Kinetic plots for PET-RAFT polymerizations of MMA in the presence of argon using Ir(ppy)3 at 1 ppm. Results of PET-RAFT polymerization of MMA in the presence of CPADB using 4DP-IPN at 1 ppm under argon.
Introduction
Previously reported structures of organic photocatalysts for PET-RAFT polymerization (reprinted with permission from reference 1. Until now, the transition metal-based catalysts have been widely used for PET-RAFT with the oxygen tolerance.
D evelopment of OPCs for PET-RAFT polymerizations
Experimental detail
- Materials
- Instrumentation
- Syntheses of 4DP-IPN
- Experimental procedure
Gated phosphorescence spectra were acquired using a cw 405 nm laser module with TTL modulation input (maximum modulation frequency 20 kHz) and appropriate triggering of the CCD. S12, its matrix elements Rij are the spectral weights of the ith SVD spectrum Us,i in the characteristic spectrum of mode j, 𝜎𝑗. Due to the linear dependence of one of the concentrations on the others, the number of signal-related SVD states in our kinetic scheme 1 is therefore 𝑛𝑠 = 𝑛𝑠𝑡− 1 = 2.
Looking at Scheme 1, we can take advantage of the fact that due to the spin selection rule, the pump pulse exclusively generates S1 states. S14 turns out to be very helpful: As we know the absolute diameter of the ground state 𝜎𝑆0(𝐸𝑝𝑟), we can find both ∆𝑠𝑇1(𝑡) and 𝜎′𝑇1(𝐸𝑆0) at the same time changing schemes by a the spectral shape 𝜎𝑇1 (𝐸𝑝𝑟) by introducing one or more positive Gaussians, taking advantage of the fact that triplet states do not show stimulated emission (formally negative TA). A 0.1 ml portion of the reaction mixture was taken via a syringe at predetermined interval times and dissolved in a vial containing CDCl3.
After that, the process is the same as above. However, due to the weakness of the small rubber septa for 4 ml vials, kinetic study in the presence of air was performed by dividing time intervals into three parts to prevent further addition of oxygen from atmosphere; 0 min ~ 1 h, 2 h ~ 6 h, 9 h ~ 18 h. A 0.1 ml aliquot of the reaction mixture was taken via a syringe at each interval and dissolved in a vial containing CDCl 3 . A 0.1 ml aliquot of the reaction mixture was removed via syringe and dissolved in a vial containing CDCl 3 .
Results and discussion
OPCs design logic
The designed OPCs based on this platform have a strong intramolecular charge transfer (CT) character in the S1 state, so the T1 state is effectively populated in OPCs. Furthermore, key catalyst parameters (e.g., ground/excited state redox potentials and triplet energies) can be systematically tuned over wide ranges by simply adjusting the D and A groups. We therefore envisioned that our strategy could be used to develop highly efficient OPC- for PET-RAFT powered by visible light with excellent oxygen tolerance.
Structures and properties of OPCs studied in the present work, selected from our OPC library. We first selected OPCs from our library by considering triplet exciton generation, visible light absorption, ground and excited state redox potentials, and triplet energies (Figure 14; for our OPC library, Figure 4 and Table 1). It should be noted that here we consider both redox potentials and triplet energies for the selection of OPCs, because two different mechanisms (i.e., photoinduced electron and energy transfer) may be involved in PET-RAFT (Figure 12).
In fact, according to recent reports, photoinduced energy transfer 49,50 is the most reliable pathway in PET-RAFT polymerization using Ir(ppy)3 or Ru(bpy)32+; however, it is still unclear for polymerization reactions using other types of catalysts. A complete description of the synthetic, UV−Vis, cyclic voltammetry (CV) and 1H-NMR approaches is found in Figures 36−39 and 43−52. Properties of CPADB and CDTPA: Molecular geometries in S0 and T1 (the latter is exclusively described by a HOMO→LUMO excitation); Corresponding border MO.
