Quantum Sized Gold Clusters as Efficient Two-Photon Absorbers. Supporting Information

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1 Quantum Sized Gold Clusters as Efficient Two-Photon Absorbers Guda Ramakrishna, Oleg Varnavski, Junhyung Kim, Dongil Lee and Theodore Goodson * Department of Chemistry, University of Michigan, Ann Arbor, MI, 4809 Department of Chemistry, Western Michigan University, Kalamazoo, MI tgoodson@umich.edu Supporting Information. Characterization of Gold clusters Investigated gold clusters with hexane thiolates as capping agent have been synthesized by following a modified Burst s synthesis published elsewhere,. Transmission electron microscope (TEM) images of the synthesized gold clusters are shown in Figure S provide the cluster diameters and the number of gold atoms comprising the nanoparticles is obtained from the analysis. It can be observed from the Figure S that all the clusters have pretty good mono-dispersity and the error bars in the sizes are pretty much small. The only difference from the previously published results, and present results is the re-assignment of cluster size of Au 38 (SR) 4 to Au 5 (SR) 8 in accordance with the recently published results of Murray and co-workers 3, Tsukuda and co-workers 4 and the theoretical efforts of Nobusada and co-workers 5. From the additional electron-spray ionization measurements, Murray and co-workers 3 have shown that the cluster that has been previously mentioned to be Au 38 (SR) 4 is actually Au 5 (SR) 6 and it was found to be extraordinarily stable which has been proposed and confirmed experimentally by Tsukuda and co-workers 4 and theoretically by Nobusada and coworkers 5. S

2 Figure S. TEM images and corresponding histograms of the core diameters of hexanethiolatecoated (a) Au 5, (b) Au 40, (c) Au 309, (d) Au 976, and (e) Au 406 MPCs. TEM images were obtained with JEOL JEM-30. Scale bar = 50 nm. S

3 . Optical absorption measurements Optical absorption measurements on the investigated samples have been carried out with Agilent (Model # 834) spectrophotometer. Shown in Figure SA and SB are the normalized optical absorption spectra and extinction coefficient spectra of different clusters investigated in the present study. It can be observed from Figure SA that only the Au 976 and Au 406 are showing some kind of surface plasmon absorption while it seems to be absent in the lower sized gold clusters. The results are in accordance with the previously reported literature, 4. Figure SB shows the extinction coefficient spectra of increasing size of gold clusters which tend to increase with increase in the size of gold clusters. Absorbance (arb.).0 Au 5 A Au 40 Au 309 Au 976 Au ext.coeff (0 6 cm - ) Au 5 Au 40 Au 309 Au 976 Au 406 B Figure S: (A) Normalized optical absorption spectra of the investigated gold clusters dissolved in hexane. (B) Extinction coefficient spectra for different gold clusters. 3. Two-photon fluorescence spectra on gold clusters and nanoparticles In order to measure the two-photon excited fluorescence spectra at 800 nm for different gold clusters, we have utilized the output of mode-locked Ti:Sapphire (MaiTai, HP from spectraphysics, 00 fs pulse width) laser as the excitation source. Fluorescence spectra has been observed for all the gold clusters with two-photon excitation at 800 nm at different pump powers as has been observed for Au 5 clusters and are shown in parts A, C, E and G of Figure S3. Corresponding pump-power dependence has been shown for all the clusters in parts B, D, F and H of Figure S3. All the clusters have indeed shown near slope dependence suggesting the emission is two-photon emission. It is interesting S3

4 to observe that the emission maxima from the clusters all fell near around 500 to 555 nm. But, it is not the same for all of them. TP Fluorescence (cps) 400 Au 40 A log (Fl.int) Slope =.98±0.04 B Au 40 TP Fluorescence Au 309 C log (Fl.int) log (power) Slope =.9±0. Au 309 D TP Fluorescence Au 3 nm 800 excitation E Log (Fl.Int.) Au 3 nm Slope =.98 ± 0.07 log (power) F Log (power) S4

