Sn(IV) Multiporphyrin Arrays as Tunable Photoactive Systems

1 day ago - A series of arrays made of a central Sn(IV) porphyrin scaffold axially connected to two free-base units has been synthesized and character...
0 downloads 0 Views 2MB Size
Download PDF
Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Sn(IV) Multiporphyrin Arrays as Tunable Photoactive Systems Agnese Amati,† Paolo Cavigli,† Nicola Demitri,‡ Mirco Natali,*,§ Maria Teresa Indelli,*,§ and Elisabetta Iengo*,† †

Department of Chemical and Pharmaceutical Sciences, University of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy Elettra−Sincrotrone Trieste, S.S. 14 Km 163.5 in Area Science Park, 34149 Basovizza − Trieste, Italy § Department of Chemical and Pharmaceutical Sciences, University of Ferrara and Centro Interuniversitario per la Conversione Chimica dell’Energia Solare, sezione di Ferrara, Via L. Borsari 46, 44121 Ferrara, Italy ‡

Downloaded via EAST CAROLINA UNIV on March 13, 2019 at 16:57:51 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: A series of four arrays made of a central Sn(IV) porphyrin as scaffold axially connected, via carboxylate functions, to two free-base porphyrins has been prepared and fully characterized. Three arrays in the series feature the same free-base unit and alternative tin-porphyrin macrocycles, and one consists of a second type of free-base and one chosen metallo-porphyrin. A thorough photophysical investigation has been performed on all arrays by means of timeresolved emission and absorption techniques. Specific focus has been given at identifying how structural modifications of the free-base and tin-porphyrin partners and/or variation of the solvent polarity can effectively translate into distinct photophysical behaviors. In particular, for systems SnTPP(Fb)2 (1) and SnOEP(Fb)2 (2), an ultrafast energy transfer process from the excited Sn(IV) porphyrin to the free-base unit occurs with unitary efficiency. For derivative SnTPP(FbR)2 (3), the change of solvent from dichloromethane to toluene is accompanied by a neat change in the intercomponent quenching mechanism, from photoinduced electron transfer to energy transfer, upon excitation of the Sn(IV) porphyrin unit. Finally, for array SnTpFP(Fb)2 (4), an ultrafast electron transfer quenching of both chromophores is detected in all solvents. This work provides a general outline, accompanied by clear experimental support, on possible ways to achieve a systematic fine-tuning of the quenching mechanism (from energy to electron transfer) of Sn(IV) multiporphyrin arrays.

■

INTRODUCTION Artificial systems featuring bioinspired light-driven functions, such as the antenna effect and charge-separation, obtained via metal-mediated strategies, have been thoroughly studied in recent years.1−3 In this context, metallo-porphyrins represent an important class of compounds to attain photoactive multicomponent arrays with the desired photoinduced properties.4−12 Sn(IV) porphyrins are an excellent choice as scaffolds for the construction of light-responsive supramolecular assemblies because of a series of peculiar and convenient properties: (i) a strong absorption in the visible region, (ii) an intense fluorescence, (iii) a reversible redox behavior, (iv) relative ease of reduction, (v) a diamagnetic nature, plus a central NMR active nucleus, and (vi) a notable preference to form two strong axial bonds with oxygen anionic ligands.13 In particular, the latter feature has been exploited by several research groups to efficiently construct multiporphyrin adducts of various complexity, via axial coordination of phenolate or benzoate functions of many different types of organic fragments, such as metallo-porphyrins and free-base porphyrins.14−24 Recently, Guldi and co-workers reported on the synthesis of multichromophoric arrays consisting of two bodipy units axially bound to a Sn(IV) porphyrin.25 An ultrafast photophysical © XXXX American Chemical Society

