Design rules for high mobility xanthene-based hole transport materials† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c9sc01491h

A set of design principles for high mobility xanthene-based organic hole transport materials are elucidated by combining multiple scales of theoretical chemistry (from virtual screening to bulk simulation) with experimental synthesis and characterization.


Optical and Electrochemical Properties
Solution UV-visible absorption spectra were recorded using a Cary 5000 UV/vis spectrophotometer. Photoluminescence (PL) spectra were recorded with a Cary Eclipse. All samples were measured in a 1 cm cell at room temperature with DCM as solvent. Concentration of 2 × 10 −5 M and 1 × 10 −5 M were used for solution UV/visible and PL, respectively. Solution-phase electrochemical data was recorded with a CHI660D potentiostat using a platinum wire counter electrode, Ag/AgCl reference electrode, and a platinum working electrode. A 0.1 M n-NBu4PF6 electrolyte solution in DCM at ambient temperature was used for all HTMs. Ag/AgCl in saturated KCl was used as the reference electrode and was calibrated versus the normal hydrogen electrode (NHE) by addition of 0.235 mV. Cyclic voltammograms (CVs) were acquired for 0.5 mM solutions of each HTM at a scan rate of 50 mV s −1 .

Conductivity Measurements
Four parallel Au electrodes with a spacing of 0.75 mm and length of 23 mm were used to measure the film conductivities in the dark and under ambient conditions. A Keithley 2400 sourcemeter was used to force current through the outer electrodes while sensing the voltage across the inner two electrodes. A linear fit of each measurement was used to determine the resistance, R, and the sample geometry was then used to calculate the conductivity, σ, according to the equation: where l is the electrode length, d is the inter-electrode spacing and t is the film thickness. The film thickness was measured using a Bruker DektakXT profilometer. Figure S1: (a) UV-Visible absorption spectra and (b) emission spectra for spiro-R series recorded in DCM.

S2 Bulk Thermal Properties of Spiro-R Series
Thermal Properties DSC (Differential Scanning Calorimetry) curves of compounds were collected using a Netzsch DSC 214 Polyma instrument under a nitrogen protective atmosphere. For each measurement, two aluminum crucibles were used, one as empty reference and the other for sample measurement. A customized heating program was developed to include a 5 min hold at the initial temperature of 50 • C and four consecutive heating-cooling cycles (Tmin = 50 • C, Tmax = 300 • C, 10 K/min). Data from the last 3 heating segments were used to identify the T m (melting point) and extrapolate the T g (glass transition temperature) of the material.
a Determined by differential scanning calorimetry (DSC). Figure S3: 1 H NMR of spiro-p-,o-OMe in CD 2 Cl 2 at ambient temperature.

S3 NMR Characterization of Spiro-R Series
S7 Figure S4: 13 C NMR of spiro-p-,o-OMe in CD 2 Cl 2 at ambient temperature. Figure S5: 1 H NMR of spiro-Me in CD 2 Cl 2 at ambient temperature.
S8 Figure S6: 13 C NMR of spiro-Me in CD 2 Cl 2 at ambient temperature.
S9 Figure S7: 1 H NMR of spiro-SMe in CDCl 3 at ambient temperature. S10 Figure S8: 13 C NMR of spiro-SMe in CDCl 3 at ambient temperature. S11 Figure S9: 1 H NMR of spiro-FOMe in CDCl 3 at ambient temperature. S12 Figure S10: 13 C NMR of spiro-FOMe in CDCl 3 at ambient temperature. S13 Figure S11: 1 H NMR of spiro-F in CD 2 Cl 2 at ambient temperature. Figure S12: 13 C NMR of spiro-F in CD 2 Cl 2 at ambient temperature. S14

Based on Functional Groups
A table of all theoretical data is provided (screened molecules homo lumo energies.csv). The molecules are represented as SMILES strings in the column "smiles." These strings can be directly copy/pasted into chemical drawing programs, such as ChemDraw or MarvinSketch to visualize the molecules. Figure S19: Plot of LUMO and HOMO (B3LYP/def2-SV(P)) as a function of functional group presence. No TPA means that functional groups are attached directly to the fluorene, xanthene, or biothiophene core. Since the functional groups are mixed in the screening library, each point can have multiple other coincident points. Figure S20: Plots of LUMO and HOMO (B3LYP/def2-SV(P)) for molecules with a single type of functional group, with the experimentally selected molecules (some of which have mixed functional groups) superimposed.

S23
S8 Frontier Molecular Orbital Energies in Vacuum and in Implicit Solvent Figure S21: Plots of correlation between HOMO and LUMO (B3LYP/def2-SV(P)) for conformers of the spiro-R series and spiro-OMeTAD in vacuum and in a PCM implicit solvent (with DCM as the implicit solvent). Though there is a non-unity slope in this plot, for this subset of the data, there is a strong correlation between the vacuum and the PCM calculation when a linear model is applied. One could screen at the PCM level after conducting the vacuum calculation, though the computational cost is higher with molecules of this size.