Ultrafast Energy Transfer Followed by Electron Transfer in a Polymeric

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Ultrafast Energy Transfer Followed by Electron Transfer in Polymeric Nanoantenna Based Light Harvesting System Bikash Jana, and Amitava Patra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06052 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 15, 2018

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The Journal of Physical Chemistry

Ultrafast Energy Transfer Followed by Electron Transfer in Polymeric Nanoantenna Based Light Harvesting System

Bikash Jana and Amitava Patra* Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India

*

Corresponding author’s e- mail: [email protected] Phone: (91)-33-2473-4971, Fax: (91)-332473-2805

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The Journal of Physical Chemistry 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

ABSTRACT Polymeric nanoantenna based light harvesting system is prospective due to capturing of more number of photons by multi-chromophoric donor and subsequently transfers its energy to the acceptor.

Here, we have designed an aqueous solution based polymeric

nanoantenna where conjugated polymer nanoparticles (donor) are attached with Au nanoparticles capped porphyrin (acceptor) encapsulated BSA protein for both energy transfer and electron transfer processes.Ultrafast fluorescence upconversion and transient absorption spectroscopic studies reveal that the ultrafast energy transfer occurs from conjugated polymer nanoparticle to porphyrin in 130 fs and followed by the electron transfer from porphyrin to Au nanoparticles in 70 ps after photoexcitation. Furthermore, the antenna effect of this light harvesting system is found to be 79 at the donor to acceptor ratio of 16:1. Analysis reveals that this polymeric nanoantenna opens up a new avenue for designing highly efficient aqueous solution based light harvesting system.


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INTRODUCTION Photosynthesis is one of the most well known processes where antenna complexes absorb sunlight and subsequently transfer its energy to reaction centres to convert light energy into chemical energy.1-5 In natural photosynthesis, multistep processes such as energy transfer process in antenna molecules occurs in the femtosecond (fs) to picoseconds (ps) time scale whereas the electron transfer process in reaction centre takes place in picoseconds to nanosecond time scale.6 Chlorophylls, β-carotene are being used as antenna molecules for photosynthetic light harvesting because of their high molar extinction coefficients, typically ~ 1 x 105 M-1 cm-1.7-8 Considerable efforts have been given on understanding the fundamental aspects of pigment-protein light harvesting systems and study reveals that the arrangement and selection of chromophores are important for energy and charge transfer processes.9-10 Therefore, the selection of chromophores, their molecular arrangement and the electronic coupling of the donor-acceptor moieties are the important parameters to control the energy and electron transfer process in an artificial light harvesting system. Considering these issues, several strategies have been taken to design artificial light harvesting systems. The majority of them are involved in organic solution based systems such as dendrimers, porphyrin assemblies and few of them are in aqueous based scaffolds systems such as DNA, virus, micelle and organic-inorganic hybrid materials.11-17 Among them, conjugated polymer nanoparticles (PNPs) are much promising antenna materials due to easy possessing in aqueous solution, highly luminescent, higher photo stability and low cytotoxicity.18-22 Basically, conjugated polymers are multichromophoric π-conjugated materials where each sub-unit acts as a chromophore with delocalized electronic structure and have larger molar extinction coefficient.23-26 Furthermore, the choice of acceptor to collect the excitonic energy from the donor is another challenging issue. In this regard, porphyrin molecules are found to be suitable because of their structural similarity with the chlorophyll molecule and high molar extinction coefficient.11, 27 Besides the energy transfer, the electron transfer in the reaction centre is another important aspect in natural photosynthesis. The most commonly used electron accepting materials are C60, cyno-substituted conjugated small molecules and polymer, various organic molecules, graphene oxide, reduced graphene oxide, metal nanoparticle etc.27-29 Among them, most are organic soluble and merely employed in aqueous environment. Contrary, easily synthesizable aqueous dispersion of Au nanoparticle (NP) is suitable due to large electron accepting property.30



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Most of the reported light harvesting systems are followed either energy transfer or electron transfer process.17, 31-34 Less emphasis has been given where both the processes are taking place in artificial light harvesting system.35-37 Recently, Ito and his co-workers have designed a light harvesting system where both energy and electron transfer processes are used.38-39 Generally, organic solution based light harvesting systems are designed, therefore, it is very challenging to construct an aqueous solution based artificial light harvesting system where both energy transfer and electron transfer are taking place. In the present work, we have designed an aqueous solution based light harvesting system where conjugated polymer nanoparticles (donor) are attached with Au NP capped porphyrin(acceptor) encapsulated BSA protein (Scheme 1) for both energy transfer and electron transfer processes. The multi-chromophoric property of the conjugated PNPs enables to capture more number of photons and deliver the light efficiently to a single acceptor, BSA encapsulated porphyrin by energy transfer process. Finally, the electron transfer process takes place from the photo excited porphyrin to Au NP. Here, ultrafast fluorescence upconversion and transient absorption spectroscopy are carried out to explore the ultrafast energy transfer and electron transfer processes in this light harvesting system. The observed energy transfer followed by electron transfer in the present artificial light harvesting system mimics the natural photosynthesis.

