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Panchromatic absorbers for solar light-harvesting† Cite this: DOI: 10.1039/c4cc06853j
Eric J. Alexy,a Jonathan M. Yuen,b Vanampally Chandrashaker,a James R. Diers,c Christine Kirmaier,b David F. Bocian,*c Dewey Holten*b and Jonathan S. Lindsey*a
Received 30th August 2014, Accepted 6th October 2014 DOI: 10.1039/c4cc06853j www.rsc.org/chemcomm
A set of panchromatic absorbers exhibiting long excited-state lifetimes in both polar and nonpolar media has been prepared. The architectures are based on a porphyrin strongly coupled electronically to 1 4 perylene– monoimides via ethyne linkers. The constructs should find utility in molecular solar-conversion systems.
An ideal light-harvesting unit with wide applicability for molecularbased solar-conversion systems should have a number of attributes. These characteristics include the following: (1) Absorption with high cross section over a broad spectral region to maximize use of the incident solar energy. (2) A lowest excited-state of relatively uniform energy for all terminal absorbers – obtained in quantitative yield – to afford efficient delivery of a given quantity of energy to the target. (3) A long intrinsic lowest excited-state lifetime regardless of medium polarity to afford (even in ‘sluggish’ systems) a high quantum yield of energy or electron transfer to a target site. (4) A synthetically tailorable structure to allow melding with other components, including incorporation into a host scaffold or attachment to partner molecular units or a surface. Systems that achieve the first of the four above-noted properties are often called panchromatic absorbers.1 For the most efficient use of solar radiation, the panchromatic absorption would span the near-ultraviolet (NUV), visible, and near-infrared (NIR) regions of the solar spectrum. Combining panchromaticity with even one of the other above-noted properties is difficult enough, let alone realizing multiple or all of these desired characteristics. While there appears to be no specific first-principles blueprint for achieving panchromatic absorption, especially in conjunction with the other desired properties, many approaches are possible. a
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, USA. E-mail: [email protected] b Department of Chemistry, Washington University, St. Louis, Missouri 63130-4889, USA. E-mail: [email protected] c Department of Chemistry, University of California, Riverside, California 92521-0403, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: Complete synthesis and characterization of all arrays; spectroscopic methods for determination of excitedstate lifetimes. See DOI: 10.1039/c4cc06853j
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Herein, we present an approach that affords a set of panchromatic absorbers based on a porphyrin chassis (Fig. 1). The porphyrin has been derivatized in a manner that gives a substantial and unexpected redistribution of NUV absorption oscillator strength into the visible, red and NIR (to B800 nm) spectral regions (Fig. 2 and 3). The four above-noted target properties have been demonstrated (Table 1). The molecular designs shown in Fig. 1 represent a redirection from the approach that we have taken previously to broaden spectral coverage of tetrapyrrole-based light-harvesting arrays by successive incorporation of one or more ‘accessory’ pigments with complementary absorption properties.2,3 Such systems primarily utilized perylenes as the accessory absorbers because of their moderately strong absorption from 450 to 550 nm,4 long excited-state lifetimes,5 photostability,4 and wide use in arrays6–9 and artificial photosynthesis.10 Linker motifs (type, length, attachment sites) were chosen to provide relatively weak electronic coupling between the components such that the absorption spectrum is essentially the sum of those of the components.3 The sum of the porphyrin and perylene spectra in Fig. 2 trace A is typical of a dyad with weak electronic coupling. The above-described approach is ultimately limiting if the goal is to achieve panchromatic absorption spanning the NUV to NIR for at least two reasons: (1) Tetrapyrroles (porphyrins, chlorins, bacteriochlorins) have relatively sharp absorption features. Thus, many such units and/or many different accessory pigments (such as perylenes) will be required, resulting in a bulky array. (2) Differences in characteristics (molecular orbital, redox) necessarily associated with compiling many units having complementary spectral (electronic) properties may give rise to substantial charge-transfer and diminished excited-state lifetimes, especially in polar media. Such considerations prompted us to alter tactics and employ strong perylene–tetrapyrrole coupling. The motivation stemmed partly from the pioneering work by Therien11 and Anderson12 on strongly coupled multi-porphyrin arrays. Our initial foray afforded a trio of dyads with an ethyne bridge between the C9-position of a perylene and meso-position of a porphyrin, chlorin or
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Fig. 1
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Arrays with 1 4 perylene per porphyrin, and benchmarks.
