Exploiting racemism enhanced organic room-temperature phosphorescence to demonstrate Wallach’s rule in the lighting chiral chromophores

The correlation between molecular packing structure and its room-temperature phosphorescence (RTP), hence rational promotion of the intensity, remains unclear. We herein present racemism enhanced RTP chiral chromophores by 2,2-bis-(diphenylphosphino)-1,1-napthalene (rac-BINAP) in comparison to its chiral counterparts. The result shows that rac-BINAP in crystal with denser density, consistent with a long standing Wallach’s rule, exhibits deeper red RTP at 680 nm than that of the chiral counterparts. The cross packing between alternative R- and S- forms in rac-BINAP crystal significantly retards the bimolecular quenching pathway, triplet-triplet annihilation (TTA), and hence suppresses the non-radiative pathway, boosting the RTP intensity. The result extends the Wallach’s rule to the fundamental difference in chiral-photophysics. In electroluminescence, rac-BINAP exhibits more balanced fluorescence versus phosphorescence intensity by comparison with that of photoluminescence, rendering a white-light emission. The result paves an avenue en route for white-light organic light emitting diodes via full exploitation of intrinsic fluorescence and phosphorescence.


Derivations of intrinsic fluorescence and phosphorescence yield.
Under the assumption that the net intersystem crossing yield (x), which includes the effect resulting from triplet-triplet annihilation (TTA), is the same in both photoluminescence (PL) and electroluminescence (EL), x can be calculated from the following equation: where denotes the ratio for radiative rate of fluorescence versus phosphorescence. IF and IP denote the PL intensity of fluorescence and phosphorescence, respectively. EF and EP denote the EL intensity of fluorescence and phosphorescence. Therefore, x can be solved by dividing eqs 1 with eqs 2. The net intersystem crossing yield (x) can be expressed in following equation: The ratio for PL of fluorescence versus phosphorescence is set to be 18.23, which is deduced from the ratio of area from the PL fluorescence and phosphorescence of rac-BINAP. As for the ratio for EL of fluorescence versus phosphorescence, the value is set to be 0.58, which is determined from the ratio for area for the EL fluorescence versus phosphorescence under 3.6V operated voltage. Under low operated voltage, TTA is hindered as much as possible.
x is thus deduced to be 9.8%. The intrinsic fluorescence and phosphorescence yield can thus be obtained by dividing the respective quantum yield with 1-x and x. S-4

Derivations for the ratio variation of fluorescence versus RTP in Electroluminescence (EL).
In EL mechanism, the energy originates from the migrations of electrons and holes, i.e., hopping between molecules. Thus, we assume that the energy transporting molecules serve as the energy source (ES). 2 On the other hand, molecules undergoing TTA (rac-BINAP in this case) serve as annihilators (A). The initial reaction pathways are depicted as follows: Here, kexc, kqf, and kqp denotes the constants of excitation rate, quenching rate of the singlet exciton and triplet exciton, respectively. 1 ES, 1 ES * , and 3 ES * represent the nonexcited energy source, singlet exciton, and triplet exciton respectively. Accordingly, the corresponding time-resolved differential equations without any energy transfer processes are depicted below: Next, we list all the possible energy transfer processes, including the annihilation pathways in triplet-triplet annihilation (TTA) and singlet-triplet annihilation (STA) processes depicted as follows: 2 SET  subscript is the rate constant of the corresponding reaction list above. Again, the corresponding time-resolved differential equations are listed below: 2 We then employ stead-state approximations for the equations list above: Followed by the above equations, the derivations are listed below: From eqs. 16 From eqs. 17 From eqs. 18 From eqs. 19 From eqs. 20 Combine eqs. 23 with 24 Then combine with eqs. 24 Considering 25% of singlet excitons and 75% of triplet excitons in EL system, the overall energy source intensity Iexc can be described as: We then combine eqs. 27 with eqs. 26 to obtain eqs. 28.
Further, we define the fluorescence intensity to be IF, which is then combined with eqs. 20.
Next, we define the phosphorescence intensity to be IP In order to simplify the correlation between Iexc and IF, we made two assumptions to conduct steady-state approximation. First, under low excitation intensity, TTA should have less effect to the phosphorescence intensity. Accordingly, the following relationship should hold.
Combining eqs 33 with eqs.20 Second, under high excitation intensity (i.e. high current density), TTA should have large effect to the phosphorescence intensity (annihilation dominant region). Accordingly, the following relationship holds.
Eqs. 31, 34 and 36 are used as the fitting functions to the experimental data shown in Fig. 5d in the main text. The fitting procedures were performed by Origin 2016.
As described above, strictly speaking, the "Quadratic" (eqs. 34) and "Linear" (eqs. 36) components of fluorescence and "Quadratic" phosphorescence (eqs. 31) intensities are not pure linear and quadratic character because of the existence of 3 ES * and 1 ES * , which are proportional to the excitation intensity.

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Supplementary Figure 14. The crystal geometry of (a) rac-BINAP, (b) R-BINAP, and (c) S-BINAP depicted with thermal ellipsoids shown at 50% (hydrogen atoms are omitted for clarity). Note that in rac-BINAP, the left molecule is R-form, and the right one is Sform with atom number in corresponding order. As for the rac-BINAP crystal, it is worth to note that R-and S-BINAP show the same structure parameters except for its chirality in the racemic environment. However, both R-and S-BINAP in racemic environment show different values with respect to their homochiral crystal (see Table S1 in detail). a All the corresponding parameters of R-/S-form is identical to each other in rac-BINAP, and the items listed is represented by R-form.

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Supplementary  Supplementary Figure 18. The excitation spectra of rac-BINAP in crystal.

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Supplementary Figure 19. R-BINAP and S-BINAP displayed subtle differences in the spectral shapes of the CPL signals (particularly, at extreme wavelengths around 500 nm ) with |gem|≈(1.46-1.65)×10 -3 that were nearly mirror images of each other.

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Supplementary    Note that NA denotes "not available". a) Rate constants deduced from fit shown Fig. 3a and Supplementary Figure 17b. b) The radiative rate (kp, r) is calculated by kp, r = Фp, intrinsic  kobs. c) The non-radiative rate (kp, nr) is calculated by kobs  kp, r. Comparing with the population decay of RTP measured in single crystals, all the decay curves of BINAPs measured in thin films show similar temporal evolution. This can be rationalized by the amorphous arrangement of BINAPs in thin films, while the crystals of BINAPs possess specific and ordered packing structure, resulting in the different population decay for the racemic versus homochiral crystals.