Low Magnetic Field Induced Surface Enhanced Transient Spin

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C: Energy Conversion and Storage; Energy and Charge Transport

Low Magnetic Field Induced Surface Enhanced Transient Spin-Trajectory Modulation of A Prototype Anticancer Drug Sanguinarine on Single Domain Superparamagnetic Nanosurface Sudeshna Das Chakraborty, Abhishek Sau, Arnab Maity, Uttam Pal, Chaitrali Sengupta, Maireyee Bhattacharya, Gourab Bhattacharjee, Sanjib Banik, Biswarup Satpati, Samita Basu, and Dulal Senapati J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06129 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018

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The Journal of Physical Chemistry

Low Magnetic Field Induced Surface Enhanced Transient

Spin-Trajectory

Modulation

of

A

Prototype Anticancer Drug Sanguinarine on Single Domain Superparamagnetic Nanosurface Sudeshna Das Chakraborty,†# Abhishek Sau,†# Arnab Maity,† Uttam Pal,† Chaitrali Sengupta,† Maireyee Bhattacharya,† Gourab Bhattacharjee,‡ Sanjib Banik,⊥ Biswarup Satpati,‡ Samita Basu,†* Dulal Senapati†* †

Chemical Sciences Division, Saha Institute of Nuclear Physics, HBNI, 1/AF, Bidhannagar,

Kolkata-700064, India ‡

Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, HBNI, 1/AF,

Bidhannagar, Kolkata-700064, India ⊥Experimental

Condensed Matter Physics, Saha Institute of Nuclear Physics, HBNI, 1/AF,

Bidhannagar, Kolkata-700064, India

ACS Paragon Plus Environment

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ABSTRACT: The role of single-domain ferrimagnetic nanostructure and the associated surface for many fold magnetic-field-induced surface enhanced transient spin-trajectory modulation is a new venture in molecular spin dynamics. Though the inter conversion between spin isomers in the excited state is often forbidden by spin selection rule, hopping between two spin trajectory surface is quite probable due to their high magnetic-field induced spin-interconversion. This spin-interconversion-induced conical intersection is mainly governed by the spin-orbit (L-S) coupling. The extent of spin-interconversion which controls their spin degeneracy or relative population beyond molecular field can also be efficiently regulated by inducing a large magnetic moment in a nearfield configuration. In the present set of experiments a large magnetic field antenna has been launched by external low magnetic-field-induced single-domain magnetic moment alignment of individual constituent atoms arranged inside ferrimagnetic nanostructure which generates a superparamagnetic nano-architecture to push for the programmable transient spin-trajectory modulation for surface adsorbed sanguinarine (Sgr), a known small molecule anticancer drug, through enhanced population of spin- modulated component. Along with spininterconversion, the distribution and the stabilization of radical formation in different solvent environments also play the pivotal role to control the overall transient spin trajectory population.


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1. INTRODUCTION Populating a molecule in excited states beyond its natural lifetime by spin-trajectory modulation can surprise by initiating different unusual optical, spectroscopic, photophysical and photochemical pathways1-12, providing new insight on molecular resource for renewable energy4 13-14

, offering flexible magnetokinetics (through radical pair mechanism)

8,15

, commencing new

dynamic trajectories,16,17 crucially expanding the field of organic spintronics13-14 and achieving abnormal optical transition.18-21 Most of the known molecular systems remain in the singlet

ground state whereas the photo-chemically energized excited state could either be singlet

; = 1, 2, 3, . or triplet ; = 1, 2, 3, . . There are reports on the formation of

different excited spin states (singlet and triplet) when one electron from the singlet ground state (S0) is being excited to a higher energy level (Sn or Tn) either by preserving (spin-conservation) 22-23

, reversal (spin-flipping) 24-25, rephasing (spin-flopping)26-27 or noncollinear spin arrangement

(spin-canting)26 of their original spin orientation.28-30 Since the spin reversal process is not an allowed transition governed by spin selection rule, opto-molecular excitations are singlet-singlet (S-S)31 type exclusively and passage to triplet excited states is primarily governed by excited state tunnelling or conical intersection which we generally define as intersystem crossing (ISC).32-33 In contrary, ferrimagnetic or ferromagnetic nanostructures and especially singledomain superparamagnetic nanosurfaces34-35 induce several different unusual spin dynamics of electrons in surface adsorbed organic molecules by applying a small external magnetic field.30 These spin dynamics include enhanced electronic and magnetic quantum efficiency,36-37 population inversion,38 introduction of trap-sites by balancing of injection and recombination of charge carriers,39 or even unusual photochemical reaction pathways40-41 to find real life applications in organic conductors and crucial biochemical reactions along with their potential


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functions as molecular ruler42 to estimate the extent of drug release for effective therapy. The spin of an electron can be represented by a vector precessing around the laboratory Z-axis with Larmour frequency, ω (Figure 1A).43 The spin vector is represented either by α or β for up (↑) and down (↓) spin respectively with respect to the reference axis. The orientations of the vectors remain opposite in singlet states (S0 and Sn are singly degenerate) with total spin angular momentum (S) as 0 and the corresponding asymmetric spin wave function can

Figure 1. (A) Direction of precession of an electron, the large arrow indicates the external magnetic field and the small arrow directs the spin angular momentum of the electron; (B) TEM images of synthesized FeNPs; (B1) HRTEM image of an individual FeNP; (B2) Acquired


