Regioselectively Halogenated Expanded Porphyrinoids as Building

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Regioselectively Halogenated Expanded Porphyrinoids as Building Blocks for Constructing PorphyrinPorphyrinoid Hetero-Dyads with Tunable Energy Transfer Qizhao Li, Chengjie Li, Jinseok Kim, Masatoshi Ishida, Xin Li, Tingting Gu, Xu Liang, Weihua Zhu, Hans Agren, Dongho Kim, Hiroyuki Furuta, and Yongshu Xie J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13148 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 8, 2019

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Journal of the American Chemical Society

Regioselectively Halogenated Expanded Porphyrinoids as Building Blocks for Constructing Porphyrin-Porphyrinoid Hetero-Dyads with Tunable Energy Transfer Qizhao Li,† Chengjie Li,† Jinseok Kim,‡ Masatoshi Ishida,§ Xin Li,ξ Tingting Gu,║ Xu Liang,║ Weihua Zhu,║ Hans Ågren,ξ Dongho Kim,*,‡ Hiroyuki Furuta,*,§ and Yongshu Xie*,† †

Key Laboratory for Advanced Materials and Joint International Research Laboratory of Precision Chemistry and Molecular Engineering, Feringa Nobel Prize Scientist Joint Research Center, School of Chemistry and Molecular Engineering, East China University of Science & Technology, 130 Meilong Road, Shanghai 200237, China

‡

Department of Chemistry and Spectroscopy Laboratory for Functional π -Electronic Systems, Yonsei University, Seoul 03722, Korea Department of Chemistry and Biochemistry, Graduate School of Engineering and Center for Molecular Systems, Kyushu University, Fukuoka 819-0395, Japan ξ School of Biotechnology, KTH Royal Institute of Technology, SE-10691 Stockholm, Sweden ║ School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China §

ABSTRACT: Expanded porphyrins have been attracting increasing attention owing to their unique optical and electrochemical properties as well as switchable aromaticity. Toward material applications, regioselective functionalization of the expanded porphyrins at their periphery is indeed challenging due to the presence of multiple reactive sites. Herein, a set of regioselective halogenated isomers (L5-Br-A/B/C) of neoconfused isosmaragdyrin (L5) are synthesized by a combination of the halogenation reaction of L5 and sequential macrocycle-to-macrocycle transformation reactions of its halogenated isomers. On this basis, the regioselectively functionalized isosmaragdyrins are utilized as building blocks for constructing multichromophoric porphyrinoids, specifically, hetero-dyads L5-ZnP-A/B/C, in which a common zinc porphyrin is linked at three different pyrrolic positions of isosmaragdyrins, respectively, by Sonogashira coupling reactions. The highly efficient energy cascade from porphyrin to isosmaragdyrin is elucidated using steady-state/time-resolved spectroscopies and theoretical calculations. Notably, the energy transfer processes from the porphyrin to the isosmaragdyrin moieties as well as the excitation energy transfer rates in L5-ZnP-A/B/C are highly dependent on the linking sites by through-bond and Förster-type resonance energy transfer mechanism. The site-selective functionalization and subsequent construction of a set of hetero-dyads of the expanded porphyrinoid would provide opportunities for developing new materials for optoelectronic applications.

INTRODUCTION

We herein report a unique selective post-functionalization approach for expanded porphyrinoids using a neo-confused isosmaragdyrin (L5) as a testbed (Chart 1). Recently, we have reported the synthesis of a pyrrolyl norrole (L1) and its conversion to isonorrole (L2) and isosmaragdyrin (L5) through the oxidative ring closure reaction of the corresponding linear pentapyrrole with two terminal confused pyrrole units, followed by macrocycle-to-macrocycle transformation reactions.17 The resulting derivative L5 contains two reactive Cα-H moieties at the two mislinked-pyrrole rings in the pentapyrrolic macrocycle core. Notably, the highly regioselective halogenation of L5 was achieved in this work by a combination of inherent electrophilic halogenation reactions of L1, L2, and L5 and sequential macrocycle-to-macrocycle transformation reactions. The resulting brominated isosmaragdyrins (L5-Br-A, L5-Br-B, and L5-Br-C) show the site-dependent absorption and redox properties, which encouraged us to synthesize hetero-arrays coupled with a πconjugated porphyrin. A meso-ethynylphenyl-substituted zinc porphyrin (termed as ZnP) was selected for the coupling reactions because of the intriguing properties in terms of the efficient energy and electron transfer abilities, single-molecule conductivity, and nonlinear optical properties.18–20 Thus, the three structural isomers L5-BrA/B/C were utilized as versatile intermediates to construct the porphyrin-isosmaragdyrin hetero-dyads L5-ZnP-A/B/C, the hybrid chromophores with tunable energy transfer behavior.

