Synthesis and Photodynamics of Fluorescent Blue BODIPY-Porphyrin

Article pubs.acs.org/JPCA

Synthesis and Photodynamics of Fluorescent Blue BODIPY-Porphyrin Tweezers Linked by Triazole Rings Antoine Eggenspiller,† Atsuro Takai,‡,∥ Mohamed E. El-Khouly,‡ Kei Ohkubo,‡ Claude P. Gros,† Claire Bernhard,† Christine Goze,† Franck Denat,† Jean-Michel Barbe,*,† and Shunichi Fukuzumi*,‡,§ †

ICMUB, UMR CNRS 6302, Université de Bourgogne, 9 Avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France Department of Material and Life Science, Graduate School of Engineering, Osaka University, and ALCA, Japan Science and Technology Agency (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan § Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea ‡

S Supporting Information *

ABSTRACT: Novel zinc porphyrin tweezers in which two zinc porphyrins were connected with π-conjugated boron dipyrromethenes (BDP meso-Por2 and BDP β-Por2) through triazole rings were synthesized to investigate the photoinduced energy transfer and electron transfer. The UV−vis spectrum of BDP β-Por2 which has less bulky substituents than BDP mesoPor2 exhibits splitting of the Soret band as a result of the interaction between porphyrins of BDP β-Por2 in the excited state. Such interaction between porphyrins of both BDP β-Por2 and BDP meso-Por2 is dominant at room temperature, while the coordination of the nitrogen atoms of the triazole rings to the zinc ions of the porphyrins occurs at low temperature. The conformational change of the BDP−porphyrin composites was confirmed by the changes in UV−vis and fluorescence spectra depending on temperature. Photodynamics of BDP meso-Por2 and BDP β-Por2 has also been investigated by laser flash photolysis. Efficient singlet−singlet energy transfer from the ZnP to the πconjugated BDP moiety of both BDP meso-Por2 and BDP β-Por2 occurred in opposite direction as compared to energy transfer from conventional BDP to ZnP due to the π-conjugation in nonpolar toluene. In polar benzonitrile, however, additional electron transfer occurred along with energy transfer.

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INTRODUCTION Ensembles of different organic dyes that can undergo efficient energy transfer and photoinduced electron transfer has attracted much current interest not only for the development of solar-energy conversion system1−3 but also for the rational design of fluorescent sensors and optoelectronic devices.4 Boron dipyrromethene (BDP) fluorophores, which are derived from 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4adiaza-s-indacene,5 have been widely employed as light-harvesting antenna and electron donor or acceptor as well as laser dyes in a variety of applications, because a large number of BDP derivatives are readily obtained by modification at carbon positions 1, 3, 5, 7, and 8 exhibiting attractive photophysical and photochemical characteristics with strong absorption bands in the visible to near-infrared region.6−10 On the other hand, porphyrinoid assemblies act as lightharvesting antenna and electron donors in the electron-transfer cascade in the natural photosynthetic system. Porphyrin assemblies associated with other organic dyes have extensively been used as light-harvesting and electron donor−acceptor composites in artificial photosynthetic systems performing energy transfer and photoinduced electron transfer.11−16 In such porphyrin assemblies, the distances and the alignment among porphyrins and associated molecules significantly affect © 2012 American Chemical Society

efficiency and kinetics of energy transfer and electron transfer.17,18 Thus, in the case of porphyrins covalently linked with other organic dyes, the linker unit of porphyrin assemblies plays important roles to control the conformation of each molecule and photodynamics.14 A five-membered triazole ring, which is formed as a result of “click chemistry” between azide and alkyne precursors,19 is regarded as a significant π-conjugated linker unit, because photodynamics of porphyrin-containing assemblies is significantly affected by the position and electronic states of the triazole ring.20 In addition, it is known that the nitrogen atom of the triazole ring can coordinate to zinc ion of a zinc porphyrin to form a stable porphyrin dimer where π−π interaction between zinc porphyrins is observed.21 Such coordination of the triazole nitrogen atom to the zinc ion of porphyrin as well as π−π interaction between zinc porphyrins is expected to induce dramatic change in conformation, photophysical properties, and photodynamics of porphyrin-containing ensembles.17a,18 The conformation change of porphyrin assemblies bridged by triazole linker can be induced by change Received: January 13, 2012 Revised: March 13, 2012 Published: March 21, 2012 3889

dx.doi.org/10.1021/jp300415a | J. Phys. Chem. A 2012, 116, 3889−3898

The Journal of Physical Chemistry A

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of temperature. However, the temperature control of the π−π interaction between porphyrins associated with the coordination of triazole nitrogen atom to zinc ion of porphyrin has yet to be reported. We report herein synthesis of covalently linked BDP− porphyrin tweezers bridged by triazole linkers, which are shown in Scheme 1, to investigate the conformation change by

