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  • Supporting information for:

    Molecular Design of a Room-Temperature

    Maser

    Stuart Bogatko,,,, Peter D. Haynes,, Juna Sathian, Jessica Wade,, Ji-Seon

    Kim,, Ke-Jie Tan, Jonathan Breeze, Enrico Salvadori,,# Andrew

    Horseld,,, and Mark Oxborrow

    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 addressedDepartment of Materials, Imperial College London, Exhibition Road, London SW7 2AZ, UKLondon Centre for Nanotechnology, Department of Materials, Imperial College London, Exhibition Road,

    London SW7 2AZ, UKThomas Young Centre, Imperial College London, Exhibition Road, London SW7 2AZ, UKDepartment of Physics, Imperial College London, Exhibition Road, London SW7 2AZ, UKCentre for Plastic Electronics, Imperial College London, Exhibition Road, London SW7 2AZ, UKLondon 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) andphenazine 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 andrelative zero-eld populations derived from simulation of the TR-EPR spectrarecorded 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 relativesimulations (red lines) of powder samples of phenazine in biphenyl and pentaceneand 6,13-diazapentacene in p-terphenyl recorded at 9GHz at room temperature.Phenazine was excited at 355 nm, the pentacene and 6,13-diazapentacene wereexcited 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 0.12Pentacene 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 samplesof phenazine in biphenyl and pentacene and 6,13-diazapentacene in p-terphenyl recordedat 9GHz at room temperature. Phenazine was excited at 355 nm, the pentacene and6,13-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 w