Design and synthesis of structurally diverse porphyrins and their regioselectively functionalized derivatives1 are essential to attain novel functions in the material applications such as catalysts,2 light-emitting materials,3 sensors,4 solar cells5 and so forth.6 Among various porphyrinoids, N-confused porphyrins,7 neo-confused porphyrins,8,9a norroles,9 and other confused porphyrinoids10 (Chart 1), which were developed by installing the peculiar mislinked-pyrrole ring(s) connected to the surrounding carbon atoms at its a- and bor pyrrolic N- and b-positions (different from the regular “a-a” mode) in the macrocyclic framework, form a unique family of carbaporphyrins.11 In fact, they have more electronically biased C-H sites in the core, and thus it would be an appropriate approach to post-functionalize the porphyrinoid scaffolds by taking advantage of the specific reactivity of the confused pyrrole rings in a controllable manner.12 In this regard, aromatic halogenation (e.g., bromination) can be considered as one of the simplest synthetic approaches for preparing useful porphyrinic intermediates suitable for the organometallic cross-coupling reactions.13 Over the past few decades, porphyrinoids with more than five pyrrole units, so-called expanded porphyrins, have attracted increasing attention because of their rich structural diversity and interesting properties which are dramatically different from porphyrins.14,15 However, because of the presence of multiple reaction sites (e.g., at their periphery), the regioselective functionalization of expanded porphyrins is relatively challenging compared with the corresponding tetrapyrrolic systems.16 ACS Paragon Plus Environment

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Chart 1. Structures of porphyrin analogues and their expanded derivatives (R = COC6F5, ZnP refers to a meso-ethynylphenylsubstituted porphyrinato zinc complex as shown in Scheme 2)

Scheme 1. Syntheses of the regioselectively halogenated L5 derivatives (R = COC6F5)

Conditions: i) NBS, CH2Cl2; ii) NaOAc, toluene, reflux; iii) NaBH4, THF/MeOH.

RESULTS AND DISCUSSION Regioselective Halogenation and Macrocycle Transformations. When L5 was treated with one equivalent of N-bromosuccinimide (NBS) in CH2Cl2 at room temperature, the b-brominated product, L5-Br-A, was obtained in a 76% yield as a light green solid (Scheme 1). The high-resolution mass spectrum (HRMS) of L5-Br-A shows a corresponding peak at m/z = 1133.9978 ([M+H]+) with a characteristic 79/81Br isotope pattern of the monobrominated structure (Figure S13 in the Supporting Information). In the 1H NMR spectrum of L5-Br-A in CDCl3, one of the b-H protons in the L5 spectrum disappeared (Figures 1a,b, and S1), indicating that L5-Br-A is monobrominated at the b-position, which was confirmed by the Xray diffraction analysis (vide infra). The second isomer L5-Br-B was obtained by the transformation of L1 to L5.17 At first, we treated norrole L1 with one equivalent of NBS in CH2Cl2 to afford the monobrominated product L1-Br-B in a 70% yield. The HRMS peak at m/z = 1155.9808 ([M+Na]+) is consistent with the proposed L1-Br-B structure (Figure S14). Likewise, the 1H NMR spectrum of L1-Br-B in CDCl3 reveals the disappearance of the a-H signal of the pyrrole ring B (Figure S2). The structure of L1-Br-B was conclusively clarified by crystallographic analysis (Figure S22). With L1-Br-B in hand, we then attempted to convert it into L5-Br-B following the reported procedure, namely, treatment with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), but in vain.17a However, to our pleasure, L5-Br-B was successfully obtained by refluxing the toluene solution of L1-Br-B overnight in the presence of 20 equivalents of sodium acetate in a 83% yield through the macrocycle expansion reaction. Again, the 1H NMR

