Modulation of Axial-Ligand Binding and Releasing Processes onto

Article pubs.acs.org/JPCB

Modulation of Axial-Ligand Binding and Releasing Processes onto the Triazole-Bearing Nickel(II) Picket-Fence Porphyrins: Steric Repulsion versus Hydrogen-Bonding Effects Juwon Oh,† Hongsik Yoon,‡ Young Mo Sung,† Philjae Kang,§ Moon-Gun Choi,§ Woo-Dong Jang,*,‡ and Dongho Kim*,† †

Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, ‡Biopolymer Laboratory and Department of Chemistry, and §Molecular Structure Laboratory and Department of Chemistry, Yonsei University, Seoul 120-749, Korea S Supporting Information *

ABSTRACT: N-(p-Methoxycarbonylbenzyl) triazole (BTz) substituents have been introduced to Ni(II) porphyrins (NiPs), in which their modulated axial-coordination processes have been investigated. For this study, the two types of ligands, neutral pyridine versus anionic cyanide, were employed to investigate an effect of BTz substituents. The unique microenvironments given by the BTz substituents provided two different effects on the axial-coordination processes of NiPs on the ground and excited states: (1) steric shielding and (2) donation of hydrogen-bonding sites. The steric shielding diminished the binding affinity of pyridine, while the cooperation of hydrogen bonds extraordinarily strengthened the binding affinity of CN−. Interestingly, it was observed that the binding of CN− with the supporting of BTz substituents accompanied nonplanar distortion of NiPs. Such conformational change perturbed the electronic structure of NiPs, which gave rise to the modulation of coordination processes of NiPs in the excited state. As a consequence, photoinudced ligand binding and releasing processes of four- and sixcoordinated NiPs were changed into the dominant photoinduced ligand releasing process.

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INTRODUCTION Metalloporphyrins possess the highly conjugated tetrapyrrolic macrocycle and the central metal, providing proper sites for axial-ligand coordination. The axial-ligand binding/releasing processes in the metalloporphyrins affect their electronic configurations, structural conformations, and the spin state of the central metal, which plays an important role in controlling their physicochemical and catalytic properties.1−5 Nature has employed various methodologies to utilize the coordination process for optimizing biological functions such as electron transfer and oxygen activation.6,7 In functional heme proteins, the microenvironment surrounding the heme pocket greatly influences the regulation of the substrate binding process.8 The elucidation of the coordination mechanism in metalloporphyrins has been regarded as an important task, which could lead to an effective and selective regulation of various catalytic functions.9,10 To date, a majority of investigations have been conducted to modulate the axial-ligand binding/releasing processes of metallopophyrins, where various approaches were employed such as the change of planarity or electron deficiency of porphyrin macrocycle and the adjustment of basicity of ligand or temperature. 11−16 In addition, a rich diversity of modifications at porphyrin periphery have been proposed as © XXXX American Chemical Society

the new approaches to control the axial-coordination chemistry, such as the control of spatial proximity and orientation of ligands by tethering them to the porphyrin backbone.17−23 In this regard, appending functional substituents to the porphyrin backbone can be an effective methodology to modify the axialcoordination behaviors of metalloporphyrins. This strategy can exploit multiple noncovalent intermolecular interactions, such as hydrogen bonding.24−27 Furthermore, these interactions may provide conformational changes, such as the allosteric activities in cofactor F430 of methylreductase.28 Accordingly, the introduction of functional substituents at the porphyrin backbone can afford new possibilities for the coordination control in metalloporphyrins. In this point of view, we studied here the effect of triazolebearing substituents for modulating the axial-ligand binding/ releasing processes in Ni(II) porphyrins (NiPs). Among various metalloporphyrins, NiPs exhibit an unambiguously distinctive coordination chemistry in their ground and excited states (Scheme 1).29−31 In the ground state, NiPs exhibit the axialcoordination processes with nitrogen-containing ligands like Received: March 30, 2015 Revised: May 14, 2015

A

DOI: 10.1021/acs.jpcb.5b03033 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

which can fruitfully modulate the axial-coordination processes of metalloporphyrins. On the basis of the above properties of NiPs and BTz substituents, we designed novel functional NiP derivatives, containing two or four BTz substituents, to modulate the axialcoordination processes. We have investigated the effect of BTz substituents on the axial-coordination dynamics of NiPs depending on the number and geometry of BTz substituents with two types of ligands, neutral pyridine versus anionic CN−. Our comparative study has illustrated the modulation of coordination processes of NiPs under the unique microenvironment given by the BTz substituents in both ground and excited states with various spectroscopic measurements and crystallographic analyses.

