Symmetric Positional Isomers of BODIPY Substituted - ACS Publications

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry 3

C-Symmetric Positional Isomers of BODIPY Substituted Triazines: Synthesis and Excited State Properties Ramesh Maragani, Michael Thomas, Rajneesh Misra, and Francis D'Souza J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02967 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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The Journal of Physical Chemistry

C3-symmetric Positional Isomers of BODIPY Substituted Triazines: Synthesis and Excited State Properties Ramesh Maragani,a Michael B. Thomas,b Rajneesh Misra,*,a and Francis D’Souzab,* a

Department of Chemistry, Indian Institute of Technology, Indore 453552, India.

b

Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, TX

76203-5017, United States. (Abstract). A series of meso-O-aryl functionalized BODIPY trimers positioned along the C3symmetric axis of triazine ring have newly been synthesized to probe the ground and excited state intramolecular type interactions between the BODIPY entities within the trimer. The developed synthetic strategy resulted in BODIPY trimers in good yields. The electron rich, meso-O-aryl functionalized BODIPYs revealed larger HOMO-LUMO gap, higher Stokes shift and fluorescence lifetimes compared to the traditional BODIPY derivatives having an aryl group attached at the meso position. The optical absorption, steady-state fluorescence and electrochemical studies revealed week, if any, intramolecular type interactions among the BODIPY entities within the trimer and the central triazine unit to be both photo- and redox-salient. The possibility of singletsinglet energy migration among the BODIPY entities was investigated using time-resolved emission and femtosecond transient absorption studies. Excitation of BODIPY entity in the trimers led to successful formation of 1BODIPY* that populated the 3BODIPY* via intersystem crossing. Among the three trimers, although much weak, only trimer 8 revealed excitation transfer to some extent. The present findings suggest that the meso-O-aryl functionalized BODIPYs due to their superior fluorescence properties are better probes to build light energy harvesting supramolecular oligomeric systems and for other applications such as sensing and imaging.

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1.

Introduction Over the last two decades research efforts have been directed to unveil the complex

excitation transfer phenomenon occurring in natural photosynthetic light-harvesting complexes.120

The covalently linked molecular arrays developed to mimic the natural light-harvesting have

the potential for building molecular photonic and artificial light-harvesting devices.21-33 Such molecularly engineered systems provide control over the spatially well-organized molecular architectures. A fundamental challenge in the photodynamics of such arrays is elucidation of excitation energy transfer between the fluorophores and the degree of the excitation delocalization. In bacterial photosynthetic systems such as R. Sphaeroides, two types of antenna complexes exist: a core light-harvesting antenna (LH1) and a peripheral light-harvesting antenna (LH2).34 The reaction center where a series of photoinduced electron transfer events occur resides within LH1.35-36

The LH2 comprised of two wheel-like structures: B800 with 9

bacteriochlorophyll a (Bchl a) entities and B850 with 18 Bchl a entities revealing different degrees of intra-dimer distances causing weak (in the case of B800) to strong (in the case of B850) interactions.37-38 In designing molecular arrays to mimic the natural photosynthetic systems, most of them have been prepared based on linear arrangement via macrocycle peripheral functionalization.1-20 Circular arrangement of fluorophores in an even manner is crucial for mimicry of natural LHarrays in view of their geometry. In the majority of the systems built for this purpose, porphyrins due to their close structural resemblance to chlorophyll and BChl, have been the primary choice of chromophores.1-6

However, in recent years, model compounds based on BF2-chelated

dipyrromethene (BODIPY) have attracted wide attention due to their synthetic diversity and welldefined and tunable spectral and photophysical properties.43-51 In the present study, we have utilized BODIPY as a fluorophore to model natural LH-arrays and synthesized cyclic trimers using triazine scaffold (see Figure 1). The BODIPYs employed here are functionalized to have an electron rich oxygen at the meso-position43 (ether type linkage, see structure of compound 10 in Figure 1) which were found to be a superior probe compared to BODIPY derivatives functionalized with an alkyl or aryl substituents at the meso-position for their large Stokes shift, fluorescence quantum yield and lifetime, thus making them an ideal sensitizer


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to build chemosensors for in vivo and in vitro imaging, to build photosynthetic donor-acceptor model compounds for light energy harvesting, and oligomeric systems to seek novel applications.21-33 The BODIPY entities in the cyclic timers synthesized in the present study are positioned around a triazine ring in a symmetric fashion. This arrangement resulted in minimal perturbation of the ground and excited state properties of BODIPY within the trimers. Timeresolved emission and femtosecond transient absorption studies revealed that in trimer 8 energy migration (energy hopping) occurs among the BODIPY entities to some extent.

Figure 1. Structures of BODIPY carrying C3-symmetric positional isomers of substituted triazines, 7-9, and control monomers, 10 and 11 investigated in the present study.

2.

Results and Discussion The BODIPY substituted triazines 79 were synthesized by the cyclotrimerization and

nucleophilic substitution reactions of hydroxyl benzonitrile derivatives (13) (Scheme 1 and experimental section for details). In BODIPY substituted triazines 79, the BODIPY moiety is attached at different positions via phenoxy spacer around the 1,3,5-triazine core (Figure 1). The key intermediates 46 were synthesized by the cyclotrimerization reaction of the corresponding hydroxyl benzonitrile derivatives 13 under strong acid condition (triflouromethanesulfonic acid) in chloroform solvent for 16 h at room temperature and Chloro BODIPY, 12 synthesized by 3 ACS Paragon Plus Environment


reported procedure.52 The triazine tri-hydroxy derivatives 4–6 were dissolved in acetone in the presence of a base (K2CO3) at room temperature, stirred for 10 minutes and treated with BODIPY 12 for 16 h, which resulted tri-BODIPY substituted triazines 79 in 50–58% yield (Scheme 1). Compound 10 was synthesized by reported procedure.52 All the BODIPY substituted triazines 710 were fully characterized by 1H, 13CNMR, and HR-MS techniques.

