Photoinduced Homolytic Bond Cleavage of the Central Si–C Bond in

Article pubs.acs.org/JPCA

Photoinduced Homolytic Bond Cleavage of the Central Si−C Bond in Porphyrin Macrocycles Is a Charge Polarization Driven Process Buddhadev Maiti,† Arun K. Manna,†,# Christopher McCleese,§ Tennyson L. Doane,§ Sudha Chakrapani,∥ Clemens Burda,*,§ and Barry D. Dunietz*,† †

Department of Chemistry and Biochemistry and Department of Chemistry, Kent State University, Kent, Ohio 44242, United States Department of Chemistry and ∥Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, Ohio 44106, United States

§

S Supporting Information *

ABSTRACT: Photoinduced cleavage of the bond between the central Si atom in porphyrin macrocycles and the neighboring carbon atom of an axial alkyl ligand is investigated by both experimental and computational tools. Photolysis and electron paramagnetic resonance measurements indicate that the Si−C bond cleavage of Si−phthalocyanine occurs through a homolytic process. The homolytic process follows a lowlying electronic excitation of about 1.8 eV that destabilizes the carbide bond of similar bond dissociation energy. Using electronic structure calculations, we provide insight into the nature of the excited state and the resulting photocleavage mechanism. We explain this process by finding that the electronic excited state is of a charge transfer character from the axial ligand toward the macrocycle in the reverse direction of the ground state polarization. We find that the homolytic process yielding the radical intermediate is energetically the most stable mechanistic route. Furthermore, we demonstrate using our computational approach that changing the phthalocyanine to smaller ring system enhances the homolytic photocleavage of the Si−C bond by reducing the energetic barrier in the relevant excited states.

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INTRODUCTION Porphyrins and related macrocycles exhibit desirable photochemical properties for applications such as photovoltaics, nonlinear optics, near-IR imaging, and photodynamic therapies.1−12 The photochemical activity can be tuned by altering the organic axial ligand or the nuclear center.8−12 In particular, photoradiation of silicon−porphyrins and their analogues allows us to functionalize the central silicon atom,13−15 whereas the majority of organosilanes are photostable.16−18 Recently Si−phthalocyanine (Si−Pc) was shown to form the reactive drug molecule Pc 4 upon 675 nm light exposure.19 Earlier studies of this interesting photochemistry have concentrated on phthalocyanine photophysical properties and applications.20−23 Radical products have also been identified upon photolysis of Li−Pcs.9 Electron paramagnetic resonance (EPR) spectra after photolysis are consistent with an uncoupled electron in the nitrogen containing inner ring of the Pc.9 Although there is substantial evidence indicating the formation of a radical intermediate in photoactivated Si−Pc systems,12,19 the mechanism, which this homolytic process follows, was not yet analyzed at the electronic structure level. DFT-based calculations were used to analyze the homolytic cleavage in similar systems such as the Co−C bond in vitamin B12 cofactors24−28 and the Fe−C bond breaking in CO ligated hemoglobin or myoglobin complex.29,30 Here, we investigate © XXXX American Chemical Society

computationally the radical formation mechanism in Si−Pc and consider means for enhancing the homolytic bond cleavage process by varying the size of the macrocycle. We find that the mechanism depends on a CT process that reverses the ground state polarization through the carbide bond. Namely, the homolysis process is enabled by the CT process that populates ring antibonding orbitals. Indeed, the CT process follows a relatively low-lying excited state that destabilizes the carbide bond. The dissociation at the excited level follows a relative small activation to yield the radical products. The homolytic cleavage of the axial Si−C bond is indicated upon 675 nm irradiation.19 The process is illustrated in Scheme 1, where Si−Pc and alkyl radical species are produced. Our calculations presented below confirm the radical as the most plausible photointermediate, whereas the heterolytic mechanism (i.e., forming cationic Si−Pc+ and an anionic alkyl) is associated with a higher potential energy surface (PES). We show below by the calculated spectra that the potential radical product is in better accordance with the measured spectral trends than the alternative ionic product. Received: June 4, 2016 Revised: September 2, 2016

