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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
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