5 TP Fluorescence Au 4 nm G log (Fl.Int.) Au 4 nm Slope =.94 ± 0.08 H log(power) Figure S3: Two-photon absorption spectra at different pump powers for Au 40 (.7 nm) (A), Au 309 (. nm) (C), Au 976 (3 nm) (E) and Au 406 (4 nm) (G) clusters after excitation at 800 nm. Power dependence of emission is also provided for all the clusters (B, D, F, H). 3. Absolute TPA cross-section estimation using Fluorescence Upconversion results Description of fluorescence upconversion used in the present investigation is provided elsewhere 6. Briefly, our upconversion system used frequency-doubled light from a mode-locked Ti-sapphire laser (800 nm) for one photon excitation. Fluorescence emitted from the sample was up-converted in a nonlinear crystal of β-barium borate using a pump beam at 800 nm, which first passed through a variable delay line. Instrument response function (IRF) was measured using Raman scattering from water. Spectral resolution was achieved by using a monochromator and photomultiplier tube. We have slightly modified the setup for case of two-photon excitation. For two-photon excitation, we detune the second harmonic crystal and use the fundamental of Ti:Sapphire for excitation and the fluorescence obtained is collected in the usual way. Details of twophoton excited upconversion are provided elsewhere 7. The equations for the absolute TPA cross-sections are obtained using the basic equations for one and two-photon excitations. For the case of two-photon excitation, I0( t) I( t) = ; I(t) photon flux after the sample, I 0 (t) - photon flux () + σ N li ( t) T 0 before the sample, N T - TPA absorber number density. We have calculated the number density of excited molecules with two-photon excitation using the gausian profile for pulse width and spatial profile and obtained the following relation for N * T : S5

6 N T π = W σ N lw 3/ 0TPE T 0TPE τ prwσ NTl = π rwτ p tpe w p tpe ( hω ) 4π 3/ r τ ( hω ) Similarly, for the case of one-photon excitation, we have obtained the number of excited molecules, N O 3/ OD πrw 0 OD π W0 OPE OD ( 0 ) πτ = I ( 0 ) τ r = ( ) 0 = I p p w 0 hω (which is expected because I = W 3 / 0 π r τ hω ) 0 w p OPE (), (3) here W 0OPE pulse energy (in J per pulse) used in OPE-configuration, ω OPE photon frequency in OPE-experiment (in our case ω ope =ω tpe ) The ratio of fluorescence signals (in photons) in one- and two- photon experiments k F : k F * 3 / OD F 4 ( ) ( 0 ) OPE N π rwτ O p hωtpe W0OPE = = = (4) * F N σ N lw ( hω ) TPE T T 0TPE ope Here, we have introduced the correction for incorrect waist matching, the collection efficiencies as 0.45 and b. Then, we obtain a relation for the TPA cross-section as, σ or 4π r τ ( hω ) W OD ( 0 ) 3/ w p tpe 0OPE = (5) 0.45bkF NTlW0TPEhωope OD 3/ OD ( 0 ) 4π rwτ p ( hω) W0OPE 0 4 = L( sec) σ = cm 0.45bkF NT 0.45blW0 phωope k F N T Where L is the laser or setup factor; it is the same for any substance. L 8.9π r τ ( hω ) W (cm 4 s) (6) 3/ w p tpe 0OPE = (7) blw0tpehωope Eqs (5), (6) are the main formulae for calculation of TPE cross-section. Using our laser parameters, we have obtained, L=9.8*0-8 (cm*s) if b= b and L have been calibrated using reference substance with known TPA cross-section. Reference substance (dye #6): To align the system, we have utilized a chromophore with large TPA crosssection and good fluorescence quantum yield that has been investigated in our group 8. S6

7 The investigated dye molecule is Trimer -6 (abbreviated as T-6) which got a crosssection of 037 GM at 800 nm as reported in our earlier paper 8 Shown in Figure S4 is the comparative one and two-photon excited fluorescence upconversion. 0 5 T-6 in THF Fl. Int (cps) = 46 Conc = 4 µm Figure S4: One and two-photon fluorescence upconversion kinetic traces of T-6 in THF with 400 and 800 nm respectively. With the optical density and the ratio of counts (k f = 46), we have obtained the crosssection as 6 GM at 800 nm. 6 σ = GM (8) b It can be compared with the accurate measurement made for this reference using steady state TPA followed by single photon counting: σ ref =037GM at 800nm 8 Comparing this with (7) we can conclude that factor b.4 What can be the origin of b?, while material factor 0 k F NT OD has been measured quite accurate (±5%) (several independent measurements have been averaged for estimation of k F ) the setup factor L contains some parameters which are not known with good precision such as TPA excitation beam spot size (intensity distribution) in the cell, exact pulse width. Also collection efficiency for the fluorescence excited in TPA vs S7

8 OPA modes can be different in UpC due to the beam waist matching change and the intensity distribution change for the gate pulse. As the setup factor is the same for all substances we can use T-6 to accurately calibrate this factor and then use it for other measurements. Taking into account b=.4 we can easily get the calibrated setup factor L r : L r = = or L r = 4.9*0-8 (cm*s).4 Using this approach we have estimated the TPA cross-section for Bu /OMe dissolved in toluene, a dye synthesized in Prof. Joseph Perry s group and compare the result with literature data 9 (JACS, 000,, 9500) 0 6 Fl. Int (cps) Bu OMein THF = 50 Conc = µm Figure S5: One and two-photon fluorescence upconversion kinetic traces of Bu OMe in toluene with 400 and 800 nm respectively. With the data obtained, we have measured the TPA cross-section to be around σ 63 GM; which is in reasonable agreement with the literature data (55 GM at 800 nm) 9. Estimation of the TPA cross-section for gold clusters Parts A to E of Figure S6 shows the comparative one and two-photon fluorescence upconversion traces for Au 5 to Au 406 clusters respectively. S8

