TripletâTriplet Excitation Transfer in Palladium PorphyrinâFullerene

Article pubs.acs.org/JPCC

Triplet−Triplet Excitation Transfer in Palladium Porphyrin−Fullerene and Platinum Porphyrin−Fullerene Dyads Christopher O. Obondi, Gary N. Lim, and Francis D’Souza* Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017, United States S Supporting Information *

ABSTRACT: Covalently linked donor−acceptor dyads involving palladium(II) and platinum(II) porphyrins as triplet sensitizers and fullerene as an acceptor have been newly synthesized. These dyads were characterized by optical absorbance, emission, and electrochemical methods. In contrast to the earlier reported zinc(II) porphyrin and free-base porphyrin-based dyads of similar structures, photoinduced electron transfer from the short-lived singlet and long-lived triplet excited metalloporphyrin to the fullerene was not observed, although these processes are energetically possible according to the energy level diagrams. That is, diagnostic transient bands corresponding to MP•+ [M = Pd(II) or Pt(II)] in the 600−650-nm range and C60•− in the 1000-nm range were absent in the femtosecond and nanosecond transient absorption spectra. Interestingly, excited energy transfer from the triplet excited metalloporphyrin to the fullerene was witnessed in both palladium and platinum porphyrin-derived dyads by nanosecond transient absorption studies. Three solvents with different polarities were employed to visualize the medium effects. The determined rate of energy transfer, kEnT, was found to be higher for the PdP-based dyad than the PtP-based dyad in a given solvent and that the rates were higher for polar solvents than for nonpolar solvent. The present investigation demonstrates how the heavy-metal ion in the porphyrin cavity modulates photoinduced processes and the solvent-dependent kinetics of these events.

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yields.45 Such luminescent sensitizers have found applications in the fields of photocatalysis,46,47 photon upconversion,48,49 electroluminescence,50−52 phosphorescent bioimaging and sensing,53,54 photodynamic therapy (PDT, through the sensitization of triplet oxygen to form singlet oxygen, 1O2),55−57 photoinitiated polymerization,46,47 and donor−acceptor systems undergoing photoinduced energy- and electron-transfer events.45,58 These applications signify the importance of triplet sensitizers in almost all areas of photochemistry. In the present study, we have built covalently linked dyads involving palladium(II) and platinum(II) porphyrins as triplet sensitizers and fullerene as an acceptor (Figure 1). Unlike their zinc(II) and free-base porphyrin-based analogues,59−61 where efficient photoinduced electron transfer leading to the formation of a charge-separated state was witnessed, in the case of both palladium porphyrin−fullerene and platinum porphyrin−fullerene dyads, excitation transfer from the triplet excited porphyrin to the fullerene was observed, as revealed by transient pump− probe techniques operating in the femto- and nanosecond time regimes. Systematic spectral and electrochemical studies were performed to elucidate the energetic and mechanistic aspects of the photochemical events occurring in these dyads.

INTRODUCTION During the past three decades, a large number of donor− acceptor linked dyads and multimodular systems have been synthesized, and photoinduced energy- and electron-transfer processes have been investigated as photosynthetic antenna and reaction center model compounds.1−24 These donor−acceptor dyads have been found useful in light-to-electricity conversion, light-to-fuel production, and construction of optoelectronic devices.25−35 In fact, the former processes have the capability to fulfill global energy needs in an environmentally friendly manner, if efficient economically feasible energy conversion devices can be built.31 Porphyrin36,37 and phthalocyanine38−40 are commonly used photosensitizers in building donor−acceptor systems. Their strong absorptions in the visible region, established synthetic methods, and tunable redox and spectral properties with the choice of peripheral substituents and central metal ion are some of their appealing features. Similarly, fullerene, C60,41,42 is a popular choice as an electron acceptor because of its facile reduction potentials43 and low reorganization energy needs in electron-transfer reactions.44 Generally, energy- and electrontransfer events originate from the singlet excited state of the porphyrin/phthalocyanine in these molecular systems.1−24 Interestingly, heavy metal incorporation into the porphyrin and phthalocyanine cavity [metal = Pd(II), Pt(II), etc.] promotes efficient intersystem crossing upon photoexcitation, resulting in the immediate formation of triplet excited states in high quantum © XXXX American Chemical Society

