Metal-Catalyzed Chemical Reaction of. Single Molecules Directly Probed by. Vibrational Spectroscopy

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1 Supporting Information to: Metal-Catalyzed Chemical Reaction of Single Molecules Directly Probed by Vibrational Spectroscopy Han-Kyu Choi, Won-Hwa Park, Chan Gyu Park, Hyun-Hang Shin, Kang Sup Lee and Zee Hwan Kim* Department of Chemistry, Seoul National University, Seoul , Korea A. Raman scattering enhancement factors for AgNP--AuTF junctions Table-S1. Comparison of theoretical and experimental enhancement factors a Physical dimensions of EM-hotspot b Model EF A (nm 2 ) N EF max EF avg Experimental EF( NO ) (a) For details of calculation, see Park et al 1. (b) The theoretically estimated values. The A is the EM-enhanced area on Au surfaces that experiences F NO larger than 0.31F NO, max, where F NO, max is the maximum field enhancement. The N is the number of s within A. The Raman enhancement factor (EF) of the AgNP--AuTF junction is estimated by comparing the NO peak intensities of the normal Raman scattering spectra (I Raman ) of the 10 mm ethanolic solution of placed on a Au thin film, and the corresponding SERS signals (I SERS ) from a AgNP--AuTF S1

2 junction, obtained under the same experimental conditions. The diffraction limited illumination/detection (confocal) volume of the solution-phase sample, V f, is calculated to be nm 3 (V f = (depth of focus) (focus area) = ( 1.4n / NA 2 ) 0.4 / 2 NA 2,where n,, NA are the refractive index of the ethanolic solution, laser wavelength, and the numerical aperture of the objective lens, respectively). The 3-dimensional finite-difference time-domain (FDTD) simulation (see Figure 1c in main text) reveals that the enhanced local field spans an area, A, of 12 nm 2. The surface density of the adsorbed molecules on the gold surface and the number density of the molecules in solution are s = 2.65 molecules / nm 2, and v = 5.28 molecules / nm 3, respectively. The experimental Raman enhancement factor (EF) of the junction is obtained by comparing the normal Raman signal from a single molecule with the SERS signal from a single molecule sandwiched between the nanoparticle and the surface, using the relationship, EF I /( A ) / I /( V ) SERS s Raman v f. We obtain the EF = for the NO peak of. For the model enhancement factor, we have separately calculated the in-coupling of the excitation light with the plasmon mode at the excitation frequency ( 0 ), E loc ( 0 )/E 0 2, and the out-coupling of the near-field Stokes-shifted Raman field radiation at NO with the far-field radiation, E loc ( NO )/E 0 2, and multiply the two to obtain the position-dependent Raman enhancement factor of NO (), using the FDTD method (FDTD Solutions, Lumerical Solutions, Inc): F NO (x, y) = E loc ( 0, x, y)/e 0 2 E loc ( NO, x, y)/e 0 2, (1) where the x and y are the Cartesian coordinate of a molecule on AuTF surface with respect to the junction center (0,0). The enhancement factor is position-dependent, and the enhancement factor listed in Table-S1 shows both the F NO, max, the maximum enhancement factor for a given hotspot (EF max ), and the enhancement factor averaged over the hotspot area (EF avg ). S2

3 B. Relative surface coverage of ABT,, and DMAB, and the reaction branching during the photoreaction To estimate the relative surface coverages of ABT,, and DMAB during the reaction, we analyze the relative SERS peak intensities at a = 1344 cm -1 (), b = 1437 cm -1 (DMAB), and c = 1571 cm -1 (, ABT, and DMAB). The intensities of a = 1344 cm -1 () and b = 1437 cm -1 (DMAB) arise from and DMAB, respectively. On the other hand, the peak at c = 1571 cm -1 come from, ABT, and DMAB. The three peak intensities (I a, I b, and I c ) are the sums of the surface coverages of the three species weighted by the Raman scattering cross sections of corresponding peaks, and such relations can be expressed as: I I I a b c 0, a, c 0 DMAB, b DMAB, c 0 0 ABT, c DMAB ABT (2), where X is the surface coverage of X =, DMAB and ABT, and X,y is the Raman scattering cross section (Supplementary Information-C) of peak y of X. Figure S1. (a) Time-resolved SERS spectra of AgNP--AuTF junctions (Figure 2b in main text), (b) the representative spectra (lower panel) sampled at t = 22 seconds (white line in (a)). (c) Surface coverages of, ABT, and DMAB estimated from the SERS spectra. S3

4 For the SERS spectrum at t = 22 second shown in Figure S1, the intensity ratio is measured to be I a : I b : I c = 1: 0.6: Inserting this ratio into equation (2) and solving the equation (2), we obtain the surface coverage ratio of: : DMAB : ABT = 1.0: 0.15: From the intensity trajectory of a = 1344 cm -1 (NO-stretching of ), we find that the surface coverage of has been decreased from,0 = 1.0 to = 0.5 during the t = 22 seconds of exposure. By combining the two results, we estimate = 0.5, DMAB = 0.08 and ABT = 0.39 at t = 22 second. The result indicates that the amount of ABTs produced ( ABT = 0.39) is comparable to the s depleted ( = ), while only a small amount of DMAB ( DMAB = 0.08) is created and destroyed during the exposure time. Similar result is also obtained from the hot trajectories. This proves that the indirect reaction path makes a minor (<10%) contribution, and most of the decays via the direct reaction path. S4

