Nov 1, 1996 - include the following: (1) Singlet excited-state energy transfer from the Zn porphyrin to the Fb porphyrin is extremely ... slowed up to 4-fold by the addition of groups to the linker that limit the ability of the linker and porphyrin t
Hole/electron hopping in all the monocations is rapid (107 s-1 or faster) on the .... Journal of the American Chemical Society 0 (proofing), .... The Journal of Physical Chemistry B 2006 110 (40), 19810-19819 ..... Simona Rucareanu, Olivier Mongin, A
Page 1 ... compounds are easily prepared using Zn and Fb porphyrin building blocks. In order to ... The challenge of creating model systems for energy-transfer.
May 10, 2006 - European Journal of Inorganic Chemistry 2012 2012 (36), 5979-5990 ... Australian Journal of Chemistry 2009 62 (7), 692 ... Exploration ofmeso-Substituted Formylporphyrins and Their Grignard and Wittig Reactions.
Jul 24, 2001 - David C. W. Reid completed his B.Sc. with Honors (1992) and Ph.D. (1998) in Chemistry in the Department of Chemistry at Massey University under the supervision of Professor Tony Burrell and Associate Professor David Officer. He is curr
Jul 24, 2001 - Anthony K. Burrell,* David L. Officer,* Paul G. Plieger, and David .... under the supervision of Professor Tony Burrell and Associate Professor.
Dec 16, 2014 - A recently reported synthetic method has been employed to prepare several arrays of free base and zinc porphyrins. In the arrays, the ...
The phosphine building blocks and a trimeric complex have also been analyzed by X-ray .... Orde Q. Munro , Greville L. Camp , Laurence Carlton. European ...
Sep 11, 2003 - Neil Feeder,â and Jeremy K. M. Sanders*,â . UniVersity Chemical Laboratory, UniVersity of Cambridge, Lensfield Road,. Cambridge, CB2 1EW ...
Excited-State Energy Flow in Covalently Linked Multiporphyrin Arrays: The Essential. Contribution of Energy Transfer between Nonadjacent Chromophores.
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 ‡
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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.
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
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.
Materials and Methods. All reagents were purchased from Sigma−Aldrich and used without further puriﬁcation, unless B
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
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 <200 ps (i.e., below the TC-SPC instrumental resolution), and is thus strongly quenched with respect to the lifetime (1.2 ns) of model compound 5. The excitation spectrum of the Fb ﬂuorescence (see Figure S29 in the Supporting Information) closely matches the absorption spectrum of array 1 clearly indicating that an energy transfer process (eq 1) from the excited Sn-based component to the free-base component occurs with a unitary eﬃciency.
components. This situation is common for this type of assemblies and allows a supramolecular approach based on a localized description of the individual subunits. In Table 1, the Table 1. Absorption Data for Arrays 1−4 in DCM λabs(ε) [nm (× 104 M−1 cm−1)]
relevant absorption features of all systems investigated in both the UV and visible regions are reported. Figure 2 depicts, as an
Figure 2. Comparison of the absorption spectra of SnOEP(Fb)2 (2) and model compounds Fb (8) and SnOEP(BA)2 (6) in DCM solution at room temperature.