Scheme 1. Synthesis of BODIPY substituted triazines, 7-9. Figure 2 shows the normalized absorption and fluorescence spectra of the investigated compounds in toluene while the spectral data are summarized in Table 1 both in polar benzonitrile and nonpolar toluene. The characteristic BODIPY peak in these meso-O-aryl functionalized compounds was located in the 460 nm range that was about 60 nm blue-shifted compared to mesoalkyl/aryl substituted BODIPYs.53-54

Compared to monomer spectra, the absorption and

fluorescence spectra of cyclic BODIPY trimers were slightly blue-shifted by up to 5 nm as a consequence of triazine functionalization.

Importantly, there was no significant spectral

broadening compared to the monomer absorption spectrum suggesting lack of intra-chromophore interactions. Concentration dependence studies revealed linear Beer-Lambert’s plots in the range 4 ACS Paragon Plus Environment

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of 0.05 mM to 2 mM suggesting lack of aggregation. The fluorescence emission for the present series of compounds was located in the 500 nm range that was also blue-shifted by about 20 nm compared to meso-alkyl/aryl substituted BODIPYs (see Figure 2b).16 Generally, BODIPYs show a relatively small Stokes shift of ~ 7 x 103 cm-1 (see Table 1). However, the calculated Stokes shift for the monomer and trimer of the present meso-O-aryl functionalized BODIPYs were in the range of 2-7 x 104 cm-1 (see Table 1). Such large Stokes shift generally helps in minimizing the inner filter effect in fluorescence studies.55 The fluorescence intensity of the trimers at the same optical

Figure 2. (a) Normalized absorption to the visible peak maxima, and (b) fluorescence spectra of the indicated compounds in toluene. Molar absorption coefficient was found to be 24244, 43853, 11713, and 6367 L mol-1 cm-1 for compounds 7, 8, 9, and 10, respectively. Fluorescence quantum yields were found to be 0.94, 0.97, 0.70, and 0.31 for compounds 7, 8, 9, and 10, respectively. Tolyl BODIPY was used as the reference (φf = 0.73). The compounds were excited at the most intense visible band peak maxima. (c) Differential pulse voltammograms of indicated compounds in benzonitrile containing 0.1 M (TBA)PF6 (V. vs. Ag/AgCl). The first oxidation process corresponding to ferrocene used as an internal reference.



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density of the monomer (OD = 1.0) were found to be within +15% of the monomer, 10. Often, electron and energy transfer events quench the fluorescence intensity of the primary sensitizer in multi-modular systems.56-59 Lack of such quenching suggest absence of intra-chromophore interactions between the BODIPY entities in the trimer. This lack of intra-chromophore interactions was also revealed by the electrochemical studies. The differential pulse voltammograms (DPV) corresponding to oxidation of BODIPY entity in the trimers are shown in Figure 2c. The oxidation process corresponding to the monomer and trimers were located in the 0.58-0.64 V range vs. Fc/Fc+ range revealed that were 80-140 mV cathodically shifted compared to the BODIPY control (see Table 1). The easier oxidation could be attributed to the presence of electron rich oxygen at the meso-position. As expected for systems Table 1. Absorption and fluorescence peak maxima and fluorescence lifetimes of investigated compounds in benzonitrile and toluene. Compound Solvent

abs, nm flu, nm

Stokes shift, , ns

Eox, V vs.

cm-1

Fc/Fc+

7

PhCN

462

503

1.76 x 104

5.56

0.64

8

PhCN

461

500

1.69 x 104

2.91

0.58

9

PhCN

460

503

1.85 x 104

4.17

0.62

10

PhCN

459

503

1.91 x 104

5.58

0.62

11

PhCN

459

501

1.82 x 104

5.58

0.61

BODIPY PhCN

501

519

6.94 x 103

2.86

0.72

7

Toluene

464

504

1.71 x 104

5.56

--

8

Toluene

463

502

1.67 x 104

5.32

--

9

Toluene

462

505

1.84 x 104

5.61

--

10

Toluene

460

503

1.85 x 104

5.89

--

11

Toluene

460

501

1.78 x 104

5.89

--

BODIPY Toluene

500

518

6.92 x 103

3.36

--

with large HOMO-LUMO gap, (optical HOMO-LUMO gap ~ 3.60 eV for the present BODIPY series), no reduction within the potential window of -1.80 V of benzonitrile was observed. 6 ACS Paragon Plus Environment

