DSpace at VNU: Sequence-specific synthesis of platinum-conjugated trichromophoric energy cascades of anthracene, tetracene, and pentacene and fluorescent "black chromophores"

Article pubs.acs.org/Organometallics Sequence-Specific Synthesis of Platinum-Conjugated Trichromophoric Energy Cascades of Anthracene, Tetracene, and Pentacene and Fluorescent “Black Chromophores” Minh-Hai Nguyen,† Van Ha Nguyen, and John H K Yip* Department of Chemistry, National University of Singapore, Science Drive 3, Singapore, 117543 S Supporting Information * ABSTRACT: A sequence-specific synthesis of trichromophoric energy cascades containing anthracenyl (A), tetracenyl (T), or pentacenyl (P) rings is achieved by coupling PtII ions and singly desilylated bis(triisopropylsilylethynyl)acenes (acene = anthracene, tetracene, or pentacene) Four sequences, T-P-T, P-T-P, A-P-T, and A-T-P, are generated Absorption spectra of the triads show intense bands due to the S0 → S1 transitions of the acenes The AP-T and A-T-P, being able to absorb strongly throughout the entire visible region, are rare examples of “black chromophores” Different sequences are shown to have different emission fingerprints Emission spectra of A-P-T and A-T-P show that excitation at the S0 → S1 band of A gives rise to emissions from the S1 excited states of A, T, and P, implying the presence of energy transfer from the excited state of A to those of T and P ■ INTRODUCTION Many multichromophoric molecules were devised as models for understanding kinetics of intramolecular energy transfer1 or electron transfer.2 A subset of the molecules is energy cascades, which have a stepwise arrangement of electronic excited states of different chromophores Energy cascades have been actively studied as molecular wires and light antenna for artificial photosynthesis.3 In this regard, various dyads and triads containing chromophores, such as porphyrins,4 RuII(2,2′bipyridine)3,5 polycyclic aromatic hydrocarbons (PAHs),6 and BODIPY7 dyes, have been devised Efficient energy transfer between chromophores depends on their distance and orientation as well as their excited state manifolds An early study of Meyer et al.8 highlighted the important role played by an intermediate excited state in mediating energy transfer from a donor to an acceptor: in a polymer containing RuII(bipy)3, OsII(bipy)3, and an anthracenyl ring, facile energy transfer from the metal-to-ligand charge-transfer excited state (3MLCT) of RuII(bipy)3 to the MLCT excited state of OsII(bipy)3 is mediated by the triplet excited state of the anthracenyl ring that is situated between the two MLCT excited states in terms of energy Anthracene (A), tetracene (T), and pentacene (P) form a homologous series that has similar electronic structures As the number of fused rings increases, the HOMO−LUMO gap decreases,9 and consequently, the energy of the lowest singlet excited states S1 of the molecules follows the order A > T > P (Scheme 1) The clear ordering allows realization of stepwise energy casades in linear arrays of the three chromophores In addition, combining the three acenes in one molecule would lead to a new excited state manifold that includes not only the S1 excited © XXXX American Chemical Society Scheme states but also other excited states such as the lowest triplet excited states T1 Schanze and his co-workers pioneered the use of transPtII(L)2(CCR)2 (L = phosphine) to connect different chromophores to form monodispersed oligomers or molecular wires that exhibit two-photon absorption,10a nonlinear optical properties,10b photoinduced charge separation,10c energy transfer,10d or exciton migration.10e Our previous studies11 showed that desilylation of 5,12-bis(triisopropylsilylethynyl)tetracene (TIPS-T)12 and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-P),13 which were first synthesized by Anthony et Received: June 20, 2013 A dx.doi.org/10.1021/om400578t | Organometallics XXXX, XXX, XXX−XXX Organometallics Article Scheme Scheme al.12,13, generates acetylides that can coordinate to the PtII ion to form binuclear [I(Et3P)2PtII]2-5,12-diethynyltetracene11a and [I(Et3P)2PtII]2-6,13-diethynylpentacene,11b and mononuclear trans-[I(Et3P)2PtII]-{5-ethynyl-12-[(triisopropylsilyl)ethynyl]tetracene} and trans-[I(Et3P) 2PtII]-{6-ethynyl-13-[(triisopropylsilyl)ethynyl]pentacene}.11c It is envisioned that, in conjugation with bis(triisopropylsilylethynyl)anthracene (TIPS-A), these molecules can be employed in building triads that contain the three chromophores shown in Schemes and The dipole of the S1 excited state of the chromophores is known to be polarized along their long molecular axes.14a−c In the linear arrangement of the chromophores depicted in Scheme 2, the dipoles would be aligned along the molecular axis This arrangement is expected to facilitate Foster resonance energy transfer.14d Apart from the excited state energy ordering, spatial arrangement of different chromophores in a molecule could affect the direction and efficiency of energy flow It is, therefore, essential to develop a programmable approach that allows specific sequences of the chromophores in the linear array to be synthesized Herein, we report the first sequencespecific synthesis of energy cascades that have different combinations of the chromophores including T-P-T, A-P-T, B dx.doi.org/10.1021/om400578t | Organometallics XXXX, XXX, XXX−XXX Organometallics Article tetracene), 9.05 (d, J = 8.2 Hz, 2H, H1-tetracene), 8.28−8.26 (m, 6H, H7-tetracene, H1,4,8,11-pentacene), 8.12 (d, J = 8.8 Hz, 2H, H10tetracene), 7.50−7.43 (m, 4H, H3,8-tetracene), 7.33−7.29 (m, 6H, H2tetracene, H2,3,9,10-pentacene), 7.23 (t, J = 7.6 Hz, 2H, H9-tetracene), 2.22−2.20(m, 24H, PCH2CH2CH2CH3), 1.89−1.88 (m, 24H, PCH2CH2CH2CH3), 1.38−1.31 (m, 45H, PCH2CH2CH2CH3, iPr), 0.83 (t, J = 7.6 Hz, 12H, PCH2CH2CH2CH3) 31P{1H} NMR (202.4 MHz, C6D6): δ 5.40 (s, 1JPt−P = 2339 Hz) MALDI-TOF-MS: m/z 2386.76, [M]+ Synthesis of T-P To a 250 mL Schlenk flask were charged Pt2P (0.2 g, 0.11 mmol), iPr2NH (5 mL), 5,12-bis(triisopropylsilylethynyl)tetracene (200 mg, 0.340 mmol), CuI (5 mg), and CH2Cl2 (30 mL) A CH2Cl2 solution (50 mL) of Bu4NF (20 mg, 0.06 mmol) was slowly added to the solution The mixture was stirred for 12 h, and the solid obtained from rotary evaporation was subjected to column chromatography (silica gel, hexane:CH2Cl2 4:1) from which the dark purple product was isolated Yield: 70 mg, 30% Anal Calcd (%) for T-P (C105H151IP4Pt2Si): C, 60.56; H, 7.31 Found: C, 60.19; H, 7.22 1H NMR (500 MHz, C6D6): δ 9.96 (s, 3H, H6-tetracene, H5,7pentacene), 9.84 (s, 2H, H12,14-pentacene), 9.71 (s, 1H, H11tetracene), 9.29 (d, J = 8.2 Hz, 1H, H4-tetracene), 9.04 (d, J = 8.8 Hz, 1H, H1-tetracene), 8.27−8.19 (m, 5H, H7-tetracene, H1,4,8,11pentacene), 8.11 (d, J = 8.9 Hz, 1H, H10-tetracene), 7.48−7.44 (m, 2H, H2,3-tetracene), 7.32−7.22 (m, 6H, H8,9-tetracene, H2,3,9,10pentacene), 2.26−2.18 (m, 24H, PCH2CH2CH2CH3), 1.88−1.86 (m, 12H, PCH2CH2CH2CH3), 1.76−1.75 (m, 12H, PCH2CH2CH2CH3), 1.38−1.30 (m, 45H, PCH2CH2CH2CH3, iPr), 0.85 (t, J = 7.6 Hz, 12H, PCH2CH2CH2CH3), 0.81 (t, J = 7.6 Hz, 12H, PCH2CH2CH2CH3) 31P{1H} NMR (202.4 MHz, C6D6): δ 5.36 (s, JPt−P = 2344 Hz), 1.64 (s, 1JPt−P = 2304 Hz) MALDI-TOF-MS: m/z 2082.42, [M]+ Synthesis of A-P-T To a 250 mL Schlenk flask were charged 9,10bis(triisopropylsilylethynyl)anthracene (0.15 g, 0.28 mmol), iPr2NH (5 mL), CuI (5 mg), and CH2Cl2 (30 mL) To the mixture was added a CH2Cl2 solution (50 mL) of Bu4NF (9 mg, 0.03 mmol) over h, and then T-P (30 mg, 0.01 mmol) was added to the solution The mixture was stirred for 12 h, and the dark brown product was collected from column chromatography (silica gel, hexane:CH2Cl2 4:1) Yield: 16 mg, 48% Anal Calcd (%) for A-P-T (C132H180P4Pt2Si2): C, 67.84; H, 7.76 Found: C, 67.42; H, 7.42 1H NMR (500 MHz, C6D6): δ 9.98 (s, 1H, H6-tetracene), 9.97 (s, 2H, H5,7-pentacene), 9.96 (s, 2H, H12,14pentacene), 9.71 (s, 1H, H11-tetracene), 9.31−9.30 (m, 3H, H4tetracene, H4,5-anthracene), 9.07−9.04 (m, 3H, H1-tetracene, H1,8anthracene), 8.28−8.24 (m, 5H, H7-tetracene, H1,4,8,11-pentacene), 8.12 (d, J = 8.8 Hz, 1H, H10-tetracene), 7.57−7.43 (m, 