investigation revealed that, in these systems, photoinduced electron and/or energy transfer processes can occur. An elegant and interesting study on cyclic porphyrins tetrads was done very recently by the Ravikanth group.26 These supramolecular systems were obtained by axial coordination of thiaporphyrins to Sn(IV) porphyrins exclusively on the basis of the “Sn−O” phenolate axial bonds. A preliminary photophysical study indicated that an efficient energy transfer process occurs from the Sn(IV) porphyrin to the thiaporphyrin units. Alongside to our previous studies,6b,23 we very recently reported on a Sn(IV) porphyrin axially bound to tyrosinate residues as the first example of a simple metallo-porphyrin amino acid conjugate capable of achieving long-lived charge separation by photoinduced proton-coupled electron-transfer.12 Still, while arrays based on other type of metalloporphyrins have been investigated to a larger extent,7,10,11,27−31 a thorough and systematic investigation aimed at elucidating possible ways of tuning the photophysical behavior of Sn(IV) porphyrin conjugates has never been addressed previously. The present work reports on the preparation of a series of four Sn(IV) multiporphyrin arrays, containing two free-base Received: December 20, 2018

A

DOI: 10.1021/acs.inorgchem.8b03542 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

otherwise stated. 2,3,6,7,12,13,16,17-Octaethylporphyrin (OEP) and 5,10,15,20-tetrapentafluorophenylporphyrin (TpFP) were purchased from Frontier Scientific. Deuterated chloroform was purchased from Sigma−Aldrich. 5,10,15,20-Tetraphenylporphyrin (TPP),32 5-(4benzoic acid)-10,15,20-phenylporphyrin (Fb),33 and 5-(4-benzoic acid)-15-(3,5-di-tert-butylphenyl)-2,8,12,18-tetra-n-hexyl-3,7,13,17tetramethylporphyrin (FbR)34 were synthesized and purified as described in the literature. Trans-dihydroxo(5,10,15,20-tetraphenylporphyrinato)-Sn(IV) (SnTPP(OH) 2 ), trans-dihydroxo(2,3,6,7,12,13,16,17-octaethylporphyrinato)-Sn(IV) (SnOEP(OH)2), trans-dihydroxo(5,10,15,20-tetrapentafluorophenylporphyrinato)-Sn(IV) (SnTpFP(OH)2), dibenzoato(5,10,15,20-tetraphenylporphyrinato)-Sn(IV) (5), dibenzoato(2,3,6,7,12,13,16,17-octaethylporphyrinato)-Sn(IV) (6), and dibenzoato(5,10,15,20-tetrapentafluorophenylporphyrinato)-Sn(IV) (7) were synthesized by using similar procedures that have been reported in the literature.13,14,35 Monodimensional and bidimensional NMR experiments (1H, 19F, H−H COSY, H−C COSY, H−Sn HMBC) were recorded on a Varian 500 spectrometer (operating at 500 MHz for 1H, 125 MHz for 13 C, 186 MHz for 119Sn, and 470 MHz for 19F). All spectra were run at room temperature; 1H and 13C chemical shifts were referenced to the peak of residual nondeuterated solvent (δ = 7.26 and 77.16 ppm for chloroform). 