Scheme 1. Photochemical Processes Associated with AxialCoordination of Ni(II) Porphyrins

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RESULTS AND DISCUSSION Structural Properties for Modulation of Axial-Coordination Process. As described above, the introduction of functional substituents on the NiPs can provide the unique microenvironment to the axial-coordination processes. To demonstrate how the intrinsic axial-ligand binding/releasing processes can be modulated by the functional substituents, we focused on the BTz groups. The BTz groups are composed of the two representative groups, Tz and MB groups. The Tz groups are known to form C−H····anion hydrogen bondings.43 Such properties of Tz groups can hold anionic ligands and facilitate the axial-coordination processes. The MB group is located at the edge in the BTz group, where the bulky MB group can effectively block the approach of ligands to the central metal. Thus, it is expected that the appending BTz group induces noticeably different coordination behaviors between anionic and neutral (or cationic) ligands with two different properties: hydrogen-bond formation and steric hindrance. With these properties, we employed the BTz groups as functional substituents and synthesized novel NiP derivatives, 1 and 2 (Figure 1). The synthesis and characterization of the newly prepared 1 and 2 are described in the Supporting

pyridine due to the thermodynamic stability of six-coordinated complexes.32 On the other hand, four- and six-coordinated NiPs undergo photoinduced coordination process (PCP) in their excited states by photoexcitation.33 Upon exposure of light, π−π* transition of porphyrin ring in four- and sixcoordinated NiPs induces a change of electronic configuration, which gives rise to an interconversion of both NiPs into each other through photoinduced ligand binding (PLB) and releasing (PLR) processes.34−36 Such ligand binding/releasing dynamics of NiPs in the ground and excited states accompany the distinct changes in the physicochemical properties, such as the optical spectral change and the coordination-induced spin crossover.37−41 Therefore, NiPs have been considered as proper systems to investigate the effect of functional substituents on the axial-coordination processes by various spectroscopic methods. For this study, we have focused on N-(p-methoxycarbonylbenzyl) triazole (BTz) groups as functional substituents. The BTz substituent is composed of a triazole (Tz) group, providing a hydrogen-bonding site, and a bulky methoxycarbonylbenzyl (MB) group, giving rise to a steric shielding effect.42 Thus, the appending BTz substituents to metalloporphyrins provide a unique microenvironment around the central metal,

Figure 1. Molecular (top) and X-ray crystal (bottom) structures of (a) 1 and (b) 2. Solvent molecules are omitted for clarity. B


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The Journal of Physical Chemistry B Information. Because of the steric limit on the rotation of BTz substituents, 1 and 2 have atropisomerism. We utilized the facing geometry of BTz substituents; 1 has two BTz substituents placed on one side of the porphyrin plane, as an αα atropisomer, and two pairs of BTz substituents are placed on both sides of the porphyrin plane in 2, as an αβαβ atropisomer. These molecular structures were clearly determined by the single crystal X-ray crystallography. The symmetric geometry of 1 and 2 can be considered to be advantageous for modulating the axial-coordination processes of NiPs via two intrinsic properties of BTz substituents: bulky size and hydrogen-bonding donor. The face-to-face geometry enables the BTz substituents to cover effectively a central nickel(II) as a steric shielding effect. Moreover, these conformations also allude that they can provide strong cooperative C−H hydrogen bondings to anions in the opposite directions,44 indicating that the anions can be more tightly fixed by the BTz substituents. Axial-Coordination Process with Pyridine in the Ground State. To investigate the effect of BTz substituents on the axial-coordination processes of NiPs, we employed the two representative ligands, pyridine for neutral coordinating ligand and cyanide for anionic ligand. Figure 2 shows the steady-state absorption spectra of 1 and 2 in toluene and pyridine, noncoordinating and neutral coordinating solvents, respectively. As compared to the absorption spectra of 1 and 2 in toluene, those of 1 and 2 in pyridine showed additional absorption bands at ∼25 nm red-shifted positions from the typical B and Q bands of NiPs.45 According to previous spectroscopic investigations, these new bands are attributable to the six-coordinated species, where the reduced energy gap between HOMO and LUMO by the axial-coordination gives rise to the red-shifted B and Q bands in the absorption spectrum.29,32 Thus, these absorption spectra indicate the formation of six-coordinated species (1·2py and 2·2py). Notably, the more distinct absorption bands of sixcoordinated complexes appear in the absorption spectrum of 1 in pyridine as compared to that of 2. The integrated ratios