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Importantly, no splitting or peak broadening of the oxidation waves, expected for strongly interacting electroactive entities, was observed thus confirming the lack of intra-chromophore interactions. The functionalized triazine was also found to be electro-inactive within the potential range. As a consequence of the directly attached electron rich oxygen atom to the BODIPY mesoposition, the fluorescence lifetime of monomer, 10 was found to be higher compared to regular BODIPY derivatives (Table 1, this also translates into higher quantum yields).16 Interestingly, the lifetime of trimers were slightly lower, especially for compound 8, in both polar and nonpolar solvents. In the absence of aggregation and charge transfer effects, this effect could be attributed to energy migration, among the BODIPY entities within the trimer, as depicted in Scheme 2. In an effort to gain support for the proposed energy migration mechanism, time-resolved emission of the monomer and trimers were recorded by varying their concentrations from 0.05 mM to 2 mM. In an event of energy migration the lifetime of the sensitizer would be lowered, and by increasing the concentration this effect should be more pronounced. Figure 3 shows the normalized decay profiles of the investigated compounds in toluene. For the monomer 10 and 11, the lifetimes were virtually same in the entire concentration range (also indicates lack of inner filter effects in the concentration range, supported from the earlier discussed steady-state emission results). This was also the case for trimers 7 and 9 where the lifetime were the same within the experimental error. However, for trimer, 8 the decay was biexponential at lower concentrations with short and long lived components. The lifetime of the long-lived component became smaller with increasing the concentration, and at 2.0 mM only the short-lived component persisted revealing a monoexponential decay profile. These observations suggest occurrence of energy migration in 8, however, in case of 7 and 9, the concentration range was perhaps too narrow to observe this trend. Changing the solvent to polar benzonitrile also revealed such a trend in the case of 8 (see Figure S1) but not for 7 and 9. It seems subtle structural changes in the trimers cause noticeable changes in their excitation transfer properties. Further, computational studies were performed to visualize the geometry and to ascertain absence of intramolecular type association between the BODIPY entities in the trimers.60 The optimized structures are shown in Figure 4. The dihedral angle in para substituted BODIPY triazine 7 between triazine core and BODIPY through oxygen atom was 123, whereas triazine 8 7 ACS Paragon Plus Environment


Scheme 2. Cartoon showing energy hopping among the BODIPY entities in the cyclic BODIPY trimer.

Figure 3. Normalized fluorescence decay profiles (log count vs. time) of the indicated compounds at the indicated concentrations (in mM) in toluene. The samples were excited using 374 nm nanoLED and monitored at their respective emission maxima. 8 ACS Paragon Plus Environment

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and 9 shows dihedral angle of 123 and 118 between triazine and BODIPY core as shown in Figure 4. Importantly, although flexible, the BODIPY entities were almost perpendicular with respect to the triazine core with the dihedral angles varying between 77-82o. In the optimized structures, the frontier HOMO and LUMO were found to be on the BODIPY entity without any contributions on the triazine entity (see Figure S19 for frontier orbitals). These observations suggest triazine being a redox-silent entity in these trimers and acts only as a scaffold. At the excitation wavelengths of 460-465 nm where the trimers are excited, the central triazine also acts as a photo-silent entity. It is also important to point out that in none of the trimers steric hindrance or intramolecular type association between the BODIPY entities within a given trimer was observed.

Figure 4. B3LYP/6-31G** optimized structures of the investigated BODIPY trimers 7-9. In an effort to seek evidence for energy migration in trimer 8, femtosecond transient spectral studies were performed in both toluene and benzonitrile. As shown in Figure 5a, immediately after excitation, monomer 10, revealed a positive peak at 884 nm due to transitions involving from the 1BODIPY* state and two negative peaks, the first one at 468 nm due to ground state bleaching and the second one at 515 nm as a shoulder due to stimulated emission. With time,



the 468 nm peak revealed a small red shift of 5-6 nm perhaps due to solvation/vibrational relaxation. The decay and recovery of the positive and negative peaks which lasted over 3 ns was accompanied by a new peak at 574 nm that was assigned to 3BODIPY* state. This was pretty much the case for timers 7 and 9 as shown in Figures 5b and d, respectively. In the case of trimer 7, positive peaks at 614 and 1075 nm and negative peaks at 465 and 502 nm (due to ground state bleach and stimulated emission) were observed. The 3BODIPY* state developed at the later delay time was at 602 nm. In the case of trimer 9, a positive peak at 1054 and negative peaks at 462 and 504 nm (due to ground state bleach and stimulated emission) were observed. The 3BODIPY* state developed at the later delay time was at 608 nm. In contrast, for trimer 8 where energy migration

Figure 5. Femtosecond transient absorption spectra at the indicated delay times of (a) monomer 10, (b) trimer 7, (c) trimer 8, and (d) trimer 9 in benzonitrile. The solution concentration was held at 0.1 mM. The samples were excited at the corresponding visible peak maxima.


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among the BODIPY entities was predicted, the transient spectra revealed distinct changes (Figure 5b). The two negative peaks were located at 466 and 500 nm, in agreement with the absorption and emission peak maxima of the trimer. However, the intensity of the near-IR peak in the 1045 nm range involving optical transitions (singlet-singlet) from the 1BODIPY* state was rather weak. This was also the case for the peak corresponding to 3BODIPY* expected in the 600-620 nm range (see Figure S20 for nanosecond transient spectral data). This could be rationalized by lesser availability of 1BODIPY* due to rapid energy hopping. It may be pointed out here that the energy migration in trimer 8 is not as efficient as that reported for porphyrin cyclic oligomers in literature.1-6 Changing solving to nonpolar toluene also revealed such spectral features (see Figure S2 in SI).

3.

Summary The results of the present study bring out several interesting observations. The meso-O-

aryl functionalized BODIPYs revealed larger HOMO-LUMO gap and Stokes shift compared to BODIPY derivatives having an aryl group attached at the meso position. This was also the case for singlet excited state lifetimes, which were found to be almost double as compared to the traditional BODIPYs because of directly attached electron rich oxygen atom at the meso position. The BODIPY entities in the trimer positioned symmetrically around a triazine ring revealed minimal ground state interactions between them as shown by spectral, electrochemical and computational studies. Concentration dependent time-resolved studies revealed presence of short and long-lived components for trimer 8 in which the long-lived component disappeared at higher concentration supporting the occurrence of excited state energy migration at higher concentration lowering the lifetime, and such an effect is not due to any charge transfer type interactions between BODIPY and the substituents. The performed femtosecond transient absorption studies also supported such a mechanism, although the overall energy migration efficiency is lower than that reported for cyclic porphyrin systems. The present study brings out the importance of molecular structure controlled energy migration in BODIPY oligomers. Further studies along this line are in progress in our laboratories.