8H, H2,3,6,7anthracene, H2,3,8,9-tetracene), 7.31−7.28 (m, 4H, H2,3,9,10-pentacene), 2.22−2.16 (m, 24H, PCH2CH2CH2CH3), 1.89−1.85 (m, 24H, PCH2CH2CH2CH3), 1.40−1.27 (m, 66H, PCH2CH2CH2CH3, iPr), 0.85−0.81 (m, 36H, PCH2CH2CH2CH3) 31P{1H} NMR (202.4 MHz, C6D6): δ 5.39 (s, 1JPt−P = 2337 Hz), 5.33 (s, 1JPt−P = 2337 Hz) MALDI-TOF-MS: m/z 2336.76, [M]+ Synthesis of P-T-P Pt2T (50 mg, 0.03 mmol), iPr2NH (5 mL), 6,13-bis(triisopropylsilylethynyl)pentacene (180 mg, 0.281 mmol), CuI (5 mg), and CH2Cl2 (30 mL) were added to a 250 mL Schlenk flask To the mixture was added a CH2Cl2 solution (50 mL) of Bu4NF (21 mg, 0.067 mmol) over h The resulting solution was stirred for 12 h, and the dark blue product was isolated by column chromatography (silica gel, hexane:CH2Cl2 2:1) Yield: 31 mg, 44% Anal Calcd (%) for P-T-P (C140H184P4Pt2Si2): C, 68.99; H, 7.61 Found: C, 68.70; H, 7.54 1H NMR (500 MHz, C6D6): δ 10.00 (s, 2H, H6,11-tetracene), 9.96 (s, 4H, H5,7-pentacene), 9.70 (s, 4H, H12,14pentacene), 9.35 (dd, J = 3.1, 6.9 Hz, 2H, H1,4-tetracene), 8.34 (dd, J = 3.1, 6.9 Hz, 2H, H7,10-tetracene), 8.21 (d, J = 8.8 Hz, 4H, H4,8pentacene), 8.05 (d, J = 8.8 Hz, 4H, H1,11-pentacene), 7.63 (dd, J = 3.1, 6.9 Hz, 2H, H2,3-tetracene), 7.37 (dd, J = 3.1, 6.9 Hz, 2H, H8,9tetracene), 7.26 (t, J = 8.8 Hz, 4H, H3,9-pentacene), 7.16 (overlapped, 2H, H2,10-pentacene), 2.22−2.20 (m, 24H, PCH2CH2CH2CH3), 1.90−1.88 (m, 24H, PCH2CH2CH2CH3), 1.43−1.29 (m, 66H, PCH2CH2CH2CH3, iPr), 0.83 (t, J = 7.6 Hz, 24H, PCH2CH2- A-T-P, and P-T-P (Schemes and 3) and the electronic spectroscopy of the complexes The A-P-T and A-T-P are black chromophores strongly absorbing throughout the entire visible region and display near-infrared emissions ■ EXPERIMENTAL SECTION General Methods All syntheses were carried out in a N2 atmosphere All the solvents used for synthesis and spectroscopic measurements were purified according to the literature procedures.15 trans-Pt(PnBu3)2I2,16 9,10-bis(triisopropylsilylethynyl)anthracene,17 5,12-bis(triisopropylsilylethynyl)tetracene,12 and 6,13-bis(triisopropylsilylethynyl)pentacene13 were prepared according to reported procedures Physical Methods The UV/vis absorption and emission spectra of the complexes were recorded on a Hewlett-Packard HP8452A diode array spectrophotometer and a PerkinElmer LS-50D fluorescence spectrophotometer, respectively Emission lifetimes were recorded on a Horiba Jobin-Yvon Fluorolog FL-1057 fluorescence spectrometer Cresyl violet was used as a standard in measuring the emission quantum yields.18 1H and 31P{1H} NMR spectra were recorded on a Bruker ACF 500 spectrometer All chemical shifts are quoted relative to SiMe4 (1H) or H3PO4 (31P) Elemental analyses of the complexes were carried out in the microanalysis laboratory in the Department of Chemistry at the National University of Singapore MALDI-TOF mass spectra were recorded on an Autoflex III TOF/TOF mass spectrometer using α-cyano-3-hydroxycinnamic acid as the matrix Synthesis of Pt2P.11b To a 250 mL Schlenk flask were charged trans-Pt(PnBu3)2I2 (400 mg, 0.47 mmol), iPr2NH (5 mL), Bu4NF (100 mg, 0.32 mmol), CuI (10 mg), and CH2Cl2 (30 mL) A CH2Cl2 solution (100 mL) of 6,13-bis(triisopropylsilylethynyl)pentacene (46 mg, 0.07 mmol) was added to the mixture over h The resulting solution was stirred for 12 h, and then the solvent was removed by rotary evaporation The solid was subjected to column chromatography (silica gel, hexane:CH2Cl2 4:1) from which the dark green product was collected Yield: 62 mg, 48% Anal Calcd (%) for Pt2P (C74H120I2P4Pt2): C, 50.00; H, 6.80 Found: C, 50.21; H, 7.01 1H NMR (500 MHz, CDCl3): δ 9.31 (s, 4H, H5,7,12,14), 7.90 (dd, J = 3.1, 6.9 Hz, 4H, H1,4,8,11), 7.31 (dd, J = 3.1, 6.9 Hz, 4H, H2,3,9,10), 2.23−2.20 (m, 24H, PCH2CH2CH2CH3), 1.68−1.67 (m, 24H, PCH2CH2CH2CH3), 1.41−1.34 (m, 24H, PCH2CH2CH2CH3), 0.86−0.83 (t, 36H, PCH2CH2CH2CH3) 31P{1H} NMR (202.4 MHz, CDCl3): δ 1.26 (s, 1JPt−P = 2300 Hz) ESI-MS: m/z 1777.3, [M]+ Synthesis of Pt2T To a 250 mL Schlenk flask were charged transPt(PnBu3)2I2 (1.8 g, 2.1 mmol), iPr2NH (5 mL), Bu4NF (0.42 g, 1.3 mmol), CuI (10 mg), and CH2Cl2 (40 mL) To the mixture was added a CH2Cl2 solution (60 mL) of 5,12-bis(triisopropylsilylethynyl)tetracene (0.20 g, 0.34 mmol) over h The resulting solution was stirred for 12 h and was reduced to dryness, and the dark red product was collected from column chromatography (silica gel, hexane: CH2Cl2 4:1) Yield: 0.30 mg, 51% Anal Calcd (%) for Pt2T (C70H118I2P4Pt2): C, 48.67; H, 6.88 Found: C, 48.74; H, 6.67 1H NMR (500 MHz, C6D6): δ 9.82 (s, 2H, H6,11), 9.16 (dd, J = 3.1, 6.9 Hz, 2H, H1,4), 8.22 (dd, J = 3.1, 6.3 Hz, 2H, H7,10), 7.51 (dd, J = 3.1, 6.9 Hz, 2H, H2,3), 7.28 (dd, J = 3.1, 6.3 Hz, 2H, H8,9), 2.22−2.19 (m, 24H, PCH2CH2CH2CH3), 1.71−1.69 (m, 24H, PCH2CH2CH2CH3), 1.34−1.30 (m, 24H, PCH2CH2CH2CH3), 0.87−0.84 (t, J = 7.6 Hz, 36H, PCH2CH2CH2CH3) 31P{1H} NMR (202.4 MHz, C6D6): δ 1.59 (s, 1JPt−P = 2307 Hz) ESI-MS: m/z 1727.4, [M]+ Synthesis of T-P-T To a 250 mL Schlenk flask were charged Pt2P (50 mg, 0.03 mmol), iPr2NH (5 mL), 5,12-bis(triisopropylsilylethynyl)tetracene (0.25 g, 0.42 mmol), CuI (5 mg), and CH2Cl2 (40 mL) A CH2Cl2 solution (50 mL) of Bu4NF (21 mg, 0.067 mmol) was added to the solution over h, and the mixture was stirred for 12 h before all the solvents were removed by rotary evaporation The dark red product was collected from column chromatography (silica gel, hexane:CH2Cl2 2:1) Yield: 37 mg, 55% Anal Calcd (%) for T-P-T (C136H182P4Pt2Si2): C, 68.43; H, 7.68 Found: C, 68.67; H, 7.74 1H NMR (500 MHz, C6D6): δ 9.98 (s, 6H, H6-tetracene, H5,7,12,14pentacene), 9.71 (s, 2H, H11-tetracene), 9.30 (d, J = 8.8 Hz, 2H, H4C dx.doi.org/10.1021/om400578t | Organometallics XXXX, XXX, XXX−XXX Organometallics Article Scheme Scheme CH2CH3) 31P{1H} NMR (202.4 MHz, C6D6): δ 5.42 (s, 1JPt−P = 2339 Hz) MALDI-TOF-MS: m/z 2436.73, [M]+ Synthesis of P-T Pt2T (0.20 g, 0.116 mmol), iPr2NH (5 mL), 6,13-bis(triisopropylsilylethynyl)pentacene (150 mg, 0.235 mmol), CuI (5 mg), and CH2Cl2 (30 mL) were added to a 250 mL Schlenck flask The mixture was added to a CH2Cl2 solution (50 mL) of Bu4NF (18 mg, 0.06 mmol) The mixture was stirred for 12 h, and the dark purple product was isolated by column chromatography (silica gel, hexane:CH2Cl2 4:1) Yield: 45 mg, 19% Anal Calcd (%) for P-T (C105H151P4Pt2Si): C, 60.56; H, 7.31 Found: C, 60.27; H, 7.32 1H NMR (500 MHz, C6D6): δ 9.98 (s, 1H, H6-tetracene), 9.95 (s, 2H, H5,7-pentacene), 9.84 (s, 1H, H11-tetracene), 9.71 (s, 2H, H12,14pentacene), 9.32 (d, J = 8.5 Hz, 1H, H4-tetracene), 9.19 (d, J = 8.5 Hz, 1H, H1-tetracene), 8.30 (d, J = 8.8 Hz, 1H, H7-tetracene), 8.26 (d, J = 8.2 Hz, 1H, H10-tetracene), 8.20 (d, J = 8.8 Hz, 2H, H4,8-pentacene), 8.05 (d, J = 8.8 Hz, 2H, H1,11-pentacene), 7.60−7.55 (m, 2H, H2,3tetracene), 7.35−7.30 (m, 2H, H8,9-tetracene), 7.25 (t, J = 8.2 Hz, 4H, H3,9-pentacene), 7.16 (overlapped, 2H, H2,10-pentacene), 2.24−2.17 (m, 24H, PCH2CH2CH2CH3), 1.88−1.85 (m, 12H, PCH2CH2CH2CH3), 1.74−1.71 (m, 12H, PCH2CH2CH2CH3), 1.42−1.29 (m, 45H, PCH2CH2CH2CH3, iPr), 0.87 (t, J = 7.6 Hz, 12H, PCH2CH2CH2CH3), 0.82 (t, J = 7.6 Hz, 12H, PCH2CH2CH2CH3) 31P{1H} NMR (202.4 MHz, C6D6): δ 5.40 (s, 1JPt−P = 2339 Hz), 1.63 (s, 1JPt−P = 2312 Hz) MALDI-TOF-MS: m/z 2082.30, [M]+ Synthesis of A-T-P To a 250 mL Schlenk flask were charged 9,10bis(triisopropylsilylethynyl)anthracene (0.15 g, 0.28 mmol), iPr2NH (5 mL), CuI (5 mg), and CH2Cl2 (30 mL) A CH2Cl2 solution (50 mL) of Bu4NF (12 mg, 0.038 mmol) was slowly added to the mixture, followed by P-T (30 mg, 0.01 mmol) The mixture was stirred for 12 h and dried by rotary evaporation The dark brown product was collected from column chromatography (silica gel, hexane:CH2Cl2 4:1) Yield: 23 mg, 68% Anal Calcd (%) for A-T-P (C132H180P4Pt2Si2): C, 67.84; H, 7.76 Found: C, 67.41; H, 7.72 1H NMR (500 MHz, C6D6): δ 9.99 (s, 1H, H6-tetracene), 9.97 (s, 3H, H11-tetracene, H5,7pentacene), 9.71 (s, 2H, H12,14-pentacene), 9.35−9.29 (m, 4H, H4,5anthracene, H1,4-tetracene), 9.06 (d, J = 8.2 Hz, 2H, H1,8-anthracene), 8.34−8.29 (m, 2H, H7,10-tetracene), 8.21 (d, J = 8.8 Hz, 2H, H4,8pentacene), 8.05 (d, J = 8.8 Hz, 2H, H1,11-pentacene), 7.65−7.59 (m, 2H, H2,3-tetracene), 7.56−7.50 (m, 4H, H2,3,6,7-anthracene), 7.36−7.32 (m, 2H, H8,9-tetracene), 7.26 (t, J = 8.2 Hz, 2H, H3,9-pentacene), 7.16 (overlapped, 2H, H2,10-pentacene), 2.22−2.19 (m, 12H, PCH2CH2CH2CH3), 2.16−2.13 (m, 12H, PCH2CH2CH2CH3), 1.89−1.81 (m, 24H, PCH2CH2CH2CH3), 1.43−1.28 (m, 66H, PCH2CH2CH2CH3, i Pr), 0.86−0.81 (m, 36H, PCH2CH2CH2CH3) 31P{1H} NMR (202.4 MHz, C6D6): δ 5.42 (s, 1JPt−P = 2342 Hz), 5.24 (s, 1JPt−P = 2347 Hz) MALDI-TOF-MS: m/z 2336.72, [M]+ ■ RESULTS AND DISCUSSION Syntheses As shown in Schemes and 3, the chromophores in the dyads and triads were joined by Pt−C bonds derived from the ethynyl group of the chromophores and platinum iodide complexes via copper-catalyzed crosscoupling An inherent difficulty in the syntheses is that the sequence-specific arrangement of the chromophores requires coupling to take place to only one end of the symmetric building blocks 9,10-bis(triisopropylsilylethynyl)anthracene, 5,12-bis(triisopropylsilylethynyl)tetracene, and 6,13-bis(triisopropylsilylethynyl)pentacene, and Pt2P and Pt2T The reactions require single desilyation of the bis(triisopropylsilylethynyl)acenes to form A-SiH, T-SiH, and P-SiH (Schemes and 3), which can be achieved by reacting large excess of the acenes with Bu4NF in ratios of up to 10:1 (Scheme 4) The unreacted acenes were easily recovered from column chromatography The monosilylated ethynylacenes were unstable and were coupled to the Pt ion without being isolated While double desilylation of bis(triisopropylsilylethynyl)acenes is unavoidable, the tetraplatinum products arising from coupling of diethynylacenes and Pt2P or Pt2T were not observed It could be due to the small amount of the products and the poor solubility of the large molecules Similarly, the formation of the unsymmetrical T-P and P-T required reacting T-SiH and P-SiH with excess Pt2P and Pt2T, respectively It is found that the PnBu3 ligands on the Pt ions are important for increasing the solubility of the triads in organic solvents such as CH2Cl2 The analogous complexes of PEt3 are sparingly soluble or insoluble in most of the organic solvents Structures Despite numerous attempts, we failed to obtain single crystals of the complexes for X-ray diffraction Nonetheless, the proposed structures were confirmed by NMR spectroscopy, elemental analysis, and high-resolution mass spectrometry All the 1H NMR spectra show signals in the aliphatic (isopropyl and n-butyl) and aromatic regions (acene) The D dx.doi.org/10.1021/om400578t | Organometallics XXXX, XXX, XXX−XXX Organometallics Article Figure (a) MALDI-TOF-MS cluster peak and (b) calculated isotopic distribution for the [A-T-P]+ ion structures can be rather easily determined from the number of aromatic signals of each acene in the molecules Scheme shows the numbering schemes of the protons For example, the central tetracenyl ring of the symmetrical P-T-P exhibits only signals for H2,3, H1,4, H6,11, H17,10, and H8,9, whereas the tetracenyl ring in the unsymmetrical A-T-P displays 10 signals Consistent with the symmetric structures of the complexes, the spectra of P-T-P and T-P-T show five and six signals for the central tetracenyl ring (H1,4, H2,3, H6,11, H7,10, H8,9) and pentacenyl ring (H1,4, H2,3, H 5,14 , H7,12, H 8,11, H9,10), respectively The H6,11 of the former and the H5,7,12,14 of the latter are singlet, while the other protons are doublet or double doublets The loss of C2-symmetry of the two pentacenyl rings in P-T-P and the two tetracenyl rings in T-P-T are characterized by the presence of 12 and 10 signals for the rings in the spectra of the complexes, respectively Both spectra of A-T-P and A-P-T show 8, 10, and 12 signals for the anthracenyl, tetracenyl, and pentacenyl rings, respectively The 31 P{ H} NMR spectra of the symmetrical T-P-T and P-T-P show a singlet at δ 5.40 (1JPt−P = 2339 Hz) and δ 5.42 (1JPt−P = 2339 Hz), respectively In accord with the unsymmetrical structures of the molecules, two singlets are found in the spectra of A-P-T and A-T-P at δ 5.33 (1JPt−P = 2337 Hz) and δ 5.39 (1JPt−P = 2337 