example, the absorption spectrum of 2 at room temperature in DCM solution (see Figures S26−S28 in the Supporting Information for the complete collection of UV-vis absorption spectra). For all systems, the spectrum is dominated by the superposition of the Q-bands and Soret bands of the corresponding molecular components. No additional absorptions are detected, thus supporting the weak electronic interactions previously envisioned and the supramolecular nature of the multiporphyrin arrays.4−6 The absorption spectra remain almost unchanged by varying the solvent, consistently with the π−π* nature of the electronic transitions involved, as typically observed for porphyrins of the regular type.42 In the following section, the photophysical properties of each assembly will be ﬁrst discussed separately. Attention will be paid to evaluate (i) the photoinduced processes and the quenching mechanism, particularly upon excitation of the tin(IV) porphyrin unit; (ii) within the same array, the eﬀect of
Fb‐1*SnTPP‐Fb → 1*Fb‐SnTPP‐Fb
In order to obtain kinetic information about this process, ultrafast absorption measurements were performed upon excitation at 550 nm, a convenient wavelength for substantial (ca. 50%) excitation of the SnTPP component. The spectral variations are shown in Figure 5A. The diﬀerential absorption spectrum, taken at 2 ps after the laser pulse, corresponds to the sum of the transient spectra of the singlet excited states of both the Sn-based and free-base porphyrins (see Figures S30 and S31 in the Supporting Information for the diﬀerential spectra of model compounds 5 and 8).23 This spectrum shows as distinctive features an intense absorption at 460 nm, a broad, featureless positive absorption throughout the visible region with superimposed bleaching of the ground-state Q-bands (515, 556, and 600 nm) and apparent bleaches at 650 nm corresponding to stimulated
Table 2. Relevant Photophysical and Electrochemical Data of Model Compounds 5−9 in DCM λabs(ε) [nm (× 104 M−1cm−1)]
Figure 3. Energy level diagram of 1. The energy of the singlet excited states was estimated from the intersection of the normalized absorption/ ﬂuorescence spectra, the energy of the triplet excited states was derived from phosphorescence data.23,43 Time constants are related to photophysical processes in air-equilibrated toluene. The excited-state lifetime of the 3*Fb was obtained by laser ﬂash photolysis experiments.
Figure 4. Emission spectra (λexc = 560 nm, optically matched solutions) of 1 (red line) and model 5 (black line) in toluene at room temperature.
ﬂuorescence emission. Within the experimental time window (ca. 1 ns), a clear temporal evolution from this initial spectrum to a new transient spectrum is observed. The main transient changes are (i) the disappearance of the 556 nm bleaching, characteristic of the excited SnTPP component, and (ii) the deepening of both the 515 and 650−725 nm bleaches, consistent with the ground-state depletion and stimulated emission, respectively, of the singlet excited state of the Fb component. These results are clearly diagnostic of an eﬃcient singlet−singlet energy transfer process from the tin-porphyrin to the free-base unit, as already indicated by stationary spectroﬂuorimetric experiments (see above). Kinetic analysis of the bleaching at 518 nm (Figure 5B) yields a time constant of 60 ps corresponding to a rate constant of k = 1.7 × 1010 s−1. Importantly, this rate is in reasonable agreement with the value of k = 3.7 × 1010 s−1 calculated using the Fö rster equation,44−46 thus suggesting a predominant contribution from a Coulombic mechanism in the electronic energy transfer process.47 Ultrafast absorption measurements were performed also by using a 400 nm excitation, where the light is absorbed mostly
Figure 5. (A) Spectral evolution obtained by ultrafast spectroscopy (excitation at 550 nm) of 1 in toluene; (B) kinetic analysis at 518 nm with related ﬁtting.
(ca. 80%) by the Fb component (see Figure S32 in the Supporting Information). The results well agree with those obtained upon 550 nm excitation, thus conﬁrming the singlet− singlet energy transfer pathway (eq 1). SnOEP(Fb)2 (2). Array 2 was investigated in DCM and in toluene. Both the absorption spectra and the emission properties (emission and excitation spectra, as well as the extent of quenching of the SnOEP ﬂuorescence) are very similar in both solvents, so that a similar photophysical F
transfer from 1*SnOEP to 1*Fb. The ﬁnal spectrum indeed appreciably matches the spectrum of 1*Fb. Kinetic analysis of the recovery of the bleach at 580 nm (Figure 6B) yields a time constant of ca. 40 ps. Similar to the previously discussed system 1, this energy transfer process (k = 2.3 × 1010 s−1) is very likely to proceed via a Coulombic Förster mechanism.44,47 The photophysical behavior is summarized in the energy level diagram reported in Figure 7. SnTPP(FbR)2 (3). The photophysical behavior of array 3 has been studied in solvents of diﬀerent polarity (DCM, THF, and toluene) and the results are strongly dependent on the medium used. In DCM, an extremely weak emission is observed, regardless of the unit excited, either the FbR component (λexc = 508 nm, Figure S36 in the Supporting Information) or the Sn-based one (λexc = 557 nm, Figure S37 in the Supporting Information). Also, TC-SPC experiments conﬁrm that, in 3, the singlet lifetime of both porphyrin species is below the instrumental resolution (<200 ps). All these data clearly indicate that in DCM both singlet excited states (1*SnTPP and 1 *FbR) are quenched by an ultrafast and eﬃcient process. The energy level diagram (Figure 8), obtained from the spectroscopic and redox potentials of the two components in DCM (Table 2), clearly suggests that the quenching mechanism is photoinduced electron transfer in both cases (see eqs 2 and 3).