Received: November 11, 2014 Revised: December 12, 2014

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DOI: 10.1021/jp511310c J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

progress of the metal insertion was monitored by optical absorbance until the disappearance of the free-base porphyrin visible bands. After purification of the metalloporphyrins, C60 was appended according to Prato’s procedure62 by the reaction of porphyrin with fullerene and sarcosine in toluene. Both dyads were fully characterized by 1H and 13C nuclear magnetic resonance (NMR) spectrometry, matrix-assisted laser desorption/ionization time-of-flight mass (MALDI-TOF-mass) spectrometry, absorption spectroscopy, and electrochemical methods (see the Supporting Information for NMR and mass spectra). The control compounds (TTP)Pd and (TTP)Pt (TTP = mesotetratoluylporphyrin) were prepared by metallating TTP using similar metalation procedures. It should be mentioned here that Pd insertion into porphyrin was required prior to appending the fullerene as the Pd salt reacted with fullerene to give some precipitate. Absorption, Emission, and Electrochemical Studies. Figure 2a,b shows the optical absorption spectra of PdP−C60 and PtP−C60 dyads along with the control compounds, (TTP)Pd and (TTP)Pt, in benzonitrile. For Pd porphyrins, a Soret band at 420 nm and a visible band at 525 nm were observed, whereas for Pt porphyrin, a Soret band at 406 nm and a visible band at 510 nm were observed. The fullerene entity in the dyads exhibited a band at 325 nm. Importantly, no significant shifts in either the Soret and visible bands upon appending fullerene were observed, indicating the absence of ground-state interactions between the porphyrin and fullerene entities in these dyads. Figure 2c,d shows the emission spectra of PdP−C60 and PtP− C60 dyads, along with the control compounds (TTP)Pd and (TTP)Pt, in benzonitrile at the Soret excitation wavelength. Similar results were observed when the excitation wavelength

Figure 1. Structures of the palladium porphyrin−fullerene (PdP−C60) and platinum porphyrin−fullerene (PtP−C60) dyads investigated in the present study.

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RESULTS AND DISCUSSION Syntheses of PdP−C60 and PtP−C60 Dyads. The synthetic details of the dyads is given in the Experimental Section. Briefly, the syntheses involved initial preparation of 5,10,15-(toluyl)-20(4-hydroxyphenyl)porphyrin by the reaction of pyrrole, 4hydroxybenzaldehyde, and tolualdehyde in a 4:1:3 stoichiometric ratio in propionic acid followed by chromatographic separation of the desired product. Next, a benzaldehyde group was appended by reacting the porphyrin with 4-carboxybenzaldehyde in CH2Cl2 in the presence of dicyclohexylcarbodiimide/ dimethylaminopyrdine (DCC/DMAP) and then performing chromatographic separation to obtain 5-{4″-formyl benzoic acid4′-phenyl ester}-10,15,20-tritoylporphyrin. Subsequently, metal insertion was carried out using palladium acetate for palladium porphyrin and PtCl2 for platinum porphyrin in toluene.36 The

Figure 2. Optimized (a,b) absorption and (c,d) emission spectra of (a,c) PdP−C60 (solid lines) and (TTP)Pd (dashed lines) and (b,d) PtP−C60 (solid lines) and (TTP)Pt (dashed lines) in benzonitrile. The samples were excited at their respective Soret band positions. B


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The Journal of Physical Chemistry C was changed to the visible band maximum. In agreement with literature reports,63 (TTP)Pd exhibited emission bands at 566, 614, 657, 709, and 776(sh) nm. The bands at 550−630 nm are attributed to fluorescence, whereas those at 630−850 nm are attributed to phosphorescence.63,64 The luminescence bands of (TTP)Pt were located at 674 and 735(sh) nm.64,65 The emission bands of PdP in the PdP−C60 dyad were quantitatively quenched (>98%), whereas in the case of PtP−C60, this quenching was over 94%, suggesting the occurrence of excited-state events in these dyads.66 Similar spectral results were observed in nonpolar toluene and less polar o-dichlorobenzene (DCB) solvents. Next, electrochemical studies were performed to evaluate the redox potentials of the dyads. Figure 3 shows differential pulse

