Selective dispersion of nanotubes in polyfluorene solutions studied by molecular dynamics E. J. F. de Carvalho and M. C. dos Santos J. Gao and M. A. Loi Departamento de Física dos Materiais e Mecânica Zernike Institute for Advanced Materials Instituto de Física, Universidade de São Paulo University of Groningen 05508-090 São Paulo, Brazil 9747AG Groningen, The Netherlands

Abstract Results It was recently demonstrated that conjugated a) a) 1.25 from the polyfluorene family are able to 1 PFO - SWNTs 1.2 wrap selectively in nanotubes having high chiral PFO 1.15 0.8 angles, especially when toluene is used as 1.1 kcal/mol) 4 . In the present work we report on the 1.05 0.6 photophysical properties of SWNT suspensions 1 ΔE(10 in toluene solutions of poly[9,9-dioctylfluor- (a.u.) PL 0.4 0.95

enyl-2,7-diyl](PFO). 0.9 7.5 8 8.5 9 9.5 10 10.5 0.2 Diameter (Å) We studied these systems by parametrizing a b) 1.25 1.2 force field in order to obtain an ensemble of PFO 0 400 450 500 550 600 conformations in the excited state and 1.15 Wavelength (nm) 1.1 calculating the average using kcal/mol) 1.05 a semiempirical quantum chemistry approach. b) 4 1 PFO - SWNTs 1

With these results it is possible to infer the ΔE(10 PFO 0.95 (experimentally inaccessible) chain 0.9 0.8 18 20 22 24 26 28 30 conformation from experimentally obtained Chiral angle (degrees)

spectra. 0.6

PL (a.u.) PL Fig. 4: ΔE = E(300 K) – E (0) of the PFO-wrapped We observe that the chain conformation plays an 0.4 SWNTs for chains aligned to the tube axis (black) and important role in nanotube solubilization rolling up as helices (red) as a function of a) diameter selectivity. 0.2 and b) chiral angle. (7,5) NT - PFO

Helix 0 1 Fig. 5. 400 450 500 550 600 Aligned Wavelength (nm) PFO-Toluene Simulated 0.8 c) 0.6 spectra of PFO Method hybrids with (7,5) • Classical Molecular Dynamics in the NVT 0.4 and (8-5) SWNTs. ensemble, T = 300 K, package Cerius2. 0.2 Normalized intensity (arb. units)

• CVFF950 force field modified to reproduce the 0 400 450 500 550 600 excited state features of PFO geometry. Wavelength (nm) (8,6) NT - PFO

• Simulated fluorescence spectra in the visible Helix 1 Aligned region obtained by the quantum chemical PFO-Toluene ZINDO-CI method as implemented in the 0.8

Gaussian03 package, using a sampling of 0.6 structures obtained from snapshots of the Time (ps) dynamics trajectories. 0.4 Fig. 2. a) PL spectra of the PFO solution and of 0.2

the PFO-CoMoCAT-grown SWNT hybrids (from J. Normalized intensity (arb. units) 0 Gao and M. A. Loi, Eur. Phys. J. B 75, 121 (2010)). 400 450 500 550 600 b) PL spectra of the PFO solution and of the Wavelength (nm) PFO-HiPCO-grown SWNT hybrids. c) PL decay of PFO and PFO-CoMoCAt-grown SWNT hybrids at 450nm. Conclusion

• PFO wrapped around the SWNTs shows better resolved PL spectrum than that of the polymer A B C D E F φ λmax GS 1.397 1.467 1.396 1.394 1.408 1.484 37.9 334.6 solution. ES 1.400 1.434 1.408 1.362 1.440 1.423 9.8 382.7 • Photoluminescence decay of the PFO wrapped CVFF 1.441 1.384 1.424 1.395 1.440 1.422 30.0 391.6 around the SWNTs shows a monoexponential FF 1.428 1.433 1.414 1.397 1.440 1.413 32.1 378.1 behavior with a lifetime τ = 268 ps, that is more than 100 ps shorter than that of the polymer Fig. 1. Ground (GS) and excited (ES) state solution (τ = 380 ps). geometrical data for bifluorene as obtained • MD results are consistent with helical wrapping from B3LYP and CIS calculations, of the polymer and consequently a more rigid respectively. FF data are for the optimized polymer structure. molecule in the modified CVFF force field. • More stable hybrids are formed at certain The labels A to F refer to bond lengths (Å) chiralities and certain diameters – (7,5) and (8,6) and φ to the torsion angle (degrees), zero are the best in the tested systems. being the planar anti structure shown above. Fig. 3. Structures of the MD simulation, showing • Simulated fluorescence spectra show better Also indicated is the wavelength of the lowest carbon nantube (gray balls), PFO (blue, yellow resolved vibronic peaks in helical polymer dipole allowed excitation energy, λ (nm), max and red structures) and toluene layer (grey rods). conformations. obtained via ZINDO/S-CI method.

Acknowledgements: Reference: J. Gao and M. A. Loi acknowledge Technologiestichting STW and NanoSci-ERA (a consortium of national J. Gao, M. A. Loi, E. J. F. de Carvalho, M. C. dos Santos funding organizations within European Research Area) for the funding of the project Nano-Hybrids for Selective wrapping and supramolecular structure of Photonic Devices (NaPhoD). E. J. F. de Carvalho is a CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) fellow polyfluorene - carbon nanotube hybrids and a visiting PhD student under CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) ACS Nano, 5, 3993-3999 (2011) fellowship. M. C. dos Santos acknowledges FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) for financial support.