Scheme 1. Schematic illustration of F8BT PNP-porphyrin encapsulated protein-Au nanoparticle hybrid for artificial light harvesting system.

EXPERIMENTAL SECTION Materials Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)]

(F8BT)

(Sigma Aldrich), hexadecylamine (HDA) (Sigma Aldrich), gold(III) chloride trihydrate


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(HAuCl4) (Merck), bovine serum albumin (BSA) (Merck), tetrahydrofuran (THF) (Merck) and HPLC water (Merck) were purchased and used without further purification. Synthesis of 5-(4-hydroxyphenyl)-10,15,20-(triphenyl)porphyrin (HPTP) To a solution of benzaldehyde (27.9 g, 0.262 mol) and 4-hydroxybenzaldehyde (10.7 g, 0.0876 mol) in propionic acid (800 mL), a solution of pyrrole (23.9 g, 0.356 mol) in propionic acid (200 mL) was added drop wise under vigorous stirring. The brown solution was then refluxed for 1 h and cooled to ambient temperature. The black precipitate thus obtained was collected and washed with methanol to get a purple coloured solid. The solid was purified by silica gel column chromatography using dichloromethane–hexane as an eluent to furnish 5-(4hydroxyphenyl)-10,15,20-triphenylporphyrin as purple crystals (1.8 g, 3.2% yield). 1H NMR (CDCl3, 500 MHz): δ 8.85 (s, 8H, pyrrole β-H), 8.2 (m, 6H, Ar-H), 8.04 (d, J = 8.5 Hz, 2H, Ar-H), 7.76 (m, 9H, Ar-H), 7.12 (d, J = 8.5 Hz, 2H, Ar-H), 2.70 (brs, 2H, NH) (Figure S1). Preparation of positively charged F8BT polymer nanoparticles Positively charged F8BT polymer nanoparticles (PNPs) were prepared using most common reprecipitation technique described elsewhere.17, 28, 30, 32 Briefly, 250 µL of 0.5 mg mL-1 F8BT in THF, 50 µL of 1 mg mL-1 hexadecylamine (HDA) in THF and 200 µL of 1 mg mL-1 THF were mixed under ultrasonication to get a clear and homogeneous solution in an anaerobic condition. Then, 500 µL of the mixed solution of polymer was rapidly injected into 10 mL of HPLC water under vigorously stirring condition. The mixed THF and water solution was immediately ultrasonicated for 30 min and thereafter THF was evaporated by partial vacuum evaporation at 60 °C for two hours. Thus, aqueous dispersed positively charged F8BT polymer nanoparticles (PNPs) (Shown in Figure S2) were formed and the prepared PNPs were quite stable for more than two months. Synthesis of Au nanoparticle (NP) Colloidal solution of Au NPs was synthesized using reported method.40-41 Briefly, 0.5 ml of 10 mM aqueous solution of HAuCl4 was added to 9.5 ml BSA-PBS buffer solution maintaining 10 µM BSA concentrations at pH 7. The mixed solution was well stirred for 5 minutes. Then freshly prepared 100 µL of ice cold NaNH4 solution (0.001 g/mL) was drop wise added to the vigorously stirring condition. As a result, pale yellow colour turned to dark brown colour indicating formation of Au NP. Formation of 3.0 ± 0.2 nm Au NPs is confirmed by the TEM image and the arisen plasmon band at ~520 nm in absorption spectrum (Figure S3). It is noted that the conformation of the BSA is not changing after the Au NP formation. In BSA capped Au NPs, HPTP is again loaded using same technique described earlier.