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Selected Area Diffraction Pattern of the nanoparticles and the different crystal planes present in those nanoparticles; (C) EDX spectra of magnetic nanoparticles acquired in STEM mode from TEM. EDX spectra clearly show the elements present in the sample are Fe and O; (D) Electron energy loss spectra (EELS) of magnetic Nanoparticles. Whit-line ratio method proves the composition of the nanoparticle as Fe3O4. be represented as χ 1,2 = − ⁄√2 where 1 and 2 represent two electrons, one in HOMO and the other in LUMO. From S1 state we can have nonradiative Internal Conversion (IC) or radiative fluorescence which again brings the molecules to S0 state. On the other hand the non-radiative ISC which is effective through intramolecular and intermolecular spin-modulation to produce T1 state (triply degenerate) with total spin angular momentum (S) as 1 and the

corresponding three symmetric spin wave functions can be represented as: χ 1,2 = αα, ββ, and + ⁄√2. Therefore, the conversion of S1 to T1 is possible if spins are

flipped or rephased by xy and z component of L-S coupling respectively. The L-S coupling is responsible not only to change the precessional frequency but also their direction due to the generated torque (Γ)44-45. Precessional motion of the magnetization vector which controls both the precessional frequency as well as their direction, directly enhances the S1 to T1 spin trajectory population through spin rephasing or flipping mechanism46-47. In the present set of experiments a large magnetic field (in the order of Tesla (T)) antenna has been generated by low external magnetic-field (0.08T)-induced single-domain magnetic moment alignment of individual constituent atoms arranged in superparamagnetic nanosurface to enforce the programmable transient spin-trajectory modulation through S1→T1 conversion of surface adsorbed Sgr molecule. Sgr is a known small molecule, which is a naturally occurring alkaloid and has received tremendous importance in the pharmaceutical industry for the last several decades48-50.


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Along with its proven anti-inflammatory, antioxidant, antifungal and antimicrobial activities, it shows potential application as anticancer drug51-61. Sgr is a toxic quaternary ammonium salt and the presence of this quaternary ammonium group helps to bind with free acid group (-COO-) on the surface of spherical FeNPs capped by trisodium citrate (TSC). Molecular structure of Sgr shows the accessibility of quaternary ammonium group for binding with FeNP.

Molecular Structure: Sgr (13-Methyl-[1,3] benzodioxo [5,6c][1,3]dioxolo[4,5-i]phenanthridin-13-ium

The mechanism of single-domain (with surface area less than 10-12m2) magnetic moment aligned large-field superparamagnetic field-driven spin interconversion has been cross checked by using diamagnetic nanosurface in presence of low external magnetic field (0.08T). The generated torque as a result of L-S coupling tends to align the magnetic dipole with the external magnetic field. In the present set of experiment this low externally applied magnetic field (0.08T) could be scaled up to several order on the surface of a small (10nm) superparamagnetic Fe3O4 nanoparticle. In presence of this induced gigantic magnetic field the magnetic dipole of the surface adsorbed Sgr will be aligned perpendicular to the superparamagnetic moment to achieve the lowest potential energy compared to the random angle between Sgr and Fe3O4 nanoparticle.

For, ! = 90 , $%! = 1; &'()Γ = * + , = *,%! = *, or the torque Γ becomes maximun : ! = 90

where µ is the magnetic dipole moment of surface adsorbed Sgr and B = induced magnetic field on Fe3O4 nanoparticle.


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As a result of this induced gigantic magnetic field, L-S coupling for Sgr in perpendicular orientation with Fe3O4 nanoparticle is the most favourable orientation to assist the projected spin interconversion. Along with this high-field-induced intramolecular spin-interconversion, there is another mechanism prevails in presence of low magnetic field even of the order 0.01 Tesla and that is known as radical pair mechanism. Perhaps this could be the first experimental observation of the theory for the modulation of radical pair dynamics by magnetic nanoparticles originally developed by Cohen30, for the specific case of a superparamagnetic system62-64. There the molecule itself on photoexcitation can generate ‘radical pair with the surrounding molecules including solvent’ either by donating electron from its LUMO or accepting electron to its HOMO. Therefore, in water in presence of citrate ion H-abstraction may be facilitated, which will produce intermolecular geminate radical pair (Sgr-H)● ●(-H_solvent). If the radical pair is formed involving excited singlet Sgr, then the orientation of two spins of the radical electrons will be of singlet type that is antiparallel. On the other hand if the radical pair is formed through triplet Sgr, then the orientation of the spins of two electrons will be of triplet type. In radical pairs two molecules are loosely bound by intervening or surrounding solvent molecules and undergo diffusion within the solvent cage. During diffusion either they can recombine (more with singlet pair) or can form free radicals escaping from solvent cage (more with triplet pair since there two free electrons are with similar spin orientation). However they can also undergo singlet-triplet interconversion (S ↔ T) at an interradical distance where exchange interaction between the two free electrons becomes negligible through hyperfine interaction present in the system. The S ↔ T phenomenon can be suppressed by application of an external magnetic field of the order of hyperfine interaction, generated through nuclear spin-electron spin coupling (I.S., of the order of 0.01 – 0.02 Tesla, low field effect) making T± energetically non-degenerate with


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respect to S and T0 through Zeeman splitting. Therefore if initially singlet radical pair is formed, in presence of magnetic field the singlet yield will increase, hence the recombination of the geminate radical pair will increase. On the other hand if initially triplet radical pair is formed, in presence of magnetic field the triplet yield will increase that is free radical formation will increase.

2. EXPERIMENTAL SECTION 2.1.

Reagents. Iron (III) chloride (FeCl3, anhydrous, power, ≥99.9% trace metal

basis), Iron (II) chloride (FeCl2, anhydrous, beads, -10 mesh, 99.9% trace metal basis), Ammonium hydroxide solution (NH4OH, ACS reagent, 28-30% NH3 basis), Sanguinarine chloride hydrate (C20H14ClNO4, ≥98%, HPLC grade) and Trisodium citrate tribasic dihydrate (HOC(COONa)(CH2COONa)2.2H2O, ACS reagent, ≥99.0%) were purchased from SigmaAldrich, India and used for synthesis without further purification. 2.2.

