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Molecular Design of a Room-Temperature Maser

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Molecular Design of a Room-Temperature Maser Supporting information for: Molecular Design of a Room-Temperature Maser Stuart Bogatko,∗,y,z,{ Peter D. Haynes,y,x Juna Sathian,y Jessica Wade,x,k Ji-Seon Kim,x,k Ke-Jie Tan,y Jonathan Breeze,y Enrico Salvadori,?,# Andrew Horseld,y,z,{ and Mark Oxborrowy Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ, UK, London Centre for Nanotechnology, Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ, UK, Thomas Young Centre, Imperial College London, Exhibition Road, London SW7 2AZ, UK, Department of Physics, Imperial College London, Exhibition Road, London SW7 2AZ, UK, Centre for Plastic Electronics, Imperial College London, Exhibition Road, London SW7 2AZ, UK, London Centre for Nanotechnology, University College London, 17-19 Gordon Street WC1H 0AH, London, UK, and School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road E1 4NS, London, UK E-mail: [email protected] S1 Experimental Sample preparation Crystals of pentacene and 6,13-diazapentacene in a p-terphenyl host lattice and phenazine in biphenyl were grown using an open system zone melting methodS1. The Pentacene and p-terphenyl were supplied by TCI Europe NV, Phenazine 98% (P13207-10G) and Biphenyl 99% (W312908-1KG) were obtained from Sigma Aldrich. The resulting concentrations of pentacene in p-terphenyl was 0.0045 mol/mol % pentacene. Concentrations of 6,13-diazapentacene and phenazine were not measured but can be constrained to less than 0.1 mol/mol % pen- tacene. UV/Vis Absorbance measurements were performed on samples of pentacene in p-terphenyl, 6,13-diazapentacene in p-terphenyl and phenazine in biphenyl (Figure 1, also appearing in Figure 3 of the manuscript). The absorbance measurements were performed using a Shimadzu UV-2550 spectrophotometer and an Agilent Cary 5000 UV-Vis-NIR Spectrophotometer which has an excellent photometric response in the 175-3300 nm range. The measurements were per- formed using the standard solid sample holder with a 1 mm aperture mask. The absorbance spectra of both pentacene and 6,13-diazapentacene solid samples were obtained using the Cary WinUV software of the spectrophotometer over the chosen wavelength 200-800 nm. Finally the concentration was calculated using Beer-Lambert law using the known value of ∗To whom correspondence should be addressed yDepartment of Materials, Imperial College London, Exhibition Road, London SW7 2AZ, UK zLondon Centre for Nanotechnology, Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ, UK {Thomas Young Centre, Imperial College London, Exhibition Road, London SW7 2AZ, UK xDepartment of Physics, Imperial College London, Exhibition Road, London SW7 2AZ, UK kCentre for Plastic Electronics, Imperial College London, Exhibition Road, London SW7 2AZ, UK ?London Centre for Nanotechnology, University College London, 17-19 Gordon Street WC1H 0AH, Lon- don, UK #School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road E1 4NS, London, UK S2 extinction coecient and the measured thickness of the sample. Figure S1: UV/Vis spectra of Pentacene and 6,13-diazapentacene in p-terphenyl (top) and phenazine in biphenyl (bottom). S3 From these spectra the peaks at 590 nm, 620 nm and 364 nm (corresponding to transition energies of 2.10 eV, 2.00 eV and 3.41 eV, respectively) are assigned to the S0 − S1 transition of pentacene, 6,13-diazapentacene and phenazine, respectively. Time-resolved EPR ZFS parameters (Table 1) of phenazine in biphenyl, pentacene and 6,13-diazapentacene (both in p-terphenyl) were obtained from simulation of X-band time resolved EPR spectra (Figure 2, also appearing in Figure 3 of the manuscript) of powder samples. The measurements were performed on a Bruker E580 pulsed EPR spectrometer equipped with a Bruker dielectric ring resonator (ER 4118X-MD5). The optical excitation was provided by a Surelite broadband OPO system (operating range 410 - 680 nm), pumped by a Surelite I-20 Q-switched Nd:YAG laser with 2nd and 3rd harmonic generators (20 Hz, pulse length: 5 ns). EPR spectra were simulated using the EasySpinS2 toolbox in MATLAB. Since the spin multiplicity n depends on S, the spin quantum number, (n = 2S + 1), the EPR spectrum of triplet states consists of two allowed EPR transitions for each molecular orientation with respect to the applied magnetic eld. In powder samples, as it is in the cases reported here, all orientations are present with the same probability and the resulting EPR spectrum is the sum of all contributions weighted over a sphere. Powder EPR spectra of organic triplet states are dominated by the ZFS interaction and electron spin polarization. For organic triplets, the ZFS interaction, which can be described with two independent parameters known as D and E, arises mostly from spinspin interaction between the mag- netic dipoles and, to a lesser extent, from spinorbit interaction. For powder samples, the magnitudes of