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Designing of HPTP loaded BSA protein scaffold Very little amount of 100 µM HPTP solution in ethanol was added to BSA solution at pH 7.0 to maintain 6.1 µM HPTP in 10 µM BSA solution. Then the mixed solution was kept for 2 hours in the dark to prevent any photo degradation and to complete evaporation of ethanol. The HPTP molecules were thus trapped in the hydrophobic pocket of BSA protein. Using this simple procedure, HPTP loaded BSA protein solution was adapted for further experimental works. Moreover, the concentration of HPTP was varied with exclusive variation of initial concentration of HPTP keeping the same concentration of BSA protein. Designing of triad for artificial light harvesting system In this work the designed triad is composed of polymer nanoparticle and HPTP loaded BSA capped Au NPs. As stated earlier, the synthesized polymer nanoparticle and BSA protein are positively and negatively charged, respectively. To construct triad, Au NPs were first synthesized by employing BSA as capping ligand. Then, in this Au NPs capped BSA protein HPTP is loaded using the as stated prior conditions. Therefore, BSA acts as a scaffold for both the HPTP and Au NPs. Then, 0.5 mL of the as-prepared HPTP loaded BSA protein capped Au NPs was added to the 2.0 mL of positively charged F8BT PNP and kept for 24 hours to get complete electrostatic attachment. Characterization A high-resolution transmission electron microscope (HRTEM; JEOL 2010,200 kV operating voltage) was used to investigate the morphological sizes of F8BT PNPs, Au NPs and PNP-Au NPs hybrid nanostructure. Zeta potential value and dynamic light scattering (DLS) were measured using a Malveron Zetasizer instrument. Room temperature optical absorption spectra were taken by UV-vis spectrophotometer (Shimadzu). Room temperature photoluminescence (PL) studies and steady state anisotropy were carried out by a FluoromaxP (Horiba JOBIN YVON) photoluminescence spectrophotometer. Steady state anisotropy, ⟨r⟩ was calculated from the following equation:42 r =

IVV − (G × IVH ) IVV + (2 × G × IVH )

(1)

Where G, the “G factor”, was G=

I HV I HH

(2)

Ixy corresponds to ‘x’ polarized excitation and ‘y’ polarized emission and ‘H’ and ‘V’ stand for vertical and horizontal polarization respectively. For the time correlated single photon counting (TCSPC) measurements, the samples were excited at 340 nm (nano-LED, pd =1 ns)


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and 405 nm (pico-second diode laser, pd < 200 ps). The repetition rate for all measurement in TCSPC was 1 MHz. The fluorescence decays were analyzed using IBH DAS6 software. The following equation was used to analyze the experimental time-resolved fluorescence decays, P(t):42 n  t  P(t ) = b + ∑ α i exp −  i  τi 

(3)

Here, n is the number of discrete emissive species, b is a baseline correction (“dc” offset), and αi and τi are the pre-exponential factors and excited-state fluorescence lifetimes associated with the ith component, respectively. For multi exponential decays the average lifetime, ⟨τ⟩ was calculated from the following equation:42 n

τ = ∑ βiτ i i =1

(4)

Where β i = α i / ∑α i and β i is contribution of the decay component. αi and τi are the preth

exponential factors and excited-state fluorescence lifetimes associated with the i component, respectively. The energy transfer efficiency, ϕET is measured by using the following equation:42

φET = 1−τ DA /τ D

(5)

Where, τDA and τD are the decay time of donor (F8BT PNP) in presence and absence HPTP encapsulated BSA, respectively. Rate of energy transfer and electron transfer are calculated using the following equation:42

k=

1

τ DA

−

1

(6)

τD

For determination of the rate of energy transfer, τDA and τD are the decay time of donor (F8BT PNP) in presence and absence HPTP encapsulated BSA, respectively. In case of determination of the rate of electron transfer, τDA and τD are the decay time of HPTP in presence and absence of Au NP, respectively. Cyclic Voltammetry (CV) measurements of F8BT PNPs and porphyrin (HPTP) was performed in 0.1 M tetrabutylammonium perchlorate aqueous solution and 0.1 M tetrabutylammonium hexafluorophosphate in DCM, respectively on PC-controlled PAR model 273A electrochemistry system under argon gas atmosphere. Platinum disk, platinum wire and Ag/AgCl/KCl (saturated) were used as the working electrode, counter electrode and reference electrode respectively.