based on the split B bands, corresponding to four- and sixcoordinated species, respectively, reflect that 50% of 1 and 30% of 2 are axially coordinated with pyridine, respectively. Such a difference in the population of 1·2py and 2·2py can be explained by the steric repulsion effect of BTz substituents. As seen in the crystal structures, perpendicularly aligned mesophenyl groups to the porphyrin macrocycle allowed the BTz substituents to be readily placed on the porphyrin plane. Moreover, the length of MB groups is similar to that of the porphyrin ring. As a result of the structural features, the BTz substituents can effectively cover the porphyrin macrocycle and hinder the approach of pyridine (Supporting Information Figure S1). In a series of 1 and 2, while 1 has two BTz substituents on one side of the porphyrin plane, two BTz substituents are placed on both sides of the porphyrin plane in 2. Consequently, the axial-binding of pyridine is restricted more severely in 2, causing the formation of six-coordinated complexes to be more diminished. Axial-Coordination Process with Cyanide in the Ground State. To explore the axial-coordination process of CN−, anionic ligand, to 1 and 2, the steady-state absorption spectra were measured under various CN− concentrations (Figure 3). Upon addition of CN−, new absorption bands at 430, 550, and 585 nm for 1 and 450, 565, and 600 nm for 2 grew with clear isosbestic points (380, 413, 488, and 519 nm for 1 and 391, 429, 500, and 545 nm for 2), respectively, which indicates the axial-binding of CN− to 1 and 2. Deeper insights into the nature of the NiP complexes with CN− were drawn by using Job’s plot and coordination equilibrium constant analyses. Job’s plot analysis of 1 and 2 determined that the stoichiometry of complex formation with CN− is 1:2, indicating the formation of six-coordinated complexes (1·2CN− and 2·2CN−) (Supporting Information Figure S2).46 The binding constants K1 and K2, corresponding to the two stepwise coordination model, were estimated to be 3.6 × 103 and 1.3 × 104 M−1 for 1 and 2.9 × 104 and 1.4 × 105 M−1 for 2, respectively.47,48 Interestingly, in contrast to the suppressed binding of pyridine, the BTz substituents promoted the coordination processes with CN−. As compared to the binding constants of

Figure 2. Steady-state absorption spectra of (a) 1 and (b) 2 in toluene and pyridine.

Figure 3. Steady-state absorption spectra of (a) 1 and (b) 2 under addition of CN− in toluene. C


