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Ultrafast Intramolecular Photo-Induced Energy Transfer Events in Benzothiazole – Borondipyrromethene Donor – Acceptor Dyads Deepak Badgurjar, Kolanu Sudhakar, Kanika Jain, Vibha Kalantri, Yeduru Venkatesh, Naresh Duvva, Seelam Prasanthkumar, Anuj Kumar Sharma, Prakriti Ranjan Bangal, Raghu Chitta, and Lingamallu Giribabu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03668 • Publication Date (Web): 08 Jul 2016 Downloaded from http://pubs.acs.org on July 10, 2016

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The Journal of Physical Chemistry

Submitted to J. Phys. Chem. C 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

Ultrafast Intramolecular Photo-induced Energy Transfer Events in Benzothiazole – Borondipyrromethene Donor – Acceptor Dyads Deepak Badgurjar,† Kolanu Sudhakar,‡ Kanika Jain,† Vibha Kalantri,† Yeduru Venkatesh,‡ Naresh Duvva,‡ Seelam Prasanthkumar,‡ Anuj K. Sharma,*,† Prakriti R. Bangal,*,‡Raghu Chitta,*,† Lingamallu Giribabu.*‡ †

Department of Chemistry, School of Chemical Sciences & Pharmacy, Central University of Rajasthan, Kishangarh, Dist. Ajmer, Rajasthan-305817, India. ‡

Inorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Tarnaka, Hyderabad-500007, Telangana, India.

Abstract Benzothiazole (BTZ)-Boron dipyrromethene (BODIPY) based dyads, Dyad 1 and Dyad 2, containing BTZ as light energy absorbing and transferring antenna while BODIPY as an energy acceptor, linked together with ethoxy and ester spacers, respectively, have been synthesized and photo-induced energy transfer (PEnT) events occurring within these systems were studied. Both the dyads were characterized by 1H NMR, MALDI-MS, UV-visible, steady-state fluorescence and femtosecond transient absorption spectroscopic as well as electrochemical methods. A comparison of the absorption spectra of the dyads with their reference compounds (i.e., BTZ-OMe and BODIPYOMe) revealed minimal ground state interactions between chromophores in Dyad 1 while more pronounced effect in case of Dyad 2. Electrochemical and computational studies revealed that the LUMO of both the dyads is located on BODIPY indicating that the reduction of BODIPY moiety is easier compared to BTZ moiety. Selective excitation on BTZ in the dyads at ~300 nm resulted in quenching of the BTZ emission followed by the appearance of BODIPY emission showing efficient energy transfer from singlet excited BTZ (1BTZ*) to BODIPY. The photo-induced energy transfer phenomenon was observed in four different solvents of varying polarity. The driving forces of energy transfer (∆GEN) in both the dyads were found to be exothermic and, in case of Dyad 1, they followed the trend toluene>DCB~DMF>acetonitrile. Transient absorption studies performed in polar solvents such as acetonitrile revealed an efficient photo-induced energy transfer (~95%) from 1BTZ* to BODIPY (kEN = 6.17 × 1012 s-1 for Dyad 1 and 2.5 × 1012 s-1 for Dyad 2 in acetonitrile) creating singlet excited BODIPY (1BTZ*-BODIPY BTZ-1BODIPY*), indicating the quenched pathway is exclusively PEnT process.

Key Words: Benzothiazole, BODIPY, Dyads, Intramolecular, Energy Transfer, Timeresolved.

Corresponding authors: [email protected], Phone: +91-1463-238535; [email protected], Phone: +91-40-27191724, Fax: +91-40-27160921; [email protected], Phone: +91-1463238535. [email protected], Phone: +91-40-27191431.

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1. Introduction Photosynthesis is a ubiquitous model in which sunlight is harvested by antenna molecules and the excitation energy is funnelled to the reaction centre (RC), where multi-step electron transfer reactions take place to generate a potential that can drive chemical reactions.1-6 The light energy harvesting antenna systems consists of chromophore arrays that transport the absorbed energy via singlet-singlet energy transfer, known as photo-induced energy transfer (PEnT), among the chromophores. The reaction centre absorbs the excitation energy and converts it to chemical energy in the form of transmembrane charge separation and achieves a long lived charge separated state via a series of photoinduced electron transfer (PET) reactions.7-15 During past decades, several synthetic donor-acceptor dyads, triads, polyads, known as, artificial light-harvesting systems, those mimic the primary events of natural photosynthetic machinery by displaying PEnT and PET processes, have been designed and their utilities in converting solar energy to electricity has been explored.16-22 Even though significant progress has been achieved in this area, research towards exploring for new systems that differ from previous ones in photoactive unit, spatial arrangement, linker etc., has a great demand because of their applications in constructing optoelectronic devices.23-26 Through a survey of literature, it can be realized that tetrapyrrolic macrocyles, such as porphyrins27-29 and phthalocyanines,30-32 have been extensively used in the artificial photosynthetic systems to perform photoinduced energy and electron transfer due to their close resemblance to natural pigments, namely, chlorophylls and bacteriochlorophylls, which act as light-harvesting antenna and electron donors in the electron transfer cascade. Among these, the systems containing porphyrins tethered to boron dipyrromethenes (BODIPYs)33 have proven to be efficient photosynthetic antenna-reaction centre mimics because BODIPYs absorbs at wavelengths, complementary to the absorption of porphyrins, with large molar extinction coefficients (ε ≈ 105 M-1cm-1), exhibit high fluorescence quantum yields (Φ ~ 0.57 in CH2Cl2), possess relatively long singlet excited state lifetimes (≈ 4-6 ns),34,35 and behave as excellent energy-absorbing and transferring antenna molecules.28,29 In order to characterize the PEnT events and to probe the efficiency and the rates of energy transfer in the donor-acceptor systems, steady state, time resolved fluorescence techniques in conjunction with the transient absorption have been systematically employed.33, 36-44

However, among these techniques, nano-or femtosecond transient absorption 2 ACS Paragon Plus Environment

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measurements have attracted the major attention because they provide an opportunity for real-time monitoring of the energy transfer processes, estimate the lifetime of the excited states and also calculate the exact rates of energy transfer from these excited states. Some of the examples include, transient absorption anisotropy studies to monitor energy transfer dynamics in prophyrin dimers and trimers40, femto-second transient absorption measurements to study the ultrafast energy transfer from 1BODIPY* to azaBODIPY in a azaBODIPYBODIPY dyad

41

and monitor singlet/triplet energy transfer studies in 2,6-diiodobodipy-

styrylbodipy dyads.42 In addition, Ribierre and co-workers have effectively used subpicosecond transient absorption studies to monitor the singlet energy transfer processes from triphenylene to perylenediimide units43 and Ravikanth and co-workers to visualize the intramolecular energy transfer dynamics in zinc porphyrin-dithiaporphyrin dyads.44 Towards the goal of constructing electron donor-acceptor (D-A) systems that behave as new nonlinear optical (NLO) materials and dyes with enhanced two-photon absorption, benzothiazoles (BTZs) and their protonated analogues, benzothiazolium salts, have attracted much attention in recent years as electron acceptors,45,46 π-centers in quasi-quadrupolar systems,47 and excited state intramolecular proton transfer chromophores.48-50 BTZ is a heterocyclic molecule with sulfur and nitrogen atoms and possesses high chemical and photophysical stability compared to most of the other heteroaromatics and often used as an electron acceptor due to its strong electron with-drawing capability. In addition to this, BTZ containing fluorescent dyes such as thioflavin-T (ThT) have been extensively used as small molecule-based probes or markers for in vivo labelling or detection of amyloid in Alzheimer’s (AD) patients.51 Despite many donor-acceptor systems containing BTZ and BODIPY separately are published in the literature, only two systems containing BTZ tethered to BODIPY moieties and amalgamating the properties of both these chromophores have been reported.52,53 These reports include the design of new BODIPY based fluorophores conjugated with two 2-(2hydroxyphenyl)benzothiazole units to utilize them as organic triplet photosensitizers for photooxidations52 and synthesis of pyrrolopyrroleaza-BODIPY chromophores containing benzothiazole framework and exploring their utility as visible/NIR absorbing dyes and in organic photovoltaics.53 However, to the best of our knowledge, donor-acceptor systems involving BTZ covalently connected to BODIPY moiety, especially with systematic variation of linker sizes, and study of PEnT events w.r.t. to solvent polarity and linker sizes, have not 3 ACS Paragon Plus Environment


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been reported till date. Such kind of a study is quite important to understand the variations in energetics of PEnT w.r.t. the type of the linker and polarity of the medium, and utilize these concepts in systematically controlling the energy transfer events in various opto-electronic devices and molecular probes. For this purpose we have synthesized two BTZ–BODIPY based donor-acceptor systems, Dyad 1 and Dyad 2, containing BTZ and BODIPY tethered together with ethoxy and ester linkages, respectively, and studied of PEnT processes in four different solvents of varying polarity. In these dyads, BTZ acts as an energy absorbing and transferring antenna, while BODIPY acts as an energy acceptor. We have selected BTZ-BODIPY dyads for the study because of the excitation wavelength selectivity; i.e., when excited at 295 nm, only the BTZ moiety of the dyads were excited predominantly (1BTZ*-BODIPY), leaving the BODIPY moiety in the ground state followed by decaying of 1BTZ* relaxed to the ground state by transferring it singlet energy to BODIPY (BTZ-1BODIPY*). All the energy transfer experiments were performed in four different solvents and the effects of solvent polarity on rates of energy transfer have been studied. In the present study, femtosecond transient absorption techniques studies along with global and target analysis comprising compartmental model were used to monitor the energy transfer dynamics in these donoracceptor dyads.