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Ultrafast Transient absorption data have been collected using Transient Absorption Spectrometer (TAS), Newport. The output from a femtosecond Ti:Sapphire one box amplifier (Spectra Physics Spitfire, 1 kHz, 5 mJ, 100 fs, 800 nm) passes through a beam splitter from which one is directed into an OPA (Light Conversion Topas Prime, 100 fs, 285-2680 nm) to generate pump pulse while the other one is passes through a second beam splitter from which 95% is dumped and 5% is delayed relative to the pump pulsed using a motorized translation stage to generate probe pulse by passing through a CaF2 crystal (330 nm – 750 nm). The pump power was kept low enough (5µJ/cm2) to avoid complications arising from multiexcitons and the pump beam diameter is slightly larger than the probe beam size. Surface Xplorer 4.1.0 has been used for the chirp correction of TA data prior to analysis of single wavelength kinetic fitting. Ultrafast spectroscopic data were investigated by femtosecond fluorescence upconversion spectrophotometer using Halcyone ultrafast setup (Coherent). The sample was excited with 400 nm and 450 nm (excitation power 5µJ/cm2) wavelength of excitation, pumped by 800 nm femtosecond (fs) (140 fs pulse width, 80 MHz repetition rate) laser pulse (4.4 W) from a Ti:sapphire oscillator (Chameleon, Coherent) coupled to a second harmonic generator (by BBO type I crystal).The emission wavelength (538 nm and 655 nm) and the gate pulse of the fundamental beam (800 nm) are upconverted by a nonlinear crystal (BBO type II). The FWHM of instrument response function is about 288.8 fs. The femtosecond time resolved decay data were fitted by Surface Xplorer 4.1.0 fitting software.

Computational details for Docking study: Before starting the docking simulation study the geometry of HPTP was optimized by Gaussian 09 program package43 using a Density Functional Theoretical (DFT) method with the B3LYP hybrid functional and 6-311++g(d,p) basis set. To find the probable binding location of HPTP inside BSA protein cavity, the docking simulation was exploited using the AutoDock Vina software package44 (http://vina.scripps.edu/) and to visualize docked conformer, PyMOL software package45 was used.

The

crystal

structure

of

BSA

was

taken

from

Protein

Data

Bank

(http://www.rcsb.org/pdb/explore.do?structureId=4F5S).46

RESULTS AND DISCUSSION Photophysical properties of conjugated polymer nanoparticle Surface modified positively charged Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) nanoparticles were prepared using the reprecipitation method. F8BT and hexadecylamine (HDA) were dissolved together in THF and then the mixed solution was rapidly injected in large amount of water. Immediately, the 8 ACS Paragon Plus Environment

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hydrophobic polymer chains (extended form) convert to PNPs (collapsed state). The hydrophobic aliphatic chain of HDA is moving inward the hydrophobic core of the PNP and NH2 groups reside outside of the PNP to make the positively charged water dispersed PNP (measured zeta potential + 21.7 mV). TEM image (Figure S2A) confirms that the average size of the nanoparticles is 40 ± 5 nm. It is to be noted that the absorption bands at ~320 nm and ~457 nm are unchanged after nanoparticles formation (Figure S2B). The band at 457 nm is due to benzothiadiazole (BTz) unit and the band at 320 nm is due to π–π* transitions of the fluorene backbone.47 The molar extinction coefficient of F8BT PNP is found to be 1.4 x 109 M-1 cm -1, indicating the light energy is efficiently absorbed by the F8BT PNP which is beneficial for efficient light harvesting system. The peak intensity of the emission band at ~538 nm is decreased (Figure S2C) after nanoparticle formation. The average decay time decreases from 2.54 ns to 0.66 ns during the formation of collapse state from the extended form (Figure S2D) due to intermolecular energy transfer.48

Photophysical properties of porphyrin (HPTP) encapsulated BSA protein It is to be noted that 5-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin (HPTP) porphyrin molecules is non fluorescent in aqueous or in PBS buffer solution due to the self quenching or agglomerated from of HPTP. However, 23 fold enhancement of PL intensity at ~655 nm is observed when it is encapsulated inside BSA protein (Figure 1A). The emission maximum at ~655 nm is due to the relaxation of Qx (0-0) band of S1 state. The molar extinction coefficient of HPTP is found to be 3.4 x 105 M-1 cm

-1

which is also potential for

designing light harvesting system. The decay time of the Qx (0-0) state of free HPTP in ethanol is 9.33 ns (Figure S4) whereas the bi-exponential decay is observed with an average lifetime of 4.33 ns in case of HPTP encapsulated BSA protein. Again, the steady state anisotropic study of HPTP indicates the restricted rotation due to enhancement of anisotropy value from 0.01 to 0.03 after encapsulation inside BSA protein. To understand the location of HPTP inside BSA protein, blind simulation based study has been performed using AutoDock Vina software.44 Analysis confirms that the preferred location is domain IIA of BSA protein as it acquires the lowest binding energy of -7.4 kcal mol-1 than the others locations (Figure 1B and 1C). The optimum concentration of HPTP is found to be 6.1 µM in 10 µM BSA (BSA: HPTP is 1.6:1) and no significant helical structural deformation of BSA is found at this concentration of HPTP.



