Control of Photoinduced Charge Transfer Lifetimes in Porphyrin

May 14, 2009 - We demonstrate that the binding of N-donor ligands (4-picoline, imidazole) or anions (chloride) to the terminal zinc(II) porphyrin of s...
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Control of Photoinduced Charge Transfer Lifetimes in Porphyrin Arrays by Ligand Addition† James A. Hutchison,‡ Paul J. Sintic,§ Peter R. Brotherhood,§ Colin Scholes,‡ Iain M. Blake,§ Kenneth P. Ghiggino,*,‡ and Maxwell J. Crossley*,§ School of Chemistry, The UniVersity of Melbourne, Victoria, 3010 Australia, and School of Chemistry, The UniVersity of Sydney, NSW, 2006 Australia ReceiVed: February 28, 2009; ReVised Manuscript ReceiVed: April 16, 2009

We demonstrate that the binding of N-donor ligands (4-picoline, imidazole) or anions (chloride) to the terminal zinc(II) porphyrin of selectively metalated triad and tetrad porphyrin arrays allows unprecedented control of photoinduced energy and charge transfer processes. Detailed electrochemical and photophysical studies of the arrays, and also of dyadic and monomeric reference compounds, show that the main effect of ligation is to alter the redox potentials of the zinc(II) porphyrin, making the final step of trans-array charge transfer (between the zinc(II) porphyrin and an adjacent free-base porphyrin) more exothermic. The result of this increased exothermicity is a reduced reversibility for this step and an increase in the trans-array charge transfer lifetime, correlating with the basicity of the ligand employed. For the tetrad, ligation with chloride extends the trans-array charge transfer lifetime to millisecond time scales in polar solvents. For the triad, chloride ligation “switches on” trans-array charge transfer in the nonpolar solvent toluene. The porphyrin arrays studied have geometries that match closely the arrangement of chromophores in natural photosynthetic reaction centers. Using metal ligation to control and optimize interporphyrin energy and electron transfer processes mimics another important aspect of natural light harvesting systems. Introduction The efficient light-induced electron transfer (ET) reactions between porphinoid pigments in the photosynthetic reaction centre (PRC) have inspired the study of porphyrin arrays as biomimetic photoactive molecular devices.1 Recently we reported the synthesis2 and photophysical characterization in pure solvents3 of a porphyrin triad (1) and tetrad (2) consisting of free-base porphyrin (FbP), zinc(II) porphyrin (ZnP), and gold(III) porphyrins (AuP(III)+) that resemble the geometry of chromophores in the PRC (Figure 1). The constituent chromophores of the tetrad (2) have V-shaped geometry and approximate C2 symmetry affording architectural homology to the L and M branches of the PRC (Figure 1),4 while the covalent bonding between macrocycles act in a similar fashion to the protein-cofactor interactions that hold pigments in a fairly rigid manner in the PRC.5 Selective metalation creates a redox gradient across the arrays such that photoexcitation of the ZnP, FbP, or AuP(III)+ moieties ultimately leads to ET from the ZnP to the AuP(III)+ in polar solvent, representing charge transfer over 35 and 50 Å in 1 and 2, respectively.3 The mechanism for trans-array charge transfer occurring in the arrays following excitation of the ZnP is shown for 1 in Figure 2. Excitation energy transfer (EET) from the photoexcited ZnP to the FbP occurs initially, which in turn initiates an ET from the FbP to the AuP(III)+ forming the ZnP-FbP+-AuP(II) state of 1 (this state can also be formed by direct excitation of the FbP or AuP(III)+).3 In the final step the ZnP moiety acts as an acceptor of a positive charge, or hole, †

Part of the “Hiroshi Masuhara Festschrift”. * To whom correspondence should be addressed. (K.P.G.) E-mail: [email protected]. Fax: (+61) 3 9347 5180. (M.J.C.) E-mail: [email protected]. Fax: (+61) 2 9351 2751. ‡ The University of Melbourne. § The University of Sydney.

from the FbP+ to form the “giant” charge-shifted (CSH) state ZnP+-FbP-AuP(II). A similar mechanistic pathway holds for the formation of the giant CSH state of 2, except that additional energy and electron transfer processes between the neighboring FbP moieties are involved.3 This mechanism is typical for trans-array ET in arrays with adjacent FbP and ZnP moieties, several of which have shown some of the longest charge-separated (CS) state lifetimes ever observed.6 As seen in Figure 2, the hole transfer (HT) from FbP+ to ZnP is a crucial step that must be fast enough to compete with recombination of the initial ET event in order for these arrays to function efficiently. However, even in polar solvents, HT between zinc(II) porphyrin and free-base porphyrin is associated with only a modest driving force (0.1-0.3 eV) and trans-array ET has never been observed for any of this class of arrays in nonpolar solvents such as toluene.6 Indeed, transient absorption studies of both 1 and 2 suggest that the giant CSH state in these arrays is very close in energy to less extended CSH states in the polar solvent benzonitrile (PhCN) and not observed at all in toluene.3 In the present work, we extend our previous study3 to now investigate the effect on ET dynamics of ligand addition to the ZnP component of the arrays 1 and 2. It is known that porphyrin metalation provides axial ligation sites that offer an opportunity to influence ET processes.7 This ability is exploited in nature where the protein matrix and associated amino acids “tune” the redox and electronic properties of porphinoid pigments through noncovalent interactions.5,8 In particular, the axial ligation of histidine to the chelated magnesium cores of the special pair of Photosystem 1 is known to play an important role in stabilizing the oxidative state of this primary donor.9 PRC mimics that incorporate axial ligation thus represent closer models of the natural system.