Figure 4. Energy level diagram showing different photochemical events occurring in the PdP−C60 dyad in benzonitrile. Solid arrows, most likely processes; dashed arrows, less likely processes. CS, charge separation, EnT, energy transfer; ISC, intersystem crossing; and T, triplet emission.

could influence the photochemical events by virtue of triplet− triplet energy transfer. To verify these possibilities, transient absorption studies using both femtosecond (events originating from 1MP*) and nanosecond (events originating from 3MP*) were systematically performed, in nonpolar and polar solvents, as discussed in the following section. Femtosecond and Nanosecond Transient Absorption Spectral Studies. Figure 5a,c shows femtosecond transient absorption spectra of the precursors, (TTP)Pd and (TTP)Pt, at different time intervals in benzonitrile. In agreement with the literature reports,69 for both of these probes, the instantaneously formed 1MP* revealed fast intersystem crossing to populate the triplet states. For (TTP)Pd, the emission maxima were located at 454, 493, and 845 nm, whereas for (TTP)Pt, they were at 446, 485, and 869 nm. The estimated time constants for singlet− triplet formation were found to be 9.3 and 5.0 ps for (TTP)Pd and (TTP)Pt, respectively. The time profiles of the 460-nm peak corresponding to 3MP* were long-lived, as shown by the 460-nm time profiles in panels b and d of Figure 5 for (TTP)Pd and (TTP)Pt, respectively. Transient spectral features for the PdP−C60 and PtP−C60 dyads were similar to those of the corresponding probes, as shown in Figure 6a,c. That is, the instantaneously formed singlet states populated the triplet states by fast intersystem crossing. The estimated time constants for singlet−triplet formation were found to be 5.7 and 4.2 ps for the PdP−C60 and PtP−C60 dyads, respectively. At the end of 3 ns, the time window of our femtosecond setup, long-lived triplet excited peaks were observed at 455, 497, and 849 nm for PdP−C60 and at 470, 490, and 869 nm for PtP−C60 (see Figure 6b,d for time profiles of the 460-nm peak). Importantly, no indication of charge separation from the singlet excited states was observed. That is, transient bands corresponding to MP•+ [M = Pd(II) or Pt(II)] in the 600−650-nm range and C60•− in the 1000-nm range were absent.70 Next, nanosecond transient absorption studies were performed to probe the photochemical events originating from the readily formed triplet excited states of the MPs. Figure 7a,c shows

Figure 3. Differential pulse voltammograms of (a) PdP−C60 and (b) PtP−C60 in benzonitrile containing 0.1 M (t-Bu4N)ClO4. Scan rate = 5 mV/s, pulse width = 0.25 s, pulse height = 0.025 V.

voltammograms (DPVs) of the PdP−C60 and PtP−C60 dyads in benzonitrile. For PdP−C60, the first four reductions at −1.01, −1.43, −1.71, and −2.01 V and the first three oxidations at 0.65, 1.00, and 1.27 V versus Fc/Fc+ were observed. For PtP−C60, the first three reductions at −1.01, −1.43, and −1.73 V and the first two oxidations at 0.71 and 1.06 V versus Fc/Fc+ were observed. From control experiments involving (TTP)Pd, (TTP)Pt, and fulleropyrrolidine, the first oxidation was assigned to porphyrin radical cation formation, and the first reduction was assigned to fulleropyrrolidine radical anion formation in these dyads. Using the redox potential values and the excited singlet- and triplet-state energies, the free energy for charge separation was evaluated according to the Rehm−Weller approach,67,68 and energy level diagram was constructed as shown in Figure 4 for the representative PdP−C60 dyad in benzonitrile. A similar energy diagram could be envisioned for the PtP−C60 dyad. In DCB and toluene, the energy level of the charge-separated state would be higher by another 150−200 mV. From these results, it is evident that the 1MP* and 3MP* moieties in both dyads carry sufficient energy to perform charge separation to yield MP•+−C60•− charge-separated states. However, the energetically low-lying 3 C60* compared to the energy level of the charge-separated state C


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Figure 5. (a,c) Femtosecond transient absorption spectra of (TTP)Pd and (TTP)Pt in Ar-saturated benzonitrile. λex = 400 nm for 100-fs laser pulses. (b,d) Time profiles of the 460-nm band corresponding to the triplet excited state of porphyrin.