Figure 1. (A) Photoluminescence of HPTP in PBS buffer (a) and inside BSA protein (b) (λex= 400 nm), (B) and (C) Red circle represents the exact location of HPTP inside BSA generated by AutoDock Vina based blind docking simulation.

Electron transfer from HPTP encapsulated BSA protein to Au NPs conjugate The influence of Au NPs on HPTP encapsulated BSA has been unveiled using both steady state and time resolved spectroscopy. The PL quenching (43%) of fluorescence intensity of HPTP is observed in presence of Au NPs (120 nM). From the TCSPC measurement, it is seen that the shortening of decay time from 4.33 ns to 2.17 ns and the appearance of ~660 ps decay in presence of Au NPs after excitation at 405 nm (Figure S5). It is to be mentioned that very small overlap between emission band of HPTP and absorption band of Au NPs (Figure S6). To clarify the energy or electron transfer process, we measured the absolute band position of HPTP by CV measurement (Figure S7) and construct the band diagram (Scheme S1) where the work function of Au NPs (-5.1 eV) is taken from the reported value.49 On the basis of band diagram, the electron transfer is highly favourable from the photo excited HPTP towards Au NPs. Furthermore, femtosecond time resolved decay traces of HPTP in presence and absence of Au NPs have been recorded with the excitation at 400 nm. No change is observed in 100 ps time scale (Figure S8).

Ultrafast energy transfer in F8BT- HPTP encapsulated BSA complex A complex of F8BT nanoparticle (PNP)- HPTP encapsulated BSA is designed by the electrostatic attraction of conjugation of BSA protein (measured zeta potential value -10.4 mV) with positively surface charged F8BT PNP and the complex formation is confirmed by the increased size (from 60 nm to 80 nm) in DLS measurements (Figure S9). Here, the concentrations of PNP and HPTP are 6.64 x 10-10 M and 1.22 µM, respectively. In this complex, we use F8BT PNP as a donor and HPTP loaded BSA protein as an acceptor because the large overlap integral value is 4.2 x 1012 M-1 cm-1 nm4 between emission of donor (PNP) to the absorption of acceptor (HPTP) indicating the energy transfer (Figure 2A). Negligible 10 ACS Paragon Plus Environment

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change in absorption spectrum of complex compared to its pristine counterparts indicating the absence of any ground state interaction (Figure S10). It is to be noted that 76 % quenching in donor emission and the significant 87 fold enhancement of acceptor emission are observed at donor excitation (340 nm) (Figure 2B). A systematic increase in PL quenching of PNP and concomitant enhancement in emission of HPTP (Figure S11) with increasing concentration of BSA loaded HPTP at the fixed concentration of PNPs confirms the energy transfer. The steady state fluorescence anisotropy of BSA loaded HPTP changes from 0.031 to 0.008 for the excitation of F8BT PNP indicates the energy transfer due to depolarised emission of the acceptor. In time correlated single photon counting (TCSPC) measurement, we excite the F8BT nanoparticle (PNP) - HPTP encapsulated BSA complex at 340 nm because HPTP is negligibly excited at 340 nm. The shortening of donor (F8BT PNP) lifetime in presence of HPTP is not well resolved in nano-scale time scale from the TCSPC result (Figure S12) which may be due to fast energy transfer process. To establish the precise time scale of energy transfer, ultrafast decay measurement through fluorescence upconversion and transient absorption have been carried out. The ultrafast fluorescence decay traces clearly reveal the shortening of decay time of F8BT PNP in presence of BSA encapsulated HPTP (Figure 2C). It is noted that the faster component of the decay traces of F8BT (at 538 nm) is reduced from 6.81 ps (14 %) to 6.14 ps (33 %) in presence of HPTP. In addition, excitation of F8BT PNP at 450 nm (where, HPTP is not excited) exhibits ~130 fs rise time of HPTP emission (at 655 nm) and confirms the prompt energy transfer from F8BT to HPTP in F8BT-HPTP loaded BSA complex (Figure 2D). The energy transfer efficiency is found to be 76% and the rate of energy transfer is 1.43 x 1010 sec-1. In transient absorption (TA) measurements, all the samples are excited at 340 nm (excitation wavelength of F8BT). The ground state bleaching (Figure S13) of F8BT PNP (at 460 nm) is due to charge carrier recombination dynamics and the kinetic profile is fitted with three components, a fast component of 819 fs (47%) and other two components are 19 ps (30%) and 777 ps (23%). The fast bleach recovery of F8BT PNP (at 460 nm) is observed in presence of HPTP (Figure S14) and the components of the kinetic profile are 156 fs (27%), 987 fs (33%) and 14.69 ps (40%). Interestingly, this fastest component (156 fs) of the kinetic profile at 460 nm is highly consistent with fluorescence upconversion data (130 fs). In F8BT-HPTP loaded BSA complex, an additional bleach signal at 420 nm is due to HPTP which starts within 200 fs indicating a very fast energy transfer process (Figure 2E). The bleaching kinetics of HPTP (at 420 nm) in F8BT-HPTP loaded BSA