bacteriochlorin.13 The deviations in spectral properties of the dyads from the sum of those of the benchmarks decreased in the order porphyrin 4 chlorin 4 bacteriochlorin. That initial work motivated the preparation of the arrays shown in Fig. 1, which contain a single porphyrin ethyne-linked to 1 4 perylenes. The modular synthesis and characterization of each array are described in detail in the ESI.† The absorption and fluorescence spectra of the arrays in toluene are given in Fig. 2 (traces B–F) and 3A. The vertical axis in these figures reflects measured molar absorptivity, with e = 150 000 M 1 cm 1 for PMI4P at 564 nm. This level is the reference for the relative values given in parenthesis along with the spectral positions in Table 1. Notable observations on the spectral properties of the perylene– porphyrin arrays are as follows: (1) Each array exhibits a significant shift of absorption intensity from the NUV Soret (Bx, By) region of a typical porphyrin into the visible (nominal Qx and Qy) region, leading to effective panchromatic absorption. (2) The panchromatic-like absorption extends to longer wavelengths (red to NIR) with each successive perylene addition. In particular, the position of the long wavelength absorption feature progresses from B670 nm for the porphyrin benchmark P to B690 nm for the monoperylene– porphyrin PMI1P, to B730 nm for trans-bisperylene–porphyrin
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PMI2TP to B760 nm for tetraperylene–porphyrin PMI4P. (3) The spectra are not particularly sensitive to solvent polarity, as can be seen by comparing the spectral characteristics for the compounds in toluene and benzonitrile (Table 1). Fig. 3B illustrates the panchromaticity provided by the perylene–porphyrins. An ideal panchromatic absorber would have a flat absorption across a region of interest, as indicated by the dashed rectangular shape over 400 800 nm with e = 100 000 M 1 cm 1. This region comprises B60% of the standard AM1.5 (photon flux density) solar spectrum from 300–1100 nm. The solid line with shading below is the best fit to the rectangular shape having available all five perylene–porphyrin basis spectra in Fig. 3A (and Fig. 2). The best fit draws intensity from the three arrays PMI2TP, PMI4P and PMI1P in ratio 0.57/0.37/ 0.06. The red-dashed curve shows a similarly good fit using only PMI2TP and PMI4P with ratio 0.63/0.37. The fit spectra cover 80% of the area under the 100 000 M 1 cm 1 extinction level. The large molar absorptivity of the arrays would give a near unity absorbance over the entire span by a 10 mM sample in a 10 mm path (or 100 mM in 1 mm). The substantial spectral differences for the strongly coupled perylene–porphyrins versus benchmarks derives primarily from
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Fig. 3 (A) Absorption spectra at room temperature of perylene–porphyrins in toluene. (B) The solid line with shading below is a least-square fit (using all five basis spectra in panel A) to an ideal panchromatic absorber from 400 800 nm (dashed rectangle) using a 0.57/0.37/0.06 PMI2TP/PMI4P/ PMI1P mixture. A similar fit is obtained using a 0.63/0.37 PMI2TP/PMI4P mixture (red dashed).