Figure 6. (a,c) Femtosecond transient absorption spectra of PdP−C60 and PtP−C60 dyads in Ar-saturated benzonitrile. λex = 400 nm for 400-fs pulses. (b,d) Time profiles of the 460-nm band corresponding to the triplet excited state of porphyrin.

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Figure 7. (a,c) Nanosecond transient absorption spectra of (TTP)Pd and (TTP)Pt in Ar-saturated benzonitrile. λex = 525 nm for (TTP)Pd and 510 nm for (TTP)Pt. (b,d) Time profiles of the 460-nm band.

Figure 8. (a,c) Nanosecond transient absorption spectra of PdP−C60 and PtP−C60 dyads in Ar-saturated benzonitrile. λex = 525 nm for (TTP)Pd and 510 nm for (TTP)Pt. (b,d) Time profile of the 460-nm band.

femtosecond transient studies, were present.8 The 3(TTP)Pd* species decayed at rates of 2.64 × 104, 1.79 × 104, and 2.35 × 104 s−1 in toluene, DCB, and benzonitrile, respectively (see Figure 7b

the nanosecond transient spectra of the probes, (TTP)Pd and (TTP)Pt, at the indicated time intervals in Ar-saturated benzonitrile. All of the triplet signals, assigned earlier from E


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studies. However, triplet−triplet energy transfer from the triplet excited metalloporphyrin to fullerene was observed in these dyads, as shown by nanosecond transient absorption studies. The determined rate of energy transfer, kEnT, was found to be higher for PdP−C60 than for PtP−C60 in a given solvent, and that the rates were higher for polar solvents. The findings of the present study reveal the significance of dyads composed of triplet sensitizers as photosynthetic antenna mimics, that is, light energy transporting agents. Further studies along these lines are in progress in our laboratory.

for the time profile). Similarly, the decay rate constants for 3 (TTP)Pt* in toluene, DCB, and benzonitrile were found to be 1.96 × 104, 2.64 × 104, and 4.63 × 104 s−1, respectively (see Figure 7d for the time profile). Figure 8 shows the nanosecond transient spectra of PdP−C60 and PtP−C60 dyads at the indicated time intervals in Ar-saturated benzonitrile, whereas the spectra in toluene and DCB are given in the Supporting Information. For both dyads, the triplet peaks of the porphyrins decayed faster than those of pristine 3MP*, with the appearance of new peaks in the 700- and 825-nm ranges corresponding to the formation of 3C60*.71 These results indicate triplet−triplet energy transfer in the studied dyads.72−78 The rates of energy transfer, kEnT, evaluated according to the standard