119Sn and 19F chemical shifts were referenced, respectively, to the internal standards SnCl4 at 0.00 ppm and CFCl3 at 0.00 ppm. Infrared spectra were recorded on a PerkinElmer FT-IR 2000 spectrometer in the transmission mode and the samples were prepared as KBr pellets. UV-vis absorption spectra were recorded on a Jasco V-570 UV/vis/NIR spectrophotometer. Emission spectra were acquired on an Edinburgh Instrument spectrofluorometer. Fluorescence lifetimes were measured using a time-correlated single photon counting (TC-SPC) apparatus (PicoQuant Picoharp 300) equipped with subnanosecond LED sources (280, 380, 460, and 600 nm, 500−700 ps pulse width) powered by a PicoQuant PDL 800-B variable (2.5−40 MHz) pulsed power supply. The decays were analyzed by means of PicoQuant FluoFit Global fluorescence decay analysis software. Transient absorption experiments in the picosecond time range were performed using a pump−probe setup based on a Spectra-Physics Hurricane Ti:sapphire laser source (fwhm = ca. 130 fs) and an Ultrafast Systems Helios spectrometer.36 Excitation pulses were generated either via SHG option (400 nm) or with an OPA (Spectra Physics 800 OPA). Probe pulses were obtained by continuum generation on a sapphire plate (useful spectral range: 450−750 nm). The effective time resolution was ca. 200 fs, and the temporal window of the optical delay stage was 0−1000 ps. The timeresolved spectral data were deconvoluted to correct for spectral chirp and thus analyzed with the Ultrafast Systems Surface Explorer Pro software. Transient measurements in the nanosecond time scale were performed with a custom laser spectrometer that consisted of a Continuum Surelite II Nd:YAG laser (fwhm 6−8 ns) with a frequency-doubled (532 nm, 330 mJ) or frequency-tripled (355 nm, 160 mJ) option, and an Applied Photophysics xenon light source including a mod. 720p resoultion 150 W lamp housing, a mod. 620p power-controlled lamp supply, and a mod. 03-102 arc lamp pulser. Laser excitation was provided at 90°, with respect to the white light probe beam. Light transmitted by the sample was focused onto the entrance slit of a 300 mm focal length Acton SpectraPro 2300i triple grating, flat field, double exit monochromator equipped with a photomultiplier detector (Hamamatsu R3896), and a Princeton Instruments PIMAX II gated intensified CCD camera, using a RB Gen II intensifier, a ST133 controller and a PTG pulser. Signals from the photomultiplier (kinetic traces) were processed by means of a TeledyneLeCroy 604Zi (400 MHz, 20 GS/s) digital oscilloscope. Preparation of Sn(IV) Porphyrin Adducts. The preparation of the Sn(IV) porphyrin adducts was performed according to this general procedure: a concentrated DCM solution of 1 equiv of each tin porphyrin was treated for 6 h at room temperature with 2 equiv of the corresponding axial ligand in the presence of anhydrous Na2SO4. The solvent was evaporated in vacuo and the solid redissolved in CHCl3. The slow diffusion of n-hexane over the CHCl3 solution induced the precipitation of the pure product as a violet micro-