behavior can be reasonably envisioned. The emission spectrum is independent of the excitation wavelength and corresponds in shape and intensity to the emission of the free-base component. In particular, upon selective (>90%) excitation of the free-base unit at λ = 515 nm, an emission with the same intensity and lifetime as model 8 is observed. On the other hand, upon preferential (70%) excitation of the SnOEP component at 538 nm, the characteristic ﬂuorescence of SnOEP is totally (>90%) quenched (see Figure S33 in the Supporting Information). The lifetime of the residual SnOEP ﬂuorescence is <200 ps (i.e., below the TC-SPC instrumental resolution). In order to investigate the nature of the quenching process, the excitation spectrum of the ﬂuorescence of the freebase component at 720 nm has been measured (see Figure S34 in the Supporting Information). The close matching between the excitation and the absorption spectra of 2 clearly indicates that an energy transfer process from the SnOEP to the freebase unit occurs with unitary eﬃciency. Importantly, the quenching mechanism has been further conﬁrmed by ultrafast measurements in toluene. Excitation has been performed at 400 nm where the two components absorb almost the same fraction of light. The transient spectral changes are shown in Figure 6A. The ﬁrst spectrum detected
FbR‐1*SnTPP‐FbR → FbR‐SnTPP− ‐FbR+
ΔG = −0.37 eV (2)
FbR‐SnTPP‐1*FbR → FbR‐SnTPP− ‐FbR+
ΔG = −0.21 eV (3)
Unfortunately, ultrafast measurements, which are required to conﬁrm the occurrence of such processes and obtain the values of the respective rate constants, were hampered since, in this solvent, partial decomposition was found to occur upon laser excitation. The charge-transfer state is then expected to recombine either to the ground state (eq 4) or to a localized triplet state (eqs 5 and 6), all these processes being indeed thermodynamically allowed. FbR‐SnTPP− ‐FbR+ → FbR‐SnTPP‐FbR
ΔG = −1.73 eV (4)
FbR‐SnTPP− ‐FbR+ → FbR‐ 3*SnTPP‐FbR
ΔG = −0.09 eV (5)
FbR‐SnTPP− ‐FbR+ → FbR‐SnTPP‐ 3*FbR
ΔG = −0.28 eV (6)
The photophysical characterization in THF gives slightly diﬀerent results. The energy level diagram, shown in Figure 8, is useful to the discussion. As in DCM, upon selective excitation of the two components (λ = 506 and 557 nm for Fb and Sn-based units, respectively) an eﬃcient quenching of the ﬂuorescence of both chromophores is observed (Figure S38 and S39 in the Supporting Information). However, in this solvent, while the SnTPP ﬂuorescence is completely quenched (τ < 200 ps), a residual emission from the free-base component is detected (τ = 0.5 ns) with the corresponding excitation spectrum (λem = 700 nm) presenting some contribution from the absorption bands of the Sn-based unit (Figure S40 in the Supporting Information). Comparison experiments clearly indicate that the excitation spectrum does not match exactly the absorption spectrum, as it lacks a large contribution from the absorption bands of the Sn-based unit
Figure 6. (A) Spectral evolution obtained by ultrafast spectroscopy of 2 in toluene at room temperature; (B) kinetic analysis at 580 nm with related ﬁtting.