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EXPERIMENTAL SECTION Chemicals. Buckminsterfullerene, C60 (99.95% purity), was obtained from SES Research (Houston, TX). Tetra-n-butyl ammonium perchlorate, (n-C4H9)4NClO4, was obtained from Fluka Chemicals. All chromatographic materials and solvents were procured from Fisher Scientific and were used as received. Synthesis of Free-Base Tetratoluylporphyrin (TTP)H2. To propionic acid (120 mL) in a round-bottom flask were added tolualdehyde (0.5 g, 4.1 mmol) and pyrrole (0.275 g, 4.1 mmol), and the mixture was refluxed for 5 h. The excess solvent was removed under vacuum, and the crude product was chromatographed over silica gel using a hexanes/chloroform (70:30 v/v) solvent mixture. Yield: 28%. 1H NMR in CDCl3, δ (ppm): 8.80 (s, 8H, β-pyrrole-H), 8.1 (d, 8H, phenyl-H), 7.45 (d, 8H, phenylH), 2.61 (s, 12H, toluyl-CH3), −2.81 (s, 2H, imino-H). Synthesis of (TTP)Pd. Tetratoluylporphyrin (TTP) (120 mg, 0.17788 mmol) and palladium(II) acetate (50.1 mg, 0.2236 mmol) were added to dry toluene (25 mL) in a round-bottom flask. The mixture was refluxed for 12 h under inert conditions. After the mixture had cooled to room temperature, the excess solvent was removed under vacuum. The crude compound was purified by column chromatography on silica gel using a CHCl3/ hexanes solvent mixture (70:30). Yield: 43.2%. 1H NMR in CDCl3, δ (ppm): 8.79 (s, 8H, β-pyrrole-H), 8.0 (d, 8H, phenylH), 7.45 (d, 8H, phenyl-H), 2.61 (s, 12H, toluyl-CH3). Synthesis of (TTP)Pt. (TTP)Pt was prepared similarly to (TTP)Pd. In this case, tetratoluylporphyrin (TTP)H2 (30 mg, 0.0447 mmol) and platinum(II) chloride (14.2 mg, 0.054 mmol) were added to dry toluene (20 mL) in a round-bottom flask. The mixture was refluxed for 12 h under inert conditions. After the mixture had cooled to room temperature, the excess solvent was removed under vacuum, and the crude compound was purified by column chromatography on silica gel using CHCl3/hexanes (60:40). Yield: 83.1%. 1H NMR in CDCl3, δ (ppm): 8.75 (s, 8H, β-pyrrole-H), 7.95 (d, 8H, phenyl-H), 7.45 (d, 8H, phenyl-H), 2.60 (s, 12H, toluyl-CH3). Synthesis of 5-(4′-Hydroxyphenyl)-10,15,20-tritoluylporphyrin. To propionic acid (120 mL) in a round-bottom flask were added 4-hydroxy benzaldehyde (0.5 g, 4.1 mmol), pyrrole (1.098g, 16.37 mmol), and tolualdehyde (1.476g, 12.3 mmol), and the mixture was refluxed for 5 h. The excess solvent was removed under vacuum, and the crude was chromatographed over silica gel using chloroform/methanol (99.5:0.5) to obtain the desired product. Yield: 13.5%. 1H NMR in CDCl3, δ (ppm): 8.6 (s, 8H, β-pyrrole-H), 7.78−7.88 (m, 8H,) 7.28−7.34 (d, 6H, s, phenyl-H), 6.90−6.98(d, 2H phenyl -H), 2.4−2.55 (s, 9H toluyl-CH3), 2.80 (s, 2H, imino-H). Synthesis of 5-{4″-Formyl benzoic acid-4′-phenyl ester}10,15,20-tritoluylporphyrin. In a round-bottom flask, 5-(4′hydroxyphenyl)-10,15,20-tritoluylporphyrin (0.3714 g, 0.552 mmol), 5 equiv of 4-formyl benzoic acid (0.414 g, 2.76 mmol), and 5 equiv of 4-dimethylaminopyrdine (DMAP) (0.414 g, 2.76

Table 1. Rate of Triplet−Triplet Energy Transfer from 3MP* to C60 and Decay Rate Constants of 3MP*a and 3C60* in MP− C60 (M = Pd or Pt) Dyads solventb

kEnTc (s−1)

toluene dichlorobenzene benzonitrile

7.75 × 104 8.58 × 104 10.96 × 104

toluene dichlorobenzene benzonitrile

4.90 × 104 4.56 × 104 4.94 × 104

kT(3MP*−C60) (s−1) PdP−C60 9.94 × 104 10.5 × 104 13.6 × 104 PtP−C60 6.86 × 104 7.20 × 104 9.57 × 104

kT(MP−3C60*) (s−1) 6.34 × 104 7.35 × 104 10.5 × 104 1.91 × 104 3.13 × 104 4.15 × 104

a

Decay rate constants for pristine 3PdP* in toluene, dichlorobenzene, and benzonitrile were found to be 2.64 × 104, 1.79 × 104, and 2.35 × 104 s−1, respectively. Decay rate constants for pristine 3PtP* in toluene, dichlorobenzene, and benzonitrile were found to be 1.96 × 104, 2.64 × 104, and 4.63 × 104 s−1, respectively. (See text for details.) b Refractive index and dielectric constant are 1.4969 and 2.379, respectively, for toluene; 1.551 and 9.93, respectively, for DCB; and 1.528 and 26.0, respectively, for benzonitrile. ckEnT = 1/τ3MP*−C60 − 1/ τ3MP*.