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The Journal of Physical Chemistry B pyridine, those of CN− to 1 and 2 demonstrated the highly enhanced axial-coordination processes with CN− ligands. Moreover, while Ni(II)tetraphenylporphyrin (NiTPP), an unsubstituted analogue, showed marginal spectral changes upon a large excess addition of CN− (Supporting Information Figure S3), 1 and 2 exhibited fast saturation of absorption changes upon addition of CN−. Such highly enhanced coordination behaviors of CN− to 1 and 2 are attributed to the C−H···anion hydrogen-bonding donor character of Tz groups in the BTz substituents.43 The flexible BTz substituents of 1 and 2 can capture the CN− ligands by the hydrogenbonding. Here, a pair of BTz substituents placed on the same porphyrin plane can cooperatively bind the CN− ligands.44 Moreover, according to the crystal structures, the protons of Tz groups heading toward the porphyrin plane effectively encompass the CN− ligands. These features enforce that the position of CN− can be determined to locate toward the porphyrin plane. Therefore, such supportive hydrogen bonding by BTz substituents established a proper orientation of CN− and restricted its motion, both of which provoked the substituent-triggered highly enhanced axial-coordination of CN− to NiPs. It should be noted that, as compared to 1·2py and 2·2py, the axial-binding of CN− to 1 and 2 showed peculiar spectral features, new absorption bands at 400−410 nm in the absorption spectra. Moreover, the broader and more redshifted spectra were simultaneously observed under CN− condition. These anomalous absorption bands are evidently distinguished from those originated from residual fourcoordinated species. Supposing that the new absorption bands arise from the residual 1 and 2, the integration of split B bands indicates that only 50% of 1 and 65% of 2 form sixcoordinated complexes. However, these results were contradictory to their binding constants because 92% and 97% of 1 and 2, respectively, were estimated to be six-coordinated under 1000 and 200 equiv of CN− condition. Moreover, the TA spectra of 1 and 2 under CN− condition demonstrated that the photoexcitation at 400 nm induced the π−π* transition of 1· 2CN− and 2·2CN− (see below). These results strongly suggest that the additional absorption bands in 400−410 nm are originated from the axial-coordination process of CN− to 1 and 2. The unexpected spectral features of 1·2CN− and 2·2CN− can be ascribed to specific conformations imposed by the BTz substituents. The BTz substituents aid the axial-binding of CN−. The minimum distance between the face-to-face protons at Tz groups can be estimated as 6.4 Å. In this geometry, because the BTz substituents bind CN− through supportive hydrogen bonding, it is expected that the axial-coordination of fixed CN− forces the BTz substituents to be pulled inside the porphyrin ring. In this process, the coordination of CN− requires a distortion of the porphyrin ring, where ruffled conformation of porphyrin by pulling of BTz substituents is conceived (Figure 4). This ring distortion is evidenced by their absorption spectra, the broad and red-shifted bands. Both features are typical spectral features of nonplanar porphyrins.28,49 The substituent-triggered distortion of porphyrin can entice a displacement of the central metal and the nitrogen atoms of pyrrolic units, where severe ruffling affects the electronic interaction between the metal and the porphyrin macrocycle.50−53 As a result of these perturbations in the electronic structures of porphyrin ring, 1·2CN− and 2·2CN− display additional absorption bands in 400−410 nm.54−57

Figure 4. Schematic illustration of the conformational change in the formation of 1·2CN‑.

