Rational Design of Photocatalysts for Controlled Polymerization: Effect of Structures on Photocatalytic Activities

Abstract

Over the past decade, the use of photocatalysts (PCs) in controlled polymerization has brought new opportunities in sophisticated macromolecular synthesis. However, the selection of PCs in these systems has been typically based on laborious trial-and-error strategies. To tackle this limitation, computer-guided rational design of PCs based on knowledge of structure-property-performance relationships has emerged. These rational strategies provide rapid and economic methodologies for tuning the performance and functionality of a polymerization system, thus providing further opportunities for polymer science. This review provides an overview of PCs employed in photocontrolled polymerization systems and summarizes their progression from early systems to the current state-of-the-art. Background theories on electronic transitions are also introduced to establish the structure-property-performance relationships from a perspective of quantum chemistry. Typical examples for each type of structure-property relationships are then presented to enlighten future design of PCs for photocontrolled polymerization.

Figure 1.

Chemical structures, frontier molecular orbitals, and their energy levels of the ground states of (A) XAN-1 (Erythrosine B), (B) XAN-2 (Eosin Y), (C) XAN-3 (Rose Bengal), and (D) XAN-4 (Phloxine B). q: percentage of the MO localized on the specified element. Reprinted from ref . Copyright 2019 American Chemical Society.

Figure 2.

(A) Planar, bent, and twisted conformations of two adjacent p-orbitals. (B) Calculated HOMO and LUMO energy levels of anthracene (as a simplified example of twisted acene derivatives) with different twist angles. Reprinted from ref . Copyright 2019 American Chemical Society.

Figure 3.

Chemical structures of POR-1 and POR-2.

Figure 4.

Chemical structures, molecular geometries, and visualized frontier molecular orbitals (isovalue = 0.03) of PDI derivatives PDI-1, PDI-2, PDI-3, PDI-4, PDI-5, and PDI-6. Reprinted with permission from ref . Copyright 2019 The Royal Society of Chemistry.

Figure 5.

Chemical structures and molecular geometries of (A) free-base phthalocyanine PHC-1, (B) free-base β-alkyloxy-substituted phthalocyanine PHC-2, (C) free-base α-alkyloxy-substituted phthalocyanine PHC-3, (D) zinc(II) phthalocyanine PHC-4, (E) zinc(II) β-alkyloxy-substituted phthalocyanine PHC-5, and (F) zinc(II) α-alkyloxy-substituted phthalocyanine PHC-6. Reprinted from ref . Copyright 2019 American Chemical Society.

Figure 6.

(A,E) Chemical structures and energy level diagrams, (B,F) visualization of HOMO, LUMO/LUMO+1 (isovalue = 0.3), (C,G) UV–vis spectra with λ_max denoted, and (D,H) plots of ln([M]₀/[M]_t) versus time revealing k_p^app and temporal control for model PET-RAFT polymerization via the oxygen-mediated reductive quenching pathway energy, corresponding to (A–D) zinc(II) phthalocyanine PHC-4 and (E–H) the zinc(II) β-alkyloxy-substituted phthalocyanine PHC-6, respectively. Percentage contribution of the dominant molecular pairs contributing to the S₀ → S₁ electronic transition is denoted in red. Reprinted with permission from ref . Copyright 2021 Nature Publishing Group.

Figure 7.

Chemical structures and HOMOs/LUMOs (and corresponding energy levels) of polynaphthalenetetracarboxylic dianhydride diimide dyes PDI-1, PDI-7, and PDI-8. Reprinted from ref . Copyright 2001 American Chemical Society.

Figure 8.

(A–C) Chemical structures, UV–vis spectra with λ_max denoted, energy level diagrams, and visualization of HOMO and LUMO/LUMO+1 (isovalue = 0.3) of (A) zinc tetramethyl tetraazaporphyrin PHC-7, (B) zinc phthalocyanine PHC-4, and (C) zinc naphthalocyanine PHC-8. Percentage contribution of the dominant molecular pairs contributing to the S₀ → S₁ electronic transition is denoted in red. Reprinted with permission from ref . Copyright 2021 Nature Publishing Group.

Figure 9.

(A,B) Chemical structures and molecular geometries of (A) PXZ-1 and (B) PXZ-2, with Φ_T denoted below. (C) Diagram of excited and ground state energy levels with alternative ISC pathways presented for PXZ-1 and PXZ-2. Less likely pathways are indicated in dashed lines. Reprinted from ref . Copyright 2018 American Chemical Society.

Figure 10.

(A–C) Chemical structures of (A) PXZ-3, (B) PXZ-4, and (C) PXZ-5 with k_F, k_IC, k_ISC, and Φ_T denoted below. (D) Energy level diagram of PXZ-3, PXZ-4, and PXZ-5 showing dominant decay pathways of excited states. Reprinted from ref . Copyright 2019 American Chemical Society.

