The Molecular Basis of Organic Chemiluminescence

Abstract

Bioluminescence (BL) and chemiluminescence (CL) are interesting and intriguing phenomena that involve the emission of visible light as a consequence of chemical reactions. The mechanistic basis of BL and CL has been investigated in detail since the 1960s, when the synthesis of several models of cyclic peroxides enabled mechanistic studies on the CL transformations, which led to the formulation of general chemiexcitation mechanisms operating in BL and CL. This review describes these general chemiexcitation mechanisms-the unimolecular decomposition of cyclic peroxides and peroxide decomposition catalyzed by electron/charge transfer from an external (intermolecular) or an internal (intramolecular) electron donor-and discusses recent insights from experimental and theoretical investigation. Additionally, some recent representative examples of chemiluminescence assays are given.

Keywords: 1,2-dioxetanes; 1,2-dioxetanones; bioluminescence; chemiluminescence; cyclic peroxides; sensors; ultrasensitive detection.

Scheme 1

Mechanisms in Organic Chemiluminescence. (a) General mechanism. (b) Direct chemiluminescence transformation. (c) Indirect CL in the presence of a fluorescent/phosphorescent energy acceptor. (d) Activated CL in the presence of an oxidizable fluorescent dye. R—reagents; P—products; HEI—high-energy intermediate; P*—excited state product; A—electronic energy acceptor; A*—excited state acceptor; ACT—activator; ACT*—excited state activator; HEI-ACT—activator linked to high-energy intermediate; P-ACT—ACT linked to the product; P-ACT*—excited state ACT linked to the product.

Scheme 2

Structures of important high-energy intermediates participating in chemiluminescent reactions. The R groups are alkyl, aryl, and, eventually, H.

Scheme 3

Reaction archetypes in organic chemiluminescence. (a) Thermal unimolecular decomposition of 1,2-dioxetanes and 1,2-dioxetanones (intramolecular, inefficient). (b) Activated/catalyzed peroxide decomposition of 1,2-dioxetanones (intermolecular, inefficient). (c) Peroxyoxalate reaction, formation of 1,2-dioxetanedione as the HEI (intermolecular, efficient). (d) Decomposition of electron-rich aryl-substituted 1,2-dioxetanes (intramolecular, efficient). The different types of peroxides are indicated by different colors; 1,2-dioxetane (blue); 1,2-dioxetanone (orange); 1,2-dioxetanone (red).

Scheme 4

Unimolecular decomposition of 1,2-dioxetanes. Isothermal decomposition kinetics are followed by emission intensity decay and determination of singlet- and triplet-quantum yields using energy transfer to 9,10-diphenylanthracene (DPA) and 9,10-dibromoanthracene (DBA), respectively. Φ_Fl: fluorescence-emission quantum yield; Φ_Fl^DPA: DPA fluorescence-emission quantum yield; Φ_Fl^DBA: DBA fluorescence-emission quantum yield; Φ_S-S^ET: singlet-singlet energy transfer quantum yield; Φ_S-T^ET: singlet-triplet energy transfer quantum yield; hν^dir: direct CL emission; hν^DPA: DPA sensitized CL emission; hν^DBA: DBA sensitized CL emission.

Scheme 5

Mechanistic models in thermal (uncatalyzed, unimolecular) 1,2-dioxetane and 1,2-dioxetanone decomposition. The different types of peroxides are indicated by different colors; 1,2-dioxetane (blue); 1,2-dioxetanone (orange).

Scheme 6

Chemically Initiated Electron Exchange Luminescence (CIEEL) mechanism in the catalyzed decomposition of 1,2-dioxetanone in the presence of an adequate activator (ACT) with low oxidation potential and high fluorescence quantum yields.

Scheme 7

Mechanism of the induced decomposition of phenoxy-substituted 1,2-dioxetanes. R₁, R₂: alkyl; R₁-R₂: spiro-adamantyl; X: -OCH₃, H; Y: protection group; k_DP: deprotection rate constant; k_ET: electron-transfer rate constant; k_CLEAV: C-C bond cleavage rate constant; k_EBT^INTRA: intramolecular electron back-transfer rate constant; k_EBT^INTER: intermolecular electron back-transfer rate constant; k_ESC: radical pair solvent-cage escape rate constant; k_Fl: fluorescence emission rate constant.

Scheme 8

Chemiluminescence probe for the detection of β-galactosidase. Adapted with permission from Ref. [126]. Copyright 2017, American Chemical Society. The * indicates that the compound is formed in its singlet-excited state.

Scheme 9

(A) The general chemiluminescence activation pathway of probes allows for detecting different analytes. (B) Structures of the chemiluminescence probes and names of detected analytes Adapted with permission from reference [130]. Copyright 2021, Chinese Chemical Society. The * indicates that the compound is formed in its singlet-excited state.

Scheme 10

Proposed mechanism of the chemiluminescence activation pathway of dioxetane−fluorophore conjugate. Adapted with permission from ref [113]. Copyright 2022, American Chemical Society. Ref [114]. Copyright 2020, American Chemical Society and ref. [132], John Wiley & Sons. The * indicates that the compound is formed in its singlet-excited state.

Scheme 11

The chemiluminescence reagent PNCL for ONOO^– detection. Adapted with permission from ref. [134]. Copyright 2018, The Royal Society of Chemistry. The * indicates that the compound is formed in its singlet-excited state.

Scheme 12

Reactions for the chemiluminescence emission of HNOCL-1. Adapted with permission from ref. [135]. Copyright 2019, John Wiley & Sons.

Scheme 13

The structure and sensing reaction for the HyCL-4-AM probe. Adopted with permission from ref. [136]. Copyright 2019, American Chemical Society.

Scheme 14

The peroxyoxalate reaction for H₂O₂ quantification in the exhaled breath condensate of asthma patients. Adopted with permission from ref. [137]. Copyright 2020, American Chemical Society.

Scheme 15

The reaction of the arylboronate-substituted 1,2-dioxetane probe for chemiluminescent H₂O₂ quantification. Adopted with permission from ref. [138]. Copyright 2020, Elsevier. The * indicates that the compound is formed in its singlet-excited state.