N O

B

S

F

N

S

F OCH3

O

N

N

Dyad 1

BTZ-OMe

N S

O

N

O

B

F

N

N

F H3 CO

N

B

F F

BODIPY-OMe

Dyad 2

Chart 1. Synthesized BTZ-BODIPY dyads, Dyad 1 & Dyad 2 and controlled compounds, BTZOMe & BODIPY-OMe.


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2. Experimental 2.1. Materials. Commercially available reagents and chemicals were procured from Sigma-Aldrich, Merck. Analytical reagent (AR) grade solvents were used for the reactions while laboratory reagent (LR) grade solvents were used for purifications and column chromatography. Dichloromethane, chloroform and acetonitrile were dried in presence of calcium hydride under nitrogen atmosphere. Hexane, toluene and tetrahydrofuran were purified by refluxing overnight with Na metal added benzophenone refluxing overnight, then distilled under vacuum and stored over 4Å molecular sieves. Triethylamine was distilled over NaOH pellets. ACME silica gel (60-120 mesh) was used for column chromatography. Thin-layer chromatography was performed on Merck-pre-coated silica gel 60-F254 plates. Either gravity or flash chromatography was performed for purification of all compounds. All the reactions were carried out under nitrogen or argon atmosphere using dry and degassed solvents.

2.2. Synthesis. 2.2.1. 2-(4-hydroxyphenyl)benzo[d]thiazole (2a). 4-Hydroxybenzaldehye (1 g, 8.38 mmol) and 2-aminobenzenethiol (1.05 g, 8.38 mmol) were dissolved in ethanol (20 mL). This mixture was refluxed for 8 h. After that, it was poured into ice cold water. The yellow precipitate obtained was washed with water, filtered, and recrystallized from methanol to get an off-white product. Yield: 0.44 g (23%). Anal.Calcd. For C13H9NOS % (227.28): C, 68.70; H, 3.99; N, 6.16. Found: C, 68.74; H, 4.00; N, 6.21. 1H NMR (CDCl3, 500 MHz): δ (in ppm): 8.04 (d, 1H, J = 8 Hz, benzothiazole-H), 7.99 (d, 2H, J = 9 Hz, phenyl-H), 7.89 (d, 1H, J = 8 Hz, benzothiazole-H), 7.48 (t, 1H, J1 = 8 Hz, J2= 7 Hz, benzothiazole-H), 7.37 (t, 1H, J1= J2= 7.5, benzothiazole-H), 6.95 (d,2H,J = 9 Hz, phenyl-H), 6.04 (br, 1H,-OH). 2.2.2. 2-(4-Ethylcarboxyphenyl)benzo[d]thiazole (2b). 4-Formylbenzoic acid (1 g, 6.67 mmol) and 2-aminobenzenethiol (0.83 g, 6.67 mmol) were taken in a flask and added ethanol (20 mL) and the reaction was set for refluxing. Concentrated H2SO4 (5 drops) was added to it after 12h and refluxing was continued for further 12h. After that, it was cooled to room temperature. A yellowish precipitate was obtained which was filtered, washed with ethanol and recrystallized from hot ethanol to get yellowish product. Yield: 0.71 g (38%).Anal. Calcd. For C16H13NO2S % (283.34): C, 67.82; H, 4.62; N, 4.94. Found: C, 67.80; H, 4.60; N, 5 ACS Paragon Plus Environment



4.93.1H NMR (CDCl3, 500 MHz): δ (in ppm): 8.17 (br, 4H, phenyl-H), 8.12 (d, 1H, J = 8 Hz, benzothiazole-H), 7.95 (d, 1H, J = 7.5 Hz, benzothiazole-H), 7.54 (t, 1H, J1 = 8 Hz, J2 = 8 Hz, benzothiazole-H), 7.45 (t, 1H,J1 = 7.5 Hz, J2 = 10 Hz,benzothiazole-H), 4.43 (q, 2H,J1 =J3=7 Hz, J2=7.5 Hz, OCH2CH3), 1.44 (t,3H,J1 = 7.5, J2 = 7 Hz, OCH2CH3). 2.2.3. 2-(4-Carboxyphenyl)benzo[d]thiazole (2c). Compound 2b (0.15 g, 0.587 mmol) in THF (8 mL) and potassium hydroxide (2.35 g, 0.042 mol) in water (6 mL) was refluxed for 16 h. After cooling, the reaction mixture was diluted with CH2Cl2, acidified with concentrated HCl, and extracted. The organic layer was washed with saturated NaHCO3 solution and dried over anhydrous Na2SO4. Evaporation of the solvent yielded the desired compound as a pale yellow solid. Yield: 0.12 g (89%). Anal.Calcd. For C14H9NO2S % (225.04): C, 65.87; H, 3.55; N, 5.49. Found: C, 65.85; H, 3.52; N, 5.50.1H NMR (CDCl3, 500 MHz): δ (in ppm): 8.14-8.09 (m, 4H, phenyl-H (2H)&benzothiazole-H (2H)), 8.04 (d, 1H, J = 8 Hz, phenyl-H), 7.89 (d, 1H, J = 8.5 Hz, phenyl-H), 7.48 (t, 1H, J1 = 8 Hz, J2 = 7.5 Hz, benzothiazole-H), 7.38 (t, 1H, J1= 7 Hz, J2 = 7.5 Hz, benzothiazole-H). 2.2.4. 2-(4-methoxyphenyl)benzo[d]thiazole (2d). 4-Methoxybenzaldehye (1.36 g, 9.98 mmol) and 2-aminobenzenethiol (1.25 g, 9.98 mmol) were mixed in ethanol (25 mL). This mixture was refluxed for 14 h. After that, it was cooled to room temperature. The precipitate obtained was filtered, washed twice with ethanol, and dried to get the light yellow product. Anal.Calcd. For C14H11NOS % (241.31): C, 69.68; H, 4.59; N, 5.80. Found: C, 69.70; H, 4.57; N, 5.80. Yield: 1.05 g (43.4%). 1H NMR (CDCl3, 500 MHz): δ (in ppm): 8.06-8.03 (m, 3H, phenyl-H (2H)&benzothiazole-H (1H)), 7.89 (d, 1H,J = 8 Hz, benzothiazole-H), 7.48 (t, 1H, J1 = J2 = 7.5 Hz, benzothiazole-H), 7.36 (t, 1H, J1 = 7.5 Hz, J2 = 7.75 Hz, benzothiazoleH), 7.01 (d, 2H,J = 9 Hz, phenyl-H), 3.89 (s, 3H, OMe). 2.2.5. 4-(2-bromoethoxy)benzaldehyde (3a). To a solution of 4-hydroxy benzaldehyde (15g, 122.83 mmol) and anhydrous K2CO3 (50.86 g, 368.49 mmol) in DMF (70 mL) was added 1,2- dibromoethane (85 mL, 982.63 mmol). The resulting mixture was heated under reflux for 14h until the starting material disappeared. Then the reaction mixture was cooled and the solvent was evaporated. Water was added and the mixture was extracted using ethyl acetate. Evaporation of the organic extracts yielded the crude compound which was purified by silica gel column chromatography using petroleum ether/ethylacetaate (98:2, v/v) as an eluent. Evaporation of the solvent yielded the titled compound as white solid. Yield: 13 g (46%). 1H NMR (CDCl3, 500 MHz): δ (in ppm): 9.89 (s, 1H, -CHO), 7.84 (d, 2H,J= 9 Hz, phenyl-H), 6 ACS Paragon Plus Environment