spectrometry (MALDI/TOF-MS). Absolute dichloromethane (CH2Cl2, Carlo Erba) for synthesis, CH2Cl2 and toluene spectroscopic grade (Nacalai Tesque, Inc.) for analysis, DABCO (Wako Pure Chemical Industries, Ltd.) were obtained commercially and used without further purification. Benzonitrile (PhCN) was purchased from Wako Pure Chemical Industries, Ltd., and distilled over P2O5 under reduced pressure. The precursors, 5-p-tolyldipyrromethane25 and 2,3,7,8,12,18hexamethyl-5-(4-azidomethylphenyl)-13,17-diethylporphyrinato-Zn (II) 3,26 were synthesized as already described. Physicochemical and Photophysical Measurements. 1 H NMR spectra were recorded on a Bruker Avance II 300 (300 MHz) or on a Bruker Avance DRX 500 (500 MHz) spectrometers at the “Welience (Pôle Chimic Moléculaire)”; chemical shifts are expressed in parts per million relative to chloroform (7.26 ppm). UV−vis spectra were recorded on a Varian Cary 1 spectrophotometer. Mass spectra and accurate mass measurements (HR−MS) were obtained on a Bruker Daltonics Ultraflex II spectrometer in the MALDI/TOF reflectron mode using dithranol as a matrix or on a Bruker MicroTofQ instrument in electrospray ionization (ESI) mode. Both measurements were made at the “Welience (Pôle Chimie Moléculaire)”. UV−vis−NIR spectra were recorded on a Shimadzu UV−3100PC spectrometer or a Hewlett-Packard 8453 diode array spectrophotometer at various temperatures. Measurement of fluorescence quantum yields were carried out on a Hamamatsu C9920−0X(PMA-12) U6039−05 fluorescence spectrofluorometer. The fluorescence spectra at various temperatures were measured on a SPEX Fluorolog τ3 fluorescence spectrophotometer. Theoretical Calculations. Density-functional theory (DFT) calculations were performed on a 32-processor QuantumCube. Geometry optimizations were carried out using the Becke3LYP functional basis set in Gaussian 09 program, revision A.02.27 The graphics were drawn using the Gauss View software program (version 5.0) developed by Semichem, Inc. Electrochemical Measurements. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were carried out with a ALS 630B electrochemical analyzer in a deaerated solvent containing 0.10 M TBAPF6 as a supporting electrolyte at 298 K. A conventional three-electrode cell was used with a platinum working electrode and a platinum wire as a counter electrode. The redox potentials were measured with respect to the Ag/AgNO3 (1.0 × 10−2 M) reference electrode. The potential values (vs Ag/AgNO3) are converted to those vs saturated calomel electrode (SCE) by adding 0.29 V.28 Laser Flash Photolysis. The studied compounds were excited by a Panther OPO pumped by Nd:YAG laser (Continuum, SLII-10, 4−6 ns fwhm) with the powers of 1.5 and 3.0 mJ per pulse. The transient absorption measurements were performed using a continuous xenon lamp (150 W) and an InGaAs-PIN photodiode (Hamamatsu 2949) as a probe light and a detector, respectively. The output from the photodiodes and a photomultiplier tube was recorded with a digitizing oscilloscope (Tektronix, TDS3032, 300 MHz). Femtosecond transient absorption spectroscopy experiments were conducted using an ultrafast source Integra-C (Quantronix Corp.), an optical parametric amplifier TOPAS (Light Conversion Ltd.), and a commercially available optical detection system Helios provided by Ultrafast Systems LLC. The source for the pump and probe pulses were derived from the fundamental output of Integra-C (780 nm, 2 mJ/pulse, and

Scheme 1

temperature and the photophysical properties and photodynamics. The BDP is functionalized to connect two zinc porphyrin entities through triazole linkers. In these BDP− porphyrin tweezers, two styryl groups are introduced at the 3and 5-positions of the BDP core. The resulting distyryl BDP Bz2 7, having a more extended π-conjugated system, can absorb and emit at longer wavelengths (ca. 650 nm) than porphyrin moieties. Therefore, distyryl BDP works as an energy acceptor rather than an energy donor in the present system. Although energy transfer from the BDP to the zinc porphyrin moiety has been extensively examined in the past decade, energy transfer in the opposite direction from the porphyrin to π-conjugated BDP has yet to be scrutinized.16,22−24 We performed femtosecond laser flash photolysis measurements of BDP−porphyrin tweezers to reveal the energy transfer and electron-transfer dynamics in both polar benzonitrile and nonpolar toluene.

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EXPERIMENTAL SECTION Chemicals and Reagents. Silica gel (Merck; 70−120 μm) was used for column chromatography. Analytical thin layer chromatography was performed using Merck 60 F254 silica gel (precoated sheets, 0.2 mm thick). Reactions were monitored by thin-layer chromatography, UV−vis spectroscopy, and matrixassisted laser desorption−ionization time of flight mass 3890