Synthetic protocol. The magnetic nanoparticle (MNP) was synthesized by a

chemical co-precipitation method by reducing Fe2+ and Fe3+ ions (1:2 molar ratio) in presence of NH4OH following a reported protocol65 of E. Cheraghipour et al. with minor modification. Briefly in a three necked flask, 50 mL water solution of 0.10mol/L Fe2+ and 0.20mol/L Fe3+ mixture were taken and the temperature of the flask was slowly raised to 80°C in refluxing condition under Argon (Ar) atmosphere. The solution was vigorously stirred at 1200rpm speed. Once the temperature was fixed at 800C, 10mL of NH4OH (25 wt %) was added instantaneously to reduce the whole reaction mixture to form bare FeNPs. Addition of NH4OH increases the pH to 10-11. The surface of the bare MNPs was stabilized with citrate ion by incubating the FeNPs with 0.5g/mL tri-sodium citrate and stirring the mixture for 60min at 90°C. The black precipitate


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was obtained by cooling the reaction mixture to room temperature. At last the suspension was washed several times with deionized water to remove excess citrate ion. TEM and XRD were done to characterize the as-synthesized nano particles. 2.3.

Morphological and compositional characterization by TEM coupled with

EDX. Transmission electron microscopy (TEM) measurements were performed by using a FEI, Tecnai G2F30, S-Twin microscope operating at 300kV. Compositional analysis was performed by energy dispersive X-ray spectroscopy (EDS, EDAX Inc.) attached to Tecnai F30. We used simple but modified techniques for close-to-clean monolayer sample preparation. For the TEM measurements we used a 300-mesh carbon-coated copper grid and followed drop casting as well as a previously described dip-and-dry technique66 to make the sample where the TEM grid was immersed in the concentrated nanomaterial solution by using a tweezer. Hydrophobic carbon coating allows building up a monolayer sample stuck to copper mesh dried on a soft tissue paper. After complete drying, the grid was used for TEM measurements. Details about the bright field TEM imaging of our sample has been shown in Figure 1B. The average size of the particles is 10nm in diameter. In Figure 1B1 we have shown the HRTEM image of the particles, which clearly shows the crystalline nature of the magnetic nanoparticles. We have calculated the plane spacing in different direction from the HRTEM image and have shown theme in Figure 1B1. In Figure 1B2, we have shown the selected area diffraction pattern of the nanoparticles and have shown the different planes present in those nanoparticles. In Figure 1C, we have shown the EDX spectra acquired in STEM mode from TEM. From the spectra, we can clearly observe the elements presents in the sample and they are Fe and O. To find the chemical composition of the material, we have acquired the electron energy loss spectra of our sample (Figure 1D). The composition of the material can be found from the whit-line ratio method. The maxima of L3 and


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L2 peaks for Fe are located at 709eV and 722eV respectively. Using white-line ratio method we have calculated the area under the L3 and L2 peak of 2eV window around the maxima, and the measured ratio is 4.29 which proves the composition of the nanoparticle as Fe3O4.67 2.4.

Magnetization Measurements. The magnetic measurements were carried out

using SQUID-VSM (Quantum Design) having magnetic field range 0-7 Tesla and temperature range 2K- 380K. 2.5.

Ultraviolet−Visible (UV−Vis) Absorption Spectroscopy. The UV−Vis

absorption spectra of Sgr, FeNPs and their composites were recorded on a JASCO V-650 spectrophotometer by using a quartz cuvette of 1.0 cm optical path length. 2.6.

Fluorescence Spectroscopy. Fluorescence spectra were recorded in a

FluoroMax-3 (HORIBA, Jobin Yvon) fluorimeter equipped with a xenon lamp with a quartz cuvette of 1.0 cm optical path. Excitation wavelength was selected at 470nm with emission at 586nm where both the slit widths were fixed at 5nm. 2.7.

Time-Correlated Single-Photon Counting (TCSPC). Fluorescence lifetimes

were measured by using a time-correlated single-photon counting (TCSPC) fluorescence spectrometer.68 In this setup, samples were excited with a 470nm (IBH Nanoleds) pulsed picosecond diode laser (~75ps pulse width) with a repetition rate of ~1 MHz.


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Figure 2. (A) Normalized magnetization (M/MS) as a function of H/T at 200K and 300K; (B) Magnetization versus applied magnetic field dependence recorded at 300K. The red line shows Langevin function fit to the M(H) data; and (C1 & C2) XRD pattern of FeNP and FeNP-Sgr complex respectively. Appropriate band pass filters are used to block the excitation light during the fluorescence signal collection. The fluorescence signal collected at 900 angle to the excitation beam maintaining magic angle polarization (54.70) with a band pass of 4nm. Fluorescence signal is dispersed through a monochromator (IBH, model MCG-910 IB) and detected by using a cooled micro-channel plate photomultiplier (Hamamatsu, 5000-U-09). The full width at half maximum


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(FWHM) of the instrument response function (IRF) is 270ps and the resolution is 28ps/channel. The global lifetime analysis software, IBH DAS6, is used for decay processing where the deconvolution technique is used to calculate the decay of the fluorophore by using the measured decay data and the instrument response function. The excellence of the fitted data was judged from the obtained χ; and weighted residuals where all the data were analyzed, fitted and plotted by using “Origin-8.0” software.

2.8.

Femtosecond

(fs)

fluorescence

up-conversion

measurements.

The

fluorescence lifetime of the transient species was measured by a femtosecond fluorescence upconversion setup (FOG-100, CDP Corp.)69. Using the second harmonic of a mode-locked Tisapphire laser (Mai Tai, Spectra Physics), pumped by a 5W Millennia (Spectra Physics), the sample was excited at 400nm with full excitation slit width. We used a nonlinear crystal (1mm BBO, θ = 25°, ϕ = 90°) to generate the second harmonic. The fluorescence emission from the sample was obtained under the magic angle configuration and was up-converted in another nonlinear crystal (0.5mm BBO, θ = 38°, ϕ = 90°) using the fundamental beam as a gate pulse. The up-converted light is dispersed through a monochromator and detected by photon counting electronics. Using a Gaussian shape for the instrument response function having a FWHM of

∼206 fs (obtained through water Raman scattering) and a commercial software (IGOR Pro Wave Metrics, USA), the femtosecond fluorescence decays were deconvoluted.

2.9.

