hydrogen storage), and a second fuel cell to complete the whole process of solar energy conversion, storage, and utilization. Fourth and finally, SFBs can be used
1Department
of Chemistry and Biochemistry, Utah State University, Logan, UT 84322, USA *Correspondence: [email protected] https://doi.org/10.1016/j.chempr.2018.10.008