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tolyldipyrromethane, and 627 mg (5.21 mmol) of ptolualdehyde dissolved in 250 mL of deaerated CHCl3. The reaction mixture was stirred for 15 min and shielded from light, before dropwise addition of 500 μL (3.98 mmol) of boron trifluoride diethyl etherate, BF3·OEt2. Stirring was continued for 1 h 30 min. p-Chloranil (2.6 g, 10.5 mmol) was then added, and the reaction mixture was kept under stirring for 2 h. After evaporation of the solvent under reduced pressure, the residue was redissolved in a CH2Cl2/MeOH mixture (95:5) and filtered over a pad of silica to remove any polar materials. The first fraction was isolated; the solvent was evaporated under vacuum and redissolved in 250 mL of chloroform. Zinc acetate dihydrate (2.22 g, 10.0 mmol) and sodium acetate (2.4 g, 30.0 mmol) dissolved in 10 mL of absolute methanol were added, and the solution was then heated under reflux for 2 h, shielded from light. The metalation reaction was monitored by UV−vis and MALDI/TOF-MS spectrometry. The reaction mixture was then cooled to room temperature and washed three times with 750 mL of deionized water. The organic phase was dried over magnesium sulfate and evaporated under reduced pressure. After purification by column chromatography over silica using CH2Cl2/heptane as eluent (70:30), the titled zinc porphyrin 4 was isolated in 11% yield (910 mg, 1.17 mmol) as a purple solid. 1H NMR (Pyr D5, 298 K, 300 MHz): δ (ppm) = 2.60 (s, 9H, Ph-CH3), 4.76 (s, 2H, Ar-CH2-N3), 7.56 (d, J = 8.1 Hz, 6H, Htolyl), 7.72 (d, J = 7.8 Hz, 2H, Ar-CH2−N3), 8.31 (d, J = 8.1 Hz, 6H, Htolyl), 8.47 (d, J = 7.8 Hz, 2H, Ar-CH2−N3), 9.20 (m, 8H, Hβ). MS (MALDI/TOF): m/z = 773.42 [M]+•, 773.22 calcd for C48H35N7Zn. HR−MS (ESI): m/z = 773.2271 [M]•+, 773.2240 calcd for C48H35N7Zn. UV−vis (CH2Cl2): λmax (nm) (ε, 10−3 M−1 cm−1) = 420 (416), 550 (15), 592 (5). 3,5-(8,12-Diethyl-2,3,7,13,17,18-hexamethyl-20para((4-((4-vinylphenoxy)methyl)-triazole)-N-benzyl)zinc bisporphyrin)-2,6-diethyl-1,7-dimethyl-8-(p-methyl benzoyl)-BDP (5) (BDP β-Por2). Under argon and shielded from light, BDP dialkyne 2 (39.9 mg, 0.051 mmol) and zinc porphyrin 3 (86.1 mg, 0.133 mmol) were dissolved in 20 mL of dichloromethane. Copper iodide (10 mg, 0.053 mmol) and diisopropylethylamine (DIPEA, 20 μL, 0.116 mmol) were then added, and the mixture was stirred at room temperature for 7 h. The reaction mixture was diluted by addition of 100 mL of dichloromethane. The organic phase was washed 3 times with 100 mL of distilled water, dried over magnesium sulfate, and evaporated under reduced pressure. The crude product was dissolved in CH2Cl2 and purified by column chromatography over silica (CH2Cl2/CH3CN (90:10)). After evaporation of the solvent, the title compound 5 was isolated as a blue microcrystalline solid in 46% (47 mg, 0.023 mmol). 1H NMR (Pyridine-D5, 298 K, 300 MHz): δ (ppm) = 1.01 (t, J = 7.5 Hz, 6H, −CH2−CH3 bodipy), 1.33 (s, 6H, −CH3 bodipy), 1.89 (t, J = 7.5 Hz, 12H, −CH2−CH3 porph), 2.36 (s, 12H, −CH3 porph), 2.58 (q, J = 7.5 Hz, 4H, −CH2-CH3 bodipy), 3.57 ppm (s, 12H, −CH3 porph), 3.64 (s, 12H, −CH3 porph), 3.95 (s, 3H, −CO− O−CH3), 4.15 (q, J = 7.5 Hz, 8H, −CH2-CH3 porph), 5.50 (s, 4H, Ar−CH2-triazole), 5.78 (s, 4H, −O−CH2-triazole), 7.26 (d, J = 8.6 Hz, 4H, triazole-CH2-Ar-porph), 7.40 (d, J = 7.9 Hz, 4H, −CHCH-Ar-O), 7.52 (d, J = 8.5 Hz, 2H, bodipy-ArCOOMe), 7.61 (d, J = 16.4 Hz, 2H, bodipy-CHCH-Ar), 7.86 (d, J = 8.6 Hz, 4H, triazole-CH2-Ar-porph), 7.93 (d, J = 7.9 Hz, 4H, −CHCH-Ar-O), 8.10 (s, 2H, Htriazole), 8.35 (d, J = 8.5 Hz, 2H, bodipy-Ar-COOMe), 8.41 (d, J = 16.4 Hz, 2H, bodipy-CHCH-Ar), 10.41 (s, 2H, Hmeso), 10.48 ppm (s, 4H, Hmeso). See Figure S1 in Supporting Information for 1H NMR