Flash Photolysis. The transient absorption spectra were measured using a nanosecond

laser flash photolysis setup (Applied Photophysics) having an Nd:YAG laser (Lab Series, Model Lab150, Spectra Physics)70. The samples were excited by a 355nm laser light with ~8ns full width at half-maximum. Transients were monitored by a pulsed xenon lamp (150W). The output


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of the photomultiplier (R928) was fed into an Agilent Infiniium oscilloscope (DSO8064A, 600 MHz, 4 Gs/s). Using the IYONIX software, the data were transferred to the computer. The software “Origin-8.0” was used for curve fitting using the B-Spline option. The sample was purged by pure argon gas for 10 min before each experiment. Magnetic field-induced flash photolysis experiments were performed by using an external magnetic field of strength 0.08T.

2.10.

Isothermal Magnetization and the corresponding Langevin function fit. In order to

study the behavior of the synthesized Fe3O4 nanoparticles, isothermal magnetization measurements have been performed at 200K and 300K as shown in Figure 2A. Both the MH curves show negligible hysteresis (coercivity-free or remanence-free magnetic hysteresis)71 with huge saturation magnetization of 50.6emu/g and 47.3emu/g at 200K and 300K respectively. It is well known that if the size of ferrimagnetic particle reduces to the nanometer scale then they may behave like individual ferrimagnetic entity i.e. like superparamagnetic materials.72 Therefore to verify the nature of the presently prepared 10nm Fe3O4 nanoparticles, normalized magnetization (M/Msat) has been plotted with H/T and has been presented in Figure 2A. It is clear from the figure that both the curves collapsed on to a single curve which is the signature of the superparamagnetic nature of the particles. The super paramagnetic nature of the sample can also be described using the Langevin theory of paramagnetism where the magnetization can be expressed as: MH, T = M@ AcothBµC HK E TF −

KET Gµ HH C

where, µp is the magnetic moment of nanoparticle, T is the temperature and H denotes the applied magnetic field. Figure 2B shows that the measured M(H) data at 300K is


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successfully fitted with Langevin’s theory of paramagnetism for the concerned Fe3O4 nanosystem which further explains the superparamagnetic nature of the nanoparticles.

2.11.

Structural Characterization by XRD Technique. The powder x-ray diffraction

(XRD) study of FeNP and FeNP-Sgr composite have been performed at room temperature (300K) by using a RIGAKU-TTRAX-III diffractometer with Copper K-α radiation of wavelength λ=1.5406Å in the 2ϴ range of 10°–100° with step size 0.0100 as discussed before.73 Obtained XRD patterns for both the FeNP and FeNP-Sgr complex are shown in Figure 2C1 and 2C2 respectively. Obtained XRD patterns match well with Fe3O4. Moreover, unchanged XRD pattern confirms that the complexation of FeNP with Sgr does not change the redox state of the superparamagnetic nanoparticles.

3. RESULTS AND DISCUSSION 3.1. Ground state complex formation between Sgr and superparamagnetic FeNPs and its stability to govern the transient spin-trajectory followed by photo-excitation. Information about the tendency to form ground state complex between superparamagnetic nanosurface (Iron nanoparticles; FeNP) and the fluorophore (Sgr) is important to know their relative population in the excited state. To verify the formation of any ground state complex between Sgr and FeNPs we have carried out their differential absorption spectroscopy (Figure 3A & 3B), i.e., same volume of FeNPs solution in CH3CN or H2O was added in sample (Sgr) solution as well as in reference (CH3CN or H2O) cells, which helps us to eliminate the individual contribution of FeNPs (Figure 3C) in the overall absorbance. Surprisingly on gradual addition of FeNPs in Sgr solution an isosbestic point appears at 343.5nm in acetonitrile and 267nm in water, which signifies the equilibrium between Sgr and Sgr-FeNP complex. However, the appearance of


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isosbestic points at higher wavelength implies that the ground state complex is much more stable in CH3CN solvent compared to H2O. The water molecules stabilize the individual Sgr or FeNPs much more than their complex (Sgr-FeNP) which is also evident from their time-resolved fluorescence studies (Figure 4A1 & 4B1) described later in the subsequent sections.

Figure 3. (A and B) Differential absorption spectra of Sgr in presence of superparamagnetic FeNPs in Acn (CH3CN) and Water (H2O) respectively (insets indicate their respective isosbestic points); (C) UV-Vis absorption spectra of the synthesized FeNPs. 3.2. Influence of nanosurface, surface charge, donor-acceptor proximity and hydration dynamics for transient spin-trajectory modulation. Following the excitation of the ground state complex to regulate the spin-trajectory modulated excited state we have measured their triplet-triplet absorption spectra along with their extent of fluorescence quenching separately in CH3CN, H2O and ethanol (C2H5OH). It is clearly observable from Figure 4A2 & 4A3 that both the triplet (T1) yield of Sgr as a whole and the fluorescence intensity decrease with increasing concentration of FeNPs (5-75nM) in CH3CN medium. Decrement of fluorescence intensity directly measures the drop of excited singlet species (S1) population as a result of the addition of FeNPs. The corresponding Stern-Volmer plot, i.e. the plot of relative Sgr fluorescence intensity vs. FeNPs concentration (Figure 4A4) which acts as a quencher, is almost a linear fit in the lower concentration range of the quencher with which we have performed our time-resolved studies.


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Table 1. (A and B) Details about the lifetime components of Sgr as a result of adsorption on nanosurface in CH3CN and H2O respectively. (A)

Sample in Acn

τ (ns)

χ2

65µM Sgr

4.99

1.09

Sgr +5nM FeNP

4.97

1.01

Sgr +10nM FeNP

4.93

1.00

Sgr +15nM FeNP

4.91

1.01

(B) Sample in Water

τ1(ns) τ2(ns)

B1

B2

χ2

Average Lifetime (ns) 〈J〉

65µM Sgr

-

2.36

-

-

1.01

2.36

Sgr +5nM FeNP

0.08

2.33

1.50

98.50

1.00

2.33

Sgr +10nM FeNP

0.05

2.34

4.33

95.67

1.04

2.34

Sgr +15nM FeNP

0.04

2.34

5.77

94.23

1.04

2.34

The quenching due to the formation of a ground state complex which also obtained from absorption spectra of Sgr in presence of FeNPs (Figure 3A & B) is reconfirmed from their corresponding lifetime measurements. Measured lifetime remains unchanged with variable concentration of FeNPs in acetonitrile (Figure 4A1 & Table 1A). Therefore both the excited singlet and corresponding triplet yields of Sgr are reduced in presence of FeNP in acetonitrile solution due to ground state complex formation.


