Supporting Information: Vibrationally Assisted. Intersystem Crossing in Benchmark Thermally. Activated Delayed Fluorescence Molecules

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1 Supporting Information: Vibrationally Assisted Intersystem Crossing in Benchmark Thermally Activated Delayed Fluorescence Molecules Emrys W. Evans 1, Yoann Olivier 2, Yuttapoom Puttisong 1, William K. Myers 3, Timothy J. H. Hele 1, S. Matthew Menke 1, Tudor H. Thomas 1, Dan Credgington 1, David Beljonne 2, Richard H. Friend 1, Neil C. Greenham 1* 1 Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom. 2 Laboratory for Chemistry of Novel Materials, University of Mons, Place du Parc, 20, 7000 Mons, Belgium. 3 Centre for Advanced Electron Spin Resonance (CAESR), University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom. Denotes equal contribution. Corresponding Authors * ncg11@cam.ac.uk (NCG) Transient Electron Spin Resonance (TrESR) The TrESR experiments were conducted on a Bruker E680 spectrometer operating at X-band (~9.5 GHz) with a Bruker EN 4118X-MD4 resonator. For selective photoexcitation at 420 nm and 460 nm, a GW versascan-uld and uvscan OPO was employed (20 Hz rep. rate, 3 mj per pulse, 5 ns duration). The output of the OPO was fibre-coupled to the sample, leading to unpolarized light. For 355 nm excitation, the third harmonic of a Nd:YAG Continuum 1

2 Surelite laser (10 Hz rep. rate, 1 mj per pulse, 5 ns duration) was used and aligned for direct sample excitation. The 355 nm light excitation was depolarized by an achromatic depolarizer before reaching the sample. A DG645 Stanford delay generator (itself triggered by the OPO or Surelite laser) was used to trigger the spectrometer s SpecJet digitizer. The ESR spectra were obtained in direct detection mode using a transient recorder without a lock-in amplifier; the microwave power was 1 mw. After data acquisition, the 2D data (time, field) were baseline corrected in both dimensions. Samples were dissolved in toluene (ca. 0.5mg/mL), placed in 2.7mm I.D. quartz tubes and degassed by multiple freeze-pump-thaw cycles. The tubes were sealed under vacuum in order to achieve oxygen-free samples. For the TrESR measurements, the samples were flash-frozen and studied at 80 K (temperature maintained by Oxford Instruments nitrogen gas flow cryostat). In contrast to Ogiwara et al., our TrESR studies on 4CzIPN (10 µm conc. in toluene, 355 nm excitation) gave an EEEAAA timeslice at 0.4 µs (with 0.2 µs averaging window) see Fig. S1. 2

3 Figure S1. TrESR 0.4 µs timeslice for 4CzIPN (355 nm excitation). For 4CzIPN (460 nm excitation) and 2CzPN (420 nm excitation), TrESR density plots of the measured ESR signal with respect to magnetic field and time are shown in Fig. S2 left and right. Absorptive and emissive signals are indicated by orange and blue (see color scale). 4CzIPN (460 nm excitation) 2CzPN (420 nm excitation) Figure S2. TrESR contour plots for left) 4CzIPN (460 nm excitation) and right) 2CzPN (420 nm excitation). Experimental details are given in the text. 3

4 As is typical for polarized triplet ESR signals, the inner features (x-/y-canonical points from zero-field splitting, ZFS eigenframe) decay faster than the outer features (z-canonical points). This is due to the anisotropic nature of the ZFS interaction and the effect of its modulation in driving spin relaxation 1,2. For 4CzIPN, the inner signal lifetime ( mT averaged) is τ = ± 0.002μs 1 and outer signal lifetime ( mT averaged) is τ = ± 0.002μs 1. For 2CzPN, the inner signal lifetime ( mT averaged) is τ = ± μs 1 and outer signal lifetime ( mT averaged) is τ = ± μs 1. 4

5 E-Distribution: Molecular Dynamics Figure S3. Probability distributions for the zero-field splitting (ZFS) E parameter for a) 2CzPN and b) 4CzIPN, obtained from DFT calculations (UKS) using the PBE0 functional and the EPR-II basis set based on a set of 200 molecular geometries extracted from 300 K molecular dynamics simulations. Here, E is defined as E = 1 (D 2 x D y ). 5

6 ESR Simulation Details Spectra were normalized with respect to the peak maximum, and fitted by a homemade script based on MATLAB s fmincon nonlinear solver. The ESR spectra for fitting were simulated by the pepper function in the EasySpin toolbox 3,4. Fit parameters included: the g-factor; the ZFS parameters (D and E); the relative triplet sublevel populations in the ZFS eigenframe (P a, P b, P c ); and inhomogeneous broadening parameters which reflect structural variations and their impact on the ZFS Hamiltonian (Dstrain and Estrain for the FWHM Gaussian widths of D and E distributions). The intrinsic, homogeneous Lorentzian broadening parameter was fixed (lw = 0.5 MHz = mt for FWHM). Molecule g-factor D (mt) E (mt) P i=a P i=b P i=c Dstrain (mt) Estrain (mt) 2CzPN CzIPN Table S1. ESR fit parameters for 2CzPN and 4CzIPN. Evaluating Spin-Orbit Coupling by Symmetry and Group Theory Using textbook group theory considerations, the spin-orbit coupling (SOC) integral can be evaluated by inspection. The 2CzPN and 4CzIPN molecules have a C 2 -rotational axis of symmetry, and thus are ascribed to the C 2 point group. Within this group, the transition integral is evaluated by direct products: Γ SOC,i=a,b,c = Γ(T 1 ) Γ(H SOC,i=a,b,c ) Γ(S 1 ) where Γ is the irreducible representation (or irrep) for a given integral or element; a, b, c correspond to the symmetry axes assigned following C 2 convention (with c being the C 2 axis). The symmetry axes are differently related to the ZFS principal axes for prolate and oblate 6