fwhm =130 fs) at a repetition rate of 1 kHz. 75% of the fundamental output of the laser was introduced into TOPAS, which has optical frequency mixers resulting in tunable range from 285 to 1660 nm, while the rest of the output was used for white light generation. Typically, 2500 excitation pulses were averaged for 5 s to obtain the transient spectrum at a set delay time. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. All measurements were conducted at 298 K. The transient spectra were recorded using fresh solutions in each laser excitation. 2,6-Diethyl-1,3,5,7-tetramethyl-8-(p-methylbenzoyl)BDP (1). 2,4-Dimethyl-3-ethylpyrrole (1.72 g, 14 mmol) and methyl 4-formylbenzoate (1.16 g, 7 mmol) were dissolved in 700 mL of CH2Cl2. Trifluoroacetic acid (0.03 mL) was added, and the mixture was stirred at room temperature for 24 h. A solution of DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone, 1.58 g, 7 mmol) in CH2Cl2 was added to the mixture. Stirring was continued for 50 min, followed by the addition of 17 mL of triethylamine and 17 mL of BF3·OEt2, and the reaction mixture turned purple. After stirring for 1 h, the reaction mixture was washed with water and dried over sodium sulfate, and the solvent was evaporated. The residue was purified by silica gel column chromatography using CH2Cl2/hexane (5:1) as eluent and recrystallized in a mixture of CH2Cl2 and hexane to give compound 1 as a red solid in 40% yield (1.2 g, 2.7 mmol). 1H NMR (CDCl3, 298 K, 300 MHz): δ (ppm) = 0.95 (t, J = 7.5 Hz, 6H, −CH2−CH3), 1.23 (s, 6H, CH3), 2.26 (q, J = 7.5 Hz, 4H, −CH2-CH3), 2.51 (s, 6H, −CH3), 3.95 (s, 3H, -OCH3), 7.38 (d, J = 8.3 Hz, 2H, Ar-COOMe), 8.15 (d, J = 8.3 Hz, 2H, Ar-COOMe). 11B NMR (CDCl3, 298 K, 128 MHz,): δ (ppm) = 0.78 ppm (t, J = 33.2 Hz). MS (ESI): m/z = 439.23 [M+H]+, 461.22 [M+Na]+, 899.44 [2M+Na] +, 439.24 calcd for C25H30BF2N2O2. UV−vis (CH3CN): λmax (nm) (ε, 10−3 M−1 cm−1) = 524 (86.1), 496 (30.4), 379 (9.1). Anal. Calcd for C24H29BF2N2O2 + 0.1 CH2Cl2: C, 67.47; H, 6.59; N, 6.27; Found: C, 67.95; H, 6.90; N, 6.15. 1,9-(4-((4-Vinylphenoxy)methyl)-ethynyl)-2,6-diethyl1,7-dimethyl-8-(p-methylbenzoyl)-BDP (2). Compound 1 (1.0 g, 2.32 mmol) and 4-(prop-2-yn-1-yloxy)-benzaldehyde (1.1 g, 6.96 mmol) were refluxed in a mixture of toluene (50 mL), glacial acetic acid (1 mL), and piperidine (1.3 mL). Any water formed during the reaction was removed by heating overnight in a Dean−Stark apparatus. The solvent was evaporated, and the crude solid was purified by silica gel column chromatography (first EtOAc/Hexane (90:10) and then CH2Cl2/MeOH (99:1)). The blue fraction was collected, and the solvent was evaporated. The solid was washed three times with 50 mL of diethyl ether to give 2 as a solid (350 mg, 21%). 1H NMR (CDCl3, 298 K, 500 MHz): δ (ppm) = 1.13 (t, J = 7.5 Hz, 6H, −CH2−CH3), 1.27 (s, 6H, −CH3), 2.53 (t, J = 2.4 Hz, 1H, −CCH), 2.57 (q, J = 7.5 Hz, 4H, −CH2-CH3), 3.97 (s, 3H, -OCH3), 4.73 (d, J = 2.4 Hz, 2H, −CH2-CCH), 7.00 (d, J = 8.8 Hz, 4H, −CHCH-Ar-O), 7.20 (d, J = 17.2 Hz, 2H, −CHCH-Ar), 7.42 (d, J = 8.2 Hz, 2H, Ar-COOMe), 7.57 (d, J = 8.8 Hz, 4H, −CHCH-Ar-O), 7.65 (d, J = 17.2 Hz, 2H, −CHCH-Ar), 8.17 (d, J = 8.2 Hz, 2H, Ar-COOMe). HR− MS (ESI): m/z = 745.2984 [M+Na]+, 745.3019 calcd for C45H41BF2N2NaO4. UV−vis (CH2Cl2): λmax (nm) (ε, 10−3 M−1 cm−1) = 652 (80), 422 (10), 363 (6.1). 5-(4-(Azidomethyl)phenyl)-10,15,20-tritolylporphyrin Zinc (4). In a 500-mL three-neck flask equipped with a condenser and N2 bubbling were added 2.56 g (15.9 mmol) of 4-azidomethylbenzaldehyde, 5.01 g (21.2 mmol) of 5-p3891



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spectrum.29 MS (MALDI/TOF): m/z = 2008.65 [M]•+, 2008.79 calcd for C119H115BF2N16O4Zn2. HR−MS (ESI): m/ z = 1004.3966 [M]2+, 1004.3969 calcd for C119H115BF2N16O4Zn2 (see Figure S2 in Supporting Information). UV−vis (CH2Cl2): λmax (nm) (ε, 10−3 M−1 cm−1) = 406 (380), 416 (400), 547 (70), 584 (68), 665 (110). 3,5-(5,10,15-Tritolyl-20-para((4-((4-vinylphenoxy)methyl)-triazole)-N-benzyl) zinc bisporphyrin)-2,6-diethyl-1,7-dimethyl-8-(p-methylbenzoyl)-BDP (6). (BDP meso-Por2). The title compound was obtained as a blue solid in 96% yield (65 mg, 0.028 mmol), as described for 5, starting from BDP dialkyne 2 (41 mg, 0.029 mmol) and zinc porphyrin 4 (50 mg, 0.064 mmol). For this coupling reaction, 15 mg (0.078 mmol) of copper iodide and 23 μL (0.133 mmol) of DIPEA were used. 1H NMR (Pyridine-D5, 298 K, 300 MHz): δ (ppm) = 1.09 (t, J = 7.5 Hz, 6H, −CH2−CH3 bodipy), 1.33 (s, 6H, −CH3 bodipy), 2.58 (s, 18H, Ar−CH3 tolyl), 2.58 (4H, −CH2-CH3 bodipy), 3.94 (s, 3H, −CO−O−CH3), 5.46 (s, 4H, Ar−CH2-triazole), 5.93 (s, 4H, −O−CH2-triazole), 7.19 (d, J = 8.9 Hz, 4H, triazole-CH2-Ar-porph), 7.52 (m, 18H, HAr), 7.64 (d, J = 8.0 Hz, 4H, −CHCH-Ar-O), 7.77 (d, J = 8.9 Hz, 4H, triazole-CH2-Ar-porph), 8.31 (m, 18H, HAr), 8.39 (s, 2H, Htriazole), 9.00−9.25 (m, 16H, Hbeta). MS (MALDI/TOF): m/z = 2249.73 [M-F]+, 2249.76 calcd for C141H111BFN16O4Zn2. HR−MS (ESI): m/z = 2291.7399 [M+Na]+, 2291.7529 calcd for C 141 H 111 BF 2 N 16 O 4 Zn 2 Na, 1157.3613 [M+2Na] 2+ , 1157.3711 calcd for C141H111BF2N16O4Zn2Na2 (see Figure S3 in Supporting Information). UV−vis (CH2Cl2): λmax (nm) (ε, 10−3 M−1 cm−1) = 366 (50), 430 (578), 562 (32), 603 (34), 658 (46). 3,5-(4-((4-Vinylphenoxy)methyl)-triazole)-N-benzyl2,6-diethyl-1,7-dimethyl-8-(p-methylbenzoyl)-BDP (7) (BDP Bz2). The title compound was obtained as a blue solid in 63% yield (20 mg, 0.020 mmol), as described for 5, starting from 23 mg of BDP dialkyne 2 (0.032 mmol) and 9.5 mg of benzyl azide (0.071 mmol). For this coupling reaction, 18 mg (0.094 mmol) of copper iodide and 25 μL (0.145 mmol) of DIPEA were used. 1H NMR (Pyridine-D5, 298 K, 300 MHz): δ (ppm) = 1.10 (t, J = 7.5 Hz, 6H, −CH2−CH3 bodipy), 1.35 (s, 6H, −CH3 bodipy), 2.58 (q, J = 7.5 Hz, 4H, −CH2-CH3 bodipy), 3.94 (s, 3H, −CO−O−CH3), 5.39 (s, 4H, Ar−CH2-triazole), 5.70 (s, 4H, −O−CH2-triazole), 7.12 (d, J = 8.7 Hz, 4H, −CHCH-Ar-O), 7.35 (m, 10H, Hbenzyl), 7.53 (d, J = 15.8 Hz, 2H, bodipy-CHCH-Ar), 7.54 (d, J = 8.0 Hz, 2H, bodipy-ArCOOMe), 7.72 (d, J = 8.7 Hz, 4H, −CHCH-Ar-O), 8.18 (s, 2H, Htriazole), 8.31 (d, J = 15.8 Hz, 2H, bodipy-CHCH-Ar), 8.34 (d, J = 8.0 Hz, 2H, bodipy-Ar-COOMe). MS (MALDI/ TOF): m/z = 988.21 [M]•+, 988.44 calcd for C59H55BF2N8O4, 969.23 [M-F]+, 969.44 calcd for C59H55BFN8O4. HR−MS (ESI): m/z = 988.4530 [M+H]+ , 988.4516 calcd for C59H56BF2N8O4 (see Figure S4 in Supporting Information). UV−vis (CH2Cl2): λmax (nm) (ε, 10−3 M−1 cm−1) = 368 (51), 658 (66).