Particularly, as compared to 1, the four BTz substituents of 2 on both sides of the porphyrin plane can give rise to more severe ruffling in the axial-binding process of CN−, leading to a more noticeable absorption band at 410 nm. Axial-Coordination Process with Pyridine in the Excited State. NiPs are known to undergo the photoinduced coordination process (PCP).29−31 By photoexcitation, four- and six-coordinated NiPs are interconverted into each other in the excited state through photoinduced ligand binding (PLB) and releasing (PLR) processes due to a change of electronic configuration (Scheme 1).34−36 Because such binding/releasing processes of PCP accompany the large change in the optical spectra of NiPs, the effect of BTz substituents on the axialcoordination processes in the excited state of NiPs was investigated through transient absorption (TA) measurement. For selective photoexcitation of four- and six-coordinated 1 and 2 in pyridine, we employed photoexcitation pulses at 400, 430, and 440 nm, respectively (400 nm for the π−π* transition of 1 and 2 and 430 and 440 nm for 1·2py and 2·2py). Figure 5a and c shows the TA spectra of 1 and 2 in pyridine, respectively, by photoexcitation at 400 nm and their kinetic profiles at ground-state bleaching (GSB) signals. In the TA spectra, the GSB and photoinduced absorption (PIA) signals remained at 5 ns delay time, and the PIA signals in 540−600 nm were gradually red-shifted as the delay time became longer. These TA spectra in pyridine are completely distinguishable from their intrinsic transient dynamics observed in the TA spectra recorded in toluene (Supporting Information Figure S4). The distinctive spectral features in the TA spectra of 1 and 2 in pyridine by photoexcitation at 400 nm indicate the PLB cycle (Supporting Information Figure S5).35 As described in Scheme 1, photoexcitation of four-coordinated species produces sixcoordinated complexes by the PLB processes (d process in Scheme 1) and the photochemically produced six-coordinated complexes undergo the recovery process (h process in Scheme 1), where ligands are released from the complexes to achieve an equilibrium between four- and six-coordinated NiPs in the ground state.58 Because of this PLB cycle, the TA spectra of 1 and 2 in pyridine by photoexcitation at 400 nm showed spectral change as delay time increased, where the PIA signals were redshifted to the same position as the Q bands of six-coordinated complexes. Moreover, the TA spectra at 5 ns delay time are in good agreement with the difference spectra, the absorption spectra measured in pyridine minus those in toluene (Supporting Information Figure S6). These results corroborate that 1·2py and 2·2py are produced by the PLB processes. Such PLB cycles of 1 and 2 with pyridine are well described by their TA kinetic profiles at the GSB signals. In contrast to the fast decay components of 1 and 2 in toluene (Supporting Information Table S1), those in pyridine displayed two representative slow decay components, one in the order of D


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Figure 5. TA spectra of 1 and 2 in pyridine. The TA spectra and kinetic profiles (inset) of 1 in pyridine were recorded by photoexcitation at (a) 400 nm and (b) 430 nm. Those of 2 in pyridine were recorded by photoexcitation at (c) 400 nm and (d) 440 nm. (e,f) The schematic illustrations represent the related coordination processes in the excited state. In the schemes, the two BTz substituents of 2 located on the bottom of porphyrin ring are omitted for clarity.

Table 1. Summary of Representative Decay Components of 1 and 2a 1 a

b

condition (λpump)

pyridine (400 nm)

pyridine (430 nm)

τ1 τ2

300 ps >20 ns

250 ps >20 ns

1000 equiv of CN (400 nm) 100 ps 15 ns

2 −c

1000 equiv of CN (430 nm) 100 ps 15 ns

−c

pyridine (400 nm)

pyridine (440 nm)

200 equiv of CN− f (400 nm)

200 equiv of CN− f (450 nm)

200 ps >20 ns

250 ps >20 ns

50 ps 10 ns

50 ps 10 ns

d

e

a

All components were obtained with TA kinetic profiles at their GSA signals. The monitored wavelengths were 519,a 555,b 554,c 531,d 571,e and 567f nm. The TA measurements were carried out in pyridinea,b,d,e and toluene.c,f

difference spectra, the absorption spectra measured in toluene minus those in pyridine (Supporting Information Figure S6). These features indicate the PLR cycle of 1·2py and 2·2py (Supporting Information Figure S5),34−36 where the sixcoordinated complexes are converted to the four-coordinate species by the PLR process (g process in Scheme 1) and the subsequent recovery process (i process in Scheme 1) leads to a recovery to the six-coordinated complexes based on their equilibrium in the ground state. The two decay components obtained from the kinetic profiles at GSB signals are also in accordance with the TA spectral features. The faster decay components arise from the deactivation of the excited sixcoordinated complexes competing with the PLR processes, and the slower decay components arise from the tardy recovery processes in the ground state, requiring tens of nanosecond.32,60 Here, as we expected, the steric repulsion of BTz substituents suppressed the PLB processes. In the comparative analysis of TA spectra by photoexcitation at 400 nm between 1 and 2, it is found that the spectral change of PIA signal in the TA spectra of 2 was less apparent as compared to that of 1. Such tendency was more clearly confirmed in the TA spectra of NiTPP in pyridine by photoexcitation at 400 nm (Supporting Information Figure S7). Moreover, the comparison of TA kinetic profiles showed that the decay component of 2 resulting from the deactivation process, 200 ps, was faster than that of 1, 300 ps, which indicates that, in the excited state, more population of 2 undergoes the deactivation process to the ground state