Figure 11.

(A,B) Chemical structures of (A) XAN-5 and (B) XAN-6, with k_F, k_IC, k_ISC, and Φ_T denoted below. (C) Computationally derived Jablonski diagram of XAN-6 and its derivation of Φ_T from different excited state decay rate constants. Reprinted from ref . Copyright 2019 American Chemical Society.

Figure 12.

Chemical structures and representation of molecular geometries of PDI-9, PDI-10, and PDI-11, with Φ_T and k_ISC denoted below the dye labels. Reproduced with permission from ref . Copyright 2017 The Royal Society of Chemistry.

Figure 13.

(Top) Chemical structures of twisted acene derivatives Ant-Cn (n = 3–6) tethered with alkyl bridges (n = propyl bridge to hexyl bridge). (Bottom) Transient EPR spectra for Ant-Cn and the open (no alkyl bridges) reference compound. Reproduced with permission from ref . Copyright 2019 The Royal Society of Chemistry.

Figure 14.

Chemical structures of (A) fac-Ir(ppy)₃ and (B) PTZ-1 with E⁰(PC^•+/³PC*) and E⁰(PC^•+/PC) denoted below the dye label. Reproduced from ref . Copyright 2016 American Chemical Society.

Figure 15.

Chemical structures of the original chromophore core structures (A) DPZ-1 as a dihydrophenazine derivative, (B) PXZ-6 as a phenoxazine derivative, and (C) PTZ-1 as a phenothiazine derivative, with DFT-computed E⁰(PC^•+/3PC*) and E⁰(PC^•+/PC) denoted below the dye label. Reproduced from ref . Copyright 2016 American Chemical Society.

Figure 16.

Chemical structures of dihydrophenazine derivatives (A) DPZ-1, (B) DPZ-2, (C) DPZ-3, and (D) DPZ-4, with DFT-computed E⁰(PC^•+/³PC*) denoted below the label. Reproduced with permission from ref . Copyright 2016 American Association for the Advancement of Science.

Figure 17.

(A,B) Chemical structures of dihydrophenazine derivatives (A) DPZ-5 and (B) DPZ-6, with computed E⁰(PC^•+/³PC*) denoted below the label. (C,D) T₁ frontier molecular orbitals of (C) DPZ-5 and (D) DPZ-6 which visualizes the higher-lying SOMO (top) and the lower lying SOMO (bottom). Reproduced with permission from ref . Copyright 2016 American Association for the Advancement of Science.

Figure 18.

(A–D) Chemical structures of phenoxazine derivatives (A) PXZ-6, (B) PXZ-7, (C) PXZ-8, and (D) PXZ-9 with computed E⁰(PC^•+/³PC*) denoted below the label. (E–H) T₁ frontier molecular orbitals of (E) PXZ-6, (F) PXZ-7, (G) PXZ-8, and (H) PXZ-9 which visualizes the higher-lying SOMO (top) and the lower lying SOMO (bottom). Reproduced from ref . Copyright 2016 American Chemical Society.

Figure 19.

Chemical structures of (A) PXZ-10, PXZ-11, PXZ-12, PXZ-13, PXZ-5, PXZ-14, and PXZ-15 for demonstration of the electron-donating/withdrawing effect on computed E⁰(PC^•+/³PC*) and (B) PXZ-6, PXZ-9, PXZ-5 and PXZ-16 for demonstration of the effect of extended conjugation on computed E⁰(PC^•+/³PC*), with E⁰(PC^•+/³PC*), E⁰(PC^•+/PC), and E_T denoted below the label. Reproduced from ref . Copyright 2018 American Chemical Society.

Figure 20.

(A,B) Chemical structures of (A) POR-1 and (B) POR-2. (C,D) Plot of ln([M]₀/[M]_t) versus time revealing k_p^app and temporal control for model PET-RAFT polymerization via oxidative quenching catalyzed by (C) POR-1 and (D) POR-2. (E,F) Proposed photocatalytic cycles with the Gibbs free energy change (ΔG) of key steps denoted in comparison to corresponding thresholds for PET-RAFT polymerization via the oxidative quenching pathway, catalyzed by (E) POR-1 and (F) POR-2, respectively. Green stands for favorable and red for inert. Colored values are ΔG of the corresponding process in kcal/mol, in conjunction with the derived thresholds. Reproduced with permission from ref . Copyright 2021 Nature Publishing Group.

Figure 21.