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7.02 (d, 2H, J= 8.5 Hz, phenyl-H), 4.37 (t, 2H, J1= J2, = 6 Hz, -CH2CH2Br), 3.67 (t, 2H,J1= 6.5 Hz, J2 = 6 Hz, -CH2CH2Br). 2.2.6. General Procedure for the Synthesis of Dipyrromethane:54 To pyrrole (20mL, 288.48 mmol), corresponding aldehyde (57.69 mmol) was added and purged with nitrogen for 20min. Then trifluoroacetic acid (441 µL, 5.77 mmol) was added and stirred at room temperature for 15 min. The mixture was diluted with CH2Cl2, and the reaction was quenched with NaOH (20 mL, 0.1 M), and the organic layer was extracted over Na2SO4. Excess pyrrole was distilled through vacuum at room temperature, and the compound was purified by flash column chromatography on silica gel using hexanes: ethyl acetate (70:30 v/v) as eluent. 5-(4-Hydroxyphenyl)dipyrromethane (4a). White solid.Yield: 2.07 g. (15%). Anal.Calcd. For C15H14N2O % (238.28): C, 75.61; H, 5.92; N, 11.76. Found: C, 75.64; H, 5.95; N, 11.80.1H NMR (CDCl3, 500 MHz): δ (in ppm):7.95 (br, 2H, -NH), 7.08 (d, 2h,J = 8 Hz, phenyl-H), 6.77 (d, 2H,J = 8 Hz, phenyl-H), 6.71 (s, 2H, pyrrole-H), 6.17 (m, 2H, pyrrole-H), 5.92 (s, 2H,pyrrole-H), 5.43 (s, 1H, CH), 4.78 (br, 1H,OH). 5-[4-(Bromoethoxy)phenyl]dipyrromethane (4b). Yield: 1.4 g (47%). Anal.Calcd. For C17H17BrN2O % (345.23): C, 59.14; H, 4.96; N, 8.11. Found: C, 59.4; H, 4.95; N, 8.15.1H NMR (CDCl3, 500 MHz): δ (in ppm): 7.91 (br, 2H, -NH), 7.15 (d, 2H, J = 8.5 Hz, phenyl-H), 6.88 (d, 2H,J = 9 Hz, phenyl-H), 6.67 (m, 2H, pyrrole-H), 6.17 (m, 2H, pyrrole-H), 5.92 (s, 2H, pyrrole-H), 5.43 (s, 1H, CH), 4.29 (t, 2H, J1= 6.5Hz, J2 = 6.0 Hz, -CH2CH2Br), 3.64 (t, 2H,J1 = J2 = 6 Hz, -CH2CH2Br). 5-(4-Methoxyphenyl)dipyrromethane (4c). White solid.Yield: 2.7 g. (25%). Anal.Calcd. For C16H16N2O % (252.21): C, 76.16; H, 6.39; N, 11.10. Found: C, 76.14; H, 6.42; N, 11.10. 1H NMR (CDCl3, 500 MHz): δ (in ppm): 7.91 (br, 2H, -NH),7.14 (d, 2H, J = 8.5 Hz, phenylH),6.87 (d, 2H,J = 8 Hz, phenyl-H),6.67 (s, 2H, pyrrole-H),6.17 (m, 2H, pyrrole-H), 5.92 (s, 2H, pyrrole-H), 5.44 (s, 1H, -CH), 3.81 (s, 3H, -OCH3). 2.2.7. General Procedure for the Synthesis of BODIPY.55 To a solution of respective dipyrromethane(6.04 mmol) dissolved in dry CH2Cl2 (50 mL) was added DDQ (1.371 g, 6.04 mmol),and the mixture was stirred for 15 min. Triethylamine (6 mL, 42.28 mmol), was then added, followed immediately by BF3.OEt2 (5.21 mL, 42.28 mmol), and the mixture was stirred for another 1h. The reaction mixture was washed with water and dried over Na2SO4. 7 ACS Paragon Plus Environment



The compound was purified over silica gel column using CHCl3: ethyl acetate (95:05 v/v) as eluent. N,N’-Difluoroboryl-5-(4-hydroxyphenyl)dipyrrin (5a). Yield 162 mg (10%).Anal.Calcd. For C15H11BF2N2O % (284.07): C, 63.42; H, 3.90; N, 9.86. Found: C, 63.45; H, 3.95; N, 9.88.1H NMR (500 MHz, CDCl3): δ 7.93 (s, 2H, pyrrole-H),7.50 (d, 2H,J = 8.5 Hz,phenyl-H), 6.99 (m, 4H, phenyl-H (2H) &pyrrole-H (2H)), 6.56 (m, 2H, pyrrole-H), 5.66 (br, 1H, OH). N,N’-Difluoroboryl-5-[4-(bromoethoxy)phenyl]dipyrrin (5b). Orange solid, Yield 0.4 g (12%).Anal.Calcd. For C17H14BF2N2O % (391.02): C, 52.22; H, 3.61; N, 7.16. Found: C, 52.25; H, 3.65; N, 7.18.1H NMR (500 MHz, CDCl3): δ 7.94 (s, 2H, pyrrole-H), 7.56 (d, 2H,J = 8.5Hz, phenyl-H), 7.07 (d, 2H,J = 9Hz, phenyl-H), 6.97 (m, 2H, pyrrole-H),6.56 (m, 2H, pyrrole-H), 4.41 (t, 2H,J1= 6Hz, J2 = 6.5 Hz,-CH2CH2Br), 3.72 (t, 2H,J1 = 6.5 Hz, J2 = 6 Hz,CH2CH2Br). N,N’-Difluoroboryl-5-[4-(methoxy)phenyl]dipyrrin (5c). Yield 30 mg. (6%).Anal.Calcd. For C16H13BF2N2O % (298.10): C, 64.47; H, 4.40; N, 9.40. Found: C, 64.50; H, 3.65; N, 9.42.1H NMR (500 MHz, CDCl3): δ 7.93 (s, 2H, pyrrole-H), 7.56 (d, 2H, J =9 Hz,phenyl-H), 7.06 (d, 2H,J=8.5 Hz, phenyl-H), 6.99 (m, 2H, J = 4.1 Hz, pyrrole-H), 6.56 (m,2H, pyrrole-H), 3.92 (s, 3H,-CH3). N,N’-Difluoroboryl-5-(4-(2-(4-(benzo[d]thiazol-2-yl)phenoxy)ethoxy)phenyl)dipyrrin (Dyad 1). 2a(50mg, 0.22mmol) was dissolved in DMF (20 mL) and dry potassium carbonate (304.04mg, 2.199 mmol) was added and stirred under nitrogen for 30 min. Then 5b (94.87mg, 0.2640mmol) was added and the reaction mixture was stirred for 18 h. Then the reaction mixture was cooled and the solvent was evaporated. Water was added and the mixture was extracted using ethyl acetate. Evaporation of the organic extracts yielded the crude compound which was purified by silica gel column chromatography using hexane/EtOAc (95:5, v/v) as an eluent. Evaporation of the solvent yielded the titled compound as orange solid. Yield: 50 mg, (42%). Anal.Calcd. For C30H22BF2N3O2S % (537.39): C, 67.05; H, 4.13; N, 7.82. Found: C, 67.02; H, 4.15; N, 7.85. 1H NMR (500 MHz, CDCl3): δ 8.08 (d, 2H, J = 8.5 Hz, benzothiazole phenyl-H), 8.05 (d, 1H, J = 8 Hz,benzothiazole-H), 7.93 (s, 2H, pyrrole-H) 7.90 ( d, 1H, J = 8 Hz,benzothiazole-H) 7.57 (d, 2H,J = 8.5 Hz, bodipy phenyl-H), 7.49 (t, 1H, J1 = 8 Hz, J2= 7 Hz,benzothiazole-H), 7.38 (t, 1H,J1 = J2 = 7.5 Hz, benzothiazole-H), 7.12 (d, 2H, J = 9 Hz, benzothiazole phenyl-H), 7.09 8 ACS Paragon Plus Environment

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(d, 2H, J = 8.5 Hz,bodipy phenyl-H), 6.99 (m, 2H, pyrrole-H), 6.57 (m, 2H, pyrrole-H), 4.48 (s, 4H, CH2CH2). MALDI-TOF: (m/z) found 538 (M+, C30H22BF2N3O2S requires 537.39). N,N’-Difluoroboryl-5-(4-(4-(benzo[d]thiazol-2-yl)carboxyphenyl)phenyl)dipyrrin (Dyad 2). 2c (79.08 mg, 0.30 mmol), 5a (80 mg, 0.28 mmol), and 4-(dimethylamino)pyridine (171.64 mg, 1.40 mmol) were dissolved in 50 mL of dry CH2Cl2. The reaction mixture was cooled to 0°C, and N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide (269.33 mg, 1.40 mmol) was slowly added. The reaction mixture was stirred for 4 h at room temperature, and the solvent was removed. The crude compound was washed with water and extracted with CHCl3. Further purification of the compound was carried out on a silica gel column using chloroform as eluent. Yield: 56mg (38%). Anal.Calcd. For C29H18BF2N3O2S % (521.34): C, 66.81; H, 3.48; N, 7.29. Found: C, 66.85; H, 3.50; N, 7.25. 1H NMR (500 MHz, CDCl3) δ 8.37 (d, 2H, J = 8.5 Hz, benzothiazolephenyl-H), 8.30 (d, 2H, J = 8.5 Hz, benzothiazolephenyl-H), 8.15 (d, 1H,J = 8.5 Hz, benzothiazole-H), 7.98 (m, 3H, pyrrole-H (2H)&benzothiazole-H (1H)), 7.69 (d, 2H, J = 8.5 Hz,bodipyphenyl-H), 7.60 – 7.54 (t, 1H,J1 =8 Hz, J2 = 7.5 Hz,benzothiazole-H), 7.50 – 7.46 (m, 3H, bodipyphenyl-H (2H)&benzothiazole-H (1H)), 7.01 (d, 2H, J = 4.5 Hz, pyrrole-H), 6.60 (d, 2H, J = 2.5 Hz, pyrrole-H). MALDI-TOF: (m/z) found 522 (M+, C29H18BF2N3O2S requires 521.34).