distyryl derivatives corresponding to one and two condensations. Interestingly, this condensation reaction provides an easy way to functionalize the BDP unit.7 Styryl-BDP diporphyrins were easily prepared from starting BDP 1 in only two steps involving a Knoevenagel condensation8b,31 followed by a Huisgen reaction (also wellknown as “Click Chemistry”)32 as described in Scheme 2. We recently reported the efficiency of click chemistry for the elaboration of new porphyrin-tripods.26 Scheme 2

The first coupling step, the Knoevenagel condensation, was driven by the continuous removal of the water formed during the reaction, using a Dean−Stark apparatus. The key precursor 2 was obtained in 21% yield after purification by silica gel column chromatography. The two porphyrin subunits were further attached to the resulting blue BDP by CuI-catalyzed cycloaddition. The resulting biszinc β- and meso-substituted diporphyrins, 5 and 6, were easily obtained in 47 and 96% yield, respectively. HR-MS ESI-QTOF measurements were carried out to fully characterize the blue-BDP porphyrin derivatives. In both cases, the perfect match between experimental and simulated ionic patterns undoubtedly confirmed the structure of diporphyrins 5 and 6 (see Figures S2 and S3 in Supporting Information for HR-MS spectra). In the case of the β-substituted diporphyrin 5, the calculated mass for the dication [M]2+ is equal to 1004.3969 Da (C119H115BF2N16O4Zn2), agreeing well with the experimental value found at 1004.3966 Da, [M]2+. The 1H NMR spectrum of the diporphyrin 5 is informative (see Figure S1 in

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RESULTS AND DISCUSSION Synthesis and Characterization. Styryl-BDP derivatives have been prepared by direct condensation of 3,5-dimethylBDPs with aromatic aldehydes to yield red emitting fluorophores.6−8 Indeed, it has been shown that 3- and 5methyl BDP-substituents are acidic enough to participate in Knoevenagel reactions.7 With p-dialkylaminobenzaldehyde, the reaction can be restricted to one condensation,8d,30 but 4alkoxybenzaldehydes tend to give a mixture of mono- and 3892



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Supporting Information). Indeed, the absence of any signals (1H) in the 2−3 ppm range corresponding to the −CCH ethynyl proton and the presence of a singlet at 8.10 ppm (1H) are both in agreement with the formation of the triazolic linking unit. In the case of diporphyrin 5, the α-methylene protons of the azido group are shifted downfield from 4.76 ppm (in CDCl3) to 5.50 ppm (in pyridine-D5) due to the formation of the triazole macrocycle and the same trend is also observed for the singlet signal of the -CH2-CCH protons which are shifted from 4.73 ppm (in CDCl3) to 5.78 ppm (in pyridine-D5). A reference compound 7 (BDP Bz2) was also prepared for comparison in 63% yields by the reaction of BDP dialkyne 2 and benzyl azide (see Experimental Section). Energy Transfer from Porphyrin to distyryl BDP. Figure 1 shows UV−vis spectra of BDP, BDP Bz2, BDP meso-

Figure 2. Fluorescence spectra of (a) BDP β-Por2 and (b) BDP mesoPor2 in CH2Cl2 at 293 K. The excitation wavelengths of BDP β-Por2 and BDP meso-Por2 are 540 and 550 nm, respectively.