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DOI: 10.1021/acs.jpca.6b05610 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Scheme 1. Homolytic Si−C Bond Cleavage in Si−Pca

calculations, the methanol solvent environment is represented through a polarized continuum model (PCM)33−38 with switching gaussians.39,40 We corrected the calculated BDEs for basis set superposition error (BSSE)41,42 using the well accepted counterpoise correction scheme43−45 where the dissociated products (i.e., the monomers) are calculated with the atomic basis set of the bonded system (i.e., the dimer). The same dispersion-corrected functional is employed to calculate excited state energies. In representing the PESs, we use the curves that follow stretching the Si−C bond. We refer below to relaxed PES, where the molecular system is reoptimized as the Si−C bond is stretched (the optimization is achieved by a constrained optimization of the ground state with the Si−C bond distance fixed at each point of the calculated curve), and to a f ixed PES, where a potential energy curve is calculated as the Si−C bond is stretched without reoptimization (the ground state geometry serves as the origin structure). Though excited state geometry optimizations are feasible for the considered systems, we do not expect these to affect our key observations concerning the mechanism. The excited state energies are obtained at the full TDDFT and the Tamm Dancoff approximation (TDA).46 Similar to earlier studies, the TDA is found to achieve a better agreement with the measured spectra and therefore is used.47−50 TDA calculations are performed with and without PCM included, where we employ the linear response level.51−54 In all calculations, we have not found any low-lying triplet excited states that can be indicated in the spectra. All calculations are implemented using Q-Chem.55

a

The formed radical is amiable to ligand exchange at the central Si atom. In water it forms the photodynamic therapy drug Pc 4.12

The radical intermediate is stabilized by the ring system through which the single electron is delocalized. Indeed, the intermediate was found to have relatively long lifetimes in the dark,19 and though it is only metastable, it is quite amiable to further chemical reactions. Depending on the chemical environment and upon light exposure, the intermediate can undergo subsequent oxidation and reduction or bond formation. For example, in water, the ligand exchange reaction results in Pc 4 that is known as a photodynamic therapy drug.12 To better understand the mechanism and predict means to enhance the photoreactivity of the Si−C bond, we consider computationally a series of porphyrin macrocycles. The studied series of molecules includes Si−phenylphthalocyanine (Si− PhPc), Si−phthalocyanine (Si−Pc), and Si−porphyrazine (Si− Pz) that are introduced in Scheme 2. These species are Scheme 2. Alkyl Silicon Porphyrin Analogues of Varied Ring Size (Phenylphthalocyanine [PhPc], Phthalocyanine [Pc], Porphyrazine [Pz])

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RESULTS AND DISCUSSION The activity of the Si−Pc complex solvated in methanol upon irradiation is indicated in both the EPR spectrum (Figure 1)

achieved by a chemically viable change of the Pc core ring of adding or removing a fused ring. From this study, we predict that the Si−C photocleavage is energetically most preferred in the Si−Pz molecule within the considered series.

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EXPERIMENTAL AND COMPUTATIONAL METHODS Continuous-wave EPR measurements were performed using a Bruker EMX X band spectrometer equipped with a Super High Q cavity. The microwave power was set to 2.0 mW. The magnetic field modulation frequency and amplitude were set to 100 kHz and 1.0 G. A time constant and conversion time of 20.48 ms was used, and the sweep time was 20.97 s. UV−visible absorption measurements were preformed using a Varian Cary Bio 50 spectrometer. All measurements were made at room temperature. Bond dissociation energies (BDEs) are obtained by comparing the energy of the bonded system to the sum of the energies of the potential dissociation products. We use the dispersion-corrected range-separated hybrid functional ωB97XD31,32 with several basis sets up to the 6-311+G(d,p) set that includes diffuse and polarization functions. In these

Figure 1. First-derivative EPR spectrum of 0.5 mM Pc227 after short irradiation times, measured at 9.84 GHz. The field sweep range shown is 3425−3600 G averaged over 36 scans. The g-value at the center of the signal is 2.004 (peak to peak width 7.4G).

and the UV−vis spectrum (Figure 2). The emerging UV−vis spectrum shows a decrease of the Q band peak, emergence of an intermediate band at 540 nm, and the B band that remains mostly unaltered. We begin our computational analysis by considering the Si− C axial BDE associated with the homolytic and heterolytic mechanisms. In all cases, the BSSE corrections are lower than 0.2 eV. The calculated BDEs confirm the homolytic process as the energetically preferred PES. The BDE of the homolytic B

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Table 2. Excitation Energies (eV) of the Different Complex Formsa form complex Si−Pc radical cation

Si−Pc radical cation

Figure 2. UV−visible spectra of Si−Pc with increasing exposure to 675 nm light. A significant decrease in Q band intensity and to a lesser degree in B band are indicated by red arrows. Emerging absorption features are indicated with blue arrows.