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complex exhibits 2.7 ps rise time and a slow ~1 ns bleach recovery time (Figure 2F) which further confirm the ultrafast energy transfer process. Furthermore, we measure the efficiency of the light harvesting system in terms of antenna effect (AE). The ratio of emission intensity of the acceptor chromophore upon excitation of donor to the direct excitation of the acceptor is expressed by AE value. In a more simplified way, one can explore the AE as the increase in acceptor emission with addition of donor at the donor excitation. The AE is calculated by using following equation:11

AE = ( I DA340. f − I A340 ) / I A520

(7)

where, IDA340 is the emission intensity of HPTP encapsulated BSA at 340 nm in presence of F8BT and IA340 represents the emission intensity of pure HPTP encapsulated BSA at 340 nm excitation wavelength. IA520 represents the emission intensity of HPTP encapsulated BSA at 520 nm (direct excitation of HPTP). ‘f’ represents the fraction of the total fluorescence coming from the HPTP due to energy transfer process. Qu and co-workers reported that the value of antenna effect is 4 with donor: acceptor (45:1) ratio of thioflavin T to thiazole orange inside coordination PNP.25 Li et al. have reported that the antenna effect is 32.5 for donor: acceptor (125:1) ratio.50 Recently, Trofymchuk et al. have reported ~1,000 antenna effect for PMMA-MA NPs based PNP system with a donor to acceptor ratio of 10,000:1.51 Very recently, an antenna effect of supramolecular self assembly based system is found to be 28 at 200:1 donor to acceptor.52 We have reported that the value is increased up to 31 with a donor (PVK PNP) to acceptor (Nile Red) ratio of 0.82: 1.32 Now, in F8BT- HPTP encapsulated BSA complex, a very high AE value of 79.02 for the number of donor chromophores to acceptor ratio of 16:1 is calculated which is very much promising than the reported artificial light harvesting system.


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Figure 2. (A) Spectral overlap between the absorption spectrum of HPTP in BSA (a) and emission spectrum of aqueous dispersed F8BT PNPs (b), (B) Emission spectrum of water dispersed F8BT PNPs (a), F8BT (0.66 nM)- HPTP (1.22 µM) loaded BSA complex (b) upon excitation at 340 nm and HPTP (1.22 µM) loaded BSA excited at 520 nm (c) [Inset shows the photo graphs of (a), (b) and (c) under UV light irradiation (365 nm)]. Temporal profile of fluorescence up-converted signal of (C) F8BT PNP in absence (a) and presence of HPTP (1.22 µM) loaded BSA (b) (λex = 400 nm, λem = 538 nm), and (D) HPTP in F8BT- HPTP (1.22 µM) loaded BSA complex for the excitation of F8BT (λex = 450 nm, λem = 655 nm), (E) Transient absorption spectra and (F) normalized bleach recovery kinetics at 420 nm of F8BTHPTP loaded BSA complex (λex 340 nm).

