Spiro donor-acceptor TADF emitters: Naked TADF free from inhomogeneity caused by donor acceptor bridge bond disorder. Fast rISC and invariant photophysics in solid state hosts.

The molecular photophysics of 10-phenyl-10H,10′H-spiro[acridine-9,9′-anthracen]-10′-one (ACRSA) are used as an ideal molecule to probe how external factors affect the TADF and rISC mechanisms.


b. Photophysical characterisation:
Absorption spectra for all films and solutions were collected using a double beam Shimadzu UV-3600 UV/VIS/NIR spectrophotometer.Steady state photoluminescence spectra were measured using both Jobin-Yvon Fluoromax-4 and Fluorolog spectrophotometers.

c. Time-resolved Photoluminescence:
Time-resolved photoluminescence spectra and decays were measured using nanosecond gated spectrograph-coupled iCCD (Stanford) and an Nd:YAG laser emitting at 355 nm or N 2 laser at 337 nm as an excitation sources, in conjunction with a gated 4 Picos iCCD camera.Lifetimes were obtained from a kinetic decays by fitting with a multiexponential function.

Experimental energy level scheme derived for ACRSA using nomenclature used by
Lyskov and Marian.Briefly summarising our previous experimental solution state measurements, we find a very strong 1 1 B 1 exciton absorption below 350 nm, excitation of which gives rise to fast emission from this high energy excitonic singlet state associated with the donor acridine unit, in competition with internal conversion to two lower lying singlet states of A 2 symmetry and so formally one-photon forbidden states.However, both 2 1 A 2 local acceptor 1 nπ* (hereafter 1 LE) and 1 1 A 2 1 ππ* CT (hereafter 1 CT) singlet states do have weak direct absorption transitions.
Excitation of these states either directly (excitation of 355 nm) or via the upper 1 1 B 1 state (excitation 337 nm) gives rise to prompt emission from both A 2 states, simultaneously.ACRSA thus emits from three independent excited states and can be considered one of the most anti-Kasha systems reported.The main deactivation pathway from the 1 1 B 1 exciton state is however via ISC to upper triplet states.These undergo IC to the lowest triplet states that leads to delayed fluorescence (DF) through rISC to the 1 CT singlet.The 1 LE state decays rapidly by efficient ISC to the local 3 ππ* triplet state by an allowed 1 nπ* to 3 ππ* transition.This also results in eventual DF from the 1 CT state (only), where the lowest energy triplet state, 3 nπ* local triplet, acts as the mediator triplet for the non-adiabatic vibronic coupling rISC step. 3,4We find that the main vibronic coupling mode is the anthracenone C=O bond vibration.S2.S2.S2.Data is tabulated in Table S2.S1.

Figure
In zeonex, we observe complex, fast (in our first few time windows) emission at 350-400 nm (peak at 370 nm) which we previously observed in solution state measurements and assigned to emission from the 1  3) whereas the bluer one matches that seen in zeonex with 355 nm excitation, ascribed to the LE/CT mixed state.We can only really speculate as to the origin of the redder band, but from it's spectral position we think it could be from remnant molecules with distorted conformation that are hindered by the matrix from reverting back to the lowest energy molecular conformation.In general, 337 nm excitation gives slightly broader spectra indicative of the greater excess energy allowing more molecular conformations to be accessed along with hot state (vibrationally unrelaxed) emission which blurs the vibronic structure of bands, as described previously by Greene et al 5 .S1.  S1.S1.S1.

Figure S7 .S11 8 .
Figure S7.(a)-(d) Area normalised time resolved spectra ACRSA in DPEPO (1% wt), shown in the four main regimes indicated in (e).(e) Fitting of kinetic decay results at 80 K. Laser excitation at 355 nm.Data is tabulated in TableS2.

10 .
Figure S9 (a)-(c)Area normalised time resolved spectra for highly diluted ACRSA (0.01%) in zeonex shown in the three main regimes indicated in (d).(d) Fitting of kinetic decay results at 300 K. Laser excitation at 355 nm.At such high dilution, it is clear that both 1LE and 1CT emission species are present but there is no 550 nm species, which we therefore attribute to an intermolecular species.

15 .
Figure S16.(a)-(d) Area normalised time resolved spectra of ACRSA in UGH (10% wt), shown in the four main regimes indicated in (e).(e) Fitting of kinetic decay results at 80 K. Laser excitation at 355 nm.Data is tabulated in TableS2.

Figure S17 .
Figure S17.(a)-(d) Area normalised time resolved spectra of ACRSA in DPEPO (10% wt), shown in the four main regimes indicated in (e).(e) Fitting of kinetic decay results at 80 K. Laser excitation at 355 nm.Data is tabulated in TableS2.

Figure S18 .
Figure S18.(a)-(d) Area normalised time resolved spectra of neat film, shown in the four main regimes indicated in (e).(e) Fitting of kinetic decay results at 80 K. Laser excitation at 355 nm.Data is tabulated in TableS2.
Figure S20.Power dependence of delayed fluorescence spectra of ACRSA in different host matrices at 1% and 10% loading, recorded at room temperature.

Figure S21 .
Figure S21.Linear fit of the DF intensity as a function of excitation power for ACRSA in different host matrices at 1% and 10% loading, recorded at room temperature.

Figure S24 .
Figure S24.Guest concentration dependence on the time dependent emission spectra of ACRSA (normalised intensity).Time resolved emission spectra from ACRSA at 1% loading in UGH and DPEPO host matrices, measured at room temperature and 80 K, with 355 nm and 337 nm excitation.

Figure S30 .
Figure S30.(a)-(d) Area normalised time resolved spectra of ACRSA in DPEPO (1% wt), shown in the four main regimes indicated in (e).(e) Fitting of kinetic decay results at room temperature.Laser excitation at 337 nm.Data is tabulated in TableS1.

Figure S31 .
Figure S31.(a)-(d) Area normalised time resolved spectra of ACRSA in UGH (10% wt), shown in the four main regimes indicated in (e).(e) Fitting of kinetic decay results at room temperature.Laser excitation at 337 nm.Data is tabulated in TableS1.

Figure S32 .
Figure S32.(a)-(d) Area normalised time resolved spectra of ACRSA in DPEPO (10% wt), shown in the four main regimes indicated in (e).(e) Fitting of kinetic decay results at room temperature.Laser excitation at 337 nm.Data is tabulated in TableS1.

Figure S33 .
Figure S33.(a)-(d) Area normalised time resolved spectra of neat film, shown in the four main regimes indicated in (e).(e) Fitting of kinetic decay results at room temperature.Laser excitation at 337 nm.Data is tabulated in TableS1.

Table 1 .
Figure S15.(a)-(d) Area normalised time resolved spectra of neat film, shown in the four main regimes indicated in (e).(e) Fitting of kinetic decay results at room temperature.Laser excitation at 355 nm.Data is tabulated in Table 1
1B 1 bright donor exciton state (which has very strong absorption at 337 nm) and a shoulder at 395 nm (3.14 eV) which is a signature of the fast emitting 1 LE acceptor local state.Again, we also observe a very broad highly red shifted Gaussian feature centred at 520 nm (2.38 eV) which decays faster with 337 nm than 355 nm excitation.After this initial fast time period, we observe the evolution of two emitting CT bands, with an isoemissive point at ca. 490 nm in the area normalised data, having lifetimes of 1.4 μs and 8.8 μs, similar decay to those measured with 355 nm excitation, but now clearly showing two resolved emission bands.The red component has a much longer lifetime.This redder CT band matches well to the CT band observed in DPEPO (1% loading, see Figure