a

Table 1. BSSE-Corrected BDEs BDE (eV) basis set

medium

homo/heterolytic

ωB97XD ωB97XD ωB97XD B3LYP

6-31G(d) 6-31G(d) 6-311+G(d,p) 6-311+G(d,p)

gas methanol gas gas

1.7/7.6 1.8/4.0 1.9/7.2 1.2/6.3

2.18 2.18 1.74 2.05 2.10 2.11 2.16 2.16 1.60 2.02 2.08 2.09

Q(Si)

ωB97XD/6-311G(d) (0.56) (0.56) (0.16) 2.59 (1.04) (0.00) 2.76 (0.22) (0.59) (0.59) ωB97XD/6-311+G(d,p) (0.56) (0.56) (0.11) 2.54 (0.44) (0.01) 2.56 (0.74) (0.59) (0.58)

B 3.41 3.42 3.41 3.42 3.47 3.47

(0.24) (0.24) (0.09) (0.06) (0.00) (0.00)

3.39 3.40 3.56 3.70 3.58 3.68

(0.24) (0.24) (0.15) (0.12) (0.00) (0.00)

The oscillator strengths are provided in parentheses.

reproduced relatively well only by using the ωB97XD functional. The calculated B state energy is 3.4 eV and is found to be of substantial CT character, where a charge of 0.4e is transferred from the axial alkanethiol to the macrocycle in the opposite direction to the molecular dipole. The CT nature of the B states is indicated by the lower oscillator strength (OS). The OS of the B band state is 0.24 compared to 0.56 of the Q band states, which is in good agreement with the measured spectra (the measured peaks height ratio is 0.4; Figure 2). The B band CT character is also indicated by the significantly lower B3LYP energies of 2.9 eV. The tendency of B3LYP (or other LDA-based traditional Kohn−Sham functionals) to underestimate CT states energies within TDDFT is well established,57−61 as is the ability of range switching-based functionals to rectify this trend.47,60,62−69 As reference, the calculated ωB97XD Q band energies in free Pc are 2.2 eV and B band energies are 3.7 eV using the 6-311+G(d,p) basis set. We list the main MO replacements of the low-lying electronic excited states in SI Table 1. The assignment of the product to the radical form is corroborated by considering the spectral trends upon photoactivation and intermediate formation. These trends are highlighted by the arrows included in Figure 2. Importantly, the observed trend of reducing the Q band spectral peak is reproduced only for the radical form, whereas the Q band peak remains quite similar to that of the neutral form for the cation. (See excited state energies summarized in Table 2.) Also, the emergence of a spectral peak between the Q and B bands is only indicated for the radical form. The new radical band denoted Q(Si) emerges from mixing Q states and the Si center electronic structure through the singly occupied MO (SOMO). (The key orbitals for this state are illustrated in Figure 3.) The SOMO is found also to affect the low Q-band states. We next address the B band spectral peak upon photolysis. Here as well the measured trend, where the B band remains relatively unaffected, is better reproduced by the calculated spectra of the radical potential product. For both forms, the calculated OSs of the B band are lower than those of the neutral form, but the OSs of the B states of the radical form are substantial, whereas the corresponding OSs of the ionic form are vanishing small. All these calculated spectral trends are confirmed with both 6-311G(d) and 6-311+G(d,p) basis sets.

process, which yields radicals, is 1.8 eV in methanol and 1.7 eV in the gas phase, whereas the BDE of the heterolytic cleavage, which yields ionic species, is 4.0 eV in methanol and 7.6 eV in the gas phase (Table 1). In all computational levels reported in

functional

Q

Table 1, the Si axial ligand bond lengths at the equilibrium geometry are within 1.93−1.95 Å for the Si−C bond and 1.71− 1.73 Å for the Si−O bond. The Si−O BDE is higher with 3.7 eV for the homolytic process compared to 1.7 eV for the carbide bond at the same level. As the Si−C is confirmed to be the weaker bond, we consider it to be the photoactivated axial bond. The products of the photoactivation found experimentally are consistent with the interpretation of the Si−C bond being activated.56 Furthermore, our calculated BDEs are consistent with earlier computational studies of a Si−porphyrin complex with methyl and phenyl axial ligands.11 As shown in Figure 2, the measured absorption peaks are at 1.8 eV (675 nm; Q band) and 3.8 eV (328 nm; B band). Upon continued exposure to 675 nm irradiation, the photoproduct is indicated by the decrease in the Q band oscillator strengths, though the B band intensity remains relatively unaffected. In addition, a new band emerges between the Q- and B bands. As presented next, these observed spectral trends are (much) better reproduced by the calculated radical photointermediate spectra than by those of the ionic alternative potential product. The calculated electronic excited state energies that determine the absorption spectra of the complex are listed in Table 2. The low-energy and nearly degenerate Q states are the most intense absorbing states at 2.2 eV, in relatively good agreement with the measured spectra. (The Q band states involve electronic promotion from the HOMO to LUMO and LUMO+1.) The B band states due to their CT character are C