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Figure 4. (A1 and B1) Change of fluorescence decay profile of surface adsorbed Sgr with increasing concentration of FeNP in Acn and Water respectively; (A2 and B2) FeNP concentration dependent T-T absorption spectra of surface adsorbed Sgr in CH3CN and H2O respectively; (A3 and B3) Trend and extent of fluorescence quenching from S1 state of Sgr in presence of variable concentration of FeNPs in Acn and water respectively; (A4 and B4) SternVolmer plot of Sgr-FeNP complex as a function of concentration of FeNPs for CH3CN and H2O respectively; (C) Linear and nonlinear Stern-Volmer plot of Sgr-FeNP complex as a function of low concentration FeNPs in CH3CN and H2O respectively.


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It is observable from Figure 4B2 and 4B3 that both the triplet (T1) yield of Sgr as a whole and the excited singlet (S1) (measured by fluorescence quenching) population in water decrease with increasing concentration of FeNPs as it was observed in CH3CN medium too. If we compare Figure 4A2 and 4B2, it is clear from earlier reports42 that the excited Sgr bears a signature of H abstraction which leads to the generation of H-abstracted Sgr, (Sgr-H)•, by developing two significant hump near 400nm (395 and 420nm) in the transient T-T absorption spectra in aqueous medium. Since the significant absorption strength near 400nm appears in absence of FeNPs, we can consider that the excited Sgr abstracts one H• from the water matrix to generate intermolecular geminate radical pair (Sgr-H)••OH and the extent of this radical pair reduces with increasing concentration of FeNPs due to the intramolecular (Sgr-FeNP-NaOOCCyt) hydrogen transfer between citrate ion and geminate radical pair [(Sgr-H)••(OOC-Cyt)] on same FeNP-surface. Though we cannot disregard the possibility of the formation of geminate radical pair (Sgr-H)••(N=C=CH2) in CH3CN as evident from the very weak humps near 400nm in the transient T-T absorption spectra, the formed ion pair will recombine very fast due to the lack of solvent stabilization. Moreover, the extent of H abstraction is less in case of CH3CN due to the lesser stability of the solvent separated radical pair in absence of hydrogen bonding (also supported by MD simulations; Figure S1, Video S1). Lack of geminate radical pair (SgrH)••(N=C=CH2) stability coupled with unchanged intramolecular (Sgr-FeNP-COONa-Cyt) hydrogen transfer reduces the 400nm humps in a much faster rate as clearly observable from Figure 4A2 and Figure 5 respectively that supports our mechanistic explanation. Moreover 2-

times more average fluorescence life time, 〈J〉, of Sgr in CH3CN compared to H2O (Table 1A

and 1B) indicates more stable nature of excited singlet state of Sgr in an aprotic solvent. The non-linear Stern-Volmer plot and biexponential decay profiles of fluorescence lifetime (Figure


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4B4 and 4B1) in water which generate two lifetime values, τ1 and τ2, also support H-abstraction followed by geminate recombination along with static fluorescence quenching of Sgr in presence of lower concentration FeNPs (Table 1B) with which we have performed our time-resolved studies. The τ2 component with longer lifetime as well as higher percentage contribution does not change substantially with the addition of FeNPs, therefore it bears the signature of static quenching. On the other hand the τ1 component in the order of several picoseconds with very low percentage contribution probably depicts the dynamic quenching due to the fluctuation between Sgr and the geminate recombined form, Sgr-(H••OOC-)Cyt, in S1 states and the contribution of geminate radical pair, (Sgr-H)••OH, in T1 state is negligible due to its expected longer lifetime. The relative contribution of dynamic part compared to the static part of total fluorescence quenching increases with the increased concentration of FeNPs (as quencher) as clearly observable from Figure 4B4 which indirectly indicates the enhanced rate of conversion from geminate radical pair, (Sgr-H)••OH, in the T1 state to geminate recombined state, Sgr(H••OOC-)Cyt, in S1 state. Again, due to favourable transition between S1 and S0 in presence of increased amount of FeNPs, we observed a gradual reduction of the faster component (τ1) of the excited state life time. Kinetics of geminate radical recombination (S1) is not only much faster than the corresponding internal conversion (IC)-based relaxation74, τ1 depends heavily on the close proximity between donor [D: (Sgr-H)•] and acceptor [A: •(OOC-Cyt)]. As the amount of FeNPs increases, proximity between D and A increases to increase the extent of geminate recombined S1 state (increased from 1.5% to 5.8%) to relax in a faster rate (allowed S-S transition) which is in agreement75 with the obtained fluorescence lifetime data (reduction in lifetime) in Table 1B. To verify this phenomenon, the experiments were repeated using femtosecond fluorescence up conversion technique. The fluorescence decay profiles obtained for


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Sgr in absence and in presence of variable concentration of FeNP in CH3CN and H2O are depicted in Figure 5A1 and 5B1 respectively. The corresponding lifetime components in CH3CN and H2O are listed in Table 2A1 and B1 respectively.