Hz), and δ 5.40 (1JPt−P = 2342 Hz) and δ 5.24 (1JPt−P = 2347 Hz), respectively The 1JPt−P values (2304− 2364 Hz) are consistent with a trans-orientation of the phosphines.19 All the complexes have been characterized by MALDI-TOF mass spectrometry The spectra show prominent peaks for singly charged parent ions [M]+ (see Figures S1−S5 in the Supporting Information for the MS spectra of the other complexes) The observed isotopic distributions in the cluster peaks are essentially identical with the ones calculated according to the molecular formula Figure shows the cluster peak of the parent ion [A-T-P]+ (m/z = 2336.72) and the calculated isotopic distribution Electronic Absorption Spectroscopy All the complexes show deep colors that reflect combinations of the acenes in them The dyads P-T and T-P are violet, whereas T-P-T and PT-P are purple and Prussian blue Both A-P-T and A-T-P are deep brown The absorption spectra and the colors of the solutions of the complexes are depicted in Figures and 3, and Figure UV−vis absorption spectra and colors of CH2Cl2 solutions of T-P-T (brown), T-P (pink), and A-P-T (blue) at room temperature Figure UV−vis absorption spectra and colors of CH2Cl2 solutions of P-T-P (green), P-T (red), and A-T-P (black) at room temperature the spectral data are summarized in Table The absorption spectra of the building blocks TIPS-P, TIPS-T, and TIPS-A can be found in the Supporting Information (Figures S6−S8) E dx.doi.org/10.1021/om400578t | Organometallics XXXX, XXX, XXX−XXX Organometallics Article Table UV−vis Absorption Spectroscopic Dataa complex band (ε, 104 M−1 cm−1) P-T-P 697 (4.89), 638 (3.00) 692 (4.21), 635 (2.47) 697 (4.23), 637 (2.49) 659 (3.91) P-T 652 (2.18) A-T-P 654 (2.20) T-P-T T-P A-P-T band (ε, 104 M−1 cm−1) 567 (4.57), 533 (4.24), 497 (2.35) 568 (2.49), 533 (2.21), 497 (1.20) 568 (2.31), 533 (2.10), 586 (3.84), 542 (2.48), 504 (1.13) 578 (3.94), 537 (2.65), 504 (1.21) 582 (4.55), 540 (3.18) band (ε, 104 M−1 cm−1) band (ε, 104 M−1 cm−1) 470 (3.71), 444 (3.29), 415 (1.84) 469 (3.69), 442 (3.23), 415 (1.94) (s) band (ε, 104 M−1 cm−1) 350 (5.11) 319 (27.65), 297 (31.24) 347 (3.79), 338 (3.92) (s) 348 (3.35), 337 (3.61) 318 (28.72), 297 (19.44) 360 (4.77), 338 (5.00) 319 (25.59), 297 (19.24), 279 (16.53) 316 (47.50), 304 (26.30) 357 (3.79), 338 (3.88) 316 (29.05), 303 (20.83) 359 (3.66), 339 (3.62) 316 (29.67), 303 (23.23), 278 (16.20) a The extinction coefficients (ε) of vibronic peaks are shown and as the bands of P-T-P, P-T, and A-T-P have no vibronic peak, only the εmax values are listed Figure Emission spectra of T-P-T (red) and T-P (blue) in CH2Cl2 at room temperature (excitation wavelength = 490 nm) Inset: excitation spectra of T-P-T (red) and T-P (blue) monitored at 720 nm The absorption spectra are dominated by intense π → π* transitions of the acenes The spectra exhibit two vibronic bands in ∼600−750 nm (band 1, εmax = (2.18−4.89) × 104 M−1 cm−1) and in ∼450−620 nm (band 2, εmax = (2.31−4.57) × 104 M−1 cm−1), which correspond, respectively, to the S0 → S1 transitions of the pentacenyl and tetracenyl rings, which are the common motifs of the six complexes The spectra of A-T-P and A-P-T show an additional band in ∼400−500 nm (band 3, εmax = (3.69−3.71) × 104 M−1 cm−1), which corresponds to the S0 → S1 transition of the anthracenyl ring In addition, all spectra display a shoulder around 330−380 nm (band 4, εmax = (3.35−5.11) × 104 M−1 cm−1) and very intense sharp bands between 280 and 330 nm (band 5, εmax = (2.56−4.75) × 105 M−1 cm−1), which are due to high-energy π → π* transitions characteristic of the acenes.14a It is noted that all the absorption bands, especially the S0 → S1 transitions, are red-shifted from the corresponding transitions of TIPS-A, TIPS-T, and TIPS-P by 30−50 nm A similar red shift has been observed in the spectra of binuclear PtII complexes of diethynyltetracene and diethynylpentacene and has been attributed to the perturbation of the Pt ions on the electronic structures of the acenes, mainly via metal−ligand π-interactions.11 Notably, the triads A-T-P and