hundred picosecond and the other in nanosecond order (Table 1). These kinetic profiles can be explained by the PLB cycle of NiPs. As described in Scheme 1, the photoexcitation of NiPs induces π−π* transition of porphyrin macrocycle (a process in Scheme 1). Subsequently, the excited NiPs undergo a rapid internal conversion from the lowest 1(π,π*) state to Ni(II)excited 3(d,d) state (b process in Scheme 1).59 Because the 3 (d,d) electronic configuration of Ni(II) is favorable for ligand binding, NiPs displayed the PLB processes.35 Here, the excited 1 and 2 undergo the two competitive processes in the PLB cycle: the deactivation to the ground state and the PLB process, where the excited species having no interaction with pyridine are rapidly relaxed to the ground state (c process in Scheme 1) and dominantly produce faster decay component on hundreds picosecond time scale, being similar to their intrinsic excitedstate lifetime.15,35 On the other hand, 1·2py and 2·2py produced by the PLB process experience the tardy recovery processes, resulting in slower decay on nanosecond time scale.32,60 Figure 5b and d shows the TA spectra recorded by photoexcitations at 430 and 440 nm for selective excitation, where the GSB signals located at the same position of the Q bands of 1·2py and 2·2py, representing the transient dynamics of 1·2py and 2·2py.61 In the TA spectra, the GSB signals remained at longer delay (>5 ns), and the PIA signals at 510− 550 nm were gradually blue-shifted to the same as the Q(1,0) bands of 1 and 2 as the delay time increased. Furthermore, the TA spectra at 5 ns delay time were in good agreement with the E


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Figure 6. TA spectra of 1 and 2 under CN− condition. The TA spectra and kinetic profiles (inset) of 1 recorded in toluene with 500 equiv of CN− by photoexcitation at (a) 400 nm and (b) 430 nm. Those of 2 recorded in toluene with 100 equiv of CN− by photoexcitation at (c) 400 nm and (d) 450 nm.

bands around 400−410 nm region. Because 85% and 90% of 1 and 2 were axially coordinated under 500 and 100 equiv of CN− conditions, respectively, photoexcitation at 400 nm can dominantly induce the π−π* transition of 1·2CN− and 2· 2CN−. As a result, the PLR cycle is predominant in the excited state of NiPs under CN− condition. To explore the PLB processes of 1 and 2 with CN− in more detail, we carried out fs-TA measurements under unsaturated CN− conditions (40 and 10 equiv of CN− for 1 and 2, respectively), in which only 25% and 30% of 1 and 2 exist as six-coordinated complexes, respectively. Apparently different TA spectra were observed by photoexcitation at 400 nm from those under excess CN− conditions (Figure 7). The initial TA spectra (at 5 ps delay time) by photoexcitation at 400 nm exhibited spectral features similar to those in pyridine, where the observed GSB signals were at the same position with the Q bands of 1 and 2. As the delay time increased, the positions of GSB and PIA signals, observed initially at 515 and 540 nm for 1 and 525 and 550 nm for 2, respectively, were changed to 555 and 515 nm for 1 and 560 and 525 nm for 2, respectively. On the basis of the difference spectra (Supporting Information Figure S6) and the positions of GSB and PIA signals, the TA spectra at 5 ps and 3 ns were determined to reflect the PLB and PLR cycles, respectively. This indicates that the PLB process was diminished and the PLR process became dominant as the delay time increased. Such a change of GSB and PIA signals in the TA spectra under unsaturated CN− condition is understood as quasiphotoreaction to produce 1·2CN− and 2·2CN−.61 Because the PCP is a nonequilibrium process in the excited state, the recovery of equilibrium state between four- and six-coordinated species is followed in the ground state.30 The unique microenvironment given by BTz substituents alters these PCP of NiPs. In the excited state of 1 and 2, the BTz substituents promoted the PLB processes with CN− through the supportive hydrogen bonding. In the ground state, it can be envisioned that the intensified coordination of CN− by the BTz substituents impedes the subsequent recovery processes. Such effect of BTz substituents on the PLB cycle results in a delayed