Fig. 2 Absorption (solid) and fluorescence (dashed) spectra in toluene at room temperature. Trace A is the sum of spectra of benchmarks porphyrin (P, purple) and perylene (PMI, red dots). Traces B–F are for perylene–porphyrin arrays. Absorption intensities are relative to 150 000 M 1 cm 1 for PMI4P in toluene at 564 nm. Fluorescence intensities are scaled for clarity. Fluorescence lexc (in nm) is 490 (A), 430 (B), 430 (C), 425 (D), 421 (E) and 419 (F).
the perylene absorption. Overall, the significant effects of the strongly coupled perylenes on tetrapyrrole electronic structure must derive from more than the typical substituent effects. The perturbations likely involve a change in the nature and magnitude of the interaction of the four key electronic configurations (i.e., the configuration-interaction energy) that underpin the tetrapyrrole excited-state manifold.13 Panchromatic absorption alone is not sufficient for efficient solar light-harvesting. As noted above, another requirement is a long excited-state lifetime. The lifetimes for the four perylene– porphyrin arrays in toluene range from 1.2 to 4.6 ns (2.8 ns average), with the shortest value being for PMI4P (Table 1). The lifetimes for the arrays in the highly polar benzonitrile range from 1.2 to 1.9 ns (1.5 ns average), with the value for PMI4P being the same as in toluene. Such lifetimes are more than sufficient to support efficient energy or electron-transfer processes in solar-conversion systems. Such efficiency would be retained even in polar media where charge-transfer quenching
a redistribution of the substantial oscillator strength held in the typical tetrapyrrole NUV Soret features (e B 350 000 M 1 cm 1) into the longer-wavelength regions where needed to afford effective panchromatic absorption. The spectra in Fig. 2 and 3 show that this redistribution can be extended into the NIR via strong coupling to progressively more perylene units. Despite the large redistribution of porphyrin oscillator strength, the bulk of the perylene oscillator strength is retained in its normal region (450 550 nm), as can be seen by examination of column ‘‘Abs l3’’ in Table 1. Progressing from arrays with 4/3/2/1 perylenes, the average relative peak-intensity values of 1.0/0.84/0.63/0.37 track fairly well the ideal values of 1.0/0.75/ 0.50/0.25. Deviations are expected because of contributions of the porphyrin visible (Qy, Qx) bands in this region (Fig. 2A), and there must be some effect of perylene–porphyrin coupling on Table 1
Spectral properties of perylene–porphyrin arrays and benchmarksa
Compound
Solvent
Abs l1 (nm)
P PMI PMI1P PMI1P PMI2TP PMI2TP PMI2CP PMI2CP PMI3P PMI3P PMI4P PMI4P
Toluene Toluene Toluene PhCN Toluene PhCN Toluene PhCN Toluene PhCN Toluene PhCN
434 (2.28) 430 434 430 438 425 433 421 432 420 425
(0.77) (0.77) (0.39) (0.44) (0.36) (0.38) (0.32) (0.31) (0.33) (0.37)
Abs l2 (nm) 492 497 498 475 480 450 452
(0.19) (0.20) (0.20) (0.35) (0.37) (0.36) (0.40)
Abs l3 (nm)
Abs l4 (nm)
533 526 532 534 536 540 543 546 549 552 564 562
574 (0.17)
(0.07) (0.30) (0.37) (0.37) (0.56) (0.56) (0.69) (0.69) (0.84) (0.84) (1.0) (1.0)
574b (0.69) 578b (0.65) 597 (0.98) 601b (0.85)
Abs l5 (nm)
613 617 636 641 638 648 699 692 702 703
(0.38) (0.41) (0.42) (0.44) (0.35) (0.38) (0.36) (0.42) (0.40) (0.42)
Abs l6 (nm)
Flu l (nm)
tS (ns)
FF
668 (0.06)
673 556 694 702 744 755 728 738 766 776 759 770
14 5.6 4.3 1.3 2.1 1.3 4.6 1.9 1.8 1.8 1.2 1.2
0.14 0.95 0.38 0.12 0.41 0.24 0.21 0.11 0.07 0.14 0.007 0.03
688 (0.30) 691 (0.37) 728 (0.45) 737 (0.63) 707b (0.18) 718b (0.25) 760b (0.32) 755 (0.40) 763 (0.55) 760 (0.53)
a
All data acquired at room temperature using methods described in ref. 13. PhCN is benzonitrile. The value in parenthesis along with the absorption peak wavelength is the intensity relative to that at the 564 nm peak of (PMI)4P, which has e = 150 000 M 1 cm 1. b Approximate wavelength of a shoulder on an asymmetric absorption feature.