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where the CT state becomes the lowest excited state at large enough separation. We emphasize that the repulsive B state at the equilibrium structure involves a CT character of 0.4e in the reverse direction of the polarization. Importantly this CT leads to the homolytic radical products, reversing the preexisting ground state charge polarization of 0.4e with the alkanethiol being the negative pole of the molecular dipole and therefore the donor in the CT process. More specifically, we find that the Si−C bond order reduction involves the replacement of the Si−C bonding MO (HOMO− 3) by the ring π* antibonding orbital (LUMO). Namely, the CT further populates the ring π* orbital. This explains the assignment of the photolysis process to a homolysis mechanism. A similar role of an energetically low-lying CT state in the reverse direction of the ground state polarization was recently highlighted by us in the reactivity of a trimetal cyano bridged system.70 A relatively accessible repulsive PES is found using the dispersion-corrected functional (ωB97XD). The activation energy (Ea) and stretch (dSi−C) leading to the repulsive region of the PES are 0.41 eV and 0.30 Å (Table 3). We point out that

Figure 3. HOMO, SOMO, and LUMO isosurface plots of the Si−Pc complex and of the dissociated radical and ionic forms.

B3LYP values also show the same trends. We find that the same spectral trends are obtained when TDA is calculated with PCM. Namely, in spite of the CT character of the states, the excitation energies (see below) remain quite similar between gas phase TDA and PCM-TDA calculations. (The basis set, functionality, and PCM study are provided in SI Table 1). To shine light on the photoreactivity, we calculated the relevant excited state energies as the Si−C bond is stretched. The relaxed PESs for the Si−Pc system are provided in Figure 4. (See the Experimental and Computational Methods section for defining the relaxed and f ixed PESs.) The electronic excited states at several stretches of the Si−Pc bond are resolved in Figure 4, where detachment/attachment densities of the lowlying states are illustrated at three different Si−C bond lengths (1.93, 2.30, and 2.50 Å, which are noted as geometries 1, 2, and 3, respectively). At the equilibrium geometry (#1), the lowlying Q band is of π−π* character, whereas the B band possesses CT character. At the geometry with an intermediate Si−C bond length of 2.30 Å (#2), we find strong mixing of π−π* and CT character. At longer stretches (see at 2.50 Å; geometry #3) the nature of the two low-energy states is switched: The amounts of CT from axial ligand to the Si−Pc plane of the two lowest states at the different points are 0.0 and 0.38e; 0.26 and 0.18e; and 0.20 and 0.00e, at the three geometries, respectively. We therefore find that the states cross,

Table 3. Activation Energy and Required Si−C Stretches Obtained from the Relaxed PESsa complex

dSi−C

Si−Pc Si−PhPc Si−Pz

0.30 0.50 0.05

Si−Pc Si−PhPc Si−Pz

0.15 0.35 0.01

Ea

EQ

ωB97XD/6-311G(d) 0.41 2.18 (0.56) 0.91 1.96 (0.93) 0.02 2.44 (0.12) B3LYP/6-311G(d) 0.07 2.21 (0.49) 0.34 1.92 (0.83) 0.01 2.28 (0.00)

EB 3.41 (0.24) 3.52 (0.34) 3.19 (0.18) 2.94 (0.12) 2.75 (0.00) 2.38 (0.03)

a

Corresponding PCM-TDA values are provided in SI Table 4. The Q and B band excitation energies (eV) with their oscillator strengths are provided in parentheses.

the B3LYP-calculated PES confirms an even more photoreactive scenario requiring less activation (0.07 eV and 0.15 Å). The corresponding f ixed PES is associated as expected with larger activation energy and Si−C stretch required for breaking the bond (1.36 eV and 0.60 Å).