4.

Experimental Section



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Chemicals. All the reagents were from Aldrich Chemicals (Milwaukee, WI) while the bulk solvents utilized in the syntheses were from Fischer Chemicals.

Tetra-n-butylammonium

perchlorate, (n-Bu4N)ClO4, used in electrochemical studies was from Fluka Chemicals. General Procedure for the preparation of Triazines 4-6 Synthesis of 2,4,6-tri(4’-hydroxyphenyl)-1,3,5-triazine (4). The 4-hydroxybenzonitrile (0.5g, 4.23 mmol) was dissolved in chloroform (50 ml), then reaction mass cooled to 0 oC, added tri flouro methane sulfonic acid (1.22g, 8.13 mmol) drop wise under argon atmosphere, and stirred for 24 h at rt, after completion of the reaction, the residue was neutralized with an aqueous NaHCO 3, water, methanol, acetone, and hexane in this order, and then dried under vacuum to afford 4 as an off-white solid (0.8 g, 53%); 1H NMR (400 MHz, Acetone-d6): δ = 9.14 (s, 3H), 8.68 (d, J = 8.00 Hz, 6H), 7.06

(d, J = 8.00 Hz, 6H), ppm; 13C NMR (100 MHz, Acetone-d6): δ = 206.31, 171.59, 162.53, 131.72, 128.75, 116.34 ppm; HRMS (ESI): calcd. for C21H15N3O3 358.1186 [M+H]; found 358.1182.

Synthesis of 2,4,6-tri(4’-hydroxyphenyl)-1,3,5-triazinetris(1-methoxyphenol) (5). The 4-hydroxy-3-methoxybenzonitrile (0.5g, 3.35 mmol) was dissolved in chloroform (50 ml), then reaction mass cooled to 0 oC, added tri flouro methane sulfonic acid (1.22g, 8.13 mmol) drop wise under argon atmosphere, and stirred for 24 h at rt, after completion of the reaction, the residue was neutralized with an aqueous NaHCO3, water, methanol, acetone, and hexane in this order, and then dried under vacuum to afford 5 as an off-white solid (0.65 g, 43%); 1H NMR (400 MHz, Acetone): δ = 8.38 (dd, J = 4.00 Hz, 3H), 8.31 (d, J = 4.00 Hz, 3H), 7.13 (d, J = 8.00 Hz, 3H), 4.06 (s, 9H) ppm; 13

C NMR (100 MHz, Acetone-d6): δ = 206.24, 154.58, 148.86, 126.36, 116.59, 113.52, 56.55 ppm;

HRMS (ESI): calcd. for C21H15N3O3 448.1503 [M+H]; found 448.1504.

Synthesis of 2,4,6-tri(3’-hydroxyphenyl)-1,3,5-triazine (6). The 3-hydroxybenzonitrile (0.5g, 4.23 mmol) was dissolved in chloroform (50 ml), then reaction mass cooled to 0 oC, added tri flouro methane sulfonic acid (1.22g, 8.13 mmol) drop wise under argon atmosphere, and stirred for 24 h at rt, after completion of the reaction, the residue was neutralized with an aqueous NaHCO 3, water, methanol, acetone, and hexane in this order, and then dried under vacuum to afford 6 as an off-white solid (0.6 g, 39%); 1H NMR (400 MHz, Acetone-d6): δ = 8.87 (t, 3H), 8.29 (d, J = 8.00 Hz, 6H), 7.50

(m, 3H), 7.17 (d, J = 8.00 Hz, 3H) ppm; 13C NMR (100 MHz, Acetone-d6): δ = 206.31, 171.59, 162.53, 131.72, 128.75, 116.34 ppm; HRMS (ESI-TOF):: calcd. for C21H15N3O3 358.1186 [M+H]; found 358.1185.


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Synthesis and Characterization of Triazine-BODIPYs 7-9. Triazines 4 or 5 or 6 (100 mg, 0.44 mmol) was dissolved in dry Acetone (7 mL) and K2CO3 (121 mg, 0.88 mmol) were added after 30 minutes 8-chloro BODIPY added and stirred at ∼25 °C for 16 h. Upon completion of the reaction, the mixture was evaporated to dryness and the crude product was dissolved in EtOAc, chromatographed on silica compound eluted using EtOAc/hexane (80:20) to get 7 or 8 or 9 as a yellow solid. 2,4,6-tris[4’-(4’’,4’’-difluoro-4’’-bora-3a’’,4a’’-diaza-s-indacenato-8’’-yl)oxyphenyl]-1,3,5triazine (7). Yellow crystalline solid. Yield: 47% (0.125 g). 1H NMR (CDCl3, 400 MHz, ppm): δ 8.94 (d, J = 12.00 Hz, 6H), 7.81 (s, 6H), 7.50 (d, J = 8 Hz, 6H), 6.87 (d, J = 4.00 Hz, 6H), 6.46 (d, J = 4.00 Hz, 6H). 13C NMR (DMSO-d6, 100 MHz, ppm): 172.98, 171.00, 170.21, 133.83, 130.95, 130.93, 126.01, 124.83, 116.72, 115.90, 112.91, 110.68, 110.31 ppm. HRMS (ESI-TOF): calcd. for C48H30B3F6N9O3 950.2569 [M+Na]; found 950.2569.