energy transfer from the BDP to the porphyrin moiety is observed in BDP−porphyrin dyad, because the fluorescence emission of typical BDP is overlapped with the Q-bands of porphyrin.16,22,23 The inverse energy transfer shown in this study results from the π-elongation of BDP at 3,5-positions. The fluorescence emission of porphyrin moieties is overlapped with the absorption of π-conjugated BDP, particularly in the case of BDP meso-Por2 (meso-substituted porphyrin). UV−vis spectra of BDP β-Por2 and BDP meso-Por2 in CH2Cl2 at different temperatures are shown in Figure 3. The split Soret band of BDP β-Por2 is combined into a single band at 417 nm upon lowering the temperature from 298 to 223 K. The spectrum at 223 K is similar to what is observed in benzonitrile (see Figure S6 in Supporting Information) where little interaction between porphyrins is expected. The absorbance of both the Soret and Q bands of BDP meso-Por2 increases with decreasing temperature. These results indicate that the two porphyrins of BDP β-Por2 and BDP meso-Por2 behave independently at low temperature, which may result from coordination of the N atoms of triazole rings to the Zn ions of porphyrins.21 The sharper absorption band of the πconjugated BDP unit (λmax = 660 nm) at lower temperature also suggests such coordination mode to reduce the conformational fluctuations of π-conjugated BDP. Similar UV−vis spectral changes of BDP β-Por2 were observed by the addition of 1,4-diazabicyclo[2.2.2]octane (DABCO) as shown in Figure S9 in Supporting Information, which again indicates that the coordination of N atoms of DABCO to two Zn ions of porphyrin tweezers affects the conformational flexibility of BDP units in addition to the porphyrin units. The optimized structure of BDP meso-Por2 obtained by DFT calculations is shown in Figure 4. The N atoms of triazole rings coordinate to Zn ions of porphyrins. This intramolecular coordination mode is similar to intermolecular coordination mode of two triazole−porphyrins in a crystal.21 Such closed conformation of BDP meso-Por2 is more stable by 23 kcal mol−1 than the open form where no intramolecular interaction is expected (see Figure S10 in Supporting Information).

Figure 1. UV−vis spectra of BDP (black), BDP Bz2 (green), BDP meso-Por2 (red), and BDP β-Por2 (blue) at 3.0 × 10−6 M in CH2Cl2 at 298 K.

Por2, and BDP β-Por2 in CH2Cl2 at 298 K. Comparison of each spectrum reveals that the absorption bands at around 660 nm of BDP Bz2, BDP meso-Por2, and BDP β-Por2 are attributed to π-conjugated BDP moieties. These characteristic absorption bands at longer wavelength than those typically observed for BDP (∼530 nm) result in green or blue color of BDP Bz2, BDP meso-Por2, and BDP β-Por2 (see a picture in Figure S5 in Supporting Information). Splitting of the Soret band of BDP βPor2 in CH2Cl2 indicates the π-electron overlap between porphyrins in slipped cofacial conformation.14 Such splitting was also observed in other noncoordinating solvents such as CHCl3 and toluene as shown in Figure S6 in Supporting Information. No splitting of the Soret band of BDP meso-Por2 was observed (Figure 1), indicating that there is less π-electron overlap between porphyrin rings in BDP meso-Por2 as compared to that in BDP β-Por2 because of the bulkiness of the meso substituents. Fluorescence spectra and absolute fluorescence quantum yields (Φ) of a series of BDPs and porphyrins are shown in Figure 2 and Figure S7 in Supporting Information. Even when porphyrin units of BDP meso-Por2 and BDP β-Por2 are excited, the fluorescence emission due to BDP moieties are mainly observed. In addition, the excitation spectrum of BDP β-Por2 clearly shows the contribution of porphyrin units to the emission of BDP at 680 nm (Figure S8 in Supporting Information). These observations highlight that energy transfer from the porphyrin to the BDP moiety occurs.33 Generally, 3893



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Figure 4. Optimized structure of BDP meso-Por2 based on DFT calculations at the B3LYP/6-31G(d) level: (a) top view and (b) side view. Hydrogen atoms are not shown for clarity.

Once the fluorescence emission decreases upon lowering the temperature from 293 to 258 K, the fluorescence due to the BDP unit is significantly enhanced from 258 to 198 K. The plot of fluorescence intensity at 680 nm corresponds to the

Figure 3. Temperature dependence of UV−vis spectra of (a) BDP βPor2 and (b) BDP meso-Por2 in CH2Cl2.

Temperature dependence of fluorescence spectra of BDP βPor2 and BDP meso-Por2 was also investigated as shown in Figures 5 and 6. Each isosbestic point in the temperature dependence of absorption spectra (Figure 3) was chosen as an excitation wavelength (540 and 550 nm for BDP β-Por2 and BDP meso-Por2, respectively) to excite the porphyrin units selectively. In the case of BDP β-Por2, the fluorescence due to the porphyrin unit at 605 nm increases, while the fluorescence due to the BDP unit at 676 nm decreases with lowering temperature (Figure 5a). The increase in the fluorescence emission of the porphyrin may result from (i) the increase in the viscosity of CH2Cl2 to depress thermal fluctuation34 and (ii) the coordination of the triazole nitrogen atom to the zinc ion of porphyrin to decrease forbidden transition caused by the face-to-face porphyrin interaction as illustrated in Scheme 3.14,35 The decrease in fluorescence emission of BDP indicates that the efficiency of energy transfer from porphyrin to BDP decreases because the conformational rigidity at low temperature makes porphyrin and BDP units difficult to come close to each other. The plot of fluorescence intensities at 604 and 676 nm corresponds to the absorbance change at 667 nm (parts b and c of Figures 5). On the other hand, the fluorescence spectra of BDP mesoPor2 at different temperatures change in two steps (Figure 6).