Figure 1. Partial 1H NMR spectra of L5 (a), L5-Br-A (b), L5-Br-B (c), and L5-Br-C (d) in CDCl3.

spectrum of L5-Br-B in CDCl3 does not show the a-H signal of the pyrrole ring B (Figures 1c and S3). The crystal structure of the product also unambiguously revealed the a-bromination of the pyrrole ring B in L5 (vide infra). The third isomer L5-Br-C was also obtained similarly but with a modified method. After the bromination of L2 to L2-Br-C, the attempt to convert it into L5-Br-C by refluxing the mixture of L2-BrC in toluene, the same conditions reported in our earlier work, was unsuccessful.17b After our repeated trials, we figured out that L2-BrC was converted into L1-Br-C in a high yield of 83% by treatment with NaBH4 in a mixture of THF and MeOH.21 In turn, a further reflux of the toluene solution of L1-Br-C with a base in a way similar to that for L5-Br-B afforded the desired L5-Br-C in a 85% yield (Scheme 1c). The 1H NMR spectrum of L5-Br-C in CDCl3 is apparently different from those of L5 and L5-Br-A/B; the corresponding a-H signal of the confused pyrrole ring C was diminished (i.e., no

2 ACS Paragon Plus Environment

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Journal of the American Chemical Society oselectivity for bromination, the relative stability of the corresponding bromenium cationic intermediates was calculated by the B3LYP method (Figure S23a).22a,b Based on the result, the energy trend for L2 is well consistent with the experimentally observed regioselectivity. In the case of L1, however, the relative energies between the L1Br-B and L1-Br-C (experimentally not obtained) are approximately equal (Figure S23b). Then, the calculated affinity trend for the corresponding L5 derivatives was not in line with the experimental results. Notably, the thermodynamic stability of the substituted product, L5-Br-C, is preferential among the derivatives (Figure S23a). In this way, the difference in the obtained energies for each derivative is too small to judge the selectivity in some cases. We have thus further analyzed the distinct electron richness for the scaffolds using calculated NMR shift values and HOMO density analysis to examine the regioselectivity as proposed by Jørgensen et al.22c Considering the fact that the electron rich C-H moieties could be the substrates for electrophilic halogenation reactions, the empirically calculated NMR chemical shift values for L1, L2, and L5 were used to predict the reactivity with GIAO method (Figure S23). The most up-field shifted 1H signals with the calculated chemical shift values of dcal = 4.7 and 5.3 ppm (B3LYP/6-31G** level) are associated with the appended pyrrole a-CH in L1 and the b-pyrrole CH in L5, respectively. These assignments are well consistent with the experimental NMR spectra reported.17 In the case of L2, the regiochemical outcome is consistent with the relative magnitudes of the orbital coefficients in the HOMO.17b Prominent electron density on the local bipyrrole moiety (rings B/C) lying almost perpendicularly to the macrocyclic plane (rings A/D/E) in L2 should correlate to the inherent reactivity of pyrrolic Ca-H (ring C) toward electrophiles. The moderately electron-withdrawing pentafluorophenylcarbonyl groups appended on the pyrrole ring B may partially attribute to the selectivity.22d Overall, rationalization of the regioselective reactivity of aromatic halogenation toward such complex porphyrinoids is likely still challenging. Apparently, further consideration of other factors such as steric and the nature of electrophiles (e.g., NBS, bromine, and 1,3-dibromo-5,5-di-methylhydantoin) is needed. The combination of the above computational methods should be effective to understand the regioselective reactivity of the electrophilic bromination achieved for L1, L2, and L5. Electronic Absorption and Electrochemical Properties of the Bromo Isomers. The core-distortion in L5-Br-A/B/C gave rise to the overall nonaromatic character as observed for L5.17 The dark green colored solutions of L5-Br-A/B/C exhibit nonaromatic absorption spectra, featuring a broad absorption band beyond 650 nm and a stronger absorption peak around 400 nm (Figure S24). Interestingly, L5-Br-A only exhibits an additional low-energy shoulder peak around 750 nm compared with L5-Br-B and L5-Br-C. This result is consistent with the theoretical energy gaps obtained by the density functional theory (DFT) calculations. The HOMO-LUMO gaps of L5-Br-A, L5-Br-B, and L5-Br-C were calculated to be 2.15, 2.25, and 2.26 eV, respectively, with B3LYP method (Tables S3 and S5), which indicates that the substitution at the b-position of ring A has a more pronounced influence on extending the conjugation and affecting the electronic character. To investigate the electrochemical properties of L5-Br-A/B/C, the redox potentials were determined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in CH2Cl2 using 0.1 M tetra-n-butylammonium perchlorate (TBAP) as the supporting electrolyte (Table S1 and Figure S27). The electrochemical HOMO-LUMO gaps of L5-Br-A/B/C were estimated to be 1.66,