without the PLA processes than that of 1. These features are the result of the suppressive effect from the BTz substituents. While NiTPP has no steric hindrance for axial coordination, one and both sides of the porphyrin plane are blocked by the BTz substituents in 1 and 2, respectively. As a consequence, the PLB processes become more restricted in the order from NiTPP to 1 and 2, which cause that the TA spectra by photoexcitation at 400 nm display more distinct spectral features of the PLB cycle in the order from 2 to 1 and NiTPP. Axial-Coordination Process with Cyanide in the Excited State. Because the BTz substituents provide a unique microenvironment to the central Ni(II) of NiPs, the axialbinding of CN− to 1 and 2 in the excited state was investigated by TA measurement. In the case of CN− ligand, photoexcitation pulses at 400, 430, and 450 nm were employed for selective excitation of four- and six-coordinated species, respectively. Intriguingly, regardless of selective photoexcitation of four- and six-coordinated species, all of the TA spectra exhibited similar TA spectral features, where the PIA signals in 515−540 nm were slightly blue-shifted to the same position as the Q bands of 1 and 2 as the delay time became longer (Figure 6). Although 15% and 10% of 1 and 2 remained as fourcoordinated species, respectively, under 500 and 100 equiv of CN− based on their binding constants, these features in the TA spectra were in a sharp contrast with those measured in pyridine, showing the clearly distinguished PLB and PLR cycle depending on selective photoexcitation. On the basis of a comparison of the TA spectra at 5 ns delay time with the difference spectra between the absorption spectra of four- and six-coordinated 1 and 2 (Supporting Information Figure S6) and the same position of PIA signals at 5 ns as the Q bands of 1 and 2, the TA spectra under the CN− conditions by photoexcitation at 400, 430, and 450 nm are designated as reflecting the PLR cycle. Such peculiar PCP of 1 and 2 with CN− can be interpreted by their absorption spectra under CN− condition (Figure 3). As discussed above, the substituent-triggered axial-coordination of CN− gave rise to the nonplanar distortion and electronic structural change of NiPs, giving rise to additional absorption F


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extraordinarily high binding affinities of CN−, in the order of 103−105. Furthermore, the effect of BTz substituents on the axialcoordination of CN− significantly modulates the PCP of NiPs. NiPs show the typical PCP, composed of PLB and PLR cycles (Supporting Information Figure S5), as shown in the case of pyridine ligand. However, in the case of coordination of CN− to 1 and 2, the BTz substituents provide a unique microenvironment in the coordination processes, resulting in the predominant PLR cycle in the PCP (Figure 8). This change of PCP arises from the effect of BTz substituents on the electronic structures of 1·2CN− and 2·2CN− and the recovery processes in the PLB and PLR cycles. The coordination of CN− with the aid of BTz substituents is accompanied by the nonplanar distortion of NiP, which leads to a noticeable change in the electronic structures, additional absorption bands of 1· 2CN− and 2·2CN− in the range of 400−410 nm. These electronic structural changes enable the concurrent π−π* transitions of four- and six-coordinated 1 and 2, which arouses that the PLR cycle can occur even in the photoexcitation at the wavelength region for triggering the PLB cycle. Moreover, the recovery processes in the PLB and PLR cycle are significantly affected by the BTz substituents. In the PLB cycle, 1·2CN− and 2·2CN− produced by the PLB process have the intensified coordination of CN− with the aid of BTz substituents. This tightening in the coordination of CN− suppresses the releasing of CN−, which retards the recovery processes, returning to 1 and 2. On the other hand, in the PLR cycle, the BTz substituents continuously seize CN− even after the PLR processes. This trapping effect restricts a dispersion of CN− and stimulates the subsequent recovery process, the axialbinding of CN−. Such effects of BTz substituents on the electronic structures and the recovery processes attenuate the PLB cycle and promote the PLR cycle of NiPs. Consequently, the BTz substituents enforce that the PLR cycle is dominantly operative by photoexcitation of 1 and 2 under CN− condition.