porphyrins axially bound (via carboxylate functions) to a central Sn(IV) porphyrin scaffold (1−4; see Chart 1). The Chart 1. Molecular Structures of the Arrays Studied

systems differ in the peripheral beta/meso substitution patterns at either the free-base or the metallo-porphyrin units. Some emphasis is given to the undisclosed potentials that 2D 1 H−119Sn heterocorrelation NMR experiments may offer in addressing multiporphyrin architectures based on tin-porphyrin scaffolds. A detailed photophysical investigation, by stationary and time-resolved emission and absorption spectroscopy, on the full series of three-component arrays (that can be described as pseudodyads from a functional viewpoint) and the relevant model compounds (5−9, Chart 2) is also reported. This investigation highlights how a relatively facile tuning over the type/extent of photoinduced processes can be achieved, by structural variation of the Sn(IV) porphyrin scaffold and appropriate solvent choice.

■

EXPERIMENTAL SECTION

Materials and Methods. All reagents were purchased from Sigma−Aldrich and used without further purification, unless B

DOI: 10.1021/acs.inorgchem.8b03542 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Chart 2. Molecular Structures of the Model Compounds

Scheme 1. General Synthetic Procedure Followed for the Obtainment of Systems 1−7

Synthesis of SnTPP(FbR)2 (3).37 Yield: 80%. 1H NMR (δ, 500 MHz, CDCl3): 10.11 (s, 4H, Hmeso), 9.40 (m, 8H, βSn, 4J(119SnH) = 14.8 Hz), 8.47 (d, J = 6.3 Hz, 8H, oSn), 7.91−7.82 (m, 16H, mSn + pSn + oFb), 7.78 (s, 2H, pFb), 7.12 (d, J = 8.0 Hz, 4H, b), 5.33 (d, J = 7.9 Hz, 4H, a), 3.92 (t, J = 7.5 Hz, 8H, H1), 3.81 (t, J = 7.5 Hz, 8H, H1’), 2.41 (s, 12H, CH3), 2.14 (dt, J = 15.2, 7.7 Hz, 8H, H2), 2.04 (dt, J = 15.2, 7.7 Hz, 8H, H2′), 1.78 (s, 12H, CH3′), 1.74−1.59 (m, 16H, H3,3′), 1.48 (s, 36H, HtBu), 1.46−1.27 (m, 32H, H4,4′+H5,5′), 0.89 (td, J = 7.2, 4.7 Hz, 24H, H6,6′), −2.53 (s, 2H, NH), −2.65 (s, 2H, NH). 13 C NMR (δ, 125 MHz, CDCl3, from HSQC): 134.88 (CoSn), 132.96 (CβSn), 131.06 (Cb), 128.54 (CpSn), 127.50 (CoFb), 127.06 (CmSn), 126.53 (Ca), 121.04 (CpFb), 96.75 (CHmeso), 33.21 (CH2,2′), 31.96 (CH4,4′), 31.63 (CHtBu), 29.94 (CH3,3′), 26.68 (C H1,1′), 22.70 (CH5,5′), 14.21 (CCH3′), 14.12 (CCH3), 14.07 (CH6,6′). 119Sn (δ, 186 MHz, CDCl3, from HMBC): −629.2. Selected IR bands (cm−1, KBr pellets): 1646 (νC=Oesther). Synthesis of SnTpFP(Fb)2 (4). Yield: 81%. 1H NMR (δ, 500 MHz, CDCl3): 9.50 (s, 8H, βSn, 4J(119SnH) = 13.9 Hz), 8.80−8.72 (m, 8H, βFb3,4), 8.57 (d, J = 4.6 Hz, 4H, βFb2), 8.14 (d, J = 6.7 Hz, 4H, oFb2), 8.09 (m, 12H, oFb1+βFb1), 7.78−7.64 (m, 18H, mFb+pFb), 7.16 (d, J = 8.2 Hz, 4H, b), 5.35 (d, J = 8.2 Hz, 4H, a), −3.02 (s, 4H, NH). 19F NMR (δ, 470 MHz, CDCl3): −135.32 (d, J = 16.1 Hz, 8F, oF), −149.31 (t, J = 20.8 Hz, 4F, pF), −160.19 (t, J = 17.8 Hz, 8F, mF). 13 C NMR (δ, 125 MHz, CDCl3, from HSQC): 134.28 (CoFb2), 134.23 (CoFb1), 132.95 (CβSn), 132.51 (Cb), 127.41 (CpFb), 126.43 (CmFb),