2.3. Methods and Instrumentation. 1

H-NMR spectra were recorded on a 500MHz INOVA spectrometer. Cyclic and differential

pulse voltammetric measurements were performed on a PC-controlled electrochemical analyzer (CH instruments model CHI620C). All these experiments were performed with 1 mM concentration of compounds in 1,2-dichlorobenzene at a scan rate of 100 mV s-1 in which tetrabutylammoniumperchlorate (TBAP) is used as a supporting electrolyte as documented in our previous reports.7,17 2.3.1. Absorption steady state and time resolved fluorescence measurements. The optical absorption spectra were recorded on a Shimadzu (Model UV-3600) spectrophotometer. Concentrations of solutions are ca. to be 1 x 10-6 M. Steady-state fluorescence spectra were recorded on a Fluorolog-3 spectrofluorometer (Spex model, JobinYvon) for solutions with optical density at the wavelength of excitation (λex) ≈ 0.05. Fluorescence quantum yields (Φ) were estimated by integrating the fluorescence bands and by using 2-(4-Methoxyphenyl)-benzothiazole (BTZ-OMe) (Φ = 0.032 in 9 ACS Paragon Plus Environment



CH2Cl2)55 when excited at 308 nm and N,N´-difluoroboryl-1,3,7,9-tetramethyl-5-(4carboxy)phenyldipyrrin (BDP-CO2H) (Φ = 0.57 in CH2Cl2)56 when excited at 503 nm. Fluorescence lifetime measurements were carried on a picosecond time-correlated single photon counting (TCSPC) setup (FluoroLog3-Triple Illuminator, IBH Horiba JobinYvon) employing a picosecond light emitting diode laser (NanoLED, λex = 301 and 485 nm) as excitation source. The decay curves were recorded by monitoring the fluorescence emission maxima of the dyads (λem ≈ 370nm and 520 nm). Photomultiplier tube (R928P, Hamamatsu) was employed as the detector. The lamp profile was recorded by placing a scatter (dilute solution of Ludox in water) in place of the sample. The width of the instrument response function (IRF) was limited by the full width at half maxima (FWHM) of the excitation source, ~625 ps at 301 nm and 485 nm. Decay curves were analysed by nonlinear least-squares iteration procedure using IBH DAS6 (version 2.3) decay analysis software. The quality of the fits was judged by the χ2 values and distribution of the residuals. 2.3.2. Theoretical properties. Full geometry optimization of the Dyad 1and Dyad 2 was carried out by Gaussian 09 ab initio quantum chemical software57 on the personal computer. Density Functional Theory (DFT) was used to determine the ground state properties, while time-dependent DFT (TDDFT) was employed for estimation of ground state to excited state transitions. B3LYP method58 and 6-31G (d,p) basis set59 were used to optimize the geometries of the dyads to be genuine global minimum energy structures. The geometries were used to obtain the frontier molecular orbitals (FMOs) and were also subjected to single-point TDDFT studies (First 15 vertical singlet–singlet transitions) to obtain the UV-Vis spectra of the dyes. The integral equation formalism polarizable continuum model (PCM)60 within the self-consistent reaction field (SCRF) theory was used in the TDDFT calculations to describe the solvation of the dyes in dichloromethane. The software GaussSum 2.2.5 was employed to simulate the major portions of the absorption spectra and to interpret the nature of transitions.61 The contribution percentages of individual units present in the dyes to the respective molecular orbitals were calculated.

2.3.3. Femtosecond Transient Absorption/pump-probe. The detail experimental setup of the femtosecond transient absorption (TA) spectroscopy is describe elsewhere.62 Transient absorption data were recorded using pump-probe set up of CDP Systems Corporation, 10 ACS Paragon Plus Environment

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ExciPro. In brief, the output of optical parametric amplifier (TOPAS prime) was used as pump sources at required wavelength with suitable pump energy and feed into spectrometer through synchronized chopper for 1 kHz repetition rate. A Berek's variable wave plate was used to fix the pump beam at magic angle with respect probe pulse. The amplified 800 nm pulse with 1 KHz repetition rate and 20-25 mW power focused onto a thin rotating (2-mm) CaF2 crystal window to generate a white-light continuum. A fraction of this beam was sent to a photodetector which controls speed and phase of the chopper rotation. The beam of white light was collimated with parabolic mirror (f=50 mm, 90 deg). Then this white light was reflected from a beam splitter and mirror into two identical probe and reference beams. Two concave mirrors (f=150 mm) were used to focus both probe and reference beams to the rotating sample cell. Two lenses (f=60 mm) made probe and reference images at the entrance surfaces of two optical fibers which are connected to the entrance slit of the imaging spectrometer (CDP2022i).

This spectrometer consists of UV-Vis photodiode (Si linear

photodiode) arrays with spectral response range 200-1000 nm. Quartz cells of 1 mm sample path length were used for all studies and IRF was estimated to be ≤150 fs. To minimize the solvent signal pump pulse energy was kept below 3 µJ per second and probe pulse energy was from 0.1-0.5µJ at the sample. For transient absorption spectra the group velocity dispersion compensation of white light continuum (probe beam) was done using studied solvent's two photon absorption data for few ps delay. All the samples were checked before and after taking the transient absorption to monitor the sample degradation if any. 2.3.4. Transient Data Analysis. To obtain a model-based description in terms of precisely estimated rate constants and species related spectral signature, the transient data reported in this paper were analyzed using a combination of global and target analysis.63,64 Global analysis is performed in two different approaches based on superposition principle of least number of independent exponential components and it provides a straightforward description of the data at all measured wavelengths at all time points simultaneously. The number of independent components fitted to all data is determined by gradually increasing the number of exponential components until the residuals were effectively zero. The simplest description in global analysis uses parallel kinetic model where a number of monoexponentially decaying independent components, each represented by a single rate constant (reciprocal of the lifetime) and amplitude at each recorded wavelength, yields the decay associated difference spectra (DADS). The DADSs contemplate the rise and decay of the components with their corresponding decay constants, lifetime values. A second sequential kinetic model, 11 ACS Paragon Plus Environment


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unbranched, unidirectional model, consists of successive monoexponential decays with increasing time constants estimates gross spectral evolution of the data generating evolution associated difference spectra (EADS). As for instant, the first EADS represent spectra just after excitation and it decays with first lifetime into second EADS, in turn, second EADS rises with first lifetime and decays with second lifetime, which is longer than first lifetime, into third EADS and so forth. Finally, data are fitted to a full kinetic model (compartmental scheme), target analysis, by combination of parallel and sequential kinetic model of global analysis which includes all possible branching routes and equilibria between compartments specifying the microscopic rate constants that describe the decay of the compartment as well as transfer of excitation between the compartments. This analysis estimates the real spectra of each compartment (excited species) and is termed as species associated difference spectra (SADS). The whole analysis was performed with the R package TIMP and its graphical user interface of Glotaran.65-67

3.

Results and Discussion

3.1. Synthesis The synthesis of BTZ-BODIPY dyads, Dyad 1 and Dyad 2, employed in the present study involved first the synthesis of 4-hydroxyphenyl and 4-ethylcarboxy-substituted benzothiazoles, 2a and 2b, prepared by condensation of 2-aminobenzenethiol with psubstituted benzaldehydes in ethanol (Scheme 1) followed by the base hydrolysis of 2b to obtain 4-carboxyphenyl-substituted benzothiazole2c. The benzothiazole based control compound (BTZ-OMe), 2d, was synthesized under similar condition by refluxing 2aminobenzenethiol with p-methoxybenzaldehyde in ethanol. The BODIPY precursors, 5a and 5b, were synthesized by preparing dipyrromethanes using pyrrole and 4hydroxybenzaldehyde

or

4-bromoethoxybenzaldehyde

followed

by

conversion

of

dipyrromethane to BODIPYs. Dyad 1 was synthesized by reacting 5b with 2c in presence of potassium carbonate in DMF while Dyad 2 was prepared by tethering 5a with 2c using DCC and DMAP. Preliminary characterization of the dyads and the control compounds were carried out by using elemental analyses, 1H NMR, MALDI-MS and UV-visible spectroscopic methods. The mass spectrum of Dyad 1 showed a peak at m/z = 538 (C30H22BF2N3O2S) while Dyad 2 at m/z = 522 (C29H18BF2N3O2S) ascribable to their corresponding molecular ion peaks.1H NMR spectra of compounds 2a-d, 5c, Dyad 1 and Dyad 2 were shown in the 12 ACS Paragon Plus Environment

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figures S1-S7 and the ESI mass spectra of Dyad 1 and Dyad 2 in the figures S8 and S9 in the supporting information. R SH + NH 2

Ethanol

S

Reflux

N

R

CHO

1a: R = OH 1b: R = CO2Et 1c: R = CO2 H 1d: R = OCH 3

2a: R = OH 2b: R = CO2Et KOH; reflux 2c: R = CO2H 2d: R = OCH 3 (BTZ-OMe) Br

O K2 CO 3

1a + Br

Br

DMF CHO

3a R

R R CF3CO2 H

+ CHO

1. DDQ 2. Et3 N 3. BF3 .OEt2

N H

N

NH HN

1a: R = OH 3a: R = -(CH2)2Br 3b: R = OCH 3

F

4a: R = OH 4b: R = -(CH2)2Br 4c: R = OCH 3

N

B

F

5a: R = OH 5b: R = -(CH2)2Br 5c: R = OCH 3 (BODIPY-OMe)

N

2a + 5b

K2 CO 3

B

O

F

N

S

F

O

DMF N

Dyad 1 N

2c + 5a

DMAP, EDC CH2 Cl2 , 6h, rt

B

O

S

N N

F F

O

Dyad 2

Scheme 1: Synthesis of Dyad1, Dyad 2, and controlled compounds, BTZ-OMe & BODIPY-OMe. 13 ACS Paragon Plus Environment



3.2. Optical Absorption Properties Figure 1 shows the optical absorption spectrum of the dyads, Dyad 1 and Dyad 2, along with the reference compounds, BODIPY-OMe and BTZ-OMe in 1,2-dichlorobenzene. In case of Dyad 1, the absorption bands located at 321, 390 and 502 nm were shifted marginally i.e., 1-2 nm compared to the reference compounds indicating very weak ground state interactions. However, in case of Dyad 2, as the chromophores are tethered by a shorter bridge i.e., ester linkage, the absorption bands located at 332, 350 and 506 nm were shifted at least 5-40 nm indicating strong electronic communication between the chromophores in the ground state. The wavelengths of maximum absorbance (λmax) and molar extinction coefficient (log ε) values of the dyads and reference compounds obtained from the UVvisible studies, are summarized in Table 1. Similar results were observed when the optical absorption spectra of the dyads and reference compounds were recorded in at least three other solvents, toluene, DMF and acetonitrile, of varied dielectric constants (see figures S10(a)(c)). Importantly, the absorption of BTZ moiety at 300 nm in the dyads had minimal overlap with the BODIPY absorption at these wavelengths. Hence, irradiation of the dyads at 290300 nm is expected to selectively excite the BTZ moiety of the dyads. Similarly, the stronger absorption of BODIPY moiety at 485 (shoulder), 502 and 506 nm had no overlap with the BTZ absorption and hence, allowing to selectively excite the BODIPY moiety at 485 nm when required.