2,4,6-tris[4’-(4’’,4’’-difluoro-4’’-bora-3a’’,4a’’-diaza-s-indacenato-8’’-yl)oxyphenyl]-1,3,5triazinetris(1-methoxyphenol) (8). Yellow crystalline solid. Yield: 48% (112 mg). 1H NMR (CDCl3, 400 MHz, ppm): δ 8.57 (d, J = 8.00 Hz, 3H), 7.78 (s, 6H), 7.53 (d, J = 8.00 Hz, 3H), 6.86 (d, J = 4.00 Hz, 6H), 6.43 (d, J = 4.00 Hz, 6H), 3.97 (s, 9H).

13

C NMR (DMSO-d6, 100 MHz,

ppm): 170.44, 151.79, 148.18, 130.93, 127.24, 126.29, 124.82, 115.90, 113.15, 110.79, 110.31, 55.36 ppm. HRMS (ESI-TOF): calcd. for C51H36B3F6N9O6 1040.2887 [M+Na]; found 1040.2890. 2,4,6-tris[3’-(3’’,3’’-difluoro-4’’-bora-3a’’,4a’’-diaza-s-indacenato-8’’-yl)oxyphenyl]-1,3,5triazine (9). Yellow crystalline solid. Yield: 60% (0.160 g). 1H NMR (CDCl3, 400 MHz, ppm): δ 8.81 (d, J = 12.00 Hz, 3H), 8.63 (s, 3H), 7.80 (m, 9H), 6.84 (d, 6H), 6.43 (s, 6H). 13C NMR (CDCl3, 100 MHz, ppm): 180.54, 170.89, 156.11, 141.23, 138.24, 131.09, 127.66, 126.33, 124.68, 120.48 ppm. HRMS (ESI-TOF): calcd. for C48H30B3F6N9O3 950.2569 [M+Na]; found 950.2566. Spectral measurements 1

H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded on a Bruker Avance

(III) instrument by using CDCl3, Acetone-d6 and DMSO-d6 as solvents. 1H NMR chemical shifts are reported in parts per million (ppm) relative to the solvent residual peak. Multiplicities are given as s (singlet), d (doublet), t (triplet), q (quartet), dd (doublet of doublets), and m (multiplet), and the coupling constants, J, are given in Hz.

13

C NMR chemical shifts are reported relative to the



solvent residual peak and HRMS was recorded on a Bruker-Daltonics micrOTOF-Q II mass spectrometer. The UV-visible spectral measurements were carried out with a Shimadzu Model 2550 double monochromator UV-visible spectrophotometer. The fluorescence emission was monitored by using a Horiba Yvon Nanolog coupled with time-correlated single photon counting with nanoLED excitation sources. A right angle detection method was used. Differential pulse voltammograms were recorded on an EG&G PARSTAT electrochemical analyzer using a three electrode system. A platinum button electrode was used as the working electrode. A platinum wire served as the counter electrode and an Ag/AgCl electrode was used as the reference electrode. Ferrocene/ferrocenium redox couple was used as an internal standard. All the solutions were purged prior to electrochemical and spectral measurements using nitrogen gas. Femtosecond pump-probe transient spectroscopy. Femtosecond transient absorption spectroscopy experiments were performed using an Ultrafast Femtosecond Laser Source (Libra) by Coherent incorporating diode-pumped, mode locked Ti:Sapphire laser (Vitesse) and diodepumped intra cavity doubled Nd:YLF laser (Evolution) to generate a compressed laser output of 1.45 W. For optical detection, a Helios transient absorption spectrometer coupled with femtosecond harmonics generator both provided by Ultrafast Systems LLC was used. The source for the pump and probe pulses were derived from the fundamental output of Libra (Compressed output 1.45^^W, pulse width 100 fs) at a repetition rate of 1 kHz. 95 % of the fundamental output of the laser was introduced into a TOPAS-Prime-OPA system with 290—2600 nm tuning range from Altos Photonics Inc., (Bozeman, MT), while the rest of the output was used for generation of white light continuum. In the present study, the second harmonic 400^^nm excitation pump was used in all the experiments. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. Data analysis was performed using Surface Xplorer software supplied by Ultrafast Systems. All measurements were conducted in degassed solutions at 298 K. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at: DOI: xxxx


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1

H, 13C and HRMS spectra of the conjugates, frontier orbital diagrams, nanosecond transient

spectral data on the trimers and Cartesian coordinates of the optimized structures. (PDF) AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected], [email protected] ORCID F. D’Souza: 0000-0003-3815-8949 Author Contributions The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interest.

Acknowledgment R. M. thanks the Department of Science and Technology (Project No. EMR/2014/001257), New Delhi, and INSA (Project No. SP/YSP/139/2017/2293), New Delhi, for the financial support. We are grateful to the Sophisticated Instrumentation Centre (SIC), IIT Indore and the National Science Foundation (Grant No. 1401188). References (1) Pullerits, T.; Sundstrom, V. Photosynthetic Light-Harvesting Pigment-Protein Complexes: Toward Understanding How and Why. Acc. Chem. Res., 1996, 29, 381-389. (2) Hu, X.; Schulten, K. How Nature Harvests Sunlight. Phys. Today, 1997, 50, 28-35. (3) Roszak, A.W.; Howard, T.D.; Southall, J.; Gardiner, A.T.; Law, C.J.; Isaacs, N.W.; Cogdell, R.J. Crystal Structure of the RC-LH1 Complex from Rhodopseudomonas Palustris. Science, 2003, 302, 1969-1972. (4) Scheuring, S.; Sturgis, J.N.; Prima, V.; Bernadac, A.; Levy, D.; Rigaud, J.L. Watching the Photosynthetic Apparatus in Native Membranes. Proc. Natl. Acad. Sci. U. S. A., 2004, 101, 1129311297.