signal around 6.9 ppm) (Figure 1d). This observation is consistent with the HRMS data as well as the crystal structure (vide infra). The regiospecifically brominated structures of three isomers were confirmed by the single crystal X-ray diffraction analyses (Figures 2 and S22). L5-Br-A/B/C all exhibit twisted geometries consistent with those of the corresponding parent compound L5.17a Importantly, the bromination reactions were clearly elucidated to occur at the pyrrole ring A (C25-position), ring B (C27), and ring C (C41), for L5-Br-A, L5-Br-B, and L5-Br-C, respectively (cf. labels in Figure 2). In L5-Br-A, the b-brominated pyrrole ring A and the ring E are lying nearly coplanar because of the hydrogen bonding network within the core. The ring D and the confused pyrrole rings B and C are tilted from the mean plane of ring A and ring E, with dihedral angles of 55.9°, 64.1°, and 27.6°, respectively, similar to those observed in L5.17a In contrast, the dihedral angles prominently differ in L5-Br-B and L5-Br-C; the steric hindrance between the relatively large Br atom at the a-position of ring B and neighboring ring A may cause a large dihedral angle of 75.2° between ring A and ring B. In the case of L5-Br-C, the bromo-substitution at the a-position of ring C may cause a largely tilting of ring C from ring B and ring D with dihedral angles of 61.1° and 74.8°, respectively.

Figure 2. Perspective and side views of molecular structures of the brominated L5 isomers: (a) L5-Br-A, (b) L5-Br-B, and (c) L5-Br-C in the single crystals. The thermal ellipsoids are scaled to 50% probability. Pentafluorophenyl groups, solvents, and the hydrogen atoms attached to the carbon atoms are omitted for clarity. The dotted lines represent intramolecular hydrogen bonds.

As mentioned above, it is not straightforward to predict the site selectivity of the electrophilic aromatic substitution reactions (e.g., bromination) on the complex hetero-aromatic rings. Typically, electrophilic substitution of pyrroles occurs preferably at the a-positions due to the higher stability of the corresponding bromenium cation intermediate. However, the regioselective bromination of the monosubstituted pyrroles is governed by electron donating or withdrawing nature of the substituent groups.22a To gain insight into the regi-