Figure 5. (a) Temperature dependence of fluorescence spectra of BDP β-Por2 in CH2Cl2 from 293 to 218 K. The excitation wavelength is 540 nm. (b) Plot of fluorescence changes at 604 and 676 nm. (c) Plot of absorbance change at 667 nm. 3894



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Figure 7. Plot of absorbance change at 563 nm and fluorescence change at 680 nm of BDP meso-Por2 in CH2Cl2. Figure 6. Temperature dependence of fluorescence spectra of BDP meso-Por2 in CH2Cl2: (a) from 293 to 258 K and (b) from 258 to 198 K. The excitation wavelength is 550 nm.

Table 1. One-Electron Redox Potentials (V vs SCE), Energy Levels of the Charge-Separated States (ΔGCR), and Free Energy Changes for Charge Separation (ΔGCS) for BDP, BDP Bz2, BDP β-Por2, and BDP meso-Por2 in PhCN

Scheme 3

redox potential, V (vs SCE) compound BDP BDP Bz2 BDP β-Por2 BDP mesoPor2

Por/ Por•+

BDP/ BDP•+

BDP/ BDP•−

−ΔGCR, eVa

−ΔGCS, eVb

0.50 0.58

0.98 0.69 0.70 0.68

−1.31 −1.04 −1.04 −1.05

1.54 1.63

0.54 0.40

ΔGCR = e(Eox − Ered). The electrostatic term can be neglected in a polar solvent (PhCN). b−ΔGCS = ΔE0−0 − ΔGCR, where ΔE0−0 is the energy of the lowest excited states of BDP 1β-Por2* (2.08 eV) and BDP 1meso-Por2* (2.03 eV). a

compounds in benzonitrile (PhCN). The redox data are summarized in Table 1. The BDP unit exhibits both oneelectron oxidation and reduction processes. The elongation of π-conjugation of BDP unit (BDP Bz2, BDP β-Por2, and BDP meso-Por2) makes their one-electron oxidation potentials lower and their one-electron reduction potentials higher than those of BDP. The first one-electron oxidation potential of porphyrin unit is lower than that of BDP unit. Thus, the first one-electron oxidation of BDP-Por2 5 and 6 occurs at the porphyrin unit, while the first one-electron reduction occurs at the BDP unit. These results collectively suggest that zinc(II) porphyrins being electron rich and BDP being electron deficient in these molecular systems. The molecular orbitals of BDP meso-Por2 based on DFT calculations (B3LYP/6-31G(d) basis set) are shown in Figure S13 in Supporting Information. The highest occupied molecular orbital (HOMO) and the HOMO−1 are delocalized over the porphyrin moieties, while the lowest unoccupied molecular orbital (LUMO) is localized to the BDP framework. This result is consistent with the electrochemical data of BDP meso-Por2 shown in Table 1 and Figure S12 in Supporting Information. The energy levels of the charge-separated states are also listed in Table 1. By comparing these energy levels of the charge-separated states with the energy levels of the excited states, the driving forces of charge separation (ΔGCS) were also evaluated.

absorbance change at 563 nm (Figure 7). Only the increase in fluorescence emission is observed for BDP and BDP Bz2 with decreasing the temperature (see Figure S11 in Supporting Information). These results indicate that the conformational change of BDP meso-Por2 causes decreased fluorescence emission in the same manner as BDP β-Por2, while restriction of the free rotation of phenyl rings around the BDP unit causes increased fluorescence emission in the same manner as BDP and BDP Bz2.34 The latter is expected to be dominant below 245 K to increase the fluorescence intensity of BDP unit. Such absorption/fluorescence spectral changes can be observed only in this type of BDP−porphyrin tweezer composites where energy transfer from the porphyrin to the BDP unit occurs efficiently. Electrochemistry and Energy Levels. To establish the energy levels of BDP−porphyrin tweezers, electrochemical studies using cyclic voltammetry were performed. Figure S12 in Supporting Information shows CV voltammograms of the BDP β-Por2 5 and BDP meso-Por2 6 as well as the reference 3895



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Photodynamics of BDP meso-Por2 and BDP β-Por2. Photodynamics of BDP meso-Por2 and BDP β-Por2 were then investigated. Time-resolved transient absorption spectra of BDP meso-Por2 and BDP β-Por2 were recorded by femtosecond and nanosecond laser flash photolysis in deaerated solvents. The femtosecond transient absorption spectra of the ZnP references (see Figures S14 and S15 in Supporting Information) revealed the instantaneous formation of the ZnP singlet-excited state features. Here, the transient absorption spectra exhibited the absorption bands in the visible region with a maximum at 460 nm, which decayed slowly (4.0 × 108 s−1) to populate the corresponding triplet manifold with a maximum absorption at 480 nm. Upon excitation of BDP meso-Por2 at 426 nm in toluene, the absorption spectrum of BDP β-Por2 at 1 ps after the laser pulse shows characteristic bands assigned to the singlet excited state of the porphyrin (Figure 8). With time, the ZnP singlet emission peak at 460 nm

ZnP unit to the singlet ground state of BDP in a polar media (kENT = 6.2 × 1011 s−1). Interestingly in benzonitrile, the energy transfer product of the BDP meso-Por2 revealed faster decay than that observed for the BDP control compound (Figure 9

Figure 9. Differential absorption spectra obtained upon femtosecond flash photolysis (426 nm) of BDP β-Por2 6 in benzonitrile at 2 and 55 ps. Inset: Time profile of differential absorbance at 670 nm.