(5) Sundstrom, V.; Pullerits, T.; van Grondelle, R. Photosynthetic Light Harvesting: Reconciling Dynamics and Structure of Purple Bacterial LH2 Reveals Function of Photosynthetic Unit. J. Phys. Chem. B., 1999, 103, 2327-2346. (6) Nakamura, Y.; Aratani, N.; Osuka, A. Cyclic Porphyrin Arrays as Artificial Photosynthetic Antenna: Synthesis and Excitation Energy Transfer. Chem. Soc. Rev., 2007, 36, 831-845. (7) Hori, T.; Peng, X.; Aratani, N.; Takagi, A.; Matsumoto, T.; Kawai, T.; Yoon, Z.S.; Yoon, M.-C.; Yang, J.; Kim, D.; Osuka, A. Synthesis of Nanometer-Scale Porphyrin Wheels of Variable Size. Chem. Eur. J., 2008, 14, 582-595. (8) Aratani, N.; Kim, D.; Osuka, A. Discrete Cyclic Porphyrin Arrays as Artificial LightHarvesting Antenna. Acc. Chem. Res., 2009, 42, 1922-1934. (9) Yang, J.; Yoon, M.C.; Yoo, H.; Kim, P.; Kim, D. Excitation Energy Transfer in Multiporphyrin Arrays with Cyclic Architectures: Towards Artificial Light-Harvesting Antenna Complexes. Chem. Soc. Rev., 2012, 41, 4808–4826. (10) Anderson, S.; Anderson, H.; Sanders, J.K.M. Expanding Roles for Templates in Synthesis. Acc. Chem. Res., 1993, 26, 469-475. (11) McCallien, W.J.; Sanders, J.K.M. Dioxoporphyrins as Supramolecular Building Blocks: Oligomer Synthesis via Preassembly on a Ligand Template. J. Am. Chem. Soc., 1995, 117, 66116618. (12) Li, J.; Ambroise, A.; Yang, S.I.; Diers, J.R.; Seth, J.; Wack, C.R.; Bocian, D.F.; Holten D.; Lindsey, J.S. Template-Directed Synthesis, Excited-State Photodynamics and Electronic Communication in a Hexameric Wheel of Porphyrins. J. Am. Chem. Soc., 1999, 121, 8927-8940. (13) Flamigni, L.; Ventura, B.; Oliva, A.I.; Ballester, P. Energy Migration in a Self-Assembled Nonameric Porphyrin Molecular Box. Chem. Eur. J., 2008, 14, 4214-4224. (14) Satake A.; Kobuke, Y. Dynamic Supramolecular Porphyrin Systems. Tetrahedron, 2005, 61, 13-41. (15) Kuramochi, Y.; Sandanayaka, A.S.D.; Satake, A.; Araki, Y.; Ogawa, K.; Ito, O.; Kobuke, Y. Energy Transfer Followed by Electron Transfer in a Porphyrin Macrocycle and Central Acceptor Ligand: A model for a photosynthetic Composite of the Light-Harvesting Complex and Reaction Center. Chem. Eur. J., 2009, 15, 2317-2327. (16) Cho, H.S.; Rhee, H.; Song, J.K.; Min, C.K.; Takase, M.; Aratani, N.; Cho, S.; Osuka, A.; Joo, T.; Kim, D. Excitation Energy Transport Processes of Porphyrin Monomer, Dimer, Cyclic 16 ACS Paragon Plus Environment

Page 16 of 21

Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Trimer, and hexamer Probed by Ultrafast Fluorescence Anisotropy Decay. J. Am. Chem. Soc., 2003, 125, 5849-5860. (17) Yang, J.; Kim, D. Single Molecule Spectroscopic Investigation on Various Multiporphyrin Systems as Molecular Photonic Devices. J. Mater. Chem., 2009, 19, 1057-1062. (18) Kim, P.; Lim, J.M.; Yoon, M.C.; Aimi, J.; Aida, T.; Tsuda, A.; Kim, D. Excitation Energy Migration Processes in Self-Assembled Porphyrin Boxes Constructed by Conjugated Porphyrin Dimers. J. Phys. Chem. B, 2010, 114, 9157-9164. (19) Langton, M.J.; Matichak, J.D.; Thompson, A.L.; Anderson, H.L. Template-Directed Synthesis of π-Conjugated Porphyrin [2]Rotaxanes and a [4]Catenane Based on a Six-Porphyrin Nanoring. Chem. Sci., 2011, 2, 1897-1901. (20) O’Sullivan, M.C.; Sprafke, J.K.; Kondratuk, D.V.; Rinfray, C.; Claridge, T.D.W.; Saywell, A.; Blunt, M.O.; O’Shea, J.N.; Beton, P.H.; Malfois, M.; Anderson, H.L. Vernier Templating and Synthesis of a 12-Porphyrin Nano-ring. Nature, 2011, 469, 72-75. (21) Holten, D.; Bocian, D.F.; Lindsey, J.S.; Probing Electronic Communication in Covalently Linked Multiporphyrin Arrays. A guide to the Rational Design of Molecular Photonic Devices. Acc. Chem. Res. 2002, 35, 57-69. (22) Hambourger, M.; Moore, G.F.; Kramer, D.M.; Gust, D.; Moore, A.L.; Moore, T.A. Biology and Technology for Photochemical Fuel Production. Chem. Soc. Rev. 2009, 38, 25-35. (23) Calzaferri, G.; Bossarto, O.; Bruhwiler, S.; Huber, C.; Leiggener, C.; Van Veek, M.L.; Ruiz, A.Z. Light-Harvesting Host-guest Antenna Materials for Quantum Solar Energy Conversion Devices. C. R. Chim. 2006, 9, 214-225. (24) Alstrum-Acevedo, J.H.; Brennaman, M.K.; Meyer, J. Chemical Approaches to Artificial Photosynthesis, 2. Inorg. Chem. 2005, 44, 6802-6827. (25) Wasielewski, M.R. Self-Assembly Strategies for Integrating Light Harvesting and Charge Separation in Artificial Photosynthetic Systems. Acc. Chem. Res. 2009, 42, 1910-1921. (26) Concepcion, J.J.; Jurss, J.W.; Brennaman, M.K.; Hoertz, P.G.; Patrocinio, A.O.T.; Murakami Iha, M.Y.; Templeton, J.L.; Meyer, T.J. Making Oxygen with Ruthenium Complexes. Acc. Chem. Res. 2009, 42, 1954-1965. (27) Kelly, R.F.; Lee, S.J.; Wilson, T.M.; Nakamura, Y.; Tiede, D.M.; Osuka, A.; Hupp, J.T.; Wasielewski, M.R. Intramolecular Energy Transfer within Butadiyne-Linked Chlorophyll and Porphyrin Dimer-Faced, Self-Assembled Prisms. J. Am. Chem. Soc. 2008, 130, 4277-4284. 17 ACS Paragon Plus Environment