5 C. Peak assignment of and DMAB molecule Table-S2. Peak assignment for the Raman and SERS spectra for and DMAB a (in cm -1 ) Peak No. Normal Raman b SERS Peak assignment c AgNP// AgNP/DMAB/ DMAB DMAB AuTF AuTF CC+CC+NN CCCC CCCCCN CH CH CH CHCC CC+CH; (b 2 ) CH+CN+CC s (NO) CC+CH CC+CH CC+CN+CH CC CC CC (a) All of the Raman and SERS spectra are obtained with ex = nm. (b) The Raman spectra of microcrystalline (s) and DMAB(s) (c) Vibrational peak assignments and notations following refs. 2-5 stretch; in-plane bend; out-of-plane bend; wagging S5

6 D. Estimation of relative Raman cross sections of, DMAB, and ABT molecules Figure S2. Normal Raman spectra of microcrystalline DMAB (red), (black), and ABT (blue) obtained with ex = nm with the same light intensities. The relative Raman cross sections of DMAB,, and ABT are obtained from the normal Raman scattering spectra (shown in Figure S2) obtained from each samples in pure microcrystalline states. The and ABT samples are obtained from Sigma-Aldrich and the DMAB sample is obtained from Medigen Inc (Daejeon, Korea; see Supporting Information-I for analytical data). The Raman cross section of each species is obtained from the relation, I MW / d, where is Raman cross section, I is intensity of peak in normal Raman spectrum, MW is molecular weight, and d is the density (in gram/m 3 ). The resulting relative Raman scattering cross sections of major vibrational peaks of the three species are summarized in Table-S2. Table-S3: Relative Raman cross sections of DMAB, and ABT Peak position (cm -1 ) Relative intensity Densities (g/cm 3 ) a Relative cross section DMAB ABT (a) Sigma Aldrich online catalog; For the DMAB, the density of azobenzene is used. S6

7 E. Kinetic rate law analysis of photoreduction E.1. Setup of kinetics model Here we assume a simplified kinetics model in which a short-lived intermediate (I) is formed by the plasmon-assisted reaction, and the intermediate decays via the direct and indirect pathways: k ki I ABT (direct pathway) (3) k 2 I kdmab 2I DMAB 2ABT (indirect pathway) (4) The rate equations can be expressed as: k ; 2 I k kii k2ii ; 2 DMAB k2ii kdmab DMAB (5),where X s are the surface coverage of X on Au surface. As shown in Supporting Information-B, the direct channel is a dominant reaction pathway. Therefore, the equations (5) can be further simplified as: k ; I k ki I ; 2 DMAB k2ii kdmab DMAB (6) Integration of the equations (6) yields: (7) k t kdmabt 2 t e and ( k DMAB A e e ),where 1 2 k A k2i k DMAB 2k, and a full monolayer coverage of is assumed at t = 0. k I k More generalized form of (7) is: k t k 3 e and ( 2 t k t DMAB A e e ). (8) As shown in Figure S3a, a majority of and DMAB trajectories could be satisfactorily fitted with the equations (7) and (8), and the k 3 is found to be close to 2k (Figure S3b). S7

8 Figure S3. (a) The time-trace of NO (1347 cm -1, upper trajectories in each panels, ) and 3 (1445 cm -1, lower trajectories in each panels, DMAB) along with the fit to the equations (7). (, blue line; DMAB, red line). The data in upper and lower panels are a hot and mild trajectories with k = 0.02 s -1 and s -1, respectively. (b) Log-log plot of k versus k 3 (grey dot) extracted from various trajectories, showing the correlation k 3 = 2k (log 10 k 3 = log 10 k , black line). E.2. Junction-to-junction variation in rate constants Figure S4. Correlation among k 2, k, and I,0 for each junction (logarithmic scales). (a) k versus k 2. (b) The initial intensity of NO (I,0 ) versus k. Also shown as straight lines are the results of linear fits. We assume that that initial (un-normalized) intensity of NO, I, 0, i from a plasmonic junction, i, is approximately proportional to the 4 th power of local field (E loc ) averaged over the i th junction area, multiplied by laser power density, P. I 4 A E E P, (9), 0, i loc / 0 i,where A is a constant,... is the spatial averaging around junction i, and E i 0 is the incident laser field. If we assume that the -decay (with a rate constant of k ) and DMAB-decay (k 2 = k DMAB ) steps are 1- S8