(measured at a time delay of 2 ps) is consistent with a mixture of transient spectra of both singlet excited states of the two chromophores (namely, 1*SnOEP and 1*Fb).48 The transient changes subsequently observed between 2 and 310 ps (Figure 6A) show the following distinctive features: a deepening of the free-base 515 nm ground-state bleaching, an increase of the stimulated free-base emission at 650 and 720 nm, and the recovery of the bleaching signals at 538 and 580 nm corresponding to the loss of 1*SnOEP. These changes can be attributed to the occurrence of a singlet−singlet energy G
Figure 7. Energy level diagram for 2. The energy of the singlet excited states was estimated from the intersection of the normalized absorption/ ﬂuorescence spectra, the energy of the triplet excited states was derived from phosphorescence data.42,43 Time constants are related to photophysical processes in air-equilibrated toluene. The excited-state lifetime of the 3*Fb was obtained by laser ﬂash photolysis experiments.
Figure 8. Energy level diagram for 3. The energy of the singlet excited states was estimated from the intersection of the normalized absorption/ ﬂuorescence spectra, and the energy of the triplet excited states was derived from phosphorescence data.24,43 The energy of the charge-separated state was obtained from the redox potentials in DCM and neglecting the electrostatic work terms. Apart from the energy position of the charge transfer state, this energy level diagram can be qualitatively used to describe the photophysical processes in THF, but not in toluene (see text). Time constants (black) and eﬃciencies (gray) are related to air-equilibrated THF.
the singlet excited state of the FbR component as measured in model 9 (see Figure S42 in the Supporting Information). The subsequent transient changes have a biphasic behavior: (i) In the 3−65 ps time range, the distinctive features are a growth of a broad absorption band in the 650−750 nm and a slight decrease of the absorption signals at λ < 650 nm (Figure 9A); (ii) in the longer time range, 65−600 ps (Figure 9B), the formation of a bleaching at 560 nm and a further increase of the absorption band at ∼720 nm are observed. These spectral changes are consistent with the formation of a chargeseparated state, where the Sn-based unit is reduced and the free-base counterpart is oxidized (FbR-SnTPP−- FbR+).49−51 The presence of two time components (see kinetic analysis in
(ca. 70% from spectral simulation, see Figure S41 in the Supporting Information). This may suggest that, with respect to DCM, in the less polar THF solution singlet−singlet energy transfer from the 1*SnTPP (eq 7) can compete with the photoinduced electron transfer (eq 2). FbR‐1*SnTPP‐FbR → FbR‐SnTPP‐1*FbR
In order to get a deeper insight into the photophysical processes in THF, ultrafast absorption measurements were performed upon excitation at 400 nm (at this wavelength absorption by the FbR unit is ca. 80%). The transient spectra obtained are shown in Figure 9. The initial spectrum at ca. 3 ps after the laser pulse is mainly consistent with the spectrum of H
Figure 9. Spectral evolution obtained by ultrafast spectroscopy of 3 in THF at room temperature in the (A) 2.6−65 ps and (B) 65−572 ps time windows.
Figure 10. Energy level diagram for 4. The energy of the singlet excited states was estimated from the intersection of the normalized absorption/ ﬂuorescence spectra, the energy of the triplet excited states was derived from phosphorescence data.43,53 The energy of the charge-separated state was obtained from the redox potentials in DCM and neglecting the electrostatic work terms. Apart from the energy position of the charge-transfer state, this energy level diagram can be qualitatively used to describe the photophysical processes in both toluene and THF (see text). Time constants (black and red for air-equilibrated toluene and THF, respectively) and eﬃciencies (gray and pink for air-equilibrated toluene and THF, respectively) are also indicated.
The photophysical characterization was also performed in toluene where completely diﬀerent results were obtained. Steady-state emission measurements by selective excitation of the free-base component indicate that the singlet state of the latter is practically unquenched (Figure S45 in the Supporting Information). This result is conﬁrmed by the fact that the lifetime is comparable with that of the model compound 9 (∼10 ns), thus suggesting the absence in toluene of any intercomponent processes at the singlet excited-state level of FbR. On the other hand, upon selective excitation of SnTPP at 558 nm, a strong quenching of the intensity and lifetime of the Sn-based ﬂuorescence is observed (Figure S46 in the Supporting Information). These data indicate that an energy transfer process from the Sn-based to the FbR unit occurs. The close match of the excitation spectrum with the absorption spectrum of the array clearly demonstrates that the energy transfer process occurs with unitary eﬃciency . Ultrafast spectroscopy of 3 in toluene (Figure S47 in the Supporting Information) allows for an estimation of a time constant of ca. 18 ps for such an energy transfer process, corresponding to a rate constant of k = 5.5 × 1010 s−1.