procedure8 are listed in Table 1, along with the decay rates of 3 MP* and 3C60*. Importantly, similarly to 1MP*-originated events in the dyads, no indication of charge separation from 3 MP* to C60 was observed in either dyad. That is, peaks characteristic of the MP•+−C60•− charge-separated state were not obvious in any of the employed solvents. An examination of data from Table 1 reveals the following: (i) Between the PdP−C60 and PtP−C60 dyads, energy-transfer rates were higher for Pd porphyrin-derived dyads in a given solvent. (ii) The kEnT values increased with increasing solvent polarity, more so for the PdP−C60 dyad than that for the PtP−C60 dyad. (iii) As expected for triplet-sensitizer-derived excitation transfer, the magnitude of kEnT was much lower than those reported for singlet excited-state-induced energy-transfer processes.79,80

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SUMMARY Syntheses and characterizations of new donor−acceptor dyads by utilizing triplet metalloporphyrin sensitizers, namely, palladium(II) and platinum(II) porphyrins, and fullerene have been accomplished. Photoinduced electron transfer from the short-lived singlet or long-lived triplet excited metalloporphyrin to the fullerene was not observed because of the lack of diagnostic transient bands corresponding to MP•+ and C60•− during femtosecond and nanosecond transient absorption F


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Synthesis of 5-[4″-(2-(N-Methyl) fulleropyrrolidine) benzoic acid-4′-phenyl ester]-10,15,20-tritoluylporphyrinato Platinum(II) (PtP−C60). In a 100 mL round-bottom flask, 5{4″-formyl benzoic acid-4′-phenyl ester}-10,15,20-tritoluylporphyrinato platinum(II) (40.8 mg, 0.041 mmol), 3 equiv of C60 (88.32 mg, 0.123 mmol), and 5 equiv of sarcosine (18.1 mg, 0.2 mmol) were added to 25 mL of dry toluene, and the solution was refluxed for 12 h. Solvent was removed under vacuum, and the crude compound was adsorbed on silica gel and purified on silica gel column with toluene and hexane (95:5 v/v) as eluent; the yield 30.0 mg 41.8%. 1H NMR in CS2/CDCl3 (1:1 v/v), δ (ppm): 8.6- 8.74 (m, 8H, β-pyrrole-H), 8.46−8.52 (d, 2H, ophenyl-H), 8.18−8.24 (d, 2H, phenyl-H), 7.90- 7.98 (d, 6H, phenyl), 7.44−7.50 (d, 6H), 7.34−7.42 (d, 2H,), 7.16−7.18 (d, 2H, phenyl-H), 7.04−7.1(d, 2H, phenyl-H) 4.85- 4.90 (d 1H, pyrrolidine-H), 4.8 (s, 1H, pyrrolidine-H), 4.2 (d, 1H, pyrrolidine-H), 2.80 (s, 3H, N-methyl H), 2.7 (s, 9H, methyl H). 13C NMR (500 MHz, CS2/CDCl3) δ 147.28, 147.19, 146.20, 146.10, 145.92, 145.31, 144.38, 142.57−142.11, 141.91−141.09, 140.94, 140.17, 139.97, 138.37, 137.481, 134.69, 134.18, 133.89, 131.22, 130.97−130.00, 129.06, 128.63, 127.63, 125.37, 122.63, 122.12, 82.92, 39.99, 29.96, 21.67. MS (MALDI): Calcd, 1745.71 [M+]; found 1704.2, 1695.3, 1621.3, 1605.3, 847.3 (M+ − carboxy−C60). Spectral Measurements. UV−visible and near-IR spectral measurements were carried out with a Shimadzu 2550 UV−vis spectrophotometer or a Jasco V-670 spectrophotometer. The steady-state fluorescence emission was monitored using a Varian (Cary Eclipse) fluorescence spectrophotometer or a Horiba Jobin Yvon Nanolog spectrofluorimeter equipped with photomultiplier tube (PMT) (for UV−visible) and InGaAs (for nearIR) detectors. A right-angle detection method was used for fluorescence measurements at room temperature. All solutions were purged prior to spectral measurements using argon gas. The 1 H NMR studies were carried out on a Varian 400 MHz spectrometer. Tetramethylsilane (TMS) was used as an internal standard. Electrochemistry. Differential pulse voltammetry was peformed on a model 263A Princeton Applied Research potentiostat/galvanostat using a three-electrode system. A platinum button electrode was used as the working electrode, a platinum wire served as the counter electrode, and a Ag/AgCl electrode was used as the reference electrode. The ferrocene/ ferrocenium redox couple was used as an internal standard. All solutions were purged prior to electrochemical measurements with nitrogen gas. Femtosecond Laser Flash Photolysis. Femtosecond transient absorption spectroscopy experiments were performed using an Ultrafast Femtosecond Laser Source (Libra) by Coherent incorporating a diode-pumped, mode-locked Ti:sapphire laser (Vitesse) and a diode-pumped intracavity-doubled Nd:YLF laser (Evolution) to generate a compressed laser output of 1.45 W. For optical detection, a Helios transient absorption spectrometer coupled with a femtosecond harmonics generator, both provided by Ultrafast Systems LLC, was used. The sources for the pump and probe pulses were derived from the fundamental output of the Libra system (compressed output, 1.45 W; pulse width, 100 fs) at a repetition rate of 1 kHz. Ninety-five percent of the fundamental output of the laser was introduced into a harmonic generator that produced second and third harmonics of 400 and 267 nm, respectively, in addition to the fundamental of 800 nm for excitation, whereas the rest of the output was used for the generation of a white light continuum. In the present study, the