Figure 1: Absorption spectra of the indicated compounds in 1,2-dichlorobenzene. 14 ACS Paragon Plus Environment

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Table 1: UV-visible and electrochemical data. Potential V vs. SCEa

Absorption, Compound

λmax, nm (log ɛ, M-1cm-1)a 392

501

(4.23)

(4.87)

-

-

321

390

502

(4.33)

(4.77)

(4.54)

332

350

506

(4.46)

(4.45)

(4.70)

BODIPY-OMe

BTZ-OMe

321

Reduction -0.79

Oxidation -1.78

1.01

-1.14

-1.29

1.07

-0.82

-1.06

-1.60

1.02

-0.76

-1.15

-1.60

1.07

(4.84) Dyad 1

Dyad 2

a

Solvent: 1,2-dichlorobenzene, Error limits: λmax, ± 1 nm, log ε, ± 10%. b1,2-dichlorobenzene, 0.1 M TBAP; Glassy carbon as working electrode, Standard calomel electrode is reference electrode, Pt electrode is auxillary electrode. Error limits, E1/2±0.03 V.

3.3. Electrochemical Properties Electrochemical studies using the differential pulse and cyclic voltammetric techniques were performed in 1,2-dichlorobenzene to arrive at the redox potentials of the investigated compounds. Figure 2 depicts the differential pulse voltammograms and figure S10(d) shows the cyclic voltammograms of the dyads and control compounds respectively. The redox potential data of all the compounds are presented in Table 1. BODIPY-OMe exhibited a reversible one-electron reduction at E½ = -0.79 V vs SCE in 1,2-dichlorobenzene. Additional irreversible redox waves at Epc = -1.78 V and Epa = 1.01 V were also observed under the experimental conditions. Under the similar experimental conditions, BTZ-OMe exhibited two reductions at -1.14, -1.29 and an oxidation at 1.07 V. In the Dyad 1, the first reduction of BODIPY moiety was located at Epc = -0.82 V along with other reduction waves corresponding to the reduction of BTZ and BODIPY moieties at -1.06 V and -1.60 V, respectively. Similarly, in case of Dyad 2, the first reduction appeared at -0.76 was assigned to BODIPY moiety reduction while the reductions at -1.15 V and -1.60 V were assigned to BTZ and BODIPY moieties respectively. In addition, a one-electron reversible wave at E½ = 1.02 V, in case of Dyad 1, and an irreversible wave at Epa = 1.07 V, in case of Dyad 2, were observed. As BODIPY-OMe and BTZ-OMe displayed the oxidation potentials at almost 15 ACS Paragon Plus Environment


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similar potentials (∆E = 60 mV), it was difficult to assign the oxidations observed in the dyads to either of the BODIPY or BTZ moieties.

Dyad 1

Dyad 1

Dyad 2

Dyad 2

BTZ-OMe

BODIPY-OMe

1.2

1.0

0.8

0.6

0.4

0.2

0.0 0.0

BTZ-OMe

BODIPY-OMe

-0.5

-1.0

-1.5

-2.0

E vs SCE

E vs SCE

Figure 2: Differential pulse voltammetry of Dyad 1, Dyad 2, BTZ-OMe, and BODIPYOMe in 1,2-dichlorobenzene containing 0.1 M (n-C4H9)4NClO4 and the concentrations of the compounds were held at 1 mM; scan rate = 100 mVs-1.

3.4. Theoretical Calculations To visualize the geometry, electronic structure, and optical properties of the dyads, computational studies involving DFT and TDDFT calculations using B3LYP/6-31G(d,p) level were performed. For this, the dyads were optimized on a Born-Oppenheimer potential energy surface, and a global minimum for each dyad was obtained. The energy-optimized structures and the molecular electrostatic potential (MEP) maps for the Dyad 1, and Dyad 2, are shown in Table 2. For the investigated dyads, it is observed that the BTZ moiety, in its neutral state is planar and is located perpendicular to the BODIPY moiety as shown in optimized structures in Table 2.


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Table 2: Optimized structures, electrostatic potential maps and isodensity (0.02) plots of FMOs. Dyad 1 Dyad 2

Optimized structure

Electro static potential map

LUMO+1

LUMO

HOMO

HOMO-1



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Table 3: B3LYP/6-31G (d,p)-Optimized distances between BTZ and BODIPY moieties and related orbital energies in the investigated dyads. a

E,

d b

c

Dyad 1

K.cal./mol -1314010

5.985

16.208

Dyad 2

-1288280

3.699

13.895

Re-e, Å

Rc-c, Å

d

d

-5.907

-5.839

-2.827

-1.529

3.012

-6.397

-5.917

-2.838

-2.375

3.079

HOMO (H)

LUMO (L)

d

H-L gap

d

HOMO-1

LUMO+1

The edge-to-edge (Re-e) and centre-to-centre distances (Rc-c) between the BTZ and BODIPY moieties in the dyads were estimated and summarized in Table 3. In the MEP maps, for both the dyads, the positive electrostatic potential was at the benzothiazole and spacer connecting the chromophores, while the negative potential was concentrated at the pyrrolic – NH and –BF2 groups of BODIPY. Frontier highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) for the Dyad 1 and Dyad 2 are shown in Table 2. In both the dyads, HOMO was located on BTZ moiety and LUMO on the BODIPY π-system, while the HOMO-1 and LUMO+1 were located on the BODIPY and BTZ moieties respectively. In addition, in case of Dyad 2, the frontier orbitals are partially distributed onto ester linkage and phenyl connected meso- of the BODIPY moiety, showing a stronger communication between the chromophores in the ground state as observed in the absorption studies (vide infra). For the Dyad 1 and Dyad 2, the calculated gas phase HOMO–LUMO gap was found to be 3.01 and 3.08 eV, respectively. It should be mentioned here that the location of LUMO on BODIPY moiety in both the dyads is consistent with the electrochemistry results and confirms that the first reduction in both the dyads is mainly from the BODIPY moiety of the dyads. Based on the experimental observations, TDDFT studies of these molecules were carried using B3LYP energy functional with the 6-31G(d,p) basis set in order to gain a deeper understanding of the excited-state transitions with the framework of the polarizable continuum model (PCM) in dichloromethane as the solvent. These results are in reasonable agreement with the experimental values. Table S1 in thesupporting information shows singlet state properties of maximum wavelength absorbance, oscillator strength (f), excited state energy (E) in eV and the percentage contribution of molecular orbital of both dyads by means of absorption spectra. Theoretical absorption spectra of Dyad 1, and Dyad 2 of each dyad segment has been computed from the frontier molecular orbitals by using the GaussSum software and plotted as shown in Figure S11 in the supporting information. 18 ACS Paragon Plus Environment

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3.5. Energy Transfer from the Singlet Excited State of BTZ to BODIPY moieties in the dyads 3.5.1. Excitation at 300 nm Unlike the case with the ground state properties, major differences have been noticed in the singlet state activities of the dyads, when compared to their monomeric chromophores. When equimolar solutions of the dyads and the control compounds were excited at 295 nm in toluene i.e., the λmax at which BTZ moiety absorbs predominantly, an emission peak (λmax: 364 nm), corresponding to the BTZ moiety, was observed to be totally quenched in case of the dyads when compared to their individual constituent, BTZ-OMe (see figure 3). A similar behavior was observed when the experiments were performed in three other solvents of varied polarity i.e., toluene, DMF, and acetonitrile. Corresponding emission maxima and quantum yields have been collected in Table 4. It should be mentioned here the possibility of self-aggregation of either BTZ or BODIPY, which leads to quenching of fluorescence intensity (as a result of non-radiative path in the excited state) can be ruled out as all experiments were carried out with very dilute solutions (~10-6 M). The E0-0 (0-0 spectroscopic transition energy) values of the BTZ (3.6±0.05 eV) and BODIPY(2.41±0.05 eV) moieties of the dyads, as estimated from an overlap of their absorption and emission spectra, were found to be in the same range as the E0-0 values of BTZ-OMe and BODIPYOMe, respectively. In 1,2-dichlorobenzene, excitation of BTZ-OMe at 295 nm revealed an intense BTZ moiety emission with λmax centered at 368 nm and shoulder peaks at 347, 382, 413, and 444 nm (Figure 3b). Upon excitation of the equimolar solutions of the dyads at 295 nm, the emission corresponding to BTZ moiety was efficiently quenched and new emission bands corresponding to BODIPY moiety around 520 nm were observed. Under these conditions, when equimolar solution of only BODIPY-OMe was excited at 295 nm, very weak emission of BODIPY at 520 nm was observed. This suggests that the irradiation of the dyads at 295 nm would not excite the BODIPY moiety predominantly; as mentioned the section 3.2. These control experiments and the diminished intensity of the BTZ moiety and the appearance of new emission bands corresponding to the BODIPY emission in the dyads clearly indicate the occurrence of photo-induced energy transfer. Similar experiments,