1.78, and 1.73 eV respectively. The energy gap sequence is roughly consistent with that of the onset wavelengths23 observed in the absorption spectra and that obtained from the DFT calculations (vide supra). Construction of the Hetero-dyads. It is expected that brominated neo-confused isosmaragdyrins L5-Br-A/B/C may act as effective acceptors in the targeting porphyrin-expanded porphyrinoid heterodyads with various linking modes and tunable photophysical properties. When L5-Br-A was subjected to the Sonogashira coupling reaction with ethynyl-substituted porphyrin ZnP in the presence of Pd2(dba)3/AsPh3 catalyst in THF, the L5-ZnP-A dyad was obtained in a moderate yield of 54% (Scheme 2). Similarly, L5-ZnP-B was obtained in a 81% yield, using Pd(PPh3)4/AsPh3 in toluene/Et3N solution. In contrast, no hetero-dyad was obtained from L5-Br-C under the above conditions. Thus, we have prepared iodo-congener L5-I-C (see the Supporting Information) and subsequently coupled it with ZnP to obtain L5-ZnP-C in a 72% yield. Three kinds of hetero-dyads were characterized by 1H/13C/19F NMR spectroscopy and FT-ICR-MS spectrometry (Figures S7–S12 and S19–S21). Energy Transfer within the Hetero-Dyads: DFT Calculations, Absorption Spectra, and Electrochemistry. The molecular structures of these dyads are visualized by B3LYP (LANL2DZ) calculations (Figure S29). Depending on the linkages, the isosmaragdyrin cores are orientated in quasi-parallel to the ZnP plane for L5-ZnPA, lying perpendicularly for L5-ZnP-B, and adopting a twisted gable type conformation for L5-ZnP-C. These structural differences result in the absorption spectral profiles of the hetero-dyads that exhibit the characteristic features of both the intact ZnP unit and corresponding substituted L5 unit (Figures 3 and S24). Due to the intervening phenyl ring between the chromophore cores, electronic communication is likely weak. However, through the intrinsic electronic effect, the absorption band edge of L5-ZnP-A is remarkably extended to 850 nm, compared with those of L5-ZnP-B and L5ZnP-C. In the MO diagrams, the HOMOs are delocalized on the ZnP units, whereas the LUMOs are located on the L5 parts (Figure 4). Although there is not much difference in the energy levels of HOMOs/LUMOs (Table S4), a different behavior of L5-ZnP-A was observed in the reduction potential (vide infra). The electrochemical properties of L5-ZnP-A/B/C and ZnP were investigated in CH2Cl2 (Figure S28 and Table S2). L5-ZnPA/B/C exhibit similar oxidation potentials, with very weak, irreversible first oxidation waves at 0.92, 0.94 and 0.96 V, respectively. They were slightly anodically shifted compared with the well-defined, reversible wave of ZnP at 0.89 V. The second irreversible oxidation waves of the dyads were all split into two peaks with much stronger intensity at ca. 1.10 and 1.20 V, respectively. In the reduction side, the first reduction potentials of the dyads determined by DPV were –0.51, –0.63, and –0.59 V, respectively, in a sequence same to those observed for L5-Br-A/B/C though all peaks were slightly cathodically shifted. In contrast, the second and third reduction potentials of L5-ZnP-A/B/C were nearly identical. Thus, it is obvious that the oxidation and first reduction waves for the dyads are predominantly governed by ZnP and L5 moieties, respectively. These observations are consistent with the contour plots of frontier molecular orbitals of the dyads (Figure 4). Based on these data, the electrochemical HOMO-LUMO energy gaps of L5-ZnP-A/B/C were thus estimated to be 1.43, 1.57, and 1.55 eV, respectively, which are much narrower than the corresponding values of ZnP (2.06 eV) and L5 (1.73 eV).

Scheme 2. Syntheses of zinc porphyrin (ZnP)-isosmaragdyrin (L5) hetero-dyads by Sonogashira coupling reactions (R = COC6F5, ZnP refers to a meso-ethynylphenyl-substituted porphyrinato zinc complex)

Conditions: i) [(Pd2 (dba)3], AsPh3, THF/Et3N, 50 °C, 12 h; ii) [(Pd(PPh3)4], AsPh3, toluene/Et3N, 80 °C, 18 h.

The donor-acceptor type hetero-arrays based on the porphyrinexpanded porphyrinoid are expected to show the energy/electron transfer upon photoexcitation. Actually, as compared to the emissive ZnP with a fluorescence quantum yield (QY) of 9.5%, the singlet excited states of the dyads L5-ZnP-A/B/C were significantly quenched, showing low QYs of ca. 0.05%, 0.14% and 0.16%, respectively, with a lowest QY observed for L5-ZnP-A (Figures 3b,d), indicating that the energy transfer within the b-linked24 dyad L5-ZnPA is more efficient compared with the a-linked ones (L5-ZnP-B/C) though the influence is subtle. The possible energy transfer is facilitated by the significant spectral overlap between the fluorescence spectrum of ZnP and the broad absorption band of the L5 unit (Figure 3), and thus efficient energy transfer was observed with high efficiencies of ca. 99.5%, 98.5%, and 98.3% for dyads L5-ZnP-A/B/C, respectively.25 The very small partial energy loss of L5-ZnP-B/C may arise from the distorted nature of the mislinked-pyrroles B and C, respectively. Further investigations on the quenching process in more polar solvents like DMF revealed non-emissive features for all the dyads (Figure S25). With the aid of electrochemical properties of the dyads shown in Figure S28 and Table S2, the free energy changes for the charge separation (ΔGCS) were estimated to be approximately –0.2 ~ –0.3 eV in CH2Cl2 using the Rehm-Weller equation.26 The calculated energy levels suggest that the electron transfer