D and E can be estimated directly from the experimental EPR spectrum by measuring the distances between the characteristic turning points, as indicated in Figure 2 (top panel). These correspond to the canonical orientations of the zero-eld splitting tensor with respect to the applied magnetic eld and are denoted as X, Y, and Z in Figure 2; where the and + indexes refer to the mS = 1 ! mS = 0 and mS = 0 ! mS = +1 transitions, S4 respectively. On the contrary, the sign of D and E cannot be readily determined from the EPR spectrum and therefore the modulus is often reported. The electron spin polarization results from the selective population of each triplet sublevel from the excited singlet state via the intersystem crossing (ISC) mechanism. Hence, the resulting sublevel populations dier considerably from those predicted by Boltzmann distribution. Particularly, popula- tion dierences, the physical quantity measured in a TREPR experiment, are larger than Boltzmann predictions and the corresponding EPR lines at early times after light excitation appear either in emission or in enhanced absorption. Table S1: Phenazine, pentacene and 6,13-diazapentacene ZFS parameters and relative zero-eld populations derived from simulation of the TR-EPR spectra recorded at 9GHz at room temperature. D and E were assumed to be both posi- tive resulting in the energy order Px > Py > Pz.TR-EPR (black lines) and relative simulations (red lines) of powder samples of phenazine in biphenyl and pentacene and 6,13-diazapentacene in p-terphenyl recorded at 9GHz at room temperature. Phenazine was excited at 355 nm, the pentacene and 6,13-diazapentacene were excited at 590 and 532 nm respectively. A = enhanced absorption, E = emission. D (MHz) E (MHz) Px Py Pz Phenazine 2190 ± 10 326 ± 5 0.73 0.15 0.12 Pentacene 1400 ± 10 50 ± 5 0.76 0.16 0.08 6,13-diazapentacene 1370 ± 10 85 ± 5 0.60 0.21 0.19 S5 Figure S2: TR-EPR (black lines) and relative simulations (red lines) of powder samples of phenazine in biphenyl and pentacene and 6,13-diazapentacene in p-terphenyl recorded at 9GHz at room temperature. Phenazine was excited at 355 nm, the pentacene and 6,13-diazapentacene were excited at 590 and 532 nm respectively. A = enhanced absorption, E = emission. S6 Level of theory In order to determine the optimal basis set, combining best accuracy with low computational expense, an initial study was performed using available experimental geometries for benzene, naphthalene, anthracene and pentaceneS3,S4. For the larger polyacenes, anthracene and pentacene, the cc-pvdz, cc-pvtz and cc-pvqz basis sets were used. The smaller sizes of naphthalene and benzene permitted the inclusion of the cc-pv5z and, in the case of benzene, the cc-pv6z basis set. TDDFT calculations were performed on these geometries in vacuum. The low lying singlet and triplet excited state energies (in eV) are plotted as a function of basis set size in Figure 3 and Figure 4. The singlet states presented (top row) are the lowest lying singlet states (Sn) with non-zero oscillator strength. Physically, this corresponds to the lowest excitation which may be stimulated by light absorption. The 2nd, 3rd and 4th rows correspond to the 1st, 2nd and 3rd lowest triplet excited states. Solid lines indicate TDDFT calculation results using the singlet ground state electronic conguration as a reference while dashed lines indicate a triplet ground state electron conguration (performed for naphthalene and pentacene triplet states only). Note that the T1 excitation energy is found by the dierence between the singlet and triplet ground state DFT energies. The excitation energies all appear to decrease with basis set size with the exception of the naphthalene T1 triplet state and pentacene T2 triplet state with the singlet ground state as reference. The T1 state of naphthalene and T2 state of pentacene show a slight increase in excitation energy with basis set size for the cc-pv5z and cc-pvqz basis sets, respectively. Performing a TDDFT calculation with a triplet ground state electronic conguration does decrease for increasing basis set size and suggests that explicit treatment of the triplet ground state electronic structure provides an important contribution that in turn aects the higher triplet excited states. As a result, we proceed below with two sets of TDDFT calculations wherein singlet and triplet excited states are computed relative to the singlet and triplet ground state, respectively. S7 benzene naphthalene Sn T 1 T 2 T 3 Figure S3: Low lying excited states,Sn, T1, T2, T3 of benzene and naphthalene. Note that th benzene T2 and T3 states are degenerate and instead the T4 state is plotted on the 4 row. Solid lines indicate TDDFT calculation results using the singlet ground state elec- tronic conguration as a reference while dashed lines indicate a triplet ground state electron conguration S8 anthracene pentacene Sn T 1 T 2 T 3 Figure S4: Low lying excited states,Sn, T1, T2, T3 of anthracene, pentacene.
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