Figure 3: Fluorescence spectra (λex= 295 nm) of equimolar solutions of BTZ-OMe, BODIPY-OMe, Dyad 1 and Dyad 2 in toluene (~1 × 10-6 M), 1,2-dichlorobenzene (~1 × 106 M), DMF (~0.5 × 10-6 M) and acetonitrile (~0.5 × 10-6 M).

probing the possibility and efficiency of photo-induced electron transfer were performed in three solvents, toluene, DMF, and acetonitrile, of varied polarities (see figure 3a-d). In addition to the above information, from figure 3, it can be observed that the emission intensity of BODIPY moiety, obtained due to photo-induced energy transfer from singlet excited BTZ to BODIPY moiety, is higher in case of Dyad 1 as compared to Dyad 2 and the efficiency of energy transfer decreased with increase in the solvent polarity; this observation is consistent with earlier reported studies for different donor-acceptor systems.68 It should be mentioned here that as the emission of BODIPY moiety, either in Dyad 1 or Dyad 2, does not overlap with the absorption of BTZ, no energy transfer experiments by exciting BODIPY and monitoring the BTZ emission i.e., energy transfer from 1BODIPY* to BTZ have been performed. 20 ACS Paragon Plus Environment

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Table 4. Fluorescence Dataa a

λem, nm (Φ, %Q)

λex = 300 nm

λex = 485 nm

Compound Toluene

DCB

DMF

ACN

-

-

-

-

364

370

368

368

(0.014)

(0.017)

(0.113)

(0.116)

409

417

412

364

BODIPYOMe BTZ-OMe

-6

Dyad 1

(2×10 ,

(1.7×10 ,

(2.1×10 ,

(4.5×10-3,

99)

98)

98)

96)

410 (1.3×10 4

a

416 -

Dyad 2

-3

, 99)

-3

438 -3

Toluene

DCB

DMF

ACN

518

519

515

510

(0.081)

(0.011)

(0.019)

(0.031)

-

-

-

-

517

519

514

510

(0.081)

(0.011)

(0.019)

(0.031)

524

525

521

516

339 -6

-4

(1.2×10 ,

(3.6×10 ,

(9.8×10 ,

(0.049,

(0.0061,

(0.015,

(0.024,

92)

99)

99)

40)

44)

21)

22)

Spectra were measured at 293±3 K. Error limits: λem, ± 1 nm; Φf, ± 10%.

A major difference between the fluorescence data of the dyads and that of BTZ-OMe lie in the magnitude of fluorescence quantum yield, Φf, values (Table 4). As mentioned earlier, fluorescence from BTZ part of the dyads was found to be strongly quenched in comparison with the fluorescence of control compound, BTZ-OMe, in all the investigated solvents. The quenching efficiency Q can approximately estimated as follow, Q=

Φ ( ref ) − Φ ( dyad ) Φ ( ref )

k obs =

(1)

Q /(1 − Q ) τ ( ref )

(2)

and the rate of fluorescence quenching, kobs, values evaluated using the fluorescence data are given in Table 4 and Table 5 respectively. In equations 1 and 2, Φ(ref) and Φ(dyad) refer to the fluorescence quantum yields for BTZ-OMe and the Dyad 1 or Dyad 2 and τ(BTZ-OMe) is the singlet-state lifetime of BTZ-OMe (0.166, 0.209, 0.231, 0.186 ns, in toluene, 1,2-DCB, DMF, and CH3CN, respectively, see figure S12 in the supporting information).



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Table 5. Energy transfer data of BTZ-BODIPYa

Dyad 1

Dyad 2

Solvent

%Q

%T

kobs (109s-1)

kEN(obs) (1010 s-1)e

Jd (cm mmol-1)c

kForster (1010 s-1)d

Toluene (n = 1.496, ε = 2.38)b DCB (n = 1.551, ε = 9.93)b DMF (n = 1.430, ε = 36.7)b ACN (n = 1.344, ε = 37.5)b

99

80

596

2.44

2.21 × 10-14

1.19

98

80

234

1.89

1.89 × 10-14

0.85

98

75

212

1.30

2.21 × 10-14

8.28

96

74

129

1.56

1.87 × 10-14

11.41

Toluene (n = 1.496, ε = 2.38)b DCB (n = 1.551, ε = 9.93)b DMF (n = 1.430, ε = 36.7)b ACN (n = 1.344, ε = 37.5)b

99

69

342

1.25

3.33 × 10-14

4.52

92

65

35

0.90

2.77 × 10-14

3.13

99

63

165

0.74

3.84 × 10-14

36.15

99

64

394

0.94

3.88 × 10-14

59.80

6

a

Error limits: %Q, kobs±8%, %T, kEN(obs): ±15% bn and ε refer to refractive index and dielectric constant of the solvents, respectively. d Spectral overlap term calculated by using PhotochemCADsoftware71. dκ2 = 0.66 in all the cases and Rc-c = 16.208 Å (Dyad 1) and 13.895 (Dyad 2). ekEN(obs) calculated by using equation Tobs /(1 − Tobs ) . T , energy transfer efficiency, was calculated from the overlap of the excitation and obs τ ( BTZ − OMe) absorption spectra.

A careful examination of figure 1 and 3, it can be observed that there is a strong overlap between the emission of BTZ and absorption of BODIPY moieties. This suggests that the quenching of the fluorescence of BTZ moiety in this D-A system can be due to an intramolecular PEnT from the excited singlet state of BTZ to the BODIPY (1BTZ*BODIPYBTZ-1BODIPY*). Indeed, excitation of approximately 10-7 M solutions of the dyads at 300 nm resulted in the appearance of well-defined BODIPY emission bands in all investigated solvents and the emission intensity of BODIPY decreased with increasing solvent polarity (figure 3). In case of Dyad 1, the quenching efficiencies were found to be in the range of 96-99%, whereas in case of Dyad 2, the quenching efficiencies in all the investigated solvents were found to be 99%. This may be due to the efficient electronic communication between the chromophores due to shorter D-A distance in Dyad 2. Furthermore, the excitation spectra of the dyads were recorded by monitoring the emission at 520 nm corresponding to BODIPY moiety. The spectra thus recorded were found similar to the absorption spectra of the dyads (Figure 4) suggesting an evidence for 22 ACS Paragon Plus Environment

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intramolecular PEnT in these bichromophoric dyads. It should be mentioned here that control experiment involving excitation (λex =300 nm) of 1,2-dichlorobenzene solution containing 1:1 (mole/mole) intermolecular mixture of BTZ-OMe and BODIPY-OMe did not have any effect on the emission intensity of BTZ, and extending the emission scan to the BODIPY emission region revealed no emission corresponding to BODIPY, suggesting no energy transfer from 1BTZ* to BODIPY moieties in an intramolecular manner.

Figure 4: Overlay of the absorption (_____) and excitation (_____) spectra of Dyad 1 and Dyad 2 at λem = 520 nm in 1,2-dichlorobenzene solvent. PEnT reactions between singlet states in bichromophoric D-A systems can occur by two different mechanisms, Förster mechanism (or dipole-dipole interaction)69 or the Dexter mechanism (or electron exchange mechanism).70 In case of singlet states possessing high absorbance and emission quantum yields, the dipole-dipole mechanism, in general, prevails whereas for poorly emitting donor or absorbing acceptors the Dexter mechanism, based on the mutual exchange of two electrons, can prevail. Förster mechanism require, among other things, the rate of energy transfer to be proportional to spectral overlap, J, of the donor emission and the acceptor absorption. For the dipole-dipole mechanism, it is possible to calculate the rate, kForster, by means of the following equation (3) k Foster =

8.8 x10 23 κ 2φ D JFoster n 4τ D R 6

(3)

in which Φ and τ are the emission quantum yield and lifetime of the isolated in various solvents (see Table 4 and Figure S12), RDA is the donor-acceptor center-to-center distance 23 ACS Paragon Plus Environment


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(from Table 3, 16.2 for Dyad 1 and 13.9 Å for Dyad 2), η is the refractive index of the specified solvent and JFörster is the overlap integral, κ2, the orientation factor, takes into account the relative orientation of the transition dipole moments of the donor and the acceptor and for randomly oriented dipoles a value of κ2 = 2/3 is generally used. The spectral overlap integral, JFörster, for the emission of the donor and absorption of the acceptor can be evaluated according to the eq 4.