Figure 5. (A1 & B1) Fluorescence lifetime decay profile of Sgr in absence and in presence of increasing concentration of FeNPs in Acn and Water medium respectively; (A2 & A3) biexponential decay profile of the excited triplet state of Sgr-FeNPs complex in absence and in presence of external magnetic field in CH3CN and (B2 & B3) in H2O medium. It is clear from the Table 2A1 & B1 that the lifetime of Sgr in the femtosecond LM and

picosecond L; time regime drastically decreases on addition of FeNPs in H2O medium. On the other hand when the experiments were done in CH3CN medium those components are barely affected by FeNPs. Therefore it may be inferred that the intervening water molecules by faster hydration dynamics76 stabilize the individual geminate recombined S1 state by intramolecular


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hydrogen transfer between citrate ion and geminate radical pair [(Sgr-H)••(OOC-Cyt)] on same FeNPs-surface. Figure 5A2-A3 and Figure 5B2-B3 show the decay profiles of Sgr in presence of FeNPs in absence and presence of magnetic field at 360nm and 410nm in CH3CN and H2O respectively. Similarly, Table 2A2-A3 and Table 2B2-B3 show the triplet lifetime of Sgr in presence of FeNPs in absence and presence of magnetic field at 360nm and 410nm in CH3CN and H2O respectively. In acetonitrile there are two components of lifetime at 5.59µs and 0.059µs and 0.053µs and 1.56µs at 360nm and 410nm respectively in absence of magnetic field. In presence of magnetic field the overall increase in lifetime time is observed for all the components. The increase in overall lifetime may be due to the L-S coupling, which induces S↔T conversion. Although it is very fast (ps), however the ultimate effect is the increase in lifetime of triplet species in presence of magnetic field as triplet (with microsecond lifetime) is energetically much more stable than the corresponding excited singlet state (with ns lifetime). On the other hand in case of water one of the components of lifetime at 410nm decreases in presence of magnetic field which is due to the decrease in formation of free radicals as discussed previously following radical pair mechanism. However the lifetime of the species at 360nm which is the signature of triplet Sgr remains almost unchanged in this low concentration of FeNPs, where radical pair mechanism predominates over L-S coupling. The magnetic field effect on geminate radical pair is the interplay among diffusion dynamics (that makes exchange interaction negligible), spin dynamics and geminate recombination (for singlet radical pair) or free ion formation (for triplet radical pair) as discussed in detail in the next section of discussion. Although this is a nanosecond phenomenon, however homogeneous recombination of radicals occurs in the microsecond time scale, which is being observed in our laser flash photolysis experiments.


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Table 2. (A and B) details about the lifetime components of Sgr as a result of adsorption on nanosurface in Acn and Water respectively.

τ

65µM Sgr in Acn

65µM Sgr in Acn+ 5nM FeNP

65µM Sgr in Acn+ 15nM FeNP

τ1(fs)

202.82

217

232

τ2 (ps)

1.13

1.01

1.17

(B1) τ

65µM Sgr in Water

65µM Sgr in Water+ 5nM FeNP

65µM Sgr in Water+ 15nM FeNP

τ1(fs)

886.43

490.48

178

τ2 (ps)

31.223

3.201

1.34

(A1)

(A2) Acn

Decay@360nm without MF Decay@360nm with MF

(B2) Water


τ1 (µs)

τ2 (µs)

5.59

0.059

8.42

0.36

τ1 (µs)

τ2 (µs)

1.96 2.04

(A3) Acn


(B3) Water


τ1 (µs)

τ2 (µs)

1.56

0.053

2.76

0.07

τ1 (µs)

τ2 (µs)

9.05

0.059

6.72

6.72

Decay profile of the excited triplet state of Sgr-FeNPs complex in absence and in presence of external magnetic field in C2H5OH is shown in the ESI section. It is clear from the decay profile of Sgr-FeNP composite in C2H5OH (Figure S2) that there is an overall increment of lifetime both for 360nm as well as 410nm in presence of magnetic field. This implies that the triplet yield increases in presence of magnetic field as we observed both for CH3CN and H2O (Figure 6A & C). This directly proves that irrespective of solvent, induced magnetic field always favours the S↔T conversion.

3.3. Highly efficient transient spin-trajectory modulation by controlling the low external magnetic-field-induced single-domain atomically-aligned large-field superparamagnetic moment-driven spin interconversion. To reduce the internal energy,77 magnetic alignment of individual constituent atoms gets randomized in a single domain ferrimagnetic nanomaterial to


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behave like a paramagnetic particle rather than a ferrimagnetic particle where the resultant magnetic moment of individual domains are aligned in a particular direction. Due to this random orientation of magnetic moment of individual atoms in a boundary-less single-domain crystalline paramagnetic material, the resultant magnetic moments for both the paramagnetic and diamagnetic nanomaterial remain comparable in absence of magnetic field and the final trajectorial fate of the transient state mostly depends on surface charge of the matrix on which the molecule of interest is adsorbed or the polarity of the medium in which we are performing the photoexcitation. In contrary, low magnetic-field-induced single-domain magnetic alignment of individual constituent atoms arranged on superparamagnetic nanosurface makes them strikingly different from diamagnetic materials to create large-field magnetic moment on nanosurface which in turn results efficient intramolecular (through L-S coupling) spin-trajectory modulation. Gradual increment of triplet optical density (O.D) in presence of magnetic field (00.08T) as observed in Figure 6B1 leads us to think about the larger contribution of L.S coupling to efficiently transfer the excited state population from S1 to T1 in presence of induced high magnetic field. Efficient S→T trajectory modulation has further been proved by observing a gradual increment of Sgr triplet yield (Figure 6A1-6A3) in CH3CN, monitored using laser flash photolysis through triplet-triplet absorption, as we increase the concentration of FeNPs in presence of low magnetic field (0.08T).


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Figure 6: (A, C) T-T absorption spectra and the corresponding modulation of transient spin state of Sgr in presence of variable FeNPs concentration under the influence of constant low external magnetic field (0.08T) in Acn and Water respectively; (B1, B2) Triplet absorbance (O.D.) as a function of low external magnetic field (in Tesla) in Acn and experimentally observed induced magnetic moment as a function of external magnetic-field (in Tesla) in vacuum at 300K, respectively.