and Figure S16 in Supporting Information). These results suggest occurrence of subsequent electron transfer from the ZnP to the 1BDP* moiety to produce the charge-separated product (ZnP•+−BDP•−), because an alternative pathway, i.e., energy transfer from 1BDP* to ZnP is energetically not feasible. On the basis of the decay rates of the 1BDP* of BDP meso-Por2 and the control BDP, the rate constant of the charge-separation process (kCS) for BDP β-Por2 was determined to be 9.2 × 109 s−1. Similar observations were also made for BDP meso-Por2 in benzonitrile as shown in Figure S18 in Supporting Information, where energy transfer from 1ZnP* to BDP (kENT) followed by electron transfer from ZnP to 1BDP* (kCS) was clearly observed. The kENT and kCS values were determined to be 1.3 × 1011 and 1.3 × 109 s−1, respectively. The energy diagram of BDP β-Por2 and BDP meso-Por2 are shown in Scheme 4. It should be noted that the strong overlap of the ZnP radical cation with the strong bleaching of the BDP entity has precluded the accurate determination of the rates of charge recombination of BDP•− β-Por2•+ and BDP•− mesoPor2•+ in polar benzonitrile. The complementary nanosecond transient absorption measurements of BDP meso-Por2 6 and BDP β-Por2 5 in polar benzonitrile with 550-nm laser excitation, which selectively excited the ZnP moiety, exhibited no absorption bands of the radical species suggesting that the charge recombination process via the singlet porphyrin is too fast to be detected in the nanosecond region (Figures S19−S21 in Supporting Information). On the basis of the energy levels of the CS states and the triplet states of BDP 7 and ZnP control compounds, one can expect that the radical species decay rapidly to populate the low-lying triplet BDP 7 (1.15 eV) as well as the ground states.36,37 Indeed, the transient absorption spectra of BDP meso-Por2 6 in benzonitrile exhibited weak absorption bands in the visible region with maxima at around 490 nm (Figure S21 in Supporting Information). These

Figure 8. Differential absorption spectra obtained upon femtosecond flash photolysis (426 nm) of BDP β-Por2 in toluene at indicated time intervals. Inset: Time profile of the singlet-excited state (1BDP* βPor2) at 670 nm.

diminished in intensity with concomitant increase of the BDP emission at 670 nm providing direct evidence of excitation transfer from the photoexcited singlet ZnP unit to the singlet ground state of BDP. The rate constant of energy transfer (kENT) from 1ZnP* to the BDP moiety was determined by exponential fitting of the decay of 1ZnP* and the rise of 1BDP* to be 8.5 × 1011 s−1. Further, the decay rate constant of 1BDP* was determined to be 4.4 × 109 s−1, which is close to the decay of the singlet excited state of BDP Bz2 control compound (see Figure 16 in Supporting Information). This indicates that the 1 BDP* is not quenched by the attached ZnP in toluene. Similar observations were also made for the investigated BDP mesoPor2 in toluene (Figure 17 in Supporting Information). The energy transfer from 1ZnP* to BDP (kENT) was clearly observed with a rate constant of 3.1 × 1011 s−1, which is considerably smaller than that of BDP β-Por2. By changing the solvent from toluene to benzonitrile, the transient absorption spectra of BDP β-Por2 exhibited diminished ZnP singlet emission peak at 458 nm with the concomitant increase of the BDP emission at 669 nm providing direct evidence of energy transfer from the photoexcited singlet 3896



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Scheme 4

temperature dependence of fluorescence spectra of BDP and BDP Bz2 (S11), cyclic voltammograms (S12), molecular orbitals of BDP meso-Por2 based on DFT calculations (S13), transient absorption spectra upon femtosecond laser excitation (S14−S18), and transient absorption spectra upon nanosecond laser excitation (S19−S21). This material is available free of charge via the Internet at http://pubs.acs.org.

absorption bands can be assigned to the triplet-excited state of BDP 2, which may populate from the charge recombination in BDP•− meso-Por2•+. The decay rate constant was determined to be 2.70 × 104 s−1. It should be noted that the electron transfer from the triplet porphyrin to BDP is not expected due to the extremely fast and efficient energy transfer from the singlet ZnP to the attached π-conjugated BDP entity.

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CONCLUSIONS The present study has revealed spectroscopic properties and photodynamics of a new class of blue π-conjugated BDP−zinc porphyrin tweezer composites. Efficient energy transfer from the photoexcited singlet zinc porphyrin to the π-conjugated BDP moiety of the composites occurred in opposite direction due to the π-conjugation as compared to energy transfer from the singlet excited state of conventional BDP to zinc porphyrins as indicated by the fluorescence spectra. The efficiency of energy transfer was changed by the conformation change due to coordination of the N atoms of triazole rings to the Zn ions of porphyrins at low temperature. The dynamics of energy transfer from the photoexcited singlet porphyrin to the BDP moiety of 5 and 6 in nonpolar toluene to populate the 1BDP* was revealed by femtosecond laser flash photolysis measurements. When the solvent was changed to more polar benzonitrile, the deactivation pathway of the energy transfer product, 1BDP* was changed to electron transfer involving ZnP to generate the charge separation product, BDP•−−Por2•+. These results provide new strategy for the design toward photoswitching devices, which can be controlled by the conformation change depending on temperature.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.F.); Jean-Michel. [email protected] (J.-M.B.). Present Address

∥ Organic Materials Group, Polymer Materials Unit, National Institute for Materials Science (NIMS), 1−2−1 Sengen, Tsukuba 305−0047, Japan

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Grants-in-Aid (Nos. 20108010 and 23750014) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan, and KOSEF/MEST through WCU project (R31-2008-000-10010-0) of Korea. A.T. appreciates a support from the Global COE program ″Global Education and Research Center for Bio-Environmental Chemistry″ of Osaka University (S.F.) and JSPS fellowship for young scientists. The Centre National de Recherche Scientifique (CNRS, UMR 6302) and “Région Bourgogne” are also acknowledged for funding.

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ASSOCIATED CONTENT

S Supporting Information *

REFERENCES

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