10.1021/jp9018553 CCC: $40.75  2009 American Chemical Society Published on Web 05/14/2009

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Figure 1. Structure of the triad 1 and tetrad 2 (tBu represents tert-butyl groups). A molecular model of 2 highlighting the interplanar angle across the Tro¨ger’s Base bridge and the general arrangement of macrocycles is included (tert-butyl groups replaced with H for clarity).

We show in the present work that through the binding of appropriate nitrogenous (4-picoline and imidazole) or anionic (Cl-) ligands to 1 and 2 we can decrease the oxidation potential of the ZnP moiety, increasing the driving force of the HT step and gaining unprecedented control of ET dynamics in the arrays. The increased exergonicity of the HT step upon ligation results in increases in the lifetime of the giant CSH state observed in the arrays in polar solvents, these increases correlating with the basicity of the ligand employed. Furthermore we show that binding of Cl- to the ZnP moiety of 1 allows the giant CSH state to be formed even in the nonpolar solvent toluene. Electrochemical and photophysical studies of the effect of ligation on the dyadic (3 and 4) and monomeric (5-7)

components of the arrays (Figure 3) are also presented to enhance understanding of the effects of ligand binding on the extended arrays. Experimental Section The synthesis of compounds 1-7 was described previously.1c,2 Unless otherwise stated, all photophysical studies were undertaken at room temperature (20 ( 2 °C), with samples prepared in 10 mm path length cuvettes. For photophysical studies, spectroscopic grade toluene (Lab-Scan) and HPLC grade PhCN (Sigma-Aldrich) were dried by passing through a neutral alumina column (Labchem, Activity 1) immediately prior to experiments. Tetrabutylammonium chloride (TBACl, Merck >98%), imida-

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Figure 2. Energetics for excitation energy transfer (EET), electron transfer (ET), hole transfer (HT), and charge recombination (CR) following photoexcitation of 1 at the zinc(II) porphyrin in PhCN and after binding of Cl- to the ZnP moiety of 1.

Figure 3. Structures of the dyadic (3 and 4) and monomeric (5-7) reference compounds (tBu indicates tert-butyl groups).

zole (ICN Biomedicals), and 4-picoline (Fluka >98%) were used without further purification. Steady-state absorption spectroscopy was performed using a Varian Cary 50 Bio UV-vis spectrophotometer. Steady-state fluorescence and phosphorescence spectra were measured using a Varian Cary Eclipse spectrophotometer. Low temperature experiments were performed using a variable temperature cryostat (Oxford Instruments Optistat). Optically dilute samples for fluorescence studies (absorbance 99% complexation of the ZnP occurred in the extended arrays (see binding studies below). None of the ligands absorb in the visible