Similar sets of experiments have been carried out using a 25nm diameter TSCcapped gold nanoparticle which is a single crystal diamagnetic material that does not show considerable increment of triplet yield in presence of comparable concentration


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(15nM) of AuNPs under the influence of similar magnitude (0.08T) of low external magnetic field (Figure S3 in ESI Section). The similarity of surface adsorption property of Sgr on both AuNPs and FeNPs can be verified from their quenching characteristics by observing their fluorescence intensity. Both gold and iron nanoparticle show almost similar pattern of fluorescence quenching with comparable quenching constant as we increase the concentration on nanoparticles. This indirectly proves that both AuNP and FeNP show similar adsorption property towards Sgr molecule. Details of the control experiments have been included in the ESI section as Figure S4. To find out the effect of magnetic field on Sgr in absence of magnetic nanoparticle, we have carried out the T-T absorption spectra both in H2O and CH3CN media and depicted as Figure S5 and S6 in the ESI section which show no noticeable effect on Sgr triplet yield in absence of FeNPs under the influence of same external magnetic field. This indirectly proves the merit of our hypothetical large magnetic moment induced enhanced triplet-trajectory population. Experimentally we have observed that the presence of 0.08T external magnetic field can generate substantial enhancement of induced magnetization within the ferrimagnetic nanoparticles (Figure 6B2). Besides its large induced magnetic field, absence of domain boundary allows these single-domain ferrimagnetic nanoparticles to develop a strong internal magnetization from exchange coupling of electrons within the domain in presence of an external magnetic field and thus becomes superparamagnetic. Superparamagnetic substances do not retain any net magnetization once the external field has been removed or in other word these single-domain ferrimagnetic particles act as super paramagnetic particles without magnetic memory. So, these types of materials are useful for switching ON-OFF of magnetic field effect.


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In continuation with the previous section when the similar experiments are performed in water medium, obviously the above phenomenon will exist i.e. the S1 to T1 transition of Sgr is enhanced in presence of a large induced magnetic field of the order of Tesla on the superparamagnetic FeNP surface which will increase the T-T absorbance of Sgr and is clearly observable from Figure 6C1-6C3. Comparison of Figure 6A and 6C clearly indicates that in presence of lower concentration of FeNPs under the influence of low external magnetic field the relative triplet yield of Sgr in CH3CN medium is much larger than that of the aqueous medium. As explained in the previous section, the H-abstraction of Sgr from the matrix to generate a T-T absorption hump at 400nm is well supported by the transient absorption spectra of Sgr without and with FeNPs in H2O which are not identical with those obtained in CH3CN where Sgr undergoes H-abstraction from the medium (CH3CN) in a much more restricted way due to the lack of stability of the geminate radical pair in absence of hydrogen bonding. For further support in favor of hydrogen abstraction mechanism for the appearance of two significant humps near 400nm (395 and 420nm) in the T-T transient spectra can be apparent from their recorded time delayed transient spectra (Figure S7) as included in the ESI section. It is clear from Figure S7 that as we increase the delay time between pump and probe pulse between 0.5 to 4.9µs, the intensity of humps responsible for radical formation through H-abstraction also decays gradually. To prove the origin of two significant humps near 400nm indeed comes from effective H abstraction and not from the signal fluctuation, we have further recorded the T-T absorption spectra of Sgr in absence of magnetic field in H2O medium at higher resolution (5nm data point) and have been compared with low resolution (10nm data point) transient spectra and plotted in Figure S8. Both the decrease of hump intensity with the increment of delay time and persistence


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of humps near 400nm at high resolution transient absorption spectra directly proves that the observed significant humps near 400nm are the signature of transient species not originating from noise otherwise they might have shown a fluctuating nature instead of gradual reduction. The polarity and hydrogen bonding ability of the surrounding solvents play major role in controlling the ultimate products. With the future target to find out the role of excited state and its lifetime for photo induced therapeutic activity, we have performed these experiments in water as the solvent medium. Moreover to explore the role of solvent polarity and hydrogen bonding ability for effective spin-trajectory modulation we have compared the results with two solvents, one is polar aprotic, i.e. acetonitrile (CH3CN), whereas the other is polar protic solvent, i.e. water (H2O). Due to lack of stability of the FeNPs associated with Sgr, it is not possible to carry out the experiments with other non polar organic solvents. Moreover due to large difference in dielectric constant between H2O and CH3CN, they account the effect of polarity in all possible polar solvent and the discussion has been limited within these two solvents to keep it compact and coherent. To prove further and to remove the ambiguity related to solvent effect, we have done the similar experiments with another protic polar solvent, i.e. ethanol (C2H5OH), which has

dielectric constant close to acetonitrlile (NOPQ OR : 37.5, NOV PW XP : 24.5, NPV X : 80.0) but has hydrogen bonding ability similar to water. We have observed that the radical formation is negligible in acetonitrile (Figure 4A2), moderate in EtOH (Figure S9) and maximum with water

(Figure 4B2). Incorporation of T-T absorption spectra of surface adsorbed Sgr in C2H5OH in Figure S9 explains the role of radical formation. To explain the role of different solvents on radical formation we have done MD calculations, which show that water can form a solvent cage surrounding Sgr, which facilitates maximum hydrogen abstraction with the formation of stable free radicals. On the other hand in actonitrile, the force due to coulomb interaction is maximum


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which favours recombination of the geminate radicals. In ethanol although the van der Waals force is maximum, however lesser hydrogen bonding ability reduces the stability of the individual radicals as the solvent cage formed surrounding Sgr is not rigid like that formed by water molecules. Contribution of van der Waals and coulombic forces in the solvation of Sgr has been tried to be estimated by MD simulations. Figure S1A1 shows the van der Waals interaction energy of Sgr with the solvents over the simulation time in a statistical ensemble. Figure S1A2 shows the contribution of coulombic energy in the solvation of Sgr. The time average data of the van der Waals and coulombic forces are plotted in the Figure S1A3 and listed in the Table S1. The MD simulations could highlight these dominant non-electrostatic forces in Sgr solvation. The van der Waals interaction energy has been found to vary significantly with the solvent. Solvent cage formation has been witnessed in the trajectory video which can be seen in Video S1. Water molecules are found to form stable hydrogen bonded solvent cage surrounding Sgr. However, no significant caging is observed in case of ethanol. On the other hand, acetonitrile being aprotic, solvent cage formation is absent. Such solvent cage formed due to hydrogen bonding may further stabilize the aggregates of Sgr. Figure S1B shows the hydrogen bonding tendencies of Sgr with the solvents. It shows significant number of H bond formation with water. For most of the simulation time, Sgr remains hydrogen bonded with water. On the other hand ethanol shows only negligible amount of hydrogen bonding transiently. The experimental observation supports facile hydrogen abstraction by Sgr from the protic solvent like H2O (two significant humps near 400 nm (395 and 420 nm)) and moderate hydrogen abstraction from C2H5OH (humps with moderate intensity) while negligible abstraction from CH3CN (humps with diminished intensity).