Ultrafast energy transfer followed by electron transfer in triad Finally, we construct a triad by the electrostatic attachment of positively surface charged F8BT PNP with Au NP capped and HPTP encapsulated negatively charged BSA protein. TEM image of the triad (Figure 3A) confirms the attachment of Au nanoparticles on the surface of PNP. There is no change in absorption spectra of triad (Figure S15) from the isolated F8BT PNPs and BSA protein (with HPTP and Au NPs), indicating no ground state interaction. However, 80% PL quenching in emission of F8BT PNP is observed in triad where as 69 % PL quenching of F8BT is observed in presence of Au and in absence of HPTP (Figure S16). In addition, 43 % PL quenching in HPTP emission (at 655 nm) is increased to 73 % in the triad in presence of Au NPs only (in HPTP loaded BSA capped Au NPs) after excitation of F8BT (Figure 3B) nanoparticles. To obtain any effect of Au NPs on the excited state dynamics of F8BT PNPs only and in triad, TCSPC, ultrafast fluorescence upconversion and transient absorption spectroscopic studies are undertaken. The ultrafast fluorescence decay dynamics of F8BT (at 538 nm) exhibits small change in decay traces in presence of only Au NPs (Figure S17). It is clearly seen that only slower component of the decay curve is reduced from 249 ps to 152 ps, whereas, the faster component of the decay (~6.8 ps) remains unaffected in presence of Au NPs. Again, we have recorded transient absorption spectra of F8BT with changing delayed time in presence of Au NP only (Figure S18). It is noted that there is no change in bleaching kinetics of F8BT (at 460 nm) (Figure S19). To elucidate the exact mechanism on shortening of decay trace, absolute band position of F8BT has been measured using CV (Figure S20). From the band diagram (Scheme S2), the energy/electron transfer may occur from the F8BT towards Au NPs in ps time scale. In case of triad, the faster component of the decay of F8BT is reduced from 6.8 ps (14%) to 6.1 ps (25%) (Figure S21) which is similar to the rate of 13 ACS Paragon Plus Environment


energy transfer from F8BT to HPTP without Au nanoparticle. Therefore, it is also indicating the very fast energy transfer from F8BT to HPTP in the triad. To understand the photo induced electron transfer from HPTP to Au NP in triad, nanosecond TCSPC measurement has been done with the excitation of donor (F8BT) (Figure 3C). In triad, the average lifetime of HPTP (at 655 nm) is reduced from 6.65 ns to 0.58 ns with an extra 200 ps component having maximum contribution. Again, ultrafast fluorescence decay of HPTP for the excitation of F8BT (at 450 nm) in triad shows the same 130 fs rise time for the donor excitation (Figure S22), confirming the ultrafast energy transfer. Furthermore, TA data of the triad have also been collected with the excitation at 340 nm. In this context, the ground state bleaching kinetics of the donor, F8BT (at 460 nm) in triad and in F8BT-HPTP loaded BSA complex (Figure S23) is similar. Contrary, a clear bleach signal at 420 nm arises for the HPTP within 200 fs in triad, which is similar with the prior result in F8BT-HPTP loaded BSA complex (Figure 3D). However, an interesting phenomenon has been found in bleaching kinetics of HPTP (at 420 nm) in triad. It is noted that the kinetics at 420 nm shows an extra 70 ps (50%) bleach recovery component without changing the other two component (2.7 ps rise time and ~1 ns recovery time) in triad (Figure 3E). It is already mentioned that the bleaching kinetics of HPTP (at 420 nm) in F8BT-HPTP loaded BSA complex shows 2.7 ps rise time and a slow ~1 ns bleach recovery time (Figure 2F). No noticeable change is observed in the bleach recovery kinetics of F8BT PNP (at 460 nm) in absence and presence of Au NPs at 460 nm (Figure S19). Therefore, the 70 ps component is exclusively assigned for the electron transfer from the HPTP to the Au NPs in the triad. The rate of electron transfer in triad from HPTP to Au NPs is 8.66 x 108 sec-1 based on the bleach recovery kinetics of HPTP at the excitation of F8BT PNP. The electron transfer from HPTP to Au nanoparticles takes place in 660 ps after direct excitation of HPTP (at 400 nm) which becomes 70 ps after the excitation of F8BT (at 340 nm). Therefore, the electron transfer process from HPTP to Au NPs in triad is found to be faster which matches well with PL quenching enhancement from 43 % to 73 % in the triad. As the energy transfer efficiency (in F8BT–HPTP encapsulated BSA complex) is not 100%, therefore the total quenching in emission of F8BT PNP in triad is a combination of energy transfer process from PNP to porphyrin and energy/electron transfer process from F8BT to Au NPs. The estimated FRET efficiency from F8BT PNP to HPTP is ~76% and PL quenching of F8BT is 76% in F8BT- HPTP loaded BSA complex. In presence of Au NP in triad, the PL quenching of F8BT is observed 80% (Figure S16). It is already mentioned that the energy transfer from PNP to HPTP occurs in 130 fs, whereas, energy/electron transfer from PNP to Au NPs (in absence of porphyrin) occurs in much slower rate. Therefore, the 14 ACS Paragon Plus Environment