Ligand Control of Charge Transfer Lifetimes region where photoexcitation of 1-7 was conducted, and no significant quenching of the excited singlet, triplet, or charge transfer states of 1-7 by diffusional interaction with free ligand was observed under these conditions. The latter was confirmed by monitoring the steady-state fluorescence of solutions of 1-7 (10-6-10-5 M) during titration of the ligands. As the concentration of ligand increased beyond that required for ∼99% complexation, no further fluorescence quenching was observed for 1-7, indicating that diffusional quenching by unbound ligand was not important in this ligand concentration regime (up to 50 mM). Thus studies of the ligand-bound arrays could be undertaken without interference from the large concentration of unbound ligand present in solution, an unavoidable consequence of complex formation. Ligand Addition to the Monomeric Porphyrins 5-7. It is well-known that Zn(II) porphyrins prefer to bind a single axial ligand to form a 5-coordinate, square pyramidal complex with simple nitrogenous ligands and anions.14 Association constants are typically in the 103-105 M-1 range in noncoordinating solvents and the binding is accompanied by red-shifts and changes in the ratios of the oscillator strength of the Q-bands of the porphyrin. These spectral changes correlate with the basicity and polarizibility of the ligand, suggesting that the effects are due to transfer of electron density from the ligand to the porphyrin ring via the metal center.14a Indeed, electrochemical studies of zinc(II) tetraphenylporphyrin (ZnTPP) confirm that ligation with N-donor ligands or simple anions modulates the redox potentials of the porphyrin.14b,c Addition of 4-picoline, imidazole, and TBACl to solutions of the zinc(II) quinoxalinoporphyrin monomer 6 caused redshifting and changes in the oscillator strength of its absorption bands (Figure 4, top). The detailed spectral changes that occur upon binding in toluene and PhCN are tabulated in the Supporting Information (SI, Table S1), but significantly the degree of red-shift of the absorption spectrum of 6 was greatest for Cl- ligation and least for 4-picoline ligation, correlating with the known basicity of the ligands.14b,c The red shift was greater in toluene compared to PhCN, as expected given that PhCN is a weak ligand for ZnPs.15 The fluorescence spectrum of 6 was similarly red-shifted in the presence of ligands (Figure 4, bottom, and SI, Table S1) while the fluorescence quantum yield and fluorescence lifetime were not strongly affected by N-donor ligand addition but were strongly reduced in the presence of TBACl (SI, Table S2). The strong quenching of 6 upon ligation with chloride is probably due to heavy atom-induced ISC (concentration dependent studies confirmed that this quenching was intramolecular and not due to collisional interaction with unbound TBACl). The lifetime of the triplet state of 6 (6*T) was 500 µs in deaerated PhCN, and 300 and 280 µs upon addition of ∼0.02 M imidazole and TBACl respectively in PhCN. As expected considering its lack of metal binding site, no shifts in the absorption and fluorescence bands and no change in the fluorescence quantum yield of the free-base bis-quinoxalinoporphyrin monomer 5 were observed upon addition of any of the ligands in up to 0.05 M concentration in either toluene or PhCN. The absorption spectrum of the gold(III) quinoxalinoporphyrin monomer 7 was also unaffected by the addition of N-donor ligands in both toluene and PhCN, confirming that these ligands do not interact with the metal center of the porphyrin. This is generally the case for gold(III) porphyrins, which are isoelectronic with Pd(II) and Pt(II) porphyrins that prefer a square planar coordination environment.16 However, the addition of

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Figure 4. Absorption spectra (top) and fluorescence spectra (bottom) of 6 in toluene (a) and in toluene after addition of 0.02 M imidazole (b).

TABLE 1: Association Constants (Ka) for the Binding of 4-Picoline, Imidazole, and Chloride to 5-7 in Toluene and PhCNa Ka, 4-picoline

Ka, imidazole

Ka, chloride

compound

PhCN

toluene

PhCN

toluene

PhCN

toluene

5 6 7

0 4270 0

0 17400 0

0 17000 0

0 35500 0

0 34700 4170

0 11500 269000

a

Ka is quoted in units of M-1.

TBACl to a solution of 7 caused red shifts in the absorption spectrum of 1 and 3 nm in PhCN and toluene, respectively. The interaction between chloride ions and 7 is driven by strong electrostatic attraction. Unlike PF6-, Cl- is poorly solvated in organic solvents, driving the formation of a tight ion pair with AuP(III)+. Crystallographic studies of isolated AuP(III)-Cl complexes show that the AuP(III)-Cl bond length amounts to the sum of the radii of Cl- and AuP(III)+.17 Association constants (Ka) for ligand binding were determined by UV-vis spectrophotometric titration with 6 and 7 (see the Experimental Section for details). The ZnP monomer 6 bound each ligand strongly in toluene and PhCN (Ka values >103 M-1, Table 1). Generally the Ka values for 6 were higher in toluene than in PhCN, in part because PhCN is competitive for the axial binding site of ZnP.15 The exception to this trend was for the addition of TBACl to 6. The weaker binding of Cl- to 6 in toluene compared to PhCN reflects the increased electrostatic attraction between the tetrabutylammonium cation and the Clin the more nonpolar solvent. Strong electrostatic attraction between Cl- and AuP(III)+ accounts for the high association

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1/2 1/2 TABLE 2: Half-Wave Reduction (ERed ) and Oxidation (EOx ) Potentials of 5-7 in PhCN and Toluene and in the Presence of ∼2 mol equiv of 4-Picoline, Imidazole, and TBACla

neat solvent 1/2 5 ERed 1/2 EOx 5 1/2 ERed 6 1/2 EOx 6 1/2 7 ERed 1/2 EOx 7 a

PhCN

toluene

-1.23 0.79 -0.40 1.68

-0.85 1.17 -1.16 1.02 -0.18 1.74

4-picoline PhCN

toluene

0.78

imidazole PhCN

0.72

toluene

TBACl PhCN

toluene

-1.21 0.65 -0.35 1.73

-0.87 1.19 -1.19 0.65 -0.18 1.76

Potentials are quoted in volts versus the Ag+/AgCl couple.