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In presence of lower concentration of FeNPs in H2O medium under the influence of low external magnetic field (0.08T) it is observed that the absorbance around 400nm decreases, however there prevails the signature of overall increase in triplet yield throughout other wavelength regions as obtained in CH3CN medium. Since the stability of geminate radical (SgrH)••OH) recombined state (S1) is more stable in H2O, production of geminate radical pair (T1) in the triplet state and hence the T-T transient absorbance at 400nm will always be less compared to CH3CN as clearly observed from our experimental results (Figure 6A and 6C). Decay profile (for the excited radicals by exciting at 360 and 400nm, close to the radical extinction maximum) of the excited triplet state of Sgr-FeNP complex in Figure 5A2-A3 and 5B2-B3 shows dramatic reduction in relaxation (increment in triplet life time) in presence of external magnetic field in CH3CN. In other word, applied magnetic field is capable to change the trajectorial fate of solvent dynamics by selectively stabilizing either geminate radical pair or geminate recombinant. Compared to CH3CN, in presence of lower concentration of FeNPs, the decay profile in H2O medium shows no change in triplet life time upon the application of magnetic field due to the greater stability of the geminate recombined state in water which favours the magnetic field induced singlet-triplet interconversion (S↔T) following radical pair mechanism, which tells that if the spin correlation of the geminate radical pair is singlet then in presence of magnetic field the corresponding triplet yield, i.e. formation of free radical will decrease. However in presence of excess FeNPs, enhancement of triplet yield of Sgr through L-S coupling predominates over the decrease in free radical formation through radical pair mechanism. Therefore, an overall increase in triplet-triplet absorption of Sgr is observed in presence of FeNP as depicted in Figure 6C3. Therefore, high magnetic field induction within the single domain ferrimagnetic nanoparticle (or boundary less paramagnetic nanoparticle) which acts as a superparamagnetic nanosurface


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influences the switchable on-off spin rephasing of the surface adsorbed molecules through highly efficient spin-orbit (L-S) coupling to control their overall transient spin trajectory distribution with a measurable contribution from solvent polarity. This high throughput spin-trajectory modulation may find several extraordinary applications which include electron-hole recombination in organic conductors78-79, molecular state life time to control effective drug release efficiency, role of radical reaction and their separation and stability to control crucial biochemical reactions.

4. CONCLUSIONS Effective manipulation of transient spin trajectory of a surface adsorbed molecular (here Sgr) system has easily been achieved by utilizing the low external magnetic field (less than 0.1T) induced single-domain magnetic moment alignment of the constituent atoms on a ferrimagnetic single crystalline nanoparticle surface which acts like a magnetic memory less superparamagnetic particle for programmable and error free magnetic field ON-OFF switching. Obtained results show that the high-field-induced L-S coupling with a measurable contribution from solvent polarity controls the overall transient spin trajectory distribution. Relative spin trajectory distribution and the resultant modulation not only depend on the localized gigantic magnetic field strength but can also be fine-tuned by controlling the solvent polarity as well as the hydrogen bonding induced solvent stabilization. Obtained results provide one of the rarest experimental observations of transient spin trajectory modulation in differential magnetic field strength in a nearfield configuration.


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ACKNOWLEDGEMENTS

We would like to express our sincere gratitude to the BARD project (PIC No. 12R&D-SIN-5.04-0103), DAE, Govt. of India for their generous funding. Our sincere acknowledgement to Prof. Krishnakumar S. R. Menon, Mr. Sukanta Barman and Prof. Satyaban Bhunia from SP&MS Division, SINP for useful discussion related to magnetic field induction, Mr. Anish Karmahapatra from ECMP Division, SINP for XRD recording and Prof. Indranil Das and his group for helping us in XRD analysis.

ASSOCIATED CONTENT Control experiments to prove the negligible role of external magnetic field in absence of single-domain superparamagnetic nanoparticle (FeNP) both in H2O and CH3CN medium; the definitive role of solvent polarity and hydrogen bonding ability on transient spin-trajectory modulation; comparison of surface adsorption property of Sgr on AuNP and FeNP; T-T absorption spectra of Sgr (only) in presence and in absence of magnetic field; time-delayed transient absorption spectra of Sgr in H2O medium along with high resolution T-T absorption spectra to prove the formation of radical pairs during photoexcitation and MD simulation to understand the solvation dynamics & extent of radical formation in different solvents have been presented and elaborated in the Supporting Information section. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Samita Basu: [email protected]


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*Dulal Senapati: [email protected] Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. / #These authors contributed equally. Funding Sources We would like to express our sincere gratitude to the BARD project (PIC No. 12-R&DSIN-5.04-0103), DAE, Govt. of India for their generous funding. ACKNOWLEDGMENT Our sincere acknowledgement to Prof. Satyaban Bhunia and Prof. Krishnakumar S. R. Menon from SP&MS Division, SINP for useful discussion related to magnetic field induction, Mr. Anish Karmahapatra from ECMP Division, SINP for XRD recording and Prof. Indranil Das & his group for helping us in XRD analysis.. REFERENCES

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SYNOPSIS TOC: The role of single-domain ferromagnetic nanostructure (superparamagnetic behavior) and the associated surface for many fold magnetic-field-induced surface enhanced transient spin-trajectory modulation has been explored by adsorbing Sgr drug molecule on FeNP surface and hypothesized by conceptualizing high magnetic-field induced spin-interconversion.


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Low Magnetic Field Induced Surface Enhanced Transient Spin

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