Polymerization studies under Argon
Results for PET-RAFT polymerization of MMA in the presence of different PCs at room temperature under argon (Ar) and air; 3 W blue LED illumination (455 nm, approx. 2.5 mW/cm2) (Reprinted with permission from Reference 1. Copyright 2019.. a) Negative control PET-RAFT MMA experiments in the presence of argon and air using CPADB. Results of PET-RAFT MMA with selected OPCs are summarized in Table 2; here we used 5 ppm OPC for polymerizations, as the results for 1 ppm were not satisfactory (Table 3). In particular, 4DP-IPN allowed a rather high conversion of 62% with excellent control (Đ = 1.11 and I* = 0.91), comparable to those of Ir(ppy)3; for other OPCs, the polymers were mostly obtained in less than approx.
Using 4DP-IPN, the number average molecular weight (Mn) of PMMA can also be modulated in the range of 4.2-27 kg/mol with a narrow molecular weight distribution (Đ < 1.20), high initiator efficiency (I* > 0.70) ) , and reasonable conversions (Table 5). Polymerization kinetics were then monitored by 1 H-NMR and GPC for 4DP-IPN (5 ppm) under Ar (Figure 19a,b); kinetics for Ir(ppy)3 (1 ppm) were performed for comparison (Table 6). As expected, the chain extension of PMMA with MMA was also successful, resulting in the synthesis of PMMA-b-PMMA with a narrow molecular weight distribution (Đ = 1.12, Figure 21).
Control of the molecular weight of PMMA in the presence of argon using CPADB and 4DP-IPN of 5 ppm. Chain extensions of PMMA in the presence of argon using Ir(ppy)3 of 1 ppm and 4DP-IPN of 5 ppm. GPC traces of PMMA and diblock of PMMA-b-PMMA and 1H-NMR spectra of the washed first chain (middle) and in-situ (bottom) and the second chain in-situ (top) using (a) Ir (ppy)3 of 1 ppm and (b) 4DP-IPN of 5 ppm under argon.
Oxygen tolerance
1H-NMR spectra of polymerization mixtures using CPADB and 4DP-IPN (5 ppm) at different reaction times. Kinetic curves for PET-RAFT polymerization of MMA in the presence of air using Ir(ppy)3. The polymerization of MMA was first performed in the presence of 4DP-IPN (5 ppm) and CDTPA in an Ar atmosphere using a 3 W blue LED (2.5 mW/cm2).
The chain extension experiment further confirms that polymers can be prepared in a controlled manner, even under air (Figure 27). a) Results of PET-RAFT polymerization of MMA in the presence of CDTPA using 4DP-IPN at 5 ppm under argon. Polymerizations under air were performed in a sealed 4 ml vial with a total liquid volume of 3 ml. b) 1H NMR spectra of the reaction mixtures obtained after 6-hour PET-RAFT polymerizations without purifications under air (top) and Ar (bottom) in the presence of CDTPA and MMA with/without 4DP-IPN at 5 ppm (reprinted with permission from reference 1. Copyright 2019.. a) Chain extensions of PMMA in the presence of air using CDTPA and 4DP-IPN at 5 ppm. Finally, we investigated the photophysical and electrochemical properties of 4DP-IPN to understand its excellent catalyst performance.
Emission under continuous wave excitation and gated emission of 4DP-IPN in CH3CN at 65K. PL Decay of 4DP-IPN in CH3CN at 65K, experiment (red), biexponential fits (black). 1H NMR spectra and GPC curves of the molecular weight monitored PMMA in the presence of argon using CPADB and 4DP-IPN at 5 ppm.
In-situ 1H-NMR of PMMA in the presence of CPADB, MMA and 4DP-IPN at 5 ppm under argon in CDCl3 at r.t. In-situ 1H-NMR of PMMA in the presence of CPADB, MMA and 4DP-IPN at 5 ppm under air in CDCl3 at r.t.