constant following addition of TBACl to 7 in toluene (∼105 M-1) compared to PhCN (∼103 M-1). The redox potentials for the first one-electron reduction and oxidation of the monomers 5-7 were measured in the presence of stoichiometric quantities of ligand to achieve a 1:1 complex in PhCN and toluene (Table 2, see the Experimental Section). The oxidation potential of the ZnP monomer 6 was modulated in the expected way depending on the basicity of the ligand.14b,c Shifts in oxidation potential of -0.01, -0.07, and -0.14 V were observed upon 4-picoline, imidazole, and TBACl addition, respectively, in PhCN. The oxidation potential of 6 in toluene was shifted by -0.37 V in the presence of TBACl. The reduction potential of the AuP(III)+ monomer 7 was shifted by +0.05 eV upon TBACl addition in PhCN. As ligand binding causes modulation to both the spectral and redox properties of the ZnP and AuP(III)+ monomer porphyrins, both ET and EET processes involving these moieties are expected to be affected upon ligation of the extended arrays. The following sections outline photophysical studies of reference dyads 3 and 4 that assess the effect of ligation on the photophysical reactions occurring between ZnP or AuP moieties and a neighboring FbP moiety. Ligand Addition to the FbP-AuP(III)+ Dyad 3. Since TBACl was shown to bind to and alter the redox potential of the AuP(III)+ monomer 7 in PhCN, it was important to assess the effect of TBACl addition on the interaction between the Au(III)P+ and the neighboring FbP in the extended arrays. The biquinoxalinyl-linked FbP/AuP(III)+ dyad 3 was studied in neat solvents elsewhere as a reference system for these interactions (Figure 3).3 Photoexcitation of the FbP or AuP(III)+ moiety was found to initiate a CSH reaction to form the FbP+-AuP(II) state in quantitative yield in solvents of widely varying polarity. The effect of TBACl addition on the free energy (∆G°) for this CSH reaction in 3 can be estimated using the Weller equation18

∆Go ) e[EOX(D) - ERED(A)] - E00 -

e2 4πε0εSRC-C

(1) where EOX(D) is the first oxidation potential of the donor, and ERED(A) is the first reduction potential of the acceptor, e is the charge on an electron, ε0 is the permittivity of a vacuum, εS is the static dielectric constant of the solvent, RC-C is the center-center separation of the chromophores, and E00 is the energy of the excited state from which ET occurs. If the ET reaction is a CSH, in which there is no overall change in charge, the Coulombic stabilization term on the right of eq 1 can be neglected. For photoexcitation of the FbP, the ∆GCSH was calculated to be ca. -0.5 eV for 3 in THF and toluene (5 is not soluble in PhCN).3 Upon addition of TBACl, the E00 value and EOX(D) for the FbP 5 did not change, whereas the reduction potential

Figure 5. Absorption spectra of 4 in PhCN (a) and in PhCN with 0.02 M TBACl (b).

of the AuP(III)+ 7 was shifted by +0.05 V in PhCN and was unchanged in toluene. Thus the ∆GCSH for excitation of the FbP in 3 is estimated to be changed by less than -0.05 eV in both PhCN and toluene in the presence of TBACl. Fluorescence studies of 3 in PhCN in the presence of TBACl were conducted with excitation at the FbP. FbP fluorescence was quenched by 99% in the neat solvent, and by 90% in the presence of TBACl. Thus the efficiency of CSH in 3 remains very high upon Cl- ligation. Transient absorption measurements in PhCN showed that the yield of the resulting CSH state of 3, FbP+-AuP(II), was hardly changed upon Cl- ligation and that the lifetime of the CSH state was lengthened slightly from 153 to 170 ns. These small modulations to the efficiency of the CSH reactions in 3 upon ligand binding are most likely due to the fact that the redox changes in the AuP(III)+ are at most ∼10% of the overall driving force for the reaction. The CSH reactions between FbP and AuP(III)+ moieties in the extended arrays are expected to be similarly unaffected by ligand addition. Ligand Addition to the ZnP-FbP Dyad 4. The ZnP-FbP dyad 4 was studied previously in neat solvents to model the interactions between the ZnP and FbP moieties in 1.2,3 Excitation at the ZnP resulted in EET to the FbP with >0.9 quantum yield in a range of solvents. It will be shown here that excitation at the FbP of 4 results in HT from the photoexcited FbP to the ZnP. The effect of ligand binding to the ZnP in 4 on both these HT and EET processes is investigated. Addition of the ligands to solutions of 4 caused the expected selective shifts in the absorption and emission peaks of the dyad. Figure 5 shows the absorption spectrum of 4 in PhCN in the absence and presence of TBACl. The Q-band absorptions predominantly due to FbP at 537 and 668 nm are virtually unchanged upon ligation, while the peak due to ZnP at 576 nm is red-shifted. The band at 616 nm in the neat solvent is a

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Figure 6. Absorption spectrum (a) and corrected fluorescence excitation spectrum (b) of 4 in PhCN in the presence of 0.02 M TBACl, normalized at 537 nm. The emission wavelength for the excitation spectrum was 743 nm.

