Panchromatic Ternary Polymer Dots Involving Sub-Picosecond Energy and Charge Transfer for Efficient and Stable Photocatalytic Hydrogen Evolution

Panchromatic ternary polymer dots (Pdots) consisting of two conjugated polymers (PFBT and PFODTBT) based on fluorene and benzothiadiazole groups, and one small molecular acceptor (ITIC) have been prepared and assessed for photocatalytic hydrogen production with the assistance of a Pt cocatalyst. Femtosecond transient absorption spectroscopic studies of the ternary Pdots have revealed both energy and charge transfer processes that occur on the time scale of sub-picosecond between the different components. They result in photogenerated electrons being located mainly at ITIC, which acts as both electron and energy acceptor. Results from cryo-transmission electron microscopy suggest that ITIC forms crystalline phases in the ternary Pdots, facilitating electron transfer from ITIC to the Pt cocatalyst and promoting the final photocatalytic reaction yield. Enhanced light absorption, efficient charge separation, and the ideal morphology of the ternary Pdots have rendered an external quantum efficiency up to 7% at 600 nm. Moreover, the system has shown a high stability over 120 h without obvious degradation of the photocatalysts.


DLS measurement of Pdots
DLS measurement is based on random thermal Brownian motion, that is modelled by the Stokes-Einstein equation: eq S1 = 6 ℎ Where is the diffusion coefficient, is Boltzmann coefficient (1.38 ×10 -23 kg.m 2 .s -2 .K -1 ), T is an absolute temperature, and is the viscosity of the medium, is the hydrodynamic radius of a hypothetical ℎ sphere that diffuses at the same rate as particle under investigation. 1      According to absorption spectrum of ternary Pdots, shown in Figure S13a, 80% of photon was absorbed by D 1 under excitation of 460 nm. The kinetic traces of GSB recovering for D 1 are shown in Figure S15, the kinetics were fitted by using a sum of exponential functions convoluted with the instrumental response function. Within the first picosecond time range, a fast recovering of D 1 GSB at the peak of 475 nm was observed in systems both with and without Pt NPs, accompanying with an increase in the GSB intensity of D 2 (peak at 575 nm) and ITIC (625nm to 725 nm), shown in Figure S14a & c, indicate an energy transfer (EnT) process from D 1 * to D 2 and ITIC. Kinetic traces of D 1 GSB recovering indicate that at least 85% of photon energy absorbed by D 1 was transferred to D 2 and ITIC in the time components of 600 fs or less.
Similar kinetics of D 1 GSB recovering in systems both with and without Pt cocatalyst were found, this may suggest no direct reaction between D 1 and Pt within the timescale we studied.

S15
Under the excitation of 710nm, ITIC was selectively excited, according to the energy level of three components, photoinduced hole transfer (HT) from ITIC* to D 2 is then the only photophysical pathway in ternary system. The observation for this ternary Pdots was similar with the binary D 2 /ITIC system, that immediately after the excitation, the GSB of D 2 appeared along with ITIC, with rise time (τ ≈ 100 fs) below our instrument response function (IRF, τ ≈ 200 fs), as shown in Figure S16 b & c, indicate an ultrafast hole transfer process from ITIC* to D 2 in the ternary Pdots. Similar D 2 GSB recovering kinetics were observed for systems with and without Pt NPs.
Under the excitation of 460 nm, 80% of photon is absorbed by D 1 , therefore D 2 and ITIC GSB intensity enhancement is mainly a result from EnT of the excited D 1 (D 1 *). Again, similar with binary Pdots, both the recovery dynamics of D 1 GSB and the formation dynamics of the excited ITIC and D 2 reflect the EnT rate. The observed rise time of D 2 GSB shows 200 fs in both systems with and without Pt NPs, as shown in Figure S16 e & f, respectively. Which may indicate a fine phase intermix between D 1 and D 2 in ternary Pdots.
Under the excitation of 550 nm, the kinetic traces at 575 nm indicate the recovering of excited D 2 and oxidized D 2 (a result of both electron transfer from D 2 * to ITIC and hole transfer from ITIC* to D 2 ). A faster recombination is observed in the system with Pt (Figure S16i) compare to system without Pt ( Figure   S16h). The possible mechanism of this faster recombination was explained detail in the main text.  Figure S16f. For the convenient of a direct comparison, kinetic traces were normalized, as shown in Figure S17a. A slight slower recombination for system under ascorbic acid was observed (Figure S17a purple empty circle) compare to system without ascorbic acid (Figure S17a blue circle). For the system under ascorbic acid, the actual kinetics curve shown in Figure S17b. A slight enhancement in lifetime at picosecond timescale was observed. S17 Figure S18. Energy and charge transfer pathways between D 1 , D 2 , ITIC and Pt cocatalyst.
Same photophysical pathways were found as indicated in the primary framework. Under excitation of 460 nm, EnT (include both Dexter EnT and FRET) from D 1 * to D 2 ; secondly, one step ET from D 2 * to ITIC; and/or two-step process involving FRET from D 2 * to ITIC, followed with HT from ITIC* to D 2 . In parallel, EnT from D 1 * to ITIC and followed with HT from ITIC* to D 2 is also possible. With excitation of 550 nm, again, both one step of ET from D 2 * to ITIC, and/or two-step process are possible. While under excitation of 710 nm, photoinduced HT is the only reaction that can happen from the ITIC* to D 2 . Figure S19. UV-Vis of solution washed from reaction solution and oxidized ascorbic acid as reference.

S18
After the fourth cycle of photocatalytic reaction, ternary Pdots was washed with water by using centrifuge tube with membrane size of MWCO 15 kDa. Ternary Pdots was remained in the top, within the membrane.
While small molecules such as ascorbic acid or oxidized ascorbic acid can be washed away and collected in the bottom of the tube. Samples were washed with 100 times of volume in total in order to remove (oxidized) ascorbic acid completely.