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within many multi-chromophore complexes can compromise utility. The perylene–porphyrins described herein raise a number of questions of fundamental interest. One regards the lifetime of PMI4P in toluene being reduced by only a factor of 2 4 to 1.2 ns from 4.3 ns for PMI1P and 2.1 ns for PMI2TP, even though the yield of nominal porphyrin fluorescence (Fig. 2) of PMI4P drops by a factor of B60 to 0.007 from 0.38 for PMI1P and 0.41 for PMI2TP (Table 1). The latter fluorescence yields themselves are of interest because they are quite high for porphyrinic systems. Apparently the appended perylenes result in compensating changes in the rate constants for the fluorescence, intersystem crossing, and internal conversion decay of the lowest singlet excited state to preserve a long lifetime. Future studies will address such issues as well as what and how changes in electronic structure give rise to the optical properties of these systems. The architectures described herein afford panchromatic absorption that extends from 400 to 700 nm for the monoperylene– porphyrin to 800 nm with four perylenes. The arrays exhibit long excited-state lifetimes that are similar in polar and nonpolar media. Elaboration using other rylenes7,14 such as terylenes may extend the absorption deeper into the NIR to even better utilize that photon-rich region of the solar spectrum. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award numbers DE-FG02-05ER15660 (D.F.B.), DE-FG02-05ER15661 (D.H. & C.K.) and DE-FG02-96ER14632 (J.S.L.). E.J.A. was supported by an Ernest Eliel NC-ACS undergraduate award.
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Notes and references 1 (a) J.-H. Yum, B. E. Hardin, E. T. Hoke, E. Baranoff, S. M. Zakeeruddin, M. K. Nazeeruddin, T. Torres, M. D. McGehee and ¨tzel, ChemPhysChem, 2011, 12, 657–661; (b) J. Bartelmess, M. Gra ´, A. R. M. Soares, M. V. Martı´nez-Dı´az, M. G. P. M. S. Neves, A. C. Tome J. A. S. Cavaleiro, T. Torres and D. M. Guldi, Chem. Commun., 2011, 47, 3490–3492; (c) J. Warnan, Y. Pellegrin, E. Blart and F. Odobel, Chem. Commun., 2012, 48, 675–677; (d) J. M. Ball, N. K. S. Davis, J. D. Wilkinson, J. Kirkpatrick, J. Teuscher, R. Gunning, H. L. Anderson and H. J. Snaith, RSC Adv., 2012, 2, 6846–6853; (e) L.-L. Li and E. W.-G. Diau, Chem. Soc. Rev., 2013, 42, 291–304. 2 D. Holten, D. F. Bocian and J. S. Lindsey, Acc. Chem. Res., 2002, 5, 57–69. 3 (a) C. Kirmaier, H.-E. Song, E. K. Yang, J. K. Schwartz, E. Hindin, J. R. Diers, R. S. Loewe, K.-Y. Tomizaki, F. Chevalier, L. Ramos, R. R. Birge, D. F. Bocian, J. S. Lindsey and D. Holten, J. Phys. Chem. B, 2010, 114, 14249–14264; (b) E. Yang, J. Wang, J. R. Diers, D. M. Niedzwiedzki, C. Kirmaier, D. F. Bocian, J. S. Lindsey and D. Holten, J. Phys. Chem. B, 2014, 118, 1630–1647. 4 H. Langhals, Helv. Chim. Acta, 2005, 88, 1309–1343. 5 S. I. Yang, R. K. Lammi, S. Prathapan, M. A. Miller, J. Seth, J. R. Diers, D. F. Bocian, J. S. Lindsey and D. Holten, J. Mater. Chem., 2001, 1, 2420–2430. ¨rthner, Chem. Commun., 2004, 1564–1579. 6 F. Wu 7 X. Zhan, A. Facchetti, S. Barlow, T. J. Marks, M. A. Ratner, M. R. Wasielewski and S. R. Marder, Adv. Mater., 2011, 23, 268–284. 8 C. Huang, S. Barlow and S. R. Marder, J. Org. Chem., 2011, 76, 2386–2407. 9 C. Li and H. Wonneberger, Adv. Mater., 2012, 24, 613–636. 10 M. R. Wasielewski, Acc. Chem. Res., 2009, 42, 1910–1921. 11 (a) V. S.-Y. Lin, S. G. DiMagno and M. J. Therien, Science, 1994, 264, 1105–1111; (b) T. N. Singh-Rachford, A. Nayak, M. L. Muro-Small, S. Goeb, M. J. Therien and F. N. Castellano, J. Am. Chem. Soc., 2010, 132, 14203–14211. 12 (a) H. L. Anderson, Chem. Commun., 1999, 2323–2330; (b) H. L. Anderson, A. P. Wylie and K. Prout, J. Chem. Soc., Perkin Trans. 1, 1998, 1607–1611. 13 J. Wang, E. Yang, J. R. Diers, D. M. Niedzwiedzki, C. Kirmaier, D. F. Bocian, J. S. Lindsey and D. Holten, J. Phys. Chem. B, 2013, 117, 9288–9304. ¨llen, Angew. 14 T. Weil, T. Vosch, J. Hofkens, K. Peneva and K. Mu Chem., Int. Ed., 2010, 49, 9068–9093.