Figure 4. Left: Si−Pc relaxed PESs calulated at the ωB97XD level. The red color indicates the onset of the repulsive nature in the lowest excited state. Right: detachment (bottom)/attachment (top) densities in Si−Pc of the two low-lying states along the relaxed PES (1, 1*, 2, 2*, and 3, 3*). D

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functions as the acceptor and the alkanethiol as the donor of the 0.4e charge. Overall, similar trends of activation energy and stretching lenghts are confirmed using the B3LYP functional, as detailed in Table 3. (The B3LYP-calculated relaxed PESs are provided in SI Figure 3. We also provide the values calculated using f ixed PESs in SI Table 3 with the PESs illustrated in SI Figure 4.) Similar activation energies and stretching lengths are found for the three systems with PCM-TDA calculation that address the effect of the polar solvent on the excited states. (See values listed in SI Table 4 and PESs illustrated in SI Figure 5.)

We next consider the prospect of enhancing the photoactivity by modifying the ring structure. We add or remove the fused phenyl rings in Pc resulting in Si−PhPc and Si−Pz, respectively (Scheme 2). The excited states energies for the three systems are listed in Table 3 (B3LYP results are included in SI Table 2). The relaxed PESs of Si−PhPc and Si−Pz are illustrated in Figure 5. For the Si−PhPc with the added fused

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CONCLUSIONS We studied the Si−C photocleavage process by a combination of experimental measurements and computational analysis. Homolytic bond breaking upon irradiation in alkyl−silicon− phthalocyanines is identified as the preferred mechanism. The mechanism depends on a CT process that reverses the ground state polarization, where the alkanethiol is the charge donor. The CT process follows an excitation that activates the Si carbide bond by populating the ring π* orbital, in this way a relatively low excited state of about 2.0 eV leads to a dissociative PES to yield the corresponding radical products. Our calculations predict that the porphyrazine has the smallest activation energy for Si−C homolytic cleavage, followed by the phthalocyanine, whereas phenylphthalocyanine involves the largest activation energy. Namely, the photoreactivity increases as the conjugation of the macrocycle decreases. The indicated photoproduced metastable radical is of wide interest, as it can react to form imaging probes, singlet oxygen sensors, and photodynamic therapy agents.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b05610. Electronic excited state data calculated at various functionals and basis sets (excitation energies, activation energies and Si−C stretches, HOMO and LUMO orbital pictures, relaxed and fixed geometry PESs for the different complexes, and Cartesian coordinates of the complex series (PDF)

Figure 5. Si−PhPc (upper) and Si−Pz (lower) relaxed PESs at the ωB97XD level. The red color indicates the onset of repulsive nature in the lowest excited state.

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rings a lower photolysis activity is predicted. The increased conjugation in Si−PhPc stabilizes the Q band states and therefore widens the Q−B energy gap at the equilibrium geometry. As a result, the activation energy and required bond stretch increase in Si−PhPc, reflecting reduced photoreactivity (the PES is shown in Figure 5). The necessary stretch for the dissociation increases to 0.5 Å with 0.91 eV activation energy (compare to 0.3 Å and 0.41 eV in Si−Pc). This computational prediction of reduced activity for Si−PhPc agrees with reported measurements, where no Si−C bond cleavage in naphthalocyanines is achieved upon irradiation with 675 nm light.71 On the contrary, the smaller Si−Pz system is predicted to present enhanced photoreactivity with a barrierless reaction upon excitation (Table 3). The excited state PES of the Si−Pz complex is repulsive at the equilibrium geometry (the relaxed PES is shown in Figure 5). The vanishing activation energy results from the substantial decrease in the Q−B energy difference. Here, the Q band is only 1 eV lower than the B band states. The CT character of the Si−Pz excited state is reflected in the involved orbitals illustrated in SI Figure 2, where the Pz

AUTHOR INFORMATION

Corresponding Authors

*C. Burda. E-mail: [email protected]. Phone: 216-368-5918. Fax: 216-368-3006. *B. D. Dunietz. E-mail: [email protected]. Phone: 330-6722032. Fax: 330-672-3816. Present Address #

Work done at KSU, currently at Weizmann Inst. of Science, IL. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.D.D. acknowledges support from NSF grant CHE-1362504. We are also grateful to generous resource allocations on the Ohio Supercomputer Center and the Kent State University, College of Arts and Sciences Computing Cluster. The authors gratefully acknowledge the generous donation of Pc227 from E

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Prof. Malcolm E. Kenney at Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106, USA.

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