A-P-T absorb intensely (ε > 104 M−1 cm−1) throughout the entire visible region and part of the UV region (280−750 nm), making them examples of “black chromphores” that efficiently absorb all visible lights.20 Emission Spectroscopy The complexes are weakly emissive with the quantum yield of (0.6−7.7) × 10−3 The solution emission and excitation spectra of the complexes are shown in Figures 4−6, and lifetime and quantum yield are listed in Table All the emissions are fluorescence, which is Table Emission Spectroscopic Data of the Complexes complex T-P-T T-P A-P-T P-T-P P-T A-T-P emission maxima (λmax/nm) 601, 599, 491, 770 600, 490, 727 720 600, 723 lifetimes (τ/ns) 7.0c, 0.5 7.1c, 0.6 4.7d, 4.0c, 0.5 a 770 604, 778 8.4c, a(770 nm) 4.4d, a(604 nm), a(778 nm) quantum yields (φ × 10−3)b 7.7 4.6 7.4 0.6 3.7 0.8 Not determined bExcitation wavelength (λem) is 490 nm cλem = 483 nm dλem = 373 nm a confirmed by lifetimes of nanosecond order Despite the presence of the heavy Pt center, no phosphorescence was observed Our previous studies of cyclometalated PtIIanthracene,21 Pt2T,11a and Pt2P11b show no phosphorescence, and this apparent absence of a heavy atom effect is attributed to the characteristic large S1−T1 gap of the alternant hydrocarbons,22 which imposes a large Franck−Condon barrier for S1 → T1 intersystem crossing The spectra of T-P-T and T-P F dx.doi.org/10.1021/om400578t | Organometallics XXXX, XXX, XXX−XXX Organometallics Article Figure Emission spectra of a CH2Cl2 solution of A-T-P excited at 400 nm (brown) and 490 nm (purple) at 298 K Inset: excitation spectra obtained by monitoring emission at 600 nm (purple) and 750 nm (brown) (the peak at 375 nm is an artifact) Figure Emission spectra of a CH2Cl2 solution of A-P-T excited at 400 nm (brown) and 490 nm (purple) at 298 K Inset: excitation spectra obtained by monitoring emission at 600 nm (purple) and 730 nm (brown) lie entirely in the near-infrared region (>700 nm) Excitation at 400 nm, where the absorption of the anthracenyl ring dominates (ε = 1.36 × 104 M−1 cm−1), gives rise to tetracenyl and pentacenyl emissions, indicating energy transfer from the S1(A) to the S1(T) and S1(P), and stepwise excited state manifold in the triads Excitation at 490 nm where the absorption is mainly due to S0 → S1 of the tetracenyl rings leads to both tetracenyl and pentacenyl emissions Monitoring the tetracenyl and pentacenyl emissions gives excitation spectra (insets of Figures and 5) that are similar to the absorption spectra, indicating the presence of S1(A) → S1(T), S1(A) → S1(P), and S1(T) → S1(P) energy transfer The relative intensity of the three emissions of A-T-P is different from that of A-P-T For A-T-P, the emission intensity follows the order of A > P ≫ T On the other hand, the order is P > A > T for A-P-T The difference could be due to different energy transfer rates or different intrinsic photophysics (i.e., nonradiative and radiative decay rates) of the chromophores in the two triads The relative intensity of the emission, which is characteristic of each triad, can be taken as the spectroscopic fingerprint of the different sequences of the three chromophores (Figure 4) and P-T (Figure S9 in the Supporting Information) exhibit two bands at ∼600 and ∼720 nm that are S1 → S0 fluorescent emissions of the tetracenyl ring and the pentacenyl ring, respectively Similar emissions were displayed by Pt2diethynyltetracene (λmax = 596 nm)10a and Pt2-diethynylpentacene (λmax = 726 nm).10b Irradiating the complexes at 490 nm, where the absorption is mainly due to the S0 → S1 transition of T, leads invariably to pentacenyl emission, suggesting energy transfer from S1(T) to the S1(P) It is further corroborated by the excitation spectra of the complexes obtained by monitoring the pentacenyl emission at 720 nm (Figure 4, inset, and Figure S10, Supporting Information), which resemble the absorption spectra The complexes show different relative intensity of the two emissions in the complexes For instance, the pentacenyl emission of T-P is more intense than its tetracenyl emission but