9 photon processes (and possibly 1-electron transfer process as well), the k,i and k DMAB,i of junction-i are given by: 2 4 1/ 2 X,i k0,x Eloc k0,x Eloc / E0 P (10) i i k, where X = or DMAB and k 0,X is a constant. For a fixed P, junction-to-junction variation in k X,i and I 0,,i will correlate as: i 1/2, 0, i Eloc / 4 1/2 0 i k I E. (11) We have measured the I,0, k, and k 2 for 39 junctions showing continuous trajectory for DMAB (hot junctions) with a fixed laser power density, and examined the correlation among them (see Figure S4). The correlation yields k = ai,0 1/2 and k 2 = 0.26k, confirming the hypothesis that the k 2 and k 2 are linearly proportional to the local field intensities, E. loc Figure S5. The change of the average decay rate (k ) plotted as a function of,0. The error bars represent one standard deviation of junction-to-junction variation. S9

10 F. Correlations between the step-transition frequencies of DMAB and the decay rate of Figure S6. (a) Correlations of step-up frequency (f up ) and k. (b) Correlation of step-down frequency (f down ) and k. Grey circles correspond to the raw data points derived from 24 SERS trajectories, and the black circles correspond to the binned and averaged points. The red lines are the linear fits to the data (see Figure 5 in main text for more detail). S10

11 G. Influence of blocking and unblocking the laser beam during the photocatalytic reaction Figure S7. (a) The representative SERS spectra obtained from the initial-state (blue, blue triangle in b), just before (black, black triangle in b) and just after the blocking (red, red triangle in b) of the laser light. (b) time-resolved SERS spectra, showing the effect of blocking and unblocking the laser during the reaction. (c) The time-trace of representative ( NO ) and DMAB peaks (CC+CH) sampled from (b). S11

12 H. Cross correlation analysis of SERS trajectory Figure S8. A 2D covariance map of mild junction trajectory shown in Figure 6c, displaying two peaks at 1160 cm -1 and (marked with *) that positively correlates to X = 1364 cm -1 of the intermediate HABT. The spectrum shown in green is the X = 1364 cm -1 component of the 2D covariance matrix. To further identify the vibrational peaks of short-lived intermediate, HABT, mentioned in the main text, we evaluated the covariance matrix 6 of the time-resolved SERS spectra shown in Figure 6c. The covariance matrix element, ij, between i and j'th spectral components in the time-resolved spectrum S i (t) is defined as: ij S t S S t i i j j S (12), where S k (t) is the k'th spectral component in the SERS spectrum at time t, and... denotes the timeaveraging. In the covariance map shown in Figure S7, positive (red) and negative (blue) signs of ij indicates the in-phase (positive correlation) and out-of-phase (negative correlation) temporal change of i and j spectral components, respectively. Figure S7 shows strongly positive correlations (red) among the S12

13 peaks at 1314 cm -1, 1160 cm -1 and 1364 cm -1 (marked as *), which we assign as the vibrational peaks of the short-lived intermediate, HABT, mentioned in the main text. The green trace in Figure S7 shows the X = 1364 cm -1 component of the covariance matrix, clearly showing the peak positions of the three peaks of HABT. Furthermore, the three peaks of HABT also show negative correlation (blue) to the peaks of DMAB ( 1, 2, and 3 ), indicating that the HABT is in dynamic equilibrium with DMAB. S13

14 I. Density functional theory calculation of Raman spectra of possible reaction intermediates Figure S9. Theoretical Raman spectra of possible intermediates (see Figure 1d in main text for the notation) based on (d,p) basis set of Gaussian09 package. S14

15 J. Analytical data on the 4,4 -dimercaptoazobenzene (DMAB) Figure S10. Electrospray ionization mass spectrum (ESI-MS) (a) and 1 H-NMR spectrum (b) of synthesized DMAB. Figure S9 shows the analytical data of DMAB synthesized by Medigen Inc (Daejeon, Korea). The mass spectrum is obtained using QSTAR XL+1100 series mass spectrometer (Applied Biosystems, Inc & Dionex, co. Agilent Technologies, Inc). The peak at m/z=245.0 corresponds to [DMAB] -. The 1 H NMR spectrum (Inova 400NB, Varian. Inc ;400 MHz, CDCl 3, 298K, ppm) shows characteristic peaks at (d, J HH = 8.6 Hz, 4H); (d, J HH = 7.8 Hz, 4H); (s, 2H). S15

16 REFERENCE (1) Park, W.-H.; Kim, Z. H. Nano Lett. 2010, 10, (2) Armstrong, D. R.; Clarkson, J.; Smith, W. E. J. Phys. Chem. 1995, 99, (3) Biswas, N.; Umapathy, S. J. Phys. Chem. A 1997, 101, (4) Han, S. W. Bull. Kor. Chem. Soc. 2005, 26, 463. (5) Han, S. W.; Lee, I.; Kim, K. ETRI Journal 2004, 26, 38. (6) Noda, I.; Ozaki, Y. In Two-Dimensional Correlation Spectroscopy Applications in Vibrational and Optical Spectroscopy; John Wiley & Sons, Ltd: New York, S16