(28) Serroni, S.; Campagna, S.; Pistone Nascone, R.; Hanan, G.S.; Davidson, G.J.; Lehn, J.M. Controlling the Direction of Photoinduced Energy Transfer in Multicomponent Species. Chem. Eur. J. 1999, 5, 3523-3527. (29) Cardenas, D.J.; Collin, J.P.; Gavina, P.; Sauvage, J.P.; De Cian, A.; Fischer, J.; Armaroli, N.; Flamigni, L.; Vicinelli, V.; Balzani, V. Synthesis, X-ray Structure, and Electrochemical and Excited-State Properties of Multicomponent Complexes Made of a [Ru(Tpy)2]2+ Unit Covalently Linked to a [2]-Catenate Moiety. Controlling the Energy-Transfer Direction by Changing the Catenate Metal Ion. J. Am. Chem. Soc. 1999, 121, 5481-5488. (30) Armaroli, N.; Eckert, J.F.; Nierengarten, J.F. Controlling the Energy-Transfer Direction: an Oligophenylenevinylene-phenathroline Dyad Acting as a Proton Triggered Molecular Switch. Chem. Commun. 2000, 2105-2106. (31) Indelli, M.T.; Ghirotti, M.; Prodi, A.; Chiorboli, C.; Scandola, F.; McClenaghan, N.D.; Puntoriero, F.; Campagna, S. Solvent Switching of Intramolecular Energy Transfer in Bichromophoric systems: photophysics of (2,2’-bipyridine)tetracyanorethenate(II)/pyrenyl Complexes. Inorg. Chem. 2003, 42, 5489-5497. (32) Bonifazi, D.; Scholl, M.; Song, F.Y.; Echegoyen, L.; Accorsi, G.; Armaroli, N.; Diederich, F. Exceptional Redox and Photophysical Properties of a Triply Fused Diporphyrin-C60 Conjugate: Novel Scaffolds for Multicharge Storage in Molecular Scale Electronics. Angew. Chem. Int. Ed. 2003, 42, 4966-4970. (33) Accorsi, G.; Armaroli, N. Taking Advantage of the Electronic Excited States of [60]Fullerenes. J. Phys. Chem. C. 2010, 114, 1385-1403. (34) Jungas, C.; Ranck, J.; Rigaud, J.; Joliot, P.; Vermeglio, A. Supramolecular Organization of the Photosynthetic Apparatus of Rhodobacter sphaeroides. EMBO J, 1999, 18, 534-542. (35) Ermler, U.; Fritzsch, G.; Buchanan, S.; Michel, H. Structure of the Photosynthetic Reaction Centre from Rhodobacter sphaeroides at 2.65 A Resolution: Cofactors and Protein-Cofactor Interaction . Structure 1994, 2, 925-936. (36) Hu, X.; Ritz, T.; Damjanovic, A.; Autenrieth, F.; Schulten, K. Photosynthetic Apparatus of Purple Bacteria. Q. Rev. Biophys. 2002, 35, 1-69. (37) Shreve, A. P.; Trautman, J. K.; Frank, H. A.; Owens, T. G.; Albrecht, A. C. Femtosecond Energy-Transfer Processes in the B800-850 Light-Harvesting Complex of Rhodobacter sphaeroides 2.4.1. Biochim. Biophys. Acta. 1991, 1058, 280-288. 18 ACS Paragon Plus Environment