Figure 7. TA spectra and kinetic profiles (inset) of (a) 1 and (b) 2 recorded in toluene with 40 and 10 equiv of CN−, respectively, by photoexcitation at 400 nm.

releasing of CN−. In the PLB cycle of 1 and 2, the change in position of GSB signal indicates that the recovery process was delayed longer than our excitation pulse interval time (1 ms), causing the accumulation of 1·2CN− and 2·2CN− with increasing the delay time. On the other hand, the BTz substituents gave rise to a contrasting effect on the PLR processes. The CN− ligands were consistently held by the Tz groups regardless of formation or destruction of axialcoordinate bonds. This feature can maintain the proximity and orientation of CN−, even in the uncoordinated state. Such trapping effect accelerates the recovery processes, the axialbinding of CN−, of the PLR cycle. As a result of the effect of BTz substituents on the PLB and PLR cycles, unlike the case of pyridine ligand, the PLB process dwindled and the PLR process became dominant as the delay time increased, which results in the quasi-photoreaction features in the TA spectra of 1 and 2. Effect of BTz Substituents on the Axial-Coordination Process of NiPs. The BTz substituents achieved significant modulation on the axial-coordination processes of NiPs (Supporting Information Figure S8). As discussed with the absorption spectra, the screening of central Ni(II) by BTz substituents restricted the axial-binding of pyridine to 1 and 2. On the other hand, the supportive hydrogen bonding by BTz substituents led to highly enhanced binding affinity of CN−. Here, such substituent-triggered axial-coordination of CN− to NiPs indicates that the unique microenvironment given by the BTz substituents enables NiPs to implement host−guest systems. NiPs are known to show very weak Lewis acidity, and their coordination processes are usually observed in nitrogenous solvents, that is, a large excess of ligand concentration.38,62 However, the BTz substituents facilitated the binding of CN− to NiPs, which rendered 1 and 2 to overcome the weak coordination feature of typical NiPs. As a result, in contrast to the suppressed binding of pyridine, 1 and 2 showed the characteristic behavior of the host−guest system with the

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CONCLUSION We have investigated how the BTz substituents at the porphyrin periphery modulate the axial-coordination processes of NiPs in the ground and excited states. The BTz substituents

Figure 8. Proposed model for the effects of BTz substituents on the axial-coordination processes of Ni(II) porphyrins with CN−. G


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effectively affect the axial-coordination processes of NiPs in the ground and excited states with their two intrinsic characteristics, a steric hindrance and hydrogen-bonding donor. On the basis of these two characteristics, the BTz substituents display contrasting roles in the axial-coordination of NiPs as a suppressor to pyridine ligand and as a facilitator to CN− ligand. Particularly, the substituent-triggered enhanced coordination of CN− enabled 1 and 2 to work as host−guest systems. Moreover, the BTz substituents gave rise to the nonplanar distortion of NiPs. Such conformational change led to the modulation on the excited-state coordination processes of 1 and 2 with CN−, where the PLB and PLR cycles of typical fourand six-coordinated NiPs changed into the predominant PLD cycle in 1 and 2. We believe that our investigation of the effect of BTz substituents on the axial-coordination of NiPs provides important information for controlling the axial-ligand coordination of metalloporphyrins to regulate their catalytic functions. Furthermore, our study will provide further insight into the rational design of functional group-substituted molecular systems, such as the synthetic mimicries of enzymes.

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ASSOCIATED CONTENT

S Supporting Information *

Details of synthesis, methodology, and crystallographic and spectroscopic data. CCDC-1032186 (for 1) and CCDC1032238 (for 2) contain the supplementary crystallographic data. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcb.5b03033.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +82-2-2123-7644. E-mail: [email protected]. *Tel.: +82-2-2123-2652. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Global Research Laboratory Program (2013K1A1A2A02050183) and the MidCareer Researcher Program (2014R1A2A1A10051083) funded by the Ministry of Science, ICT & Future, Korea. Graphical displays were generated with the Discovery Studio Visualizer.

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REFERENCES

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Modulation of Axial-Ligand Binding and Releasing Processes onto

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