Boosting Transport Distances for Molecular Excitons within Photo-excited Metal Organic Framework Films

Transcription

1 Supporting Information Boosting Transport Distances for Molecular Excitons within Photo-excited Metal Organic Framework Films Subhadip Goswami, a Michelle Chen, a Michael R. Wasielewski, a Omar K. Farha, a,b Joseph T. Hupp a,c * a Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, United States b Department of Chemistry, King Abdulaziz University, Jeddah 21589, Saudi Arabia c Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States *To whom correspondence should be addressed. j-hupp@u.northwestern.edu S-1

2 Section S1. Instrumentation 1. Profilometry Profilometry measurements on the thin films were measured on a Dektak 150 surface profiler with 3.2 mg stylus force and a scan duration of 20 s. 2. UV-Vis spectroscopy Ground state absorption spectroscopy of the MOF thin films for glass substrates was measured using a Cary 5000 spectrophotometer. All the spectra were corrected for baseline with blank glass substrates. 3. Emission spectroscopy Steady-state emission spectroscopy of the MOF samples were recorded using a Photon Technology International (PTI) spectrophotometer and the data was collected 45 relative to the excitation beam. 4. Atomic force microscopy (AFM) AFM measurements on the MOF containing Si substrates were measured on a Bruker Dimension FastScan atomic force microscope (Bruker Co.) under ambient conditions. 5. Time-resolved fluorescence (TRF) spectroscopy Time-resolved fluorescence data were collected using the Hamamatsu C4334 Streakscope streak camera system. The instrument response function (IRF) of the experiment is 1-2% of the window of the experiment; the measured IRFs of the 2 ns, 20 ns, and 50 ns windows are 27 ps, 270 ps, and 622 ps, respectively. Samples were excited with 420 nm laser pulses that were generated by a high repetition rate ultrafast laser system. The commercial direct-diode-pumped 100-kHz amplifier (Spirit, Spectra-Physics) produces a fundamental beam of 1040 nm (350 fs, 4.5 W), which is used to pump a noncollinear optical paramagnetic amplifier (Spirit-NOPA, S-2

3 Spectra-Physics) that delivers tunable high repetition rate pulses. The 420 nm laser pulses were generated by frequency doubling of 840 nm by using the built-in second harmonic generation module in Spirit-NOPA. The samples were excited with 420 nm, 1.0 nj pulses. Data were fit to a biexponential decay function: f(t) = emission intensity at time t = a exp(-t/τ a ) + b exp(-t/τ b ) where a designates the shorter-lived component. The ratio of a to b is about 99:1, with τ a = 70 ps and τ b = 9200 ps. Despite the coefficients, the majority of the emitted photons are associated with the longer-lived decay, reflecting the large ratio of τ b to τ a. 6. Ellipsometry Ellipsometry measurements of the MOF thin films were taken on J. A. Woollam M-2000 Spectroscopic Ellipsometer with the silicon wafer substrates. The obtained data was fitted to a B- Spline model to extract the refractive index data. For the 13 and 26 cycle thin films, the thicknesses were measured by ellipsometry. Profilometry was performed to measure the thickness for 39 and 52 cycle LbL films. Section S2. The calculation of number of layers travelled by the exciton (Shown for 52C- Zn-TCPP-DABCO) From thickness measurements 52C-Zn-TCPP-DABCO thin film is ~42 layers thick. Addition of 2 cycles of P2 results in quenching of 64% of the residual fluorescence. Making the simplifying assumption that all layers are subjected to the same intensity of light (i.e., neglecting inner-filter effects), and multiplying 42 by 0.64, we obtain an average molecular-exciton transport distance of 27 layers (requiring 26 (= 27-1) layer-layer spacings to be traversed). Correcting for inner- S-3

4 filter effects as outlined in endnote 42 in the main text, decreases the average transport distance to 22 layers. Figure S1. Polarization angle dependent absorption spectrum 39 cycles LbL film. For this measurement, the film was placed at an angle of 45 with respect to the incident light and the angle of the incident light was varied using the polarizer. Whereas no absorbance change is observed when the film was placed at 0 angle with respect to the incident light. The incident angle describes the angle between the electromagnetic field, which (for 0 ) is normal to the direction of light propagation, but in the same plane as the transition dipole moment for the chromophore. S-4

5 Figure S2. AFM height images for (a and b) 13 cycles and (c and d) 52 cycles Zn-TCPP- DABCO LbL films. Figure S3. Steady state emission spectroscopy for 5C-LBL and 8C-LBL films with quencher, without the presence of quencher and after SALE. S-5

6 Figure S4. TRF single-wavelength kinetic at 660 nm of a) 13C-Zn-TCPP-DABCO and b) 13C-Zn-TCPP-DABCO + 2C-Pd-TCPP using the 2 ns experimental window, c) both 13C-Zn- TCPP-DABCO, 13C-Zn-TCPP-DABCO + 2C-Pd-TCPP using the 20 ns experimental window. Figure S5. a) Plots of percent of emission quenched for (N-C +2C)-LbL MOF films as a function of the number of layers of emissive P1 present, with DABCO as pillaring or interdigitating ligands for three experiments. b) Data replotted to correct for residual emission from inefficient quenching of the short-lived sub-population of P1, presumably due to an S-6

7 inability to move the excitons far enough to encounter the quenching layers, i.e. the layers at the film terminus containing P2. Figure S6. Emission spectra based on time-resolved measurements. The upper spectrum corresponds to the longer-lived component, and the lower spectrum to the shorter-lived component. The line shapes, within the considerable noise, appear similar, if not identical. S-7