mmol) were added to dry CH2Cl2 (75 mL). The resulting solution was cooled to 0 °C, and then dicyclohexylcarbodiimide (DCC) (0.3416 g, 1.658 mmol) was added. The reaction mixture was stirred under nitrogen for 2 h at room temperature. Excess solvent was removed under vacuum, and the crude compound was washed with water several times and extracted with CHCl3. Purification of the crude compound was carried out on a silica gel column with toluene and chloroform (80:20 v/v) as the eluent. Yield: 34.3%. 1H NMR in CDCl3, δ (ppm): 10.1 (1H, −CHO), 8.80 (m, 8H, β-pyrrole-H), 8.45 (d, 2H, phenyl-H), 8.20 (d, 2H, phenyl-H), 8.05,(m, 8H, phenyl-H), 7.65 (d, 2H, phenyl-CHO), 7.5 (d, 6H phenyl-H) 7.1(d, 2H, phenyl-H), 2.6 (s, 9H, CH3− H), −2.81 (s, 2H, imino-H). Synthesis of 5-{4″-Formyl benzoic acid-4′-phenyl ester}10,15,20-tritoluylporphyrinato palladium(II). To dry toluene (25 mL) in a round-bottom flask were added 5-{4″-formyl benzoic acid-4′-phenyl ester}-10,15,20-tritoluylporphyrin (50.0 mg, 0.062 mmol) and palladium acetate (20.9 mg, 0.93 mmol). The reaction mixture was refluxed under nitrogen for 6 h and monitored by UV−vis absorbance until the disappearance of the free-base porphyrin visible bands. The excess solvent was removed under vacuum, and the residue was purified on a silica gel column with hexanes and chloroform (60:40). Yield: 47.8 (%). 1H NMR in CDCl3, δ (ppm):: 10.1 (1H, −CHO), 8.80 (m, 8H, β-pyrrole-H), 8.45 (d, 2H, o-phenyl-H), 8.20 (d, 2H, phenylH), 8.05,(m, 8H, phenyl-H), 7.65 (d, 2H, phenyl-CHO), 7.5 (d, 6H, phenyl-H), 7.1 (d, 2H, phenyl-H), 2.6 (s, 9H, CH3−H). Synthesis of 5-{4″-Formyl benzoic acid-4′-phenyl ester}10,15,20-tritoluylporphyrinato platinum(II). To dry toluene (25 mL) in a round-bottom flask were added 5-{4″-formyl benzoic acid-4′-phenyl ester}-10,15,20-tritoluylporphyrin (116.5 mg, 0.145 mmol) and PtCl2 (57.7 mg, 0.22 mmol). The mixture was then refluxed for 8 h and monitored by UV−vis absorbance. The excess solvent was removed, and the solid residue was purified on a silica gel column using the solvent system hexanes/ toluene (20:80). Yield: 41.6%. 1H NMR in CDCl3, δ (ppm): 10.1 (1H, −CHO), 8.80 (m, 8H, β-pyrrole-H), 8.45 (d, 2H, o-phenylH), 8.20 (d, 2H, phenyl-H), 8.05,(m, 8H, phenyl-H), 7.65 (d, 2H, phenyl-CHO), 7.5 (d, 6H phenyl-H), 7.1 (d, 2H, phenyl-H), 2.6 (s, 9H, CH3−H). Synthesis of 5-[4″-(2-(N-Methyl) fulleropyrrolidine) benzoic acid-4′-phenyl ester]-10,15,20-tritolulylporphyrinato Palladium(II) (PdP−C60). In a 100 mL round-bottom flask, 5{4″-formyl benzoic acid-4′-phenyl ester}-10,15,20-tritoluylporphyrinato palladium(II) (32.5 mg 0.0345 mmol), 3 equiv of C60 (74.6 mg,0.104 mmol), and 5 equiv of sarcosine (15.38 mg, 0.173 mmol) were added to 25 mL of dry toluene, and the solution was refluxed for 12 h. Solvent was removed under vacuum, and the crude compound was adsorbed onto silica gel and purified on a silica gel column with toluene and hexane (95:5 v/v) as the eluent. Yield: 34.4 mg (ca. 60.2%). 1H NMR in CS2/CDCl3 (1:1 v/v), δ (ppm): 8.72−8.81 (m, 8H, β-pyrrole-H), 8.64−8.66 (d, 2H, o-phenyl-H), 8.48−8.52 (d, 2H, phenyl-H), 8.20, 8.26 (d, 2H, phenyl-H), 7.92−8.0 (d, 6H, phenyl), 7.44−7.5 (d, 6H), 7.38−7.40 (d, 2H), 4.65 (d 1H, pyrrolidine-H), 4.6 (s, 1H, pyrrolidine-H), 4.2 (d, 1H, pyrrolidine-H), 2.81 (s, 3H, Nmethyl H), 2.6−2.7 (s, 9H, methyl H). 13C NMR (500 MHz, CS2/CDCl3) δ: 192.69−192.47, 165.20, 151.00, 147.11, 145.94− 145.09, 144.97, 142.401, 142.04, 141.93−141.63, 140.74, 139.96, 138.74, 138.71, 137.49, 137.45,134.38, 131.49−131.15, 130.30, 128.81, 128.56, 127.53, 127.51, 122.29, 122.09, 119.17, 82.66, 39.91, 29.78, 21.59. MS (MALDI): Calcd, 1657.05 [M+]; found, 1656.0 [M+]. G