Figure 7. Fluorescence spectra of 5 in aerated PhCN (a), of 4 in aerated PhCN (b), and of 4 in aerated PhCN in the presence of 0.02 M 4-picoline (c), imidazole (d), and TBACl (e). Solutions were optically matched at the excitation wavelength (668 nm).

composite band due to absorptions of both the FbP and ZnP. This band splits upon Cl- ligation, with the band at 614 nm due to FbP, and a red-shifted band at 641 nm due to the ligated ZnP. The observed shifts in the ZnP absorption and emission peaks of 4 were of similar magnitude to those observed for 6 in the presence of the same ligands in toluene and PhCN. Excitation of ZnP in 4. Time resolved fluorescence studies of 4 in PhCN with excitation of the ZnP showed that ZnP fluorescence remained strongly quenched in the presence of all of the ligands (see SI, Table S3 for details). The rate of quenching of photoexcited ZnP was in the range 7-9 × 109 s-1 in neat PhCN and in PhCN in the presence of 4-picoline or imidazole, but occurred at a rate of 1.5 × 1010 s-1 in the presence of TBACl. The quantum yield of EET (ΦEET) occurring after photoexcitation of ZnP in 4 in PhCN in the presence of ligands was assessed by fluorescence excitation spectroscopy. Figure 6 shows the corrected fluorescence excitation spectrum and absorption spectrum of 4 in PhCN in the presence of 0.02 M TBACl. The excitation spectrum was collected at an emission wavelength predominantly due to FbP (743 nm) and the spectra are normalized at 537 nm where ∼80% of absorption is due to FbP in the dyad. Under these conditions the ratio of the two spectra in regions dominated by ZnP absorption (e.g., the 570-590 nm region) gives an estimate of ΦEET.19 In the absence of ligands the normalized spectra are similar in the region of ZnP absorption at 580 nm,3 indicative of a ΦEET of almost unity. A similar result was found for 4 in PhCN in the presence of 4-picoline and imidazole. After chloride complexation however, the ZnP band is not obvious in the excitation spectrum of 4, indicating that the efficiency of EET is greatly reduced. It should be noted that the theoretical rate for Fo¨rster-type EET hardly changed for 4 in the presence of any of the ligands (see SI, Table S3 for details). The additional, more efficient nonradiative decay pathway quenching ZnP fluorescence in 4 in the presence of TBACl is mostly likely ET. While the presence of a ZnP+-FbP- state was not confirmed under these conditions, ET is expected to become competitive with EET as the oxidation potential of the ZnP is reduced by ligation. The driving force for ET from photoexcited ZnP is calculated to be more exothermic by 0.08 eV in PhCN upon addition of TBACl (eq 1, using RC-C ) 17.3 Å3). This takes into account the decrease in the oxidation potential of 7 upon chloride ligation (0.14 eV, Table 1), the reduction in the excited singlet state energy of 7 upon ligation (0.06 eV), and assumes no change in the reduction potential of

the FbP in the presence of TBACl. ET was calculated to become more exothermic by 0.04 and 0.00 eV upon ligation of 4 with imidazole and 4-picoline respectively. Thus one consequence of the effect of ligand binding to the extended arrays, at least in the case of strongly basic ligands like TBACl, is a reduction in the “antenna” role of the ZnP moiety in the arrays as an EET donor. States such as the ZnP+-FbP--AuP(III)+ state of 1, which might be generated by direct excitation of the ZnP of 1 in PhCN in the presence of TBACl, were not investigated here but could still function as intermediates for trans-array charge transfer. Excitation of FbP in 4. Upon excitation of the FbP in 4, ET can occur from the ZnP to the photoexcited FbP. In this case ET occurs between the HOMOs of the donor and acceptor and is defined as a hole transfer (HT) from the photoexcited species to the electron donor. The driving force for HT, ∆GHT, was estimated using eq 1, where D and A still refer to the electron donor and acceptor and the E00 is the excited state energy of the electron acceptor. The ∆GHT value for HT from FbP* to ZnP was calculated as -0.02 eV in THF (the FbP 5 is insoluble in PhCN) using eq 1 (RC-C ) 17.3 Å). The photoinduced HT process in 4 was assessed by fluorescence spectroscopy with excitation at FbP absorption bands. Figure 7 shows the fluorescence spectrum of 5 and that of 4, with and without the presence of ligands, after excitation at 668 nm in PhCN (solutions were optically matched at the excitation wavelength). The FbP fluorescence band of 4 at 746 nm was quenched by 58% compared to 5 in neat PhCN and quenched by 60%, 79%, and 92% compared to 5 in the presence of 4-picoline, imidazole, and TBACl, respectively. The observed quenching efficiency correlates with the basicity of the ligand and the quenching is attributed to an enhanced rate of HT in each case. Time-resolved fluorescence studies of FbP emission in 4 showed that the FbP fluorescence decay was shortened with respect to the decay of 5 and could be fitted with biexponential kinetics in neat PhCN (Table 3). The addition of 4-picoline or imidazole to 4 in PhCN caused shortening of the fluorescence decay profile, but the biexponential character of the decays remained. Upon addition of TBACl the fluorescence decay profile of 4 was shortened further and became almost exponential (the dominant decay component contributed >97% of the decay). In toluene, the decay of FbP fluorescence in 4 was exponential and similar to 5 upon addition of 4-picoline and