∫ F( ν)ε( ν)ε = ∫ F( ν)dν

−4

JForster

dν (4)

where FD(λ) is the fluorescence intensity of the donor with total intensity normalized to unity. The ɛA(λ) refers to the molar extinction coefficient of the acceptor expressed in units of M1

cm-1 and λ in nanometers. In the present study, JFörster for Dyad 1 and Dyad 2 in four

different solvents were calculated using software, PhotochemCAD71 and obtained values were tabulated in Table 5. In case of Dyad 1, the calculated JFörster were found to be in the range of 2.21 × 10-14 - 1.87 × 10-14 cm6 mmol-1 (or M-1 cm3) and, for the Dyad 2 they were in the range of 3.33 × 10-14- 3.88 × 10-14 cm6 mmol-1 ranging from a non-polar solvent, toluene, to a polar solvent, acetonitrile, employed for the study. Using these JFörster values, the rates of Förster energy transfer, kFörster, evaluated according to eq 4 were found to be in the range of 1.19 × 1010 s-1(toluene) – 11.41 × 1010s-1 (acetonitrile) for Dyad 1 and 4.52 × 1010s-1 (toluene) – 59.80 × 1010s-1(acetonitrile) for Dyad 2. From Table 5, it can be observed that, in case of

Dyad 1, kEN(obs) values obtained in toluene (2.44 × 1010 s-1) and 1,2-DCB (1.89 × 1010s-1) are comparable (slightly higher) to the calculated results (1.19 × 1010 s-1for toluene and ~0.9 × 1010 s-1 for 1,2-DCB) while for Dyad 2, kEN(obs) values obtained in toluene (1.25 × 1010 s-1) and1,2-DCB (0.9 × 1010s-1) are comparable (lower by ~ 3.5 times) to the calculated results (4.52 × 1010 s-1 for toluene and 3.13 × 1010 s-1 for 1,2-DCB), stating the possibility of Förster type mechanism in both the dyads in non-polar solvents. On the other hand, the calculated rates of energy transfer in more polar solvents, i.e., DMF and acetonitrile, for the dyads were found to be unusual and not comparable to the experimental values. The free-energy changes (∆GEN) accompanying energy transfer by dipole-dipole mechanism in all solvents, were calculated by eq 566 and tabulated in Table 6. (5)


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The driving forces for the photo-induced energy transfer, ∆GEN, from1BTZ* to BODIPY, for both the dyads in all solvents were found to be exothermic and this is more pronounced in case of Dyad 1 as compared to Dyad 2 (see Table 6). In case of Dyad 1, the ∆GEN was found be more exothermic in non-polar solvents than in polar solvents, as expected, and followed the trend toluene>1,2-DCB~DMF>acetonitrile. However, a similar trend was not observed in case of Dyad 2. An energy level diagram depicting the photochemical events for all the solvents in Dyad 1 is shown in figure 5 and a similar energy level diagram for Dyad 2 is shown in Fig. S13 in the supporting information.

Figure 5. Energy level diagram showing photo-induced energy transfer processes in Dyad 1 in four different solvents. Table 6. Free-energy changes (in eV) for singlet energy transfer (∆GEN) for the Dyad 1 and Dyad 2 in various different solvents. Compound

∆GEN(BTZ*-BODIPY) Toluene

DCB

DMF

ACN

Dyad 1

-1.44

-1.41

-1.41

-1.38

Dyad 2

-1.15

-1.12

-1.16

-1.16




3.5.2. Excitation at 485 nm Various radiative and non-radiative intramolecular processes can be responsible for the excited state decay of the dyads. Among these, in addition to PEnT, PET can be conceived as one of the pathways for quenching the fluorescence intensity. If PET is known to occur in a donor-acceptor (D-A) system then excitation of either the D or A would lead to fluorescence quenching of the excited chromophore and generates a charge separated state, D+.A-..72 Also, the rates of PET and the driving forces of charge separation should increase with increase in the polarity of the solvents.17 In order to monitor this, we have excited equiabsorbing solutions of BODIPY-OMe, Dyad 1 and Dyad 2 at λmax = 485 nm, the wavelength at which BODIPY absorbs predominantly, in four different solvents and monitored the emission from 1BODIPY* at 520 nm (see Figure S14). Surprisingly, it was observed that, in comparison with emission intensity of reference compound, BODIPY-

OMe, the emission of Dyad 1 was not quenched and the intensity appeared to be same (Iref/IDyad 1) in all the solvents of varying polarity (see table S2 in the supporting information). Where as in case of Dyad 2, as compared to reference compound, the emission intensity was observed to be quenched in a given solvent but the intensity of emission (Iref/IDyad 1) did not change appreciably with increase in the solvent polarity. Hence, the decrease in emission intensity in Dyad 2 cannot be attributed to PET inadvertently, and is might be because of the ground and excited state interactions between the chromophores leading to lower quantum yields of emission (vide supra). Similar results were obtained when excited state fluorescence decay measurements were performed for Dyad 1, Dyad 2, and BODIPY-OMe in four different solvents and the corresponding decay parameters are collected in Table S3. Therefore, from these studies it can be understood that Dyad 1 and Dyad 2 employed in the present study stand out as excellent photosynthetic antenna-reaction centre models that predominantly undergo photo-induced energy transfer, from 1BTZ* to BODIPY to yield BTZ-BODIPY*, and do not participate appreciably in photo-induced electron transfer events.

3.6 Ultrafast Transient Absorption Studies: 3.6.1. Transient Absorption of BTZ: In view to have detail insight into energy transfer dynamics particularly in polar solvents we have extended our investigation to femtosecond time resolved transient absorption studies for 26 ACS Paragon Plus Environment

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Dyad-1 and Dyad-2 as well as constituent moieties, BTZ-OMe and BODIPY-OMe along with model based data analysis approach comprising global and target analysis for transient data sets. Figure 6(a) shows the heat map of transient absorption of BTZ in ACN solution upon 330 nm excitation, at which the BTZ moiety absorbs predominantly. Simultaneous rise of negative signal at around 370 nm region (blue area in heat map), the region where BTZ fluorescence was observed and positive signal peaking around 440-450 nm (yellow to red area in heat map) are observed. The time evolution of these negative signal and positive signal follow the exactly similar fashion as observed in transient heat map. Hence this negative signal is safely assigned as stimulated emission (SE) from S1 of BTZ and positive signal can be assigned as excited state absorption (ESA) of BTZ from Sn←S1. A rise of new positive peak around 400 nm (yellowish green area) is observed followed by decay of SE as well as ESA. The decay of this new positive band is very slow estimated population time profiles and corresponding evolution associated difference spectra (EADS), respectively of global analysis of transient data set using sequential kinetic model. At least three sequential components are necessary to fit the data satisfactorily. The time constants of the first two components are precisely found to be 7 and 210 ps but the time constant of the third component is very large which cannot be estimated exactly within the 6 ns time window. However, it can predict to be more than 25 ns. The short component of 7 ps lifetime can be assigned as intramolecular vibrational relaxation which is associated to the decay of hot S1 state to thermally relaxed S1 state of BTZ. The lifetime (210 ps) of the second component nicely corroborated to the fluorescence lifetime of BTZ observed by TCSPC and the long lived component with lifetime more than 25 ns can be assigned to lifetime of triplet state of

BTZ. Hence, EADS1 and EADS2 are identified as hot S1 and relaxed S1 states and EADS3 as first triplet state.



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Submitted to J. Phys. Chem. C 0

0.0375

1

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0.4

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0

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50

Hot S1 S1 T1 IRF

0.0 0

1000 2000 3000 4000 5000 6000 Time (ps)

Figure 6: ∆OD heat maps as a function of probe wavelength (vertical) and probe delay (horizontal) for the isolated BTZ in ACN upon 330 nm excitation (a). As indicated in the color map, the red/yellow indicates positive signals (i.e., excited state absorption), and green/blue denote negative signals (i.e., decrease in absorption due to stimulated emission and/or ground-state bleaching). Estimated EADS (b) and population profile (c).


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3.6.2. Transient Absorption of BODIPY: To understand the transient behaviour of BODIPY we have recorded transient absorption spectra of reference (isolated) BODIPY upon two different excitations at 330 nm and 475 nm respectively. The transient absorption spectra is mostly dominated by negative signal due to ground state bleaching (GSB) and stimulated emission (SE) in the 340-580 nm spectral window (see supporting info, Figure S15-S18 for TA spectra and decay profiles).55 However, a weak positive signal at 435 nm region is observed and the dynamic of this signal exactly follows the dynamics of GSB and SE. Hence this band could be assigned to excited state absorption corresponds to Sn ←S1 transition. Upon 330 nm excitation the magnitude of this negative signal is very weak and it is 6-7 times weaker than that was observed upon 475 nm excitation. Global analysis of transient data for both the excitation shows two major components with 8 and 285 ps respectively. The 8 ps component can be assigned as intramolecular vibrational relaxation time where as the second component is assigned as S1 lifetime due to its close resemblance to the fluorescence lifetime measured by TCSPC. It is important to mention here that a very ultrafast component can be predicted when BODYPI is excited at 330 nm, higher order excited state, due to internal conversion from S1←Sn. However, this component is too fast to measure confidently with our existing set up of ~100 fs IRF.

3.6.2. Transient Absorption of Dyads: In order to compare the transient behaviours of BODIPY moiety of Dyad 1 and Dyad

2 to that of isolated BODIPY in BODIPY-OMe, we measured TA spectra of Dyad 1 and Dyad 2 upon 475 nm excitation, little higher energy of BODIPY absorption. In no surprise, the observed results are grossly parallel to that of isolated BODIPY, in term of spectral shape and their dynamics (supporting info, Figure S19-S22). Global analysis of TA data with sequential kinetic model satisfactorily fit with two components. The EADS corresponding to hot S1 and thermally equilibrium S1 for the dyads are quite similar to the respective EADS for the isolated BODIPY. The characteristic time constants of the said sates of BODIPY are 15 and 272 ps in Dyad 1 and 19 and 170 ps in Dyad 2. The lifetime of hot S1 state of BODIPY is slightly larger in both the dyads than the isolated BODIPY while lifetime of S1 state is nearly equal to that of isolated BODIPY in Dyad 1 and little lower in Dyad 2.