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second harmonic 400-nm excitation pump was used in all of the experiments. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. Data analysis was performed using Surface Xplorer software supplied by Ultrafast Systems. All measurements were conducted in degassed solutions at 298 K. Nanosecond Laser Flash Photolysis. The studied compounds were excited by an Opolette HE 355 LD laser system pumped by a high-energy Nd:YAG laser with second and third harmonics optical parametric oscillators (OPOs) (tuning range, 410−2200 nm; pulse repetition rate, 20 Hz; pulse length, 7 ns) with powers of 1.0−3 mJ per pulse. The transient absorption measurements were performed using a Proteus UV−vis−NIR flash photolysis spectrometer (Ultrafast Systems, Sarasota, FL) with a fiber-optic-delivered white probe light and either a fast-rise Si photodiode detector covering the 200−1000-nm range or an InGaAs photodiode detector covering the 900−1600-nm range. The output from the photodiodes and a photomultiplier tube was recorded with a digitizing Tektronix oscilloscope. Data analysis was performed using Surface Xplorer software supplied by Ultrafast Systems.

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ASSOCIATED CONTENT

S Supporting Information *

Nanosecond transient spectra of the dyads in toluene and DCB; MALDI-TOF-mass and 1H and 13C NMR spectra of the dyads. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 940-565-4318. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the National Science Foundation (Grant 1401188) is acknowledged.

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REFERENCES

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