Figure S43 in the Supporting Information) can be reasonably ascribed to the population of such charge transfer state from two diﬀerent excited states: the fast (minor) component with τ = 12 ps can be attributed to the photoinduced electron transfer (PET) from the singlet SnTPP excited state, whereas the long (major) component with τ = 355 ps can be attributed to PET from the singlet FbR excited state.52 Nanosecond laser ﬂash photolysis experiments (excitation at 532 nm) on 3 (Figure S44 in the Supporting Information) show that the prompt spectrum corresponds to that of the triplet excited state localized on the free-base component (τ ≈ 0.8 μs in airequilibrated solution). Also, comparative experiments conducted on isoabsorbing solutions of 3 and 9 indicate that the amount of localized triplet state formed in 3 corresponds to ca. 60% of the total absorbed photons. These ﬁndings thus demonstrate that (i) the lifetime of the FbR-SnTPP−-FbR+ charge transfer state lies below the time resolution of the laser ﬂash photolysis apparatus (i.e., <10 ns); (ii) the chargeseparated state recombines in part (ca. 60%) to the triplet state localized on the free-base component (eq 6) and partially (ca. 40%) undergoes ground-state decay (eq 4). I
Figure 11. Spectral evolution obtained by ultrafast spectroscopy of SnTpFP(Fb)2 (4) in toluene at room temperature in the (A) 1−95 ps time window and (B) 95−585 ps time window.
SnTpFP(Fb)2 (4). The photophysical behavior of array 4 was studied in diﬀerent solvents (DCM, THF, and toluene). In DCM, for both chromophores, eﬃcient quenching of both emission intensity and lifetimes (τ < 200 ps) is observed (Figure S48 and S49). The mechanism of the quenching processes is most likely electron transfer (see eqs 8 and 9). Similar to that observed with compound 3, the so-formed charge transfer state may then recombine either to the ground state (eq 10) or to a localized triplet state (eq 11). However, the absence of reliable ultrafast data, because of degradation of the sample during the measurement, prevents the direct determination of the quenching mechanism and kinetics. Fb‐1*SnTpFP‐Fb → Fb‐SnTpFP− ‐Fb+
evolution can be assigned to the formation of a chargeseparated product of the type Fb-SnTpFP−-Fb+ from the singlet excited state of the Fb unit (eq 7).49−51 In a longer time scale (95−585 ps, Figure 11B), a partial recovery of the initial spectrum occurs, possibly attributable to the backward charge recombination process. The kinetic analysis (Figure S55 in the Supporting Information) shows that the ﬁrst process has a time constant of 9 ps, while a precise value of the rate of the second one cannot be obtained, because of temporal limitations of the instrumental apparatus. In order to obtain additional information on the latter process, nanosecond laser ﬂash photolysis experiments, using 532 nm excitation light, have been performed on isoabsorbing solutions of the model Fb and 4 (Figure S56 in the Supporting Information). The fact that, in these comparative experiments, the same amount of Fb triplet state (τ ≈ 0.5 μs in airequilibrated conditions) is observed immediately after the laser pulse clearly indicates that, in the array, the charge-separated state recombines quantitatively to the triplet state localized on the Fb unit (eq 11) within ca. 10 ns. Mechanistic investigation by ultrafast measurements was also performed in THF. The spectral changes are qualitatively similar to those observed in toluene (Figure S57 in the Supporting Information) and can be therefore assigned as above: the ﬁrst process to the forward photoinduced electron transfer from the excited free-base component (Figure S57a), the second process to the backward charge recombination (Figure S57b). The time constants of these processes (Figure S58 in the Supporting Information) are 15 and 720 ps for charge separation and recombination, respectively. Interestingly, as demonstrated by comparative laser ﬂash photolysis experiments (Figure S59 in the Supporting Information), the charge-separated product in THF decays mainly (70%) to the ground state (eq 10), while only a fraction (30%) to the triplet excited state (τ ≈ 1 μs in air-equilibrated conditions) localized on the free-base component (eq 11).