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Panchromatic absorbers for solar light-harvesting† Cite this: DOI: 10.1039/c4cc06853j
Eric J. Alexy,a Jonathan M. Yuen,b Vanampally Chandrashaker,a James R. Diers,c Christine Kirmaier,b David F. Bocian,*c Dewey Holten*b and Jonathan S. Lindsey*a
Received 30th August 2014, Accepted 6th October 2014 DOI: 10.1039/c4cc06853j www.rsc.org/chemcomm
A set of panchromatic absorbers exhibiting long excited-state lifetimes in both polar and nonpolar media has been prepared. The architectures are based on a porphyrin strongly coupled electronically to 1 4 perylene– monoimides via ethyne linkers. The constructs should find utility in molecular solar-conversion systems.
An ideal light-harvesting unit with wide applicability for molecularbased solar-conversion systems should have a number of attributes. These characteristics include the following: (1) Absorption with high cross section over a broad spectral region to maximize use of the incident solar energy. (2) A lowest excited-state of relatively uniform energy for all terminal absorbers – obtained in quantitative yield – to afford efficient delivery of a given quantity of energy to the target. (3) A long intrinsic lowest excited-state lifetime regardless of medium polarity to afford (even in ‘sluggish’ systems) a high quantum yield of energy or electron transfer to a target site. (4) A synthetically tailorable structure to allow melding with other components, including incorporation into a host scaffold or attachment to partner molecular units or a surface. Systems that achieve the first of the four above-noted properties are often called panchromatic absorbers.1 For the most efficient use of solar radiation, the panchromatic absorption would span the near-ultraviolet (NUV), visible, and near-infrared (NIR) regions of the solar spectrum. Combining panchromaticity with even one of the other above-noted properties is difficult enough, let alone realizing multiple or all of these desired characteristics. While there appears to be no specific first-principles blueprint for achieving panchromatic absorption, especially in conjunction with the other desired properties, many approaches are possible. a
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, USA. E-mail: [email protected] b Department of Chemistry, Washington University, St. Louis, Missouri 63130-4889, USA. E-mail: [email protected] c Department of Chemistry, University of California, Riverside, California 92521-0403, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: Complete synthesis and characterization of all arrays; spectroscopic methods for determination of excitedstate lifetimes. See DOI: 10.1039/c4cc06853j
This journal is © The Royal Society of Chemistry 2014