(A–F) Chemical structures of (A) XAN-1, (B) XAN-6, (C) XAN-2, (D) XAN-7, (E) XAN-3, and (F) XAN-4. (G) Electronegativities of the halogen substituents versus H to represent their electron-withdrawing capability. (H) Computed E⁰(PC^•+/³PC*) of the PCs in comparison. Reproduced from ref and ref . Copyright 2019 American Chemical Society.

Figure 22.

Chemical structures, redox potentials, and polymerization efficiency for PCs used in photocationic polymerization. Triphenylpyrylium derivatives (A) PY-1, (B) PY-2, (C) PY-3, (D) PY-4, triphenylthiopyrylium derivative (E) TPY-1, Ir-based complexes (F) IR-1, (G) IR-2, (H) IR-3, and bisphosphonium salt derivatives (I) BPP-1, (J) BPP-2, and (K) BPP-3.

Figure 23.

Chemical structures of Ir-based complexes (A) IR-1, (B) IR-4, and (C) IR-5 with E⁰(PC*/PC^•−) vs SCE denoted below the label. (D) Polymerization of isobutyl vinyl ether via photocationic NMP catalyzed by IR-1 (red), IR-4 (blue), and IR-5 (orange). Reproduced with permission from ref . Copyright 2019 Wiley-VCH.

Figure 24.

Chemical structures of PCs used in photo-ROMP, their redox potentials, and portions of oxidized norbornene vs polymerization. Triphenylpyrylium derivatives (A) PY-1, (B) PY-2, (C) PY-5, and (D) PY-3 and triphenylthiopyrylium derivatives (D) TPY-1, (E) TPY-2, (F) TPY-3, and (G) TPY-4. A portion of the norbornene monomers being polymerized and being oxidized after full conversion was achieved within 150 min in photo-ROMP catalyzed by each PC, respectively.

Scheme 1.

Key Steps in Photo-Controlled Polymerization

Scheme 2.. Jablonski Diagram of Excited States^a

^aNote: A, absorption; F, fluorescence; P, phosphorescence; IC, internal conversion; ISC, intersystem crossing; and VR, vibrational relaxation.

Scheme 3.. Catalytic Cycles of (a) ET via Oxidative Quenching Pathway, (b) ET via Reductive Quenching Pathway, and (c) TET in Photocontrolled Polymerization^a

^aNote: R-X, initiator; P_n-X, polymer chain capped with an end group X; X⁻ or X^•, end group in (a)/(b,c); R^• and P_n^•, propagating radicals for radical polymerization in (a,c); R⁺ and P_n⁺, propagating cations for cationic polymerization in (b); and M, monomer.

Scheme 4.. Classification of Photocontrolled Polymerizations with Their Corresponding Monomers (Left) and Compatible Catalytic Cycles (Right)^a

^aNote: ET, electron transfer; OQP, oxidative quenching pathway; RQP, reductive quenching pathway; and TET, triplet energy transfer.

Scheme 5.. (A) Proposed Mechanism of Photo-ATRP via an Oxidative Quenching Pathway and (B) Chemical Structures of ATRP Initiators Commonly Used in Photo-ATRP^a

^aNote: ISC, intersystem crossing; M, monomer; X, bromine (Br) and chlorine (Cl) are the most commonly employed end groups (in some specific cases, iodine (I) was also employed); EBPA, ethyl-α-bromophenylacetate; EClPA, ethyl-α-chlorophenylacetate; EBiB, ethyl α-bromoisobutyrate; and BnBiB, benzyl α-bromoisobutyrate.

Scheme 6.. Activation Pathways Proposed in Photo-RDRP^a

^aNote: OSET-2-body, outer sphere electron transfer (OSET) that generates 2 bodies including the PC^•+/X⁻ ion pair and R^•; OSET-3-body, OSET followed by decomposition of (R-X)^•− into X⁻ and R^•.

Scheme 7.. Deactivation Pathways Proposed in Photo-RDRP^a

^aNote: 2-body-OSET, two bodies (the PC^•+/X⁻ ion pair and R^•) undergo OSET to recover PC and R-X; 3-body-OSET, three bodies (PC^•+, X⁻,and R^•) undergo OSET to recover PC and R-X; and 3-body-ISET, three bodies (PC^•+, X⁻ and R^•) undergo ISET to recover PC and R-X.

Scheme 8.

Chemical Structures of Representative PCs in Photo-ATRP Systems, Including (A) Transition-Metal-Based PCs, (B) Perylene, (C) Phenothiazine Derivatives, (D) Dihydrophenazine Derivatives, and (E) Phenoxazine Derivatives

Scheme 9.