ΔG = −0.56 eV (8)
Fb‐SnTpFP‐1*Fb → Fb‐SnTpFP− ‐Fb+
ΔG = −0.36 eV (9)
Fb‐SnTpFP ‐Fb → Fb‐SnTpFP‐Fb
ΔG = − 1.58 eV (10)
Fb‐SnTpFP ‐Fb → Fb‐SnTpFP‐ *Fb
ΔG = −0.24 eV (11)
Analogous stationary emission results were obtained in both toluene and THF: regardless of the excitation wavelength, for both chromophores, the emission is strongly quenched (see Figures S50 and S51 in the Supporting Information). The excitation spectrum of the residual Fb ﬂuorescence at 720 nm (Figure S52 in the Supporting Information) is lacking the bands of the Sn-based unit, thus suggesting that an energy transfer process from the SnTpFP unit to the free-base one does not occur. In order to clarify the quenching mechanism, ultrafast measurements have been performed upon 400 nm excitation, where light is absorbed mainly by the Fb unit. The energy level diagram shown in Figure 10 is useful for the interpretation of the results. In toluene (Figure 11), the ﬁrst transient spectrum, obtained using a time delay of 1 ps, perfectly matches the diﬀerential spectrum of the singlet excited state of the free-base unit (Figure S31; also see, for comparison, the transient spectrum of 1*SnTpFP in Figure S54 in the Supporting Information). This is indeed expected, considering the almost selective excitation of the Fb component with the 400 nm pump. The subsequent spectral changes have a biphasic behavior: in the time scale of 1−95 ps (Figure 11A), a deepening of the Qband bleaches and a growth of a large absorption band in the 600−700 nm region is observed. This distinctive spectral
CONCLUSION A series of four arrays made of a central Sn(IV) porphyrin connected to two free-base units, via carboxylate functions, diﬀering in the substitution patterns at the peripheral beta or meso positions, was prepared and fully characterized in solution by means of NMR spectroscopy. A detailed photophysical study was performed on the entire array series in diﬀerent solvents and produced a very detailed and intriguing picture: (i) structural variation of the peripheral porphyrin substituents causes a sharp modiﬁcation in the photophysical behavior of J
ACKNOWLEDGMENTS Financial support from the University of Ferrara (No. FAR2017) and the University of Trieste (No. FRA2016) is gratefully acknowledged.
the corresponding array; (ii) for some systems, a decrease in the solvent polarity was found to aﬀect the type of quenching mechanism (from electron to energy transfer) occurring upon excitation of the Sn(IV) porphyrin scaﬀold. The main results can be summarized as follows: (1) For 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 in both DCM and toluene; (2) For SnTPP(FbR)2 (3), a progressive decrease in the solvent polarity (from DCM to THF and ﬁnally to toluene) imparts an evident metamorphosis of the intercomponents photoinduced process from electron to energy transfer, upon excitation of the central Sn(IV) porphyrin scaﬀold. The decrease in solvent polarity increases the energy level of the charge-separated state and concomitantly converts one type of quenching mechanism into the other. (3) For SnTpFP(Fb)2 (4), in all solvents studied (DCM, THF, and toluene), an ultrafast electron transfer quenching of both chromophores is observed, with charge separation and recombination pathways being strongly dependent on the solvent nature. In particular, a quantitative recombination to the triplet state localized on the free-base porphyrin is detected in toluene, whereas, in the more polar THF, the chargetransfer state decays mainly (ca. 70%) to the ground state. In conclusion, this work clearly outlines how an extensive tuning of the energy of the charge-separated state and, as a consequence, of the photophysical mechanism (e.g., from energy to electron transfer) can be easily achieved in Sn(IV) multiporphyrin arrays via systematic structural variations of the molecular components, and appropriate choice of the solvent polarity. To this respect, Sn(IV) porphyrins represent very promising scaﬀolds for the design of supramolecular systems capable of eﬃcient and easily tunable photoinduced electron/ energy transfer processes.
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S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03542.
NMR spectra; absorption, emission, and excitation spectra not shown in the main text; ultrafast measurements of model compounds; nanosecond laser ﬂash photolysis experiments (PDF)
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