Herein, we present an approach that affords a set of panchromatic absorbers based on a porphyrin chassis (Fig. 1). The porphyrin has been derivatized in a manner that gives a substantial and unexpected redistribution of NUV absorption oscillator strength into the visible, red and NIR (to B800 nm) spectral regions (Fig. 2 and 3). The four above-noted target properties have been demonstrated (Table 1). The molecular designs shown in Fig. 1 represent a redirection from the approach that we have taken previously to broaden spectral coverage of tetrapyrrole-based light-harvesting arrays by successive incorporation of one or more ‘accessory’ pigments with complementary absorption properties.2,3 Such systems primarily utilized perylenes as the accessory absorbers because of their moderately strong absorption from 450 to 550 nm,4 long excited-state lifetimes,5 photostability,4 and wide use in arrays6–9 and artificial photosynthesis.10 Linker motifs (type, length, attachment sites) were chosen to provide relatively weak electronic coupling between the components such that the absorption spectrum is essentially the sum of those of the components.3 The sum of the porphyrin and perylene spectra in Fig. 2 trace A is typical of a dyad with weak electronic coupling. The above-described approach is ultimately limiting if the goal is to achieve panchromatic absorption spanning the NUV to NIR for at least two reasons: (1) Tetrapyrroles (porphyrins, chlorins, bacteriochlorins) have relatively sharp absorption features. Thus, many such units and/or many different accessory pigments (such as perylenes) will be required, resulting in a bulky array. (2) Differences in characteristics (molecular orbital, redox) necessarily associated with compiling many units having complementary spectral (electronic) properties may give rise to substantial charge-transfer and diminished excited-state lifetimes, especially in polar media. Such considerations prompted us to alter tactics and employ strong perylene–tetrapyrrole coupling. The motivation stemmed partly from the pioneering work by Therien11 and Anderson12 on strongly coupled multi-porphyrin arrays. Our initial foray afforded a trio of dyads with an ethyne bridge between the C9-position of a perylene and meso-position of a porphyrin, chlorin or
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Fig. 1
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Arrays with 1 4 perylene per porphyrin, and benchmarks.
bacteriochlorin.13 The deviations in spectral properties of the dyads from the sum of those of the benchmarks decreased in the order porphyrin 4 chlorin 4 bacteriochlorin. That initial work motivated the preparation of the arrays shown in Fig. 1, which contain a single porphyrin ethyne-linked to 1 4 perylenes. The modular synthesis and characterization of each array are described in detail in the ESI.† The absorption and fluorescence spectra of the arrays in toluene are given in Fig. 2 (traces B–F) and 3A. The vertical axis in these figures reflects measured molar absorptivity, with e = 150 000 M 1 cm 1 for PMI4P at 564 nm. This level is the reference for the relative values given in parenthesis along with the spectral positions in Table 1. Notable observations on the spectral properties of the perylene– porphyrin arrays are as follows: (1) Each array exhibits a significant shift of absorption intensity from the NUV Soret (Bx, By) region of a typical porphyrin into the visible (nominal Qx and Qy) region, leading to effective panchromatic absorption. (2) The panchromatic-like absorption extends to longer wavelengths (red to NIR) with each successive perylene addition. In particular, the position of the long wavelength absorption feature progresses from B670 nm for the porphyrin benchmark P to B690 nm for the monoperylene– porphyrin PMI1P, to B730 nm for trans-bisperylene–porphyrin