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Journal of the American Chemical Society selective excitation of the donor moiety at 550 nm, the TA spectra of L5-ZnP-A/B/C show the similar spectral features to the TA spectra of ZnP. As the time delay increases, the characteristic GSB and SE signals of the donor at 550, 600, and 650 nm, however, disappear in a few ps (Figures 5a and S31) and negative signals appear in the range of 580–670 nm which correspond to the GSB of the acceptor L5, and they follow the excited state dynamics of L5. These dramatic spectral evolutions indicate the efficient excitation energy transfer (EET) processes arising from the large spectral overlap between the emission spectra of ZnP and the absorption band of the L5.

Figure 3. Absorption (solid) (a, c) and emission (dotted) (b, d) spectra of L5-ZnP-A/B/C in CH2Cl2 along with ZnP and L5. Insets: partially enlarged (c) absorption and (d) emission spectra (lex = 421 nm).

Figure 4. Contour plots of frontier molecular orbitals of L5-ZnPA/B/C.

process is also energetically feasible within the dyads. However, considering the large distance between the donor ZnP and the acceptor L5 units for L5-ZnP-A/B/C, the efficient quenching phenomenon can be mainly attributed to the energy transfer mechanism. It is obvious that the normalized emission peaks at 600 and 650 nm for the ZnP donor unit in the dyads overlap well with those of the monoporphyrinic ZnP compound27 (Figure S26), which is in good agreement with the proposed energy transfer processes. Structure-Dependent Energy Transfer Probed by Transient Absorption Spectroscopy. Finally, we measured the femtosecond transient absorption (fs-TA) spectra to investigate the energy transfer processes in the hetero-dyads. The TA spectra of the donor ZnP, in the absence of the acceptor, show distinct ground state bleaching (GSB) and stimulated emission (SE) at 550, 600, and 650 nm, which decays without any spectral change in the early time window (Figure S30a). In the case of the acceptor moiety L5, the TA spectra exhibit negative GSB signal at 650 nm and excited state absorption (ESA) bands at 500 nm and in the range of 700 nm to 840 nm (Figure S30b). In contrast, the TA spectra of the L5-ZnP-A/B/C exhibit dynamic spectral evolutions, which provide strong evidence for the energy transfer processes (Figure 5a).28 In the early time delay upon

Figure 5. (a) Transient absorption spectra of L5-ZnP-A/B/C in CH2Cl2 upon photoexcitation at 550 nm and (b) their decay profiles at 710 nm.

To elucidate the energy transfer rates of L5-ZnP-A/B/C, we carefully monitored the TA decay profiles in the region where the SE signals of the donor disappear and the ESA bands of L5 appear as the EET occurs. The time constants of the EET processes are described as the rise component in the decay profile of 710 nm which can distinguish the distinct ESA bands of L5 compared to the broad ESA bands of ZnP in the TA spectra (Figure 5b). The decay profile of L5-ZnP-A, whose fluorescence is almost completely quenched in the steady-state measurements, shows the time constant of 1.7 ps (Figure S32a) with the rate of kEET = 5.8 x 1011 s–1. The time constants of the EET processes in L5-ZnP-B and L5-ZnP-C are assigned as 6.3 and 7.8 ps (Figures S32b,c) with the rates of kEET = 1.6 x 1011 s–1 and 1.3 x 1011 s–1, respectively. The EET rate of L5-ZnP-A appears to be the fastest even though it has the longest distance between donor and acceptor units. The theoretical Förster-type resonance en-