the pentacenyl emission of P-T is very much weaker (Figure S9, Supporting Information) than its tetracenyl emission The two emissions of T-P-T have similar intensities, but the tetracenyl emission of PT-P is essentially quenched and only the pentacenyl emission is observed (Figure S9) Figures and show the emission and excitation spectra of A-T-P and A-P-T, respectively Irradiating solutions of the complexes at 400 nm gives rise to three emission bands at 490, 604, and 778 nm (A-T-P), and 491, 600, and 723 nm (A-P-T) that are S1 → S0 fluorescence of A, T, and P, respectively The triads are visible and NIR emitters as the pentacenyl emissions ■ CONCLUSION In this study, we demonstrated the sequence-specific syntheses of triads of anthracene, tetracene, and pentacene While crosscoupling provides the chemistry required to link up the G dx.doi.org/10.1021/om400578t | Organometallics XXXX, XXX, XXX−XXX Organometallics Article (3) (a) Serin, J M.; Brousmiche, D W.; Frechet, J M J Chem Commun 2002, 2605 (b) Bura, T.; Retailleau, P.; Ziessel, R Angew Chem., Int Ed 2010, 49, 6659 (c) Diring, S.; Puntoriero, F.; Nastasi, F.; Campagna, S.; Ziessel, R J Am Chem Soc 2009, 131, 6108 (d) Nakamura, Y.; Aratani, N.; Osuka, A Chem Soc Rev 2007, 36, 831 (4) (a) Duan, X.-F.; Wang, J.-L.; Pei, J Org Lett 2005, 7, 4071 (b) Casanova, M.; Zangrando, E.; Iengo, E.; Alessio, E.; Indelli, M T.; Scandola, F.; 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method can be used to synthesize sequence-specific polymers of the chromophores such as A-T-P-A-T-P-A-T-P···, although the process is timeconsuming and probably the solubility of the polymer would decrease as the chain length increases The triads are energy cascades as pumping the molecules to higher excited states such as the S1 of A led to the fluorescence from a lower-energy excited state such as the S1 of T or P Different sequences have different emission spectroscopic fingerprints Finally, A-T-P and A-P-T are rare examples of “black chromophores”, which show strong absorption throughout the entire visible region ■ ASSOCIATED CONTENT S Supporting Information * MALDI-TOF mass spectra of T-P-T, T-P, A-P-T, P-T-P, and P-T and emission and excitation spectra of P-T-P and P-T This material is available free of charge via the Internet at http://pubs.acs.org ■ AUTHOR INFORMATION Corresponding Author *E-mail: chmyiphk@nus.edu.sg Present Address † Inorganic Chemistry Department, Hanoi University of Science, 19 Le Thanh Tong, Hanoi, Vietnam Notes The authors declare no competing financial interest ■ ACKNOWLEDGMENTS We are grateful to the National Environmental Agency, Environment Technology and Research Program (R-143-000547-490) for financial support ■ REFERENCES (1) (a) Bronner, C.; Veiga, M.; Guenet, A.; De Cola, L.; Hosseini, M W.; Strassert, C A.; Baudron, S A Chem.Eur J 2012, 18, 4041 (b) Jiménez, A J.; Marcos, M L.; Hausmann, A.; Rodríguez-Morgade, M S.; Guldi, D M.; Torres, T Chem.Eur J 2011, 17, 14139 (c) Cotlet, M.; Vosch, T.; Habuchi, S.; Weil, T.; Müllen, K.; Hofkens, J.; De Schryver, F J Am Chem Soc 2005, 127, 9760 (d) Du, B.; Harvey, P D Chem Commun 2012, 48, 2671 (e) Albert-Seifried, S.; Finlayson, C E.; Laquai, F.; Friend, R H.; Swager, T M.; Kouwer, P H J.; Juríček, M.; Kitto, H J.; Valster, S.; Nolte, R J M.; Rowan, A E Chem.Eur J 2010, 16, 10021 (f) Wong, W.-Y Coord Chem Rev 2005, 249, 971 (g) Aly, S M.; Ho, C.-L.; Fortin, D.; Wong, W.-Y.; 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ASSOCIATED CONTENT S Supporting Information * MALDI-TOF mass spectra of T-P-T, T-P, A-P-T, P-T-P, and P-T and emission and excitation spectra of P-T-P and P-T This material is available free of. .. Figures and 3, and Figure UV−vis absorption spectra and colors of CH2Cl2 solutions of T-P-T (brown), T-P (pink), and A-P-T (blue) at room temperature Figure UV−vis absorption spectra and colors of. .. studies of cyclometalated PtIIanthracene,21 Pt2T,11a and Pt2P11b show no phosphorescence, and this apparent absence of a heavy atom effect is attributed to the characteristic large S1−T1 gap of the