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PMI2TP to B760 nm for tetraperylene–porphyrin PMI4P. (3) The spectra are not particularly sensitive to solvent polarity, as can be seen by comparing the spectral characteristics for the compounds in toluene and benzonitrile (Table 1). Fig. 3B illustrates the panchromaticity provided by the perylene–porphyrins. An ideal panchromatic absorber would have a flat absorption across a region of interest, as indicated by the dashed rectangular shape over 400 800 nm with e = 100 000 M 1 cm 1. This region comprises B60% of the standard AM1.5 (photon flux density) solar spectrum from 300–1100 nm. The solid line with shading below is the best fit to the rectangular shape having available all five perylene–porphyrin basis spectra in Fig. 3A (and Fig. 2). The best fit draws intensity from the three arrays PMI2TP, PMI4P and PMI1P in ratio 0.57/0.37/ 0.06. The red-dashed curve shows a similarly good fit using only PMI2TP and PMI4P with ratio 0.63/0.37. The fit spectra cover 80% of the area under the 100 000 M 1 cm 1 extinction level. The large molar absorptivity of the arrays would give a near unity absorbance over the entire span by a 10 mM sample in a 10 mm path (or 100 mM in 1 mm). The substantial spectral differences for the strongly coupled perylene–porphyrins versus benchmarks derives primarily from
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Fig. 3 (A) Absorption spectra at room temperature of perylene–porphyrins in toluene. (B) The solid line with shading below is a least-square fit (using all five basis spectra in panel A) to an ideal panchromatic absorber from 400 800 nm (dashed rectangle) using a 0.57/0.37/0.06 PMI2TP/PMI4P/ PMI1P mixture. A similar fit is obtained using a 0.63/0.37 PMI2TP/PMI4P mixture (red dashed).
Fig. 2 Absorption (solid) and fluorescence (dashed) spectra in toluene at room temperature. Trace A is the sum of spectra of benchmarks porphyrin (P, purple) and perylene (PMI, red dots). Traces B–F are for perylene–porphyrin arrays. Absorption intensities are relative to 150 000 M 1 cm 1 for PMI4P in toluene at 564 nm. Fluorescence intensities are scaled for clarity. Fluorescence lexc (in nm) is 490 (A), 430 (B), 430 (C), 425 (D), 421 (E) and 419 (F).
the perylene absorption. Overall, the significant effects of the strongly coupled perylenes on tetrapyrrole electronic structure must derive from more than the typical substituent effects. The perturbations likely involve a change in the nature and magnitude of the interaction of the four key electronic configurations (i.e., the configuration-interaction energy) that underpin the tetrapyrrole excited-state manifold.13 Panchromatic absorption alone is not sufficient for efficient solar light-harvesting. As noted above, another requirement is a long excited-state lifetime. The lifetimes for the four perylene– porphyrin arrays in toluene range from 1.2 to 4.6 ns (2.8 ns average), with the shortest value being for PMI4P (Table 1). The lifetimes for the arrays in the highly polar benzonitrile range from 1.2 to 1.9 ns (1.5 ns average), with the value for PMI4P being the same as in toluene. Such lifetimes are more than sufficient to support efficient energy or electron-transfer processes in solar-conversion systems. Such efficiency would be retained even in polar media where charge-transfer quenching