7 distributions see assignment for 2CzPN and 4CzIPN in Fig.1, main text. The a,b,cdefinition is transferred from the triplet term in the integral to the SOC irrep in the direct product. Simply, the direct product for non-vanishing integrals must yield the totally symmetric irrep, A. Considering the direct product components in parts: 1) Γ(H SOC,i=a,b,c ) transforms as the irrep of rotations. i.e. Γ(R i=a,b,c ). 2) In order to evaluate Γ(T 1 ), since the electronic transition to T 1 is mainly dominated by a HOMO to LUMO transition, only the symmetry properties of the HOMO and LUMO irreps need to be considered (assuming the system is otherwise closed-shell in nature). It follows that the HOMO and LUMO are assigned to the b and a irreps, and overall Γ(T 1 ) = b a = B - see Fig. 1d,e, main text. Furthermore, the DFT triplet-derived HOMO and LUMO orbitals can be decomposed and derived with chemical intuition. The central benzene moiety in both 4CzIPN and 2CzPN acts as a bridging unit. To a good approximation, the overall HOMO constitutes a linear combination of HOMO orbitals associated with the benzene and carbazoles units. Similarly, the overall LUMO is well-described by a linear combination of the LUMO orbitals of central benzene and cyano-group components. Using these considerations, the MO diagrams for 2CzPN and 4CzIPN can be constructed as depicted in Fig. S4, with symmetry of the individual MOs assigned according to the C 2 point group. The HOMO ( LUMO ) is made up of the most anti-bonding (bonding) combination of the HOMOs (LUMOs). From inspection, it can be seen that the nature of the 2CzPN and 4CzIPN HOMO and LUMO orbitals pictured by chemical intuition (Fig. S4) reproduce the same characteristic features found by DFT calculation (Fig. 1d,e). 7

8 Figure S4. Molecular orbital diagrams for a) 2CzPN and b) 4CzIPN as defined for the C 2 point group. Coordinate designation follows a,b,c-symmetry axes and x,y,z-zfs axes. Irreps for the HOMO, LUMO, and the S 0, S 1 and T 1 states are indicated. As the singlet S 1 excited state is also expected to follow the same HOMO LUMO monoelectronic transition as the lowest-lying triplet state, it also has the same electronic symmetry, i.e. Γ(S 1 ) = B. Thus, from DFT (or even pen-and-paper) with group theory, it follows that non-zero SOC elements T 1,i=a,b,c H SOC S 1 are expected only for the ccomponent of the triplet sublevels: B A B = A. Thus, symmetry considerations provide the fundamental basis for the ISC bias to the c-sublevels observed in the polarized TrESR signals. As an aside, it is noteworthy that the derived MO diagrams can also be used to rationalize the additional absorption band at > 400 nm seen for 4CzIPN, and not 2CzPN (Fig. 1b). In 8

9 4CzIPN, the four carbazoles form three bonding and three anti-bonding combinations of hybrid HOMOs (Fig. S4b); i.e. one more anti-bonding combination (both in number and magnitude) than found in the two-carbazole 2CzPN system which comprises two bonding and two anti-bonding HOMOs (Fig. S4a). This analysis could be extended to all TADF molecules of the carbazoles-cyanobenzene class, thereby providing a more rigorous explanation for the so-called intravalence charge transfer states which appears to show favourable energy-matching between the carbazole triplet LE (T k > 1 ) and S 1 in vibronically assisted reverse ISC 5. (1) Fischer, P. H. H.; Denison, A. B. Anisotropic Saturation of the Electron Spin Resonance in the Photo-Excited Triplet State of Pyrene-D-10. Mol. Phys. 1969, 17 (3), (2) Wolfe, J. P. Direct Measurement of Spin-Lattice Relaxation Rates for the Localized Triplet State in Molecular Crystals; Evidence for One- and Two-Phonon Processes. Chem. Phys. Lett. 1971, 10 (2), (3) Stoll, S.; Schweiger, A. An Adaptive Method for Computing Resonance Fields for Continuous-Wave EPR Spectra. Chem. Phys. Lett. 2003, 380 (3 4), (4) Stoll, S.; Schweiger, A. EasySpin, a Comprehensive Software Package for Spectral Simulation and Analysis in EPR. J. Magn. Reson. 2006, 178 (1), (5) Hosokai, T.; Matsuzaki, H.; Nakanotani, H.; Tokumaru, K.; Tsutsui, T.; Furube, A.; Nasu, K.; Nomura, H.; Yahiro, M.; Adachi, C. Evidence and Mechanism of Efficient Thermally Activated Delayed Fluorescence Promoted by Delocalized Excited States. Sci. Adv. 2017, 3,