Chemical Structures of Representative PCs in O-ATRP Systems, Including (A) Dimethyl-Dihydroacridine Derivatives and (B) Organic Donor–Acceptor Scaffolds

Scheme 10.. Proposed Mechanisms for PET-RAFT Polymerization Activated by (A) ET via the Oxidative Quenching Pathway or (B) TET and (C) Chemical Structures of Thiocarbonylthio RAFT Agents Commonly Used in PET-RAFT Polymerizations^a

^aTrithiocarbonates: BTPA, BSTP, CDTPA, and CPDTC; dithiobenzoates: CPADB and CDB; xanthate. Note: M, monomer; Z, Z group of the RAFT agent.

Scheme 11.

Chemical Structures of Representative PCs Employed in PET-RAFT Polymerization Systems, Including (A) Transition-Metal-Based PCs fac-[Ir(ppy)₃] (1) and Ru(bpy)₃²⁺ (57), (B) Porphyrin Derivatives, (C) Naturally Derived Chlorophyll a (62), Bacteriochlorophyll a (63), and Pheophorbide a (64) and (D) Xanthene Derivatives

Scheme 12.. (A) Proposed Mechanism for Photocationic RAFT Polymerization and (B) Common RAFT Agents and (C) Proposed Mechanism for Photocationic NMP Polymerization, (D) Common Monomers, and (E) Common Alkoxyamine Initiators^a

^aNote: IBDTC, S-1-isobutoxyethyl N,N-diethyl dithiocarbamate; IBTTC, S-1-isobutoxylethyl S′-ethyl trithiocarbonate. M, monomer; Z, RAFT agent Z group.

Scheme 13.

Chemical Structures of Representative PCs in Photo-LCP and Photo-ROMP Systems, Including (A) Triphenylpyrylium Derivatives, (B) Triphenylthiopyrylium Derivatives, (C) Substituted Ir-Based Complexes, and (D) Bisphosphonium Salt Derivatives

Scheme 14.. (A) Proposed Mechanism for Photo-ROMP via a Reductive Quenching Pathway and (B–C) Chemical Structures of the (B) Initiators and (C) Monomers Used in Photo-ROMP^a

^aNote: ISC, intersystem crossing; DCPD, dicyclopentadiene.

Scheme 15.. Roadmap of the Photocatalytic Pathway of a Typical Photocontrolled Polymerization^a

^aNote: λ_max, maximum absorption wavelength of a given absorption peak; ε_max, maximum molar extinction coefficient of a given absorption peak; E_ex,S0–S1, excitation energy of the S₁ state; E_ex,S0-T1, excitation energy of the T₁ state; f, oscillator strength of excitation to an excited state; τ_S1, lifetime of the S₁ state; τ_T1, lifetime of the T₁ state; k_F, fluorescence rate constant; k_IC, internal conversion rate constant; k_ISC, intersystem crossing rate constant; ϕ_T, triplet quantum yield; IP, ionization potential; EA, electron affinity; E*_ox, excited state oxidation potential; E*_red, excited state reduction potential; E_USOMO, energy level of the upper singly occupied molecular orbital; and E_LSOMO, energy level of the lower singly occupied molecular orbital.

Scheme 16.

Various (A) Functional Substituents and (B) Organic Chromophore Cores Commonly Used in Designing a PC in Photocontrolled Polymerization

Scheme 17.

Overview of Key Structure-Property-Performance Relationships Known to Date in Designing a PC

Scheme 18.. Jablonski Diagram for Rate Constants of Typical Excited State Decay Pathways^a

^aNote: k_F, fluorescence rate constant; k_P, phosphorescence rate constant; k_IC, internal conversion rate constant between two electronic states of the same multiplicity; k_ISC, intersystem crossing rate constant from a singlet state to a triplet state; and k′_ISC, “reversed” intersystem crossing rate constant from a triplet state to a singlet state.

Scheme 19.. (A–B) Illustration of the Photocatalytic Cycles via (A) the Oxidative Quenching Pathway and (B) the Reductive Quenching Pathway and (C) Derivation of Redox Potentials from Gibbs Free Energies^a

^aNote: E⁰(B/A): the standard potential for the A → B half reaction of the A + C → B + D electron transfer reaction. E⁰(triplet): the triplet excitation energy in eV.

Scheme 20.

(A–B) Illustration of the Relationships among Activation/Deactivation Efficiencies, Redox Potentials, and Energy Levels of Frontier Molecular Orbitals for (A) the Oxidative Quenching Pathway and (B) the Reductive Quenching Pathway

Scheme 21.

Rational Design of a PC to Control the Performance of Photocontrolled Polymerization

Scheme 22.

A Few Examples of the PC-Initiator Pairs Designed with Better Performance in Photocontrolled Polymerization Systems to Date

Scheme 23.

Some Examples of Future Directions for Photocontrolled Polymerization