Page 18 of 21

Page 19 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(38) Joo, T.; Jia, Y.; Yu, Y.H.; Jonas, D.M.; Fleming, G.R. Dynamics in Isolated Bacterial Light Harvesting Antenna (LH2) of Rhodobacter sphaeroides at Room Temperature. J. Phys. Chem. 1996, 100, 2399-2409. (39) Ulrich, G.; Ziessel, R.; Harriman, A. The Chemistry of Fluorescent BODIPY Dyes: Versatility Unsurpassed. Angew. Chem. Int. Ed., 2008, 47, 1184-1201. (40) El-Khouly, M.E.; Fukuzumi, S.; D’Souza, F. Photosynthetic Antenna-Reaction Center Mimicry by Using Boron Dipyrromethene Sensitizers. ChemPhysChem, 2014, 15, 30-47. (41) Sharma, R.; Gobeze, H.B.; D’Souza, F.; Ravikanth, M. Panchromatic Light Capture and Efficient Excitation Transfer Leading to Near-IR Emission of BODIPY Oligomers. ChemPhysChem, 2016, 17, 2516-2524. (42) El-Khouly, M.E.; Amin, A.N.; Zandler, M.E.; Fukuzumi, S.; D’Souza, F. Near-IR Excitation Transfer and Electron Transfer in a BF2-Chealted Dipyrromethane-Azadipyrromethane Dyad and Triad, Chem. Eur. J. 2012, 18, 5329-5337. (43) Flores-Rizo, J.O.; Esnal, I.; Osorio-Martinez, C.A.; Gomez-Duran, C.F.A.; Banuelos, J.; Arbeloa, I.O.; Pannell, K.H.; Metta-Magana, A.J.; Pena-Cabrera, E. 8-Alkoxy- and 8-AryloxyBODIPYs: Straightforward Fluorescent Tagging of Alcohols and Phenols. J. Org. Chem. 2013, 78, 5867-5872. (44) Ni, Y.; Lee, S.; Son, M.; Aratani, N.; Ishida, M.; Samanta, A.; Yamada, H.; Chang, Y.T.; Furuta, H.; Kim, D.; Wu, J. A Diradical Approach Towards BODIPY-Based Dyes with Intense Near-Infrared Absorption around λ=1100 nm. Angew. Chem. Int. Ed. 2016, 55, 2815-2819. (45) Azarias, C.; Russo, R.; Cupellini, L.; Mennucci, B.; Jacquemin, D. Modeling Excitation Energy Transfer in Multi-BODIPY Architectures. Phys. Chem. Chem. Phys. 2017, 19, 6443-6453. (46) Lu, H.; Mack, J.; Nyokong, T.; Kobayashi, N.; Shen, Z. Optically Active BODIPYs. Coord. Chem. Rev. 2016, 318, 1-15. (47) Fan, C.; Wu, W.; Chruma, J.J.; Zhao, J.; Yang, C. Enhanced Triplet-Triplet Energy Transfer and Upconversion Fluorescence Through Host-Guest Complexation. J. Am. Chem. Soc. 2016, 138, 15405-15412. (48) Patalag, L.J.; Jones, P.G.; Werz, D.B. BOIMPYs: Rapid Access to a Family of RedEmissive Fluorophores and NIR Dyes. Angew. Chem. Int. Ed. 2016, 55, 13340-13344. (49) Ziessel, R.; Ulrich, G.; Haefele, A.; Harriman, A. An Artificial Light-Harvesting Array Constructed from Multiple BODIPY Dyes. J. Am. Chem. Soc. 2013, 135, 11330-13340. 19 ACS Paragon Plus Environment


Page 20 of 21

(50) Iehl, J.; Nierengarten, J.F.; Harriman, A.; Bura, T.; Ziessel, R. Artificial Light-harvesting Arrays: Electronic Energy Migration and Trapping on a Sphere and Between Spheres. J. Am. Chem. Soc. 2012, 134, 988-998. (51) Olivier, J.H.; Barbera, J.; Bahaidarah, E.; Harriman, A.; Ziessel, R. Self-Assembly of Charged BODIPY Dyes to Form Cassettes That Display Intracomplex Electronic Energy Transfer and Accrete into Liquid Crystals. J. Am. Chem. Soc. 2012, 134, 6100-6103. (52) Leen, V.; Yuan, P.; Wang, L.; Boens, N.; Dehaen, W. Synthesis of Meso-Halogenated BODIPYs and Access to Meso-Substituted Analogues. Org. Lett., 2012, 14, 6150-6153. (53) Bandi, V.; D’Souza, F.P.; Gobeze, H.B.; D’Souza, F. Multistep Energy and Electron Transfer in a “V-Configured” Supramolecular BODIPY-azaBODIPY-Fullerene Triad: Mimicry of Photosynthetic Antenna Reaction-Center Events. Chem. Eur. J. 2015, 21, 2669-2679. (54) El-Khouly, M.E.; Wijesinghe, C.A.; Nesterov, V.N.; Zandler, M.E.; Fukuzumi, S.; D’Souza, F. Ultrafast Photoinduced Energy and Electron Transfer in Multi-Modular DonorAcceptor Conjugates. Chem. Eur. J. 2012, 18, 13844-13853. (55) Principles of Fluorescence Spectroscopy, 3rd ed., Ed.: Lakowicz, J.R. Springer, Singapore, 2006. (56) KC, C.B.; D’Souza, F. Design and Photochemical Study of Supramolecular DonorAcceptor Systems Assembled Via Metal-Ligand Axial Coordination. Coord. Chem. Rev. 2016, 322, 104-141. (57) D’Souza,

F.;

Ito,

O.

Supramolecular

Donor-Acceptor

Hybrids

of

Porphyrins/Phthalocyanines with Fullerenes/Carbon Nanotubes: Electron Transfer, Sensing, Switching, and Catalytic Applications. Chem. Commun., 2009, 4913-4928. (58) Fukuzumi, S.; Kojima, T. Photofunctional Nanomaterials Composed of Multiporphyrins and Carbon-Based π-Electron Acceptors. J. Mater. Chem., 2008, 18, 1427-1439. (59) Guldi, D.M.; Rahman, A.; Sgobba, V.; Ehli, C. Multifunctional Molecular Carbon Materials – From Fullerenes to Carbon Nanotubes. Chem. Soc. Rev. 2006, 35, 471-487. (60) Gaussian 09, Revision B.01, Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Zakrzewski, V.G.; Montgomery, J.A.; Stratmann, R.E.; Burant, J.C. et. al., Gaussian, Inc., Pittsburgh PA, 2009.


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Symmetric Positional Isomers of BODIPY Substituted - ACS Publications

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