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TABLE 3: Lifetime in Nanoseconds of FbP Emission from 4 (τ) in PhCN and Toluene (neat) and in the Presence of ∼0.02 M 4-Picoline (pico), Imidazole (imid), and TBACl (Cl-)a toluene neat τ1

11.54 (1.0)

PhCN

pico

imid

Cl-

neat

pico

imid

Cl-

10.13 (1.0)

9.32 (1.0)

3.35 (1.0)

8.48 (0.57) 3.44 (0.43) 1.01 0.721 1.40

8.44 (0.37) 3.06 (0.63) 1.59 0.634 1.32

7.58 (0.09) 2.24 (0.91) 3.28 0.249 1.36

0.50 (0.97)

τ2 kHT k-HT kBET

0.0722

0.193

1/2 TABLE 4: First Oxidation Potential (EOx ) of 6 and the Calculated Equilibrium Constant (KHT) for Reversible HT in 4 in PhCN and in PhCN in the Presence of 4-Picoline and Imidazolea

0.279

2.2

19.1

a Relative contributions are included in parentheses. The emission wavelength for decay measurements was 746 nm. Calculated rates of HT (kHT), reverse HT (k-HT), and BET (kBET) following FbP excitation in 4 are also included and quoted in units of 108 s-1.

imidazole. Upon addition of TBACl, the decay of FbP emission in 4 in toluene was exponential but shortened compared to 5 (Table 3). In the cases where exponential or almost exponential decay of FbP fluorescence was observed in 4, a rate of HT (kHT) was calculated from the lifetime of the dominant component of FbP emission (Table 3, τFbP) and the lifetime of 5 (τ5, 11.02 and 12.59 ns in PhCN and toluene, respectively)

kHT )

1 1 τFbP τ5

(2)

Considering the low predicted driving force for the HT, the biexponential decay kinetics observed for the FbP emission of 4 in neat PhCN, and in PhCN in the presence of 4-picoline and imidazole, are suggestive of reversible HT in these conditions. If the HT process is only barely exothermic, thermal repopulation of the local excited singlet state of the FbP may occur in competition with decay of the ZnP+-FbP- state to the ground state. In this case the observed biexponential decay profile reflects the population and depopulation rates of each state. A well-known analysis of the biexponential decay allows the rate of forward (kHT) and reverse (k-HT) HT, and the rate of recombination to the ground state from the HT state (kCR), to be determined if the rate of relaxation to the ground state from the FbP*S state (k0) is known (Table 3, see SI for details).19,20 The change from biexponential to exponential decay behavior for the FbP emission of 4 upon addition of TBACl in PhCN suggests a loss of reversibility for the HT process due to increased exergonicity. The equilibrium constants, KHT, for the reversible HT processes measured for 4 in PhCN allow a spectroscopic estimation of the free energy for HT, ∆GHT (Table 4)19,20

KHT )

kHT k-HT

∆GHT ) -RT ln KHT

(3) (4)

In neat PhCN, the spectroscopically determined value of ∆GHT in 4 is -0.01 eV, and in the presence of 4-picoline and imidazole it becomes -0.02 and -0.07 eV respectively. The ∆GHT values for 4 in PhCN were also calculated from the electrochemical data. In this case the driving force for ΗΤ for 4 in neat PhCN was taken as the spectroscopically determined value (-0.01 eV), and the decrease in the oxidation potential of the ZnP was assumed to be the only factor affecting ∆GHT upon ligand addition (this assumes no change in the excited singlet state energy of the FbP in the presence of ligands). The

ligand

1/2 EOx ,6

KHT, 4

∆GHT (4)b, eV

none 4-pico imid Cl-

0.79 0.78 0.72 0.65

1.40 2.51 13.13

-0.01 -0.02 -0.07

∆GHT (4)c, eV

τCSH (1), ns

τCSH (2), µs

-0.02 -0.08 -0.15

150 163 182 238

59 60 150 2200d

a

The driving force for HT (∆GHT) estimated from both fluorescence and electrochemical studies are included. Giant CSH state lifetimes (1/e decay times, τCSH) for 1 and 2 in neat solvent and in the presence of 0.02 M ligand in deaerated PhCN are also included. Potentials are quoted in volts vs the Ag+/AgCl couple. b From analysis of FbP emission in 4. c Calculated using eq 1 using ∆GHT ) -0.01 eV in neat PhCN and assuming that the shift in oxidation potential of the ZnP is the only factor altering ∆GHT in 4 upon ligation. d The CSH transient decayed with second order kinetics in PhCN at room temperature. The 2.2 ms lifetime was measured in a 2-methyltetrahydrofuran/butyronitrile glass at 78 K.