a redistribution of the substantial oscillator strength held in the typical tetrapyrrole NUV Soret features (e B 350 000 M 1 cm 1) into the longer-wavelength regions where needed to afford effective panchromatic absorption. The spectra in Fig. 2 and 3 show that this redistribution can be extended into the NIR via strong coupling to progressively more perylene units. Despite the large redistribution of porphyrin oscillator strength, the bulk of the perylene oscillator strength is retained in its normal region (450 550 nm), as can be seen by examination of column ‘‘Abs l3’’ in Table 1. Progressing from arrays with 4/3/2/1 perylenes, the average relative peak-intensity values of 1.0/0.84/0.63/0.37 track fairly well the ideal values of 1.0/0.75/ 0.50/0.25. Deviations are expected because of contributions of the porphyrin visible (Qy, Qx) bands in this region (Fig. 2A), and there must be some effect of perylene–porphyrin coupling on Table 1
Spectral properties of perylene–porphyrin arrays and benchmarksa
Compound
Solvent
Abs l1 (nm)
P PMI PMI1P PMI1P PMI2TP PMI2TP PMI2CP PMI2CP PMI3P PMI3P PMI4P PMI4P
Toluene Toluene Toluene PhCN Toluene PhCN Toluene PhCN Toluene PhCN Toluene PhCN
434 (2.28) 430 434 430 438 425 433 421 432 420 425
(0.77) (0.77) (0.39) (0.44) (0.36) (0.38) (0.32) (0.31) (0.33) (0.37)
Abs l2 (nm) 492 497 498 475 480 450 452
(0.19) (0.20) (0.20) (0.35) (0.37) (0.36) (0.40)
Abs l3 (nm)
Abs l4 (nm)
533 526 532 534 536 540 543 546 549 552 564 562
574 (0.17)
(0.07) (0.30) (0.37) (0.37) (0.56) (0.56) (0.69) (0.69) (0.84) (0.84) (1.0) (1.0)
574b (0.69) 578b (0.65) 597 (0.98) 601b (0.85)
Abs l5 (nm)
613 617 636 641 638 648 699 692 702 703
(0.38) (0.41) (0.42) (0.44) (0.35) (0.38) (0.36) (0.42) (0.40) (0.42)
Abs l6 (nm)
Flu l (nm)
tS (ns)
FF
668 (0.06)
673 556 694 702 744 755 728 738 766 776 759 770
14 5.6 4.3 1.3 2.1 1.3 4.6 1.9 1.8 1.8 1.2 1.2
0.14 0.95 0.38 0.12 0.41 0.24 0.21 0.11 0.07 0.14 0.007 0.03
688 (0.30) 691 (0.37) 728 (0.45) 737 (0.63) 707b (0.18) 718b (0.25) 760b (0.32) 755 (0.40) 763 (0.55) 760 (0.53)
a
All data acquired at room temperature using methods described in ref. 13. PhCN is benzonitrile. The value in parenthesis along with the absorption peak wavelength is the intensity relative to that at the 564 nm peak of (PMI)4P, which has e = 150 000 M 1 cm 1. b Approximate wavelength of a shoulder on an asymmetric absorption feature.
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within many multi-chromophore complexes can compromise utility. The perylene–porphyrins described herein raise a number of questions of fundamental interest. One regards the lifetime of PMI4P in toluene being reduced by only a factor of 2 4 to 1.2 ns from 4.3 ns for PMI1P and 2.1 ns for PMI2TP, even though the yield of nominal porphyrin fluorescence (Fig. 2) of PMI4P drops by a factor of B60 to 0.007 from 0.38 for PMI1P and 0.41 for PMI2TP (Table 1). The latter fluorescence yields themselves are of interest because they are quite high for porphyrinic systems. Apparently the appended perylenes result in compensating changes in the rate constants for the fluorescence, intersystem crossing, and internal conversion decay of the lowest singlet excited state to preserve a long lifetime. Future studies will address such issues as well as what and how changes in electronic structure give rise to the optical properties of these systems. The architectures described herein afford panchromatic absorption that extends from 400 to 700 nm for the monoperylene– porphyrin to 800 nm with four perylenes. The arrays exhibit long excited-state lifetimes that are similar in polar and nonpolar media. Elaboration using other rylenes7,14 such as terylenes may extend the absorption deeper into the NIR to even better utilize that photon-rich region of the solar spectrum. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award numbers DE-FG02-05ER15660 (D.F.B.), DE-FG02-05ER15661 (D.H. & C.K.) and DE-FG02-96ER14632 (J.S.L.). E.J.A. was supported by an Ernest Eliel NC-ACS undergraduate award.
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