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Submitted to J. Phys. Chem. C Figure 7 shows the comparative ∆OD heat map of the dyads with BTZ in 2ps time window in ACN solution, following excitation of the BTZ moiety at 330 nm. For isolated BTZ, positive TA signal, ESA spectra of BTZ, appears immediately after excitation and hardly decays within 2 ps time window whereas, for the dyads following the excitation similar type of positive TA signal emerged and unlike isolated BTZ, it decays almost completely within 2 ps time window with concomitant appearance of negative signal above 430 nm, a exactly similar GSB and SE signal observed for isolated BODIPY or for dyads upon 475 nm excitation and these signals decay with slowly with characteristic lifetime of BODIPY. A comparative time trace of the TA signals for BTZ and both the dyads at two different wavelengths (445 nm and 495 nm) pertaining to the characteristic transient signal of BTZ and BODIPY respectively is shown in Figure 7(d). These results clearly reveal the fact that for the dyads within 100 fs following excitation a TA spectrum is formed, which displays not only the expected Sn←S1 ESA from the BTZ but also a negative photo-bleaching (PB) signal peaking at around 490-500 nm, reflecting excitation of the BODIPY moiety and thus indicating BODIPY ← BTZ excitation energy transfer (PEnT). Both the dyads follow exactly similar dynamics. However, depopulation of ESA from BTZ as well as population of excited BODIPY occurs relatively slowly in Dyad 2. 540 Wavelength (nm)

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

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Figure 7: ∆OD heat maps as a function of probe wavelength (vertical) and probe delay (horizontal) for the isolated BTZ in 2 ps time window (a), Dyad 1 (b) and the Dyad 2(c) in acetonitrile upon 330 nm excitation. As indicated in the color map, the red/yellow indicates positive signals (i.e., excited state absorption), and green/blue/black denote negative signals (i.e., decrease in absorption due to stimulated emission and/or ground-state bleaching). Time traces at selected probe wavelengths 445 and 495 nm respectively (d).


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Submitted to J. Phys. Chem. C

Hot S1

k1= (3.1 ps) -1, Dyad-1 (7.7ps) -1, Dyad-2

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

BTZ

k2= (0.162 ps) -1, Dyad-1 (0.4ps) -1, Dyad-2 BODIPY* (S1*) k3 = (15 ps) -1, Dyad-1 (13ps) -1, Dyad-2 S1 k4= (285 ps) -1, Dyad-1 (170 ps) -1, Dyad-2

S0 Figure 8: Kinetic scheme used for target analysis of the 330 nm excitation data. The estimated rate constants (in 1/ps) are indicated in the figure; the global lifetimes of Hot S1 of BTZ moiety are 154 (Dyad 1) and 385 (Dyad 2) fs.

Global and target analysis, comprising compartmental model, were used to interpret in transient data matrices for the dyads taking minimum number components that are necessary for satisfactorily fit the data. Unlike isolated BTZ or BODIPY, an extra ultrafast component is essential to globally fit the TA data of the dyads along with relatively slower components which are observed for isolated BODIPY. Figure 8 depicts the compartmental model used for target analysis for 330 nm excitation data of the dyads. Hot S1 is the species associated difference spectra (SADS) associated with BTZ, while (S1*) and S1 are associated with the BODIPY. Hot S1 is considered due to little excess energy is initially deposited in the BTZ via 330 nm pump pulse. Spectral evolutions of hot S1 → S0 of BTZ is taken into account to include the contribution of BTZ fluorescence if any, and S1* → S1 of BODIPY is taken into account in order to include internal vibrational relaxation (IVR) processes within the two species as it was observed in isolated BODIPY. The estimated population time profiles and corresponding SADSs are shown in Figure 9. In order to compare the kinetic of the dyads to those of isolated moieties, we have assumed that excited states are not significantly affected upon covalently linking the two molecules. However, based on the assumption, the first SADS can be identified as hot S1 state of the BTZ, with a strong ESA band at 410-480 nm regions for both the dyads. This SADS nearly equal to that of EADS1 of isolated BTZ. However, a closer look to the first SADS could prevail the appearance of a notch at 500 nm regions, region of GSB of BODIPY. This 31 ACS Paragon Plus Environment



observation clearly suggest that upon 330 nm excitation a fraction of BODIPY moieties are excited along with BTZ moieties as BODIPY ground state absorption extends slightly to 330 nm region. The lifetime of the hot S1 state of BTZ in Dyad 1 and Dyad 2 is found to be 154 and 385 fs respectively, which are very fast with respect to lifetime of S1 state of isolated BTZ. The Hot S1 state of the BTZ is depopulated via two channels: one (k1) decays to ground state and another channel (k2) transfers the energy populating the hot S1 of BODIPY (SADS2). The lifetime of hot S1 state of BODIPY is observed to be 15 and 13 ps for Dyad-1 and Dyad-2, respectively. This hot S1 state evolves (k3) into relaxed S1 state which subsequently decays (k4) to ground state in 285 ps (Dyad 1) and 180 ps (Dyad 2) and they are nicely parallel to their isolated BODIPY counterpart. Hence, based on this model the efficiency of energy transfer for both the dyads from S1 of BTZ to BODIPY can be estimated as ηET= k1/(k1+k2) = ~95%. The data from the transient spectroscopic studies firmly confirm the occurrence of ultrafast excitation energy transfer in the studied dyads with exact rate of energy transfer (k2) 6.2x1012 and 2.5 x1012 s-1 for Dyad-1 and Dyad-2 respectively. However, this directly observed energy transfer rate constants for both the dyads were found to be one order higher than those calculated using Förster-type energy transfer mechanism, dipoledipole interaction, in ACN solvent. These high values of rate constants could be rationalized by considering the partial involvement of Dexter energy transfer along with predominant Förster resonance energy transfer. Perhaps, due to very high rate of energy transfer no clear distance dependent rates of energy transfer can be predicted at the moment from these femtosecond transient spectroscopic data in ACN solvent. Future experiments comprising solvent dependent femtosecond transient studies with more better-regulated distances between donor and acceptor will help to exactly predict the role of distance in the rate of energy transfer between BTZ and BODIPY are planned soon.


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Submitted to J. Phys. Chem. C 0.04

SADS1 SADS2 SADS3

0.02 0.00

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SADS(OD)

SADS(OD)

0.02

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200

400

600

800

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Time (ps)

0

200 Time (ps)

400

600

Figure 9: Estimated SADS and respective population profile of Dyad-1 (A,B), Dyad-2 (C,D). SADS1 corresponds to hot S1 of BTZ, SADS2 corresponds to hot S1 of BODIPY and SADS3 corresponds to S1 of BODIPY moiety for respective dyad.

4. Conclusions In conclusion, two photosynthetic antenna-reaction center models, Dyad 1 and Dyad 2, comprised of benzothiazole (BTZ) as energy donor and boron dipyrromethene (BODIPY) as energy acceptor, have been designed, synthesized and systematically characterized using several spectroscopic techniques. In order to study the role of spacer in energy transfer studies, the BTZ and BODIPY moieties, in Dyad 1 and Dyad 2, are tethered together with ethoxyphenyl and ester linkages respectively. MALDI-MS and 1H NMR spectroscopic techniques confirmed the structural integrity of both the dyads. Absorption, electrochemical and computational studies revealed a minimal ground state interactions between the chromophores in Dyad 1 and, whereas, notable interactions in Dyad 2. Computational studies also revealed that the LUMO in both the dyads is located on BODIPY moiety indicating the BODIPY to behave as an energy acceptor. Excitation of dyads at λmax= 295 nm, the wavelength at which BTZ moiety absorbs predominantly, resulted in quenching of 33 ACS Paragon Plus Environment



emission of BTZ, compared to pristine BTZ emission followed by the appearance of BODIPY emission at around 525 nm indicating the occurrence of photoinduced energy transfer (PEnT) from 1BTZ* to BODIPY. This phenomenon was observed in four different solvents of varying polarity.Similarity of the excitation spectra of the dyads, obtained by monitoring the emission at 520 nm corresponding to BODIPY moiety, and absorption spectra of the dyads suggested an additional evidence for intramolecular PEnT in these bichromophoric dyads. The free-energy changes of energy transfer (∆GEN) in both the dyads were found to be exothermic and followed the trend and followed the trend toluene>1,2DCB~DMF>acetonitrile in case of Dyad 1. We have adopted Förster dipole-dipole mechanism to explain the PEnT events in these bichoromophoric systems and it obeys in non-polar solvents. Parallel transient absorption studies of the dyads and the reference compounds in polar solvents such as acetonitrile also indicated the fluorescence quenched pathway is exclusively PEnT process and the rates of energy transfer were found to be 6.2 × 1012 s-1 for Dyad 1 and 2.5 × 1012 s-1 for Dyad 2 in acetonitrile.

Acknowledgements Authors RC, LG, and AS are grateful to Department of Science and Technology (DST, SB/FT/CS-146/2014), (DST, SB/S1/IC-14/2014), and (DST-IFA-13, CH-97) for financial support of this work respectively. The authors DBand NDacknowledge Council of Scientific and Industrial Research (CSIR) for Junior Research Fellowship (JRF) and KJ acknowledges DST-INSPIRE for Research Fellowship.

Supporting Information Characterization data of the compounds such as 1H NMR (Figures S1-S7) and MALDI-MS spectra (Figure S8 & S9), absorption spectra of in different solvents (Figure

S10), theoretical absorption spectra (Figure S11), fluorescence behaviour (Figure S11- S14), and transient absorption spectra (Figure S15-S22) are available free of charge via the Internet at http://pubs.acs.org.

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2. Berardi, S.; Drouet, S.; Fracas, L.; Gimbert-Surinach, C.; Guttentag, M.; Richmond, C.; Stoll, T.; Llobet, A. Molecular Artificial Photosynthesis. Chem. Soc. Rev. 2014, 43, 75017519. 3.

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