∆GHT values calculated from the spectroscopic and electrochemical methods are in close agreement for the addition of 4-picoline and imidazole to 4 in PhCN (Table 4). The substantial ∆GHT value of -0.15 eV for HT in 4 in PhCN with TBACl could only be estimated by electrochemical means. This more substantial driving force would prevent significant thermal repopulation of the FbP*S, explaining the reduction in reversibility of the HT in 4 under these conditions. In toluene, the measured rate of HT in 4 is hardly competitive with the natural decay pathways of the singlet excited state of the FbP, even when 4-picoline or imidazole are added (Table 3). However, significantly, HT occurs efficiently in the presence of TBACl. Ligand Addition to the Extended Arrays 1 and 2. The studies of the dyads 3 and 4 emphasize that the most significant effect of ligand binding to the extended arrays is to influence the photophysical interactions between the ZnP and its neighboring FbP. Studies of 3 suggest that excitation of the FbP in 1 should lead with >0.9 yield to CSH to form the ZnP-FbP+-AuP(II) state in PhCN irrespective of the presence of ligands. Studies of 4 suggest that the efficiency of the ZnP as an EET donor will be reduced upon ligation in 1 and 2, but more importantly that HT from FbP+ to ZnP will be more exothermic by 0.01, 0.06, and 0.14 eV in the presence of 4-picoline, imidazole, and TBACl respectively in PhCN. The effects of this change in HT energetics on the formation and lifetime of the giant CSH state of 1, ZnP+-FbP-AuP(II), was investigated by laser flash photolysis with excitation into the predominantly FbP absorption band at 532 nm (Figure 8). The formation and decay of the giant CSH states were probed at 680 nm where strong absorption due to the ZnP radical cation (ZnP+) occurs.3,21 Where observed, the giant CSH states were formed within the laser pulse (∼8 ns). Addition of 4-picoline, imidazole, and TBACl caused the expected red-shifts in the absorption bands of 1 associated with the ZnP moiety at ca. 590 and 630 nm (SI, Figure S1). The transient spectra of 1 measured in neat toluene and PhCN were investigated previously,3 while the transient spectrum of 1 in PhCN with 0.02 M TBACl is shown in Figure 8. In toluene, the transient spectrum of 1 resembles exactly the transient spectrum of 3 under the same conditions and has almost exactly the same lifetime (∼12 µs).3 The spectrum shows strong transient absorption around 500 nm, and a broad absorption

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Figure 8. Transient absorption spectrum of 1 in PhCN with ∼0.02 M TBACl. Spectrum was taken 100 ns after excitation at 532 nm.

trailing into the infrared region. Minima in the spectrum at wavelengths corresponding to the ground state absorption of 5 indicate depletion of the FbP ground state. The identity of this transient is not clear at present, it is possibly the CSH state ZnP-FbP+-AuP(II) but the energetically similar FbP triplet state ZnP-FbP*T-AuP(III)+ has almost an identical spectrum and its involvement cannot be excluded here given the long lifetime of the transient (see ref 3 for a detailed discussion of this issue). Certainly the transient spectrum does not show ground state bleaching due to ZnP absorption bands and the formation of the giant CSH state in 1 neat toluene can be ruled out. In neat PhCN the spectrum of 1 again exhibits bleaching of ground state absorption bands due to FbP but also shows clear evidence of ZnP+ formation (enhanced absorption at 680 and 750 nm and ground state depletion at ca. 590 nm).3,21 The transient decayed exponentially at all wavelengths with a lifetime (150 ns) very similar to that of the FbP+-AuP(II) CSH state of 3 in the same solvent (153 ns). However, for 1 in PhCN in the presence of TBACl (Figure 8), ground state bleaching of FbP absorption bands is virtually absent and the spectrum is dominated by the features of the giant CSH state ZnP+-FbP-AuP(II). In this case the transient again decayed exponentially but with a lifetime of 238 ns, longer than the decay of the FbP+-AuP(II) CSH state of 3 in the same conditions (170 ns). The results of the transient absorption studies of 1 in PhCN can be understood with reference to the studies of reversible HT in 4. In neat PhCN a quasi-equilibrium between the ZnP-FbP+-AuP(II) and ZnP+-FbP-AuP(II) states of 1 is present and decay to the ground state occurs via the shorter lived ZnP-FbP+-AuP(II) state (Figure 2). Addition of ligands to 1 in PhCN leads to a more negative ∆G HT, pushing the equilibrium toward the longer-lived giant CSH state. The lifetime of the transient in 1 in PhCN increases with the increasing basicity of the ligand employed (Table 4). The HT processes in 1 and 4 are different. In 4 HT occurs from the singlet excited FbP to the ZnP. The driving force is extremely small in neat PhCN such that reversibility is expected (Table 4). In the case of 1, HT occurs spontaneously between the FbP+ and the ZnP, following the initial photogeneration of the ZnP-FbP+-AuP(II) state. In this case the ∆GHT is calculated simply as 1/2 1/2 (5) ∆GHT ) EOx (6) - EOx

(5)

By this calculation, ∆GHT in 1 is -0.15 eV from redox potentials measured in toluene and THF (5 is insoluble in

Figure 9. (a) Transient absorption (∆Abs) spectrum at room temperature of 2 in PhCN with 0.02 M TBACl and (b) transient decay at 680 nm with (inset) data plotted in second order linear form.