Oxygen tolerance of organic photoredox catalyst
Experimental validation of the outstanding performance of 4DP-IPN
While PL experiments at room temperature are only sensitive to the emission of S1 states (phosphorescence is suppressed), TA allows to detect the concentrations of both S1 and T1 states in a single experiment (Figure 35). Both PL and TA agree to the finding that at room temperature, direct relaxation of T1 to the ground state (via kPH and knr') is negligible; the only available relaxation channel is through the S1/T1 equilibrium leading to thermally activated delayed fluorescence (TADF; Figures 28c and 29−32); in fact, the observed 100 μs TADF lifetime yields a deviation between S1 and T1 of 0.19 eV, in good agreement with the experiment and the TD-DFT results (Figure 28c). The TD-DFT results further indicate that the high ISC efficiency is due to the well-balanced intramolecular CT character of S1 and the low-lying Tn states.
In fact, this is considered a central element of the design of multibranched 4DP-IPNs, allowing multiple intramolecular CT configurations, combined with good light absorption and compatible redox properties. The absence of a direct decay channel means that the triplet lifetime is controlled entirely by S1/T1. energetic compensation; subtle changes in molecular structure will lead to drastic changes in triplet lifetime. Transient absorption (TA) experiment of 4DP-IPN was performed in CH3CN at r.t. a) TA spectrum in the nanosecond time interval, after pumping with 300 ps monochromatic pump pulses at 355 nm with 2 μJ pulse energy and 500 Hz repetition rate.
The false color scale indicates green as zero signal, yellow as positive transient absorption (photo-induced absorption, PA) and blue as negative transient absorption (transient photobleach, PB). b) Kinetic model for simulating the TA dynamics. Without measuring PLQY, one cannot distinguish between these processes leading to the same PA and PB dynamics. S13) of panel (a) (black solid lines in panel (c) and reproduction (black dashed lines) by a weighted superposition of the characteristic spectra of excited singlet and triplet states (blue and orange lines, defined according to Eq. d) and (e) resulting dynamics of excited singlet and triplet states according to Eq.
UV-Vis and PL spectra, CV curves, and 1 H-NMR and 13 C-NMR spectra of selected OPCs and
In-situ 1H-NMR of PMMA in the presence of CPADB, MMA and eosin Y at 5 ppm under argon in CDCl3 at r.t. In-situ 1H-NMR of PMMA in the presence of CPADB, MMA and fluorescein at 5 ppm under argon in CDCl3 at r.t. In-situ 1H-NMR of PMMA in the presence of CPADB, MMA and 5Cz-BN at 5 ppm under argon in CDCl3 at r.t.
In-situ 1H-NMR of PMMA in the presence of CPADB, MMA and 4Cz-IPN of 5 ppm under argon in CDCl3 at r.t. In-situ 1H-NMR of PMMA in the presence of CPADB, MMA and 2DHPZ-DPS of 5 ppm under argon in CDCl3 at r.t. In-situ 1H-NMR of PMMA in the presence of CPADB, MMA and DMDP-TRZ of 5 ppm under argon in CDCl3 at r.t.
In-situ 1H-NMR of PMMA in the presence of CPADB, MMA, and 5 ppm eosin Y under air in CDCl3 at r.t. In-situ 1H-NMR of PMMA in the presence of CPADB, MMA and 5 ppm fluorescein under air in CDCl3 at r.t. In-situ 1H-NMR of PMMA in the presence of CPADB, MMA and 4Cz-IPN of 5 ppm under air in CDCl3 at r.t.
In-situ 1H-NMR of PMMA in the presence of CPADB, MMA and 2DHPZ-DPS of 5 ppm under air in CDCl3 at r.t. In-situ 1H-NMR of PMMA in the presence of CPADB, MMA and DMDP-TRZ of 5 ppm under air in CDCl3 at r.t.
Conclusion