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Notes

ergy transfer (FRET) rates calculated based on the optimized structure are well-matched with the experimental EET rates in L5-ZnPB/C whereas the FRET rate of L5-ZnP-A is relatively slower (3 orders) than the EET rate (Table S6). The most efficient EET process despite the orthogonal transition dipole moments of L5-ZnP-A can be interpreted by a result of the electronic coupling through directly linked moieties, a through-bond effect.29 The absorption spectra of L5-ZnP-A and L5-Br-A exhibit additional absorption band at 750 nm (vide supra), which indicate the electronic interaction between the donor and acceptor units. In the contour plot of HOMO, slightly extended conjugation through ethynylphenyl group of ZnP unit is observed (Figure 4). In the contour plot of LUMO, three pyrrole rings except the mislinked-pyrrole rings contribute to the p conjugation in L5. While L5-ZnP-B/C are linked with two mislinked-pyrrole rings, only L5-ZnP-A is linked with the pyrrole ring with localized MO, resulting in the electronic coupling via the s-framework and efficient energy transfer by the through-bond effect. These TA results fully support the efficient energy transfer processes from the donor to the acceptor moieties and well describe the tunable EET rates which vary with the linking sites by through-bond effect and FRET mechanism in L5-ZnP-A/B/C.

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work at ECUST was financially supported by Shanghai Municipal Science and Technology Major Project (Grant No.2018SHZDZX03) and the international cooperation program of Shanghai Science and Technology Committee (17520750100), NSFC (21772041, 21702062, 21811530005), the Program for Professor of Special Appointment (Eastern Scholar, GZ2016006) at Shanghai Institutions of Higher Learning, Shanghai Pujiang Program (17PJ1401700), the Fundamental Research Funds for the Central Universities (WK1616004, 222201717003), and Program of Introducing Talents of Discipline to Universities (B160170). Part of the work in Kyushu was supported by Grants-in-Aid (JP15K13646 to H.F., JP16K05700, and JP17H05377 to M.I.) from the Japan Society for the Promotion of Science (JSPS). The work at Yonsei was supported by the Global Research Laboratory (GRL) Program funded by the Ministry of Science, ICT & Future, Korea (2013K1A1A2A02050183). The authors thank Research Center of Analysis and Test of East China University of Science and Technology for the help on the characterization.