crystalline powder that was collected by filtration, washed with nhexane, and vacuum-dried. The compounds were characterized by NMR (1H, 13C, 19F, and 119Sn), and infrared (IR) spectroscopy. Synthesis of SnTPP(Fb)2 (1). Yield: 82%. 1H NMR (δ, 500 MHz, CDCl3): 9.37 (s, 8H, βSn, 4J(119SnH) = 15.0 Hz), 8.77 (s, 8H, βFb3,4), 8.61 (d, J = 4.5 Hz, 4H, βFb2), 8.43 (d, J = 6.2 Hz, 8H, oSn), 8.19 (d, J = 4.5 Hz, 4H, βFb1), 8.16 (d, J = 6.9 Hz, 4H, oFb2), 8.11 (d, J = 6.9 Hz, 8H, oFb1), 7.87−7.78 (m, 12H, mSn + pSn), 7.78−7.64 (m, 18H, mFb + pFb), 7.24 (d, J = 8.0 Hz, 4H, b), 5.32 (d, J = 8.0 Hz, 4H, a), −2.98 (s, 4H, NH). 13C NMR (δ, 125 MHz, CDCl3, from HSQC): 134.80 (CoSn), 134.44 (CoFb2), 134.35 (CoFb1), 132.88 (CβSn), 132.70 (Cb), 127.94 (CpSn), 127.52 (CpFb), 127.12 (CmSn), 126.58 (CmFb), 125.32 (Ca). 119Sn (δ, 186 MHz, CDCl3, from HMBC): −628.9. Selected IR bands (cm−1, KBr pellets): 1645 (νC=Oesther). Synthesis of SnOEP(Fb)2 (2). Yield: 83%. 1H NMR (δ, 500 MHz, CDCl3): 10.80 (s, 4H, Hmeso), 8.80−8.73 (m, 8H, βFb3,4), 8.59 (d, J = 4.7 Hz, 4H, βFb2), 8.15 (d, J = 6.7 Hz, 4H, oFb2), 8.13−8.05 (m, 12H, oFb1 + βFb1), 7.81−7.66 (m, 18H, mFb + pFb), 7.09 (d, J = 8.1 Hz, 4H, b), 5.01 (d, J = 8.1 Hz, 4H, a), 4.36 (q, J = 7.5 Hz, 16H, CH2), 2.06 (t, J = 7.6 Hz, 24H, CH3), −2.98 (s, 4H, NH). 13C NMR (δ, 125 MHz, CDCl3, from HSQC): 134.42 (CoFb1,2), 132.50 (Cb), 127.31 (CmFb), 126.34 (CpFb), 125.21 (Ca), 97.86 (CHmeso), 19.89 (CCH2), 18.46 (CCH3). 119Sn (δ, 186 MHz, CDCl3, from HMBC): − 630.3. Selected IR bands (cm−1, KBr pellets): 1640 (νC=Oesther). C

DOI: 10.1021/acs.inorgchem.8b03542 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. 1H NMR spectra (CDCl3) of 8 (top) and 4 (bottom). See Chart 1 for the proton labeling scheme. 125.30 (Ca). 119Sn (δ, 186 MHz, CDCl3, from HMBC): −628.4. Selected IR bands (cm−1, KBr pellets): 1643 (νC=Oesther).

the 119Sn chemical shift and 1H−119Sn coupling constant values stems as ideal, as these are found to be largely conservative upon variations of the porphyrin macrocycle and, at the same time, strongly sensitive to the nature of the axial ligands,39,40 with available comprehensive reference datasets.40 In practice, the use of 1D 119Sn NMR analysis is hampered, given the low sensitivity of the tin nucleus14,40 and the dilution effect resulting from the embedding of a tin-porphyrin component into large arrays of consistent molecular weights. The use of 1 H−119Sn 2D heterocorrelation experiments, introduced by the group of Crossley in the recent past,35 is most certainly a nowadays-accessible, viable alternative, since these experiments permit the indirect detection of δSn values of possible tinporphyrin components, in short acquisition times. Surprisingly, clear relevance to the application of this analytical tool is only given in two following reports from the same group.41 In the present work, we performed a comparative 1H−119Sn 2D HMBC analysis (limited to the detection of signal correlations between the metallo-porphyrin beta protons and the tin nucleus) on the series of arrays, the corresponding Sn(IV) porphyrin starting materials, selected reaction intermediates, and model compounds. The derived 119Sn chemical shifts, fully congruent with literature reference systems, and the corresponding 2D NMR spectra are reported in the Supporting Information (Table S1 and Figures S15−S25). We believe that this investigation, while offering clear proof on the purity of the derivatives under study, may promote further applications of this proton-detected heteronuclear NMR method to address tin-porphyrin systems of increased complexity. Spectroscopic and Photophysical Properties. A spectroscopic and photophysical investigation was performed on the new arrays and, for comparison, on appropriate model compounds (see Charts 1 and 2) in different solvents by both stationary and time-resolved emission and absorption spectroscopy. Although the Sn(IV)−OCOR coordination bond is considered to be stable and inert in organic media and in the absence of competing O-donor ligands,12−15 the photophysical study was performed in all solvents at concentration of ≥5 × 10−6 M, at which the spectrophotometric dilution measurements indicated the integrity of the arrays. As documented for similar systems previously studied,4−6 the binding motif between the carboxylate-functionalized units and the central metal is such that, at the ground-state level, very weak electronic interactions are expected between the porphyrin