9 Fl. Int (cps) 00 Au 5 A 0 = 9. Conc = 9.89µM (O.D) 400 = Fl. Int (cps) 0 Au 40 B = 4.6 Conc =.µm (O.D) 400 = Fl. Counts (cps) Au C = 9.5 Conc =.66 µm O.D = 0.35 Fl. Counts (cps) 0 0. Au = 6. Conc =.5 µm O.D = 0.34 D Au 406 E Fl. Counts (cps) 0 = 7.4 Conc = 0.55 µm (O.D) 400 = Figure S6: One and two-photon fluorescence upconversion kinetic traces of Au 5 (A), Au 40 (B), Au 309 (C), Au 976 (D) and Au 406 (E) with one and two-photon excitation at 400 nm (blue) and 800 nm (red) respectively. With the data obtained from the measurements, TPA cross-sections have been measured. For Au 5, the cross-section has been estimated to be around, 47,000 GM and for Au 40, it was around 87,000 GM. For Au 309, the determined cross-section is,476,000 GM, and for Au 976, it was around 905,00 GM while it was around 3,45,000 S9

10 GM for Au 406. The errors measured for the TPA cross-section are in the range of to 5%. However, the relative error bars between the clusters are less than 0% since the measurements are carried out under identical conditions. Absorption spectra of the gold clusters are taken for the gold clusters before and after the laser excitation with both one and two-photon and they looked similar. Thus, there is no significant photo-degradation observed for the investigated clusters and are quite photo stable. 4. References. (a) Kim, J.; Lee, D. J. Am. Chem. Soc. 006, 8, 458. (b) Kim, J.; Lee, D. J. Am. Chem. Soc. 007, 9, Donkers, R. L.; Lee, D.; Murray, R. W. Langmuir 004, 0, (a) Tracy, J. B.; Kalyuzhny, G.; Crowe, M. W.; Balasubramanian, R.; Choi, J-P.; Murray, R. W. J. Am. Chem. Soc. 007, 9, (b) Tracy, J. B.; Crowe, M.C.; Parker, J. F.; Hampe, O.; Fields-Zinna, C. A.; Dass, A.; Murray, R. W. J. Am. Chem. Soc. 007, 9, (a) Shichibu, Y.; Negishi, Y.; Tsukuda, T.; Teranishi, T. J. Am. Chem. Soc. 005, 7, (b) Ikeda, K.; Kobayashi, Y.; Negishi, Y.; Seto, M.; Iwasa, T.; Nobusada, K.; Tsukuda, T.; Kojima, N. J. Am. Chem. Soc. 007, 9, 730. (c) Negishi, Y.; Chaki, N. K.; Shichibu, Y.; Whetter, R. L.; Tsukuda, T. J. Am. Chem. Soc. 007, 9, (a) Iwasa, T.; Nobusada, K. J. Phys. Chem. C. 007,, 45. (b) Iwasa, T. Nobusada, K. Chem. Phys. Lett. 007, 44, 68. (c) Negishi, Y.; Tsunoyama, H.; Suzuki, M.; Kawamura, N.; Matsushita, M. M.; Maruyama, K.; Sugawara, T.; Yokoyama, T.; Tsukuda, T. J. Am. Chem. Soc. 006, 8, Varnavaski, O. P.; Ostrowski, J. C.; Sukhmolinova, L.; Tweig, R. J.; Bazan, G. C.; Goodson, T. III J. Am. Chem. Soc. 00, 4, Varnavski, O.; Bauerle, P.; Goodson, T. III Opt. Lett. 007, 3, Bhaskar, A.; Ramakrishna, G.; Lu, Z.; Twieg, R.; Hales, J. M.; Haga, D. J.; Van Stryland, E.; Goodson, T. III J. Am. Chem. Soc. 006, 8, Rumi, M.; Ehrlich, J. E.; Heikal, A. A.; Perry, J. W.; Barlow, S.; Hu, Z. Y.; McCord- Maughon, D.; Parker, T. C.; Rockel, H.; Thayumanavan, S.; Marder, S. R.; Beljonne, D.; Bredas, J. L. J. Am. Chem. Soc. 000,, S0