REFERENCES

CONCLUSIONS

(1) (a) Vairaprakash, P.; Yang, E.; Sahin, T.; Taniguchi, M.; Krayer, M.; Diers, J. R.; Wang, A.; Niedzwiedzki, D. M.; Kirmaier, C.; Lindsey, J. S.; Bocian, D. F.; Holten, D. Extending the short and long wavelength limits of bacteriochlorin near-Infrared absorption via dioxo- and bisimide-functionalization. J. Phys. Chem. B 2015, 119, 4382−4395. (b) Xiong, R.; Arkhypchuk, A. I.; Kovacs, D.; Orthaber, A.; Borbas, K. E. Directly linked hydroporphyrin dimers. Chem. Commun. 2016, 52, 9056−9058. (c) Mandal, A. K.; Diers, J. R.; Niedzwiedzki, D. M.; Hu, G.; Liu, R.; Alexy, E. J.; Lindsey, J. S.; Bocian, D. F.; Holten, D. Tailoring panchromatic absorption and excited-state dynamics of tetrapyrrole-chromophore (bodipy, rylene) arrays—Interplay of orbital mixing and configuration interaction. J. Am. Chem. Soc. 2017, 139, 17547−17564. (d) Taniguchi, M.; Lindsey, J. S. Synthetic chlorins, possible surrogates for chlorophylls, prepared by derivatization of porphyrins. Chem. Rev. 2017, 117, 344−535. (2) (a) Groves, J. T.; Viski, P. Asymmetric hydroxylation by a chiral iron porphyrin. J. Am. Chem. Soc. 1989, 111, 8537−8538. (b) Ho, C.-M.; Zhang, J. L.; Zhou, C. Y.; Chan, O. Y.; Yan, J. J.; Zhang, F. Y.; Huang, J. S.; Che, C. M. A water-soluble ruthenium glycosylated porphyrin catalyst for carbenoid transfer reactions in aqueous media with applications in bioconjugation reactions. J. Am. Chem. Soc. 2010, 132, 1886−1894. (c) Li, G.; Dilger, A. K.; Cheng, P. T.; Ewing, W. R.; Groves, J. T. Selective C-H halogenation with a highly fluorinated manganese porphyrin. Angew. Chem., Int. Ed. 2018, 57, 1251−1255. (3) (a) Endo, A.; Ogasawara, M.; Takahashi, A.; Yokoyama, D.; Kato, Y.; Adachi, C. Thermally activated delayed fluorescence from Sn4+−porphyrin complexes and their application to organic light emitting diodes—A novel mechanism for electroluminescence. Adv. Mater. 2009, 21, 4802−4806. (b) Li, B.; Li, J.; Fu, Y.; Bo, Z. Porphyrins with four monodisperse oligofluorene arms as efficient red light-emitting materials. J. Am. Chem. Soc. 2004, 126, 3430−3431. (c) Kwong, R. C.; Sibley, S.; Dubovoy, T.; Baldo, M.; Forrest, S. R.; Thompson, M. E. Efficient, saturated red organic light emitting devices based on phosphorescent platinum (II) porphyrins. Chem. Mater. 1999, 11, 3709−3713. (4) (a) D’Urso, A.; Mammana, A.; Balaz, M.; Holmes, A. E.; Berova, N.; Lauceri, R.; Purrello, R. Interactions of a tetraanionic porphyrin with DNA: From a Z-DNA sensor to a versatile supramolecular device. J. Am. Chem. Soc. 2009, 131, 2046−2047. (b) Ishida, M.; Naruta, Y.; Tani, F. A porphyrin-related macrocycle with an embedded 1, 10-phenanthroline moiety: fluorescent magnesium (II) ion sensor. Angew. Chem., Int. Ed. 2010, 49, 91−94. (5) (a) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers. Nat. Chem. 2014, 6,

In summary, a set of regioselectively halogenated isomers of neoconfused isosmaragdyrin, L5-X-A/B/C (X = Br/I), were successfully synthesized by a combination of the aromatic halogenation reactions and sequential macrocycle-to-macrocycle transformation reactions. Although there are many potential reactive sites at the periphery of L5, regioselective functionalization was achieved by taking advantage of the inherent electrophilic halogenation reactivities of L1, L2, and L5 and the sequential macrocycle-to-macrocycle transformation reactions among these structural isomers. On this basis, three novel expanded porphyrinoid-porphyrin conjugates, L5-ZnP-A/B/C, were successfully synthesized, which can be regarded as a class of porphyrin-expanded porphyrin hybrid chromophores with unique structures and tunable energy transfer behaviors. The efficient energy transfer processes in the hetero-dyads L5-ZnP-A/B/C were elucidated based on their steady-state and time-resolved photophysical spectroscopies (electronic absorption, fluorescence, and transient absorption), and DFT calculations. The current approach of combining the regioselective aromatic halogenation reactions with sequential macrocycle-to-macrocycle transformation reactions would be a new protocol for the development of expanded porphyrin-based multichromophoric molecules, which should contribute to the broad research fields such as porphyrin chemistry, supramolecular chemistry, and materials chemistry.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental procedures, characterization data (1H, 13C, 19F NMR, HRMS, etc.), crystallographic details (CIF) for L1-Br-B/C, L5-BrA/B/C, transition absorption decay profiles, theoretical calculations for all new compounds (PDF)

AUTHOR INFORMATION Corresponding Author [email protected], [email protected], [email protected]


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Regioselectively Halogenated Expanded Porphyrinoids as Building Blocks for Constructing Porphyrin-Porphyrinoid Hetero-Dyads with Tunable Energy Transfer Qizhao Li,† Chengjie Li,† Jinseok Kim,‡ Masatoshi Ishida,§ Xin Li,ξ Tingting Gu,║ Xu Liang,║ Weihua Zhu,║ Hans Ågren,ξ Dongho Kim,*,‡ Hiroyuki Furuta,*,§ and Yongshu Xie*,†

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