PhCN).3 Furthermore ∆GHT becomes -0.16, -0.21, and -0.29 eV in the presence of 4-picoline, imidazole, and TBACl, respectively, in PhCN. A driving force of 0.15 eV for HT in 1 in neat PhCN is certainly small enough to argue for some reversibility at room temperature and it is possible that the actual driving force is much smaller (the uncertainty in the electrochemical potential values is (0.05 V). Alternatively the involvement of an energetically close lying free-base porphyrin triplet state cannot be excluded. Nevertheless, it is clear that by stabilizing the oxidation potential of the ZnP, formation of the giant CSH state in 1 is favored, encouraging slower charge recombination to the ground state rather than reverse HT to form the shorter-lived states (Figure 2). The effect of ligand addition on the transient spectrum of 2 in neat PhCN was similar to that for 1. The spectrum in the neat solvent showed bleaching of both ZnP and FbP ground state absorption bands, and decayed exponentially over a range of wavelengths with a lifetime of 59 µs.2,3 Upon addition of TBACl, only the features due to giant CSH state formation were visible (Figure 9a). The ligand addition increases the lifetime of the transient formed in 2 in PhCN in the expected way, but the effect is far more dramatic than for 1 (Table 4). For the case of Cl- ligation, the giant CSH state decayed over milliseconds and with second order kinetics in PhCN, indicative of an intermolecular annihilation decay process (Figure 9b).6a In a low temperature solvent glass (deaerated 2-methyltetrahydrofuran/ butyronitrile at 78 K) in which collisional interactions are prevented, the giant CSH state of 2 decayed exponentially with a lifetime of 2.2 ms (SI, Figure S2).

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Hutchison et al. transfer in 4; Figure S1: Absorption spectra of 1 in PhCN in the presence of each ligand; Figure S2: Transient absorption decays of 2 in 2-methyltetrahydrofuran/ butyronitrile at 115 K and at 78 K. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 10. Transient absorption spectrum of 1 in toluene in the presence of ∼0.02 M TBACl, 150 ns after excitation at 532 nm.

Finally, no evidence of giant CSH state formation was observed for 1 in toluene even in the presence of nitrogenous ligands, consistent with other reports of extended arrays in strongly nonpolar solvents.6 The same observations were made for 2. However after addition of Cl- to 1 in toluene, clear evidence of giant CSH state formation was observed (Figure 10). As was observed for HT in 4, the large (-0.37 V) shift in the oxidation potential of the ZnP moiety upon Cl- binding in toluene presumably makes the HT process in 1 competitive with other processes deactivating the ZnP-FbP+-AuP(II) state. This is an important result as charge recombination is usually slower in nonpolar environments, where low reorganization energies push the process into the Marcus “inverted” region.22 In this case the giant CSH state of 1 in toluene with TBACl was clearly formed in combination with other transient states involving the FbP, and the transient decayed with a lifetime of 277 ns. Conclusion The addition of simple nitrogenous and anionic ligands provides remarkable control of photoinduced EET and ET behavior in the porphyrin arrays 1 and 2. Ligation allowed CSH lifetimes up to the millisecond time scale to be attained in polar solvents, and also for the antenna function of the ZnP as an EET donor to be controlled. Chloride ligation allowed trans-array charge transfer to occur in the triad 1 in a strongly nonpolar environment, the first observation of this for any array containing adjacent FbP and ZnP moieties. The triad 1 in toluene in the presence of Cl- is an all-porphyrin array showing directional, multistep ET in a hydrophobic environment and incorporating ligation at the primary donor, thus exhibiting many of the key features of the natural PRC. Acknowledgment. We thank the Australian Research Council for Discovery Grants to K.P.G. and M.J.C. which funded this research. J.A.H. acknowledges an Australian Postgraduate Award. This project is supported under International Science Linkages established under the Australian Government’s Backing Australia’s Ability. Supporting Information Available: Table S1: Absorption and emission maxima of 6 in PhCN and toluene in the presence of each ligand; Table S2: Fluorescence quantum yield and lifetime data for 6 in toluene and PhCN in the presence of each ligand; Table S3: Fluorescence quenching data for the zinc(II) porphyrin of 4 in the presence of each ligand and calculation of theoretical Forster-type energy transfer rates; Analysis of biexponential fluorescence decays for the case of reversible hole

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