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excess PL quenching in F8BT PNP is ascribed due to the energy/electron transfer from the F8BT PNP to Au NPs. Furthermore, we have varied the concentration of porphyrin in F8BT PNP- BSA capped Au NPs hybrid (Figure S24A) with a fixed concentration of F8BT PNPs (6.64 x 10-10 M) and Au NPs (120 nM). At the fixed concentration of PNPs and Au NPs, a systematic enhancement in emission of HPTP (inset of figure S24A) with increasing concentration of HPTP confirms that the energy transfer is faster than electron transfer from HPTP to Au NPs. In addition, the concentration of Au NPs is also varied in F8BT PNPs-BSA complex (Figure S24B) with a fixed concentration of F8BT PNPs (6.64 x 10-10 M) and HPTP (1.22 µM). In this case, very little change in emission of F8BT (at 538 nm) (inset of figure S24B) is observed with increasing the concentration of Au NPs, indicating the negligible energy/electron transfer from F8BT PNP to Au NPs. Contrary, PL quenching of HPTP (at 655 nm) increases with increasing the concentration of Au NPs. Therefore, the electron transfer from HPTP to Au NPs occurs with the increasing concentration of Au NPs. Thus, 80 % PL quenching of F8BT PNP in triad is the major contribution of the energy transfer from F8BT to HPTP and the energy/electron transfer from F8BT to Au NPs is the minor contribution of the quenching. Thus, the energy transfer occurs from F8BT to HPTP in 130 fs and followed by electron transfer from HPTP to Au NPs in 70 ps (Figure 3F) in the designed triad which is resembled with the natural photosynthesis.

Figure 3. (A) TEM image of triad, (B) Emission spectres of F8BT- HPTP (1.22 µM) loaded BSA complex (a) and triad (b) for the excitation of F8BT (λex = 340 nm), (C) Fluorescence decay profiles of HPTP (λem = 655 nm) in F8BT- HPTP (1.22 µM) loaded BSA complex (a) and triad (b) the excitation of F8BT (λex = 340 nm), (D) Transient absorption spectra of the triad (λex 340 nm) and (E) normalized bleach recovery kinetics at 420 nm of (a) F8BT- HPTP 15 ACS Paragon Plus Environment


loaded BSA complex and (b) triad, (F) Schematic representation of the energy transfer followed by electron transfer in triad for the excitation of donor, F8BT.

CONCLUSIONS In conclusion, we are trying to mimic two most essential consecutive processes of natural photosynthesis within an animal protein, BSA. In the designed artificial light harvesting system with simple electrostatic attachment of positively surface charged F8BT PNP and HPTP loaded BSA capped Au NPs, the absorbed light energy by the F8BT PNP is efficiently transferred towards HPTP loaded BSA protein, and further initiates the electron transfer from HPTP towards Au NPs inside the same BSA protein scaffold. The efficient energy transfer is described by the high antenna effect value of 79. Both the ultrafast fluorescence upconversion and transient absorption spectroscopic studies reveal the very fast (130 fs) energy transfer and followed by the electron transfer (70 ps) process. Therefore, this work may pave us to open a new avenue in future for designing highly efficient aqueous solution based artificial light harvesting system.

Supporting Information 1

H NMR of HPTP, TEM image, absorption spectra, emission spectra, time resolved emission

decays of F8BT in THF and F8BT PNP dispersed in water, TEM image and absorption spectra of Au NP, absorption spectra and time resolved emission decays of HPTP in different environment, cyclic voltammogram of HPTP, relaxation pathway of HPTP, temporal profiles of fluorescence up-converted signal of HPTP loaded BSA and HPTP loaded BSA capped Au NPs, DLS distribution of F8BT PNP, BSA protein and F8BT-BSA complex, absorption spectra, emission spectra and TCSPC of and F8BT-HPTP loaded BSA complex, transient absorption spectra and bleach recovery kinetics of F8BT PNP, absorption spectra, emission spectra, temporal profiles of fluorescence up-converted signal, transient absorption spectra and bleach recovery kinetics of F8BT PNP in triad, cyclic voltammogram of F8BT PNP, band position of F8BT PNP, tables for TCSPC, fluorescence upconversion decay and bleach recovery kinetics, emission spectra of triad with the variation of HPTP and Au NPs. This materials is available free of charge via the internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS "DST-TRC" is gratefully acknowledged for financial support. BJ thanks CSIR for awarding fellowship. Dr. Sandip K. Nayak is acknowledged for providing of 5-(4-hydroxyphenyl)10,15,20-(triphenyl)porphyrin. Prof. S. Goswami and Mr. Rajib Pramanick are acknowledged for their generous help in the CV study.


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Ultrafast Energy Transfer Followed by Electron Transfer in a Polymeric

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