■

RESULTS AND DISCUSSION Synthesis and Characterization. The Sn(IV) multiporphyrin arrays 1−4, and the corresponding model compounds 5−7, were obtained in nearly quantitative yield as microcrystalline solids with a common reaction method (Scheme 1), by slow diffusion of n-hexane over chloroform solutions of the crude products. All the compounds were unambiguously characterized by NMR and IR analysis (see Figure 1, as well as Figures S1−S25 in the Supporting Information). In Figure 1, the 1H NMR spectrum (CDCl3) of 4 is reported, compared to that of 8 (full assignments were done by means of 2D H−H and H−C NMR experiments). The number and pattern of the proton resonances in the 1H NMR spectrum of 4, together with their relative integration, confirm the nature and the purity of the array. As expected, the main consequence of the axial coordination to the metallo-porphyrins, is a remarked upfield shift of the proton resonances of the axially bound porphyrin, compared to the parent free system, with an extent that decreases as the proton distance from the Sn(IV) porphyrin plane increases (e.g., Δδ = −3.08, Ha; −1.15, Hb; −0.71, Hβ1; and −0.29, Hβ2), as already described previously for similar side-to-face assemblies.6a,12,22 The 19F NMR spectrum of 4 (CDCl3 Figure S14) is not particularly informative, with features practically unvaried as compared to those found for the parent SnTpPF(OH)2 porphyrin. Note that, while the ease and straightforward synthetic preparation of such type of multiporphyrin conjugates is welldocumented, scarce attention is normally posed to the difficulty in tracing possible (recurring) side products or unreacted materials.38 Instead, the presence of secondary unwanted tin-porphyrin species surely represents a serious pitfall for a meaningful comprehension and development of these multichromophoric systems. Detections of these side species by means of mass analysis is critical, as partial or total fragmentation, with detachment of the axial ligands is a recurrent issue.6b In addition, one-dimensional (1D) 1H NMR analysis may be not sufficient, as the concomitance of various porphyrin components generates extended signal overlaps in the aromatic region. On the other hand, a screening based on D

DOI: 10.1021/acs.inorgchem.8b03542 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

changing the solvent polarity on the operating quenching processes; and (iii) among different arrays, the effect of varying the peripheral substituents of both porphyrins on the quenching mechanism. A thorough comparison of the results obtained for all supramolecular systems will be then presented in the Conclusion section. Table 2 summarizes all the relevant spectroscopic and electrochemical data of model compounds 5−9, which are useful for the understanding of the photophysical results of the arrays, as well as for the construction of the energy level diagrams (see below). SnTPP(Fb)2 (1). The photophysical behavior of 1 was studied in toluene only (evaluation of the photophysical properties in other media was indeed hampered by solubility issues). In order to discuss the results, it is useful to consider the energy level diagram shown in Figure 3. This diagram can be built by simple superimposition of the diagrams of the Sn-porphyrin and free-base components. The important results are as follows: (i) upon selective excitation (λ = 515 nm) of the free-base unit, the system shows the same emission pattern, intensity, and lifetime (9.8 ns) of the Fb model 8; (ii) upon excitation at 560 nm, where the light is preferentially (70%) absorbed by the SnTPP component, the Sn-based fluorescence is strongly quenched whereas a fluorescence with the same shape and lifetime of the freebase unit is observed (Figure 4). The lifetime of the residual Sn-porphyrin emission, measured at ca. 600 nm, is 90%) quenched (see Figure S33 in the Supporting Information). The lifetime of the residual SnOEP fluorescence is