Chemiluminescence defines a chemical reaction which generates an electronically excited species that emits light. Very few substrates are only known to undergo such processes which find applications in the efficient detection of trace quantities of chemicals (e.g., luminol test for iron, nitric oxide test, DNA sequencing, Western blot). Many organisms utilize this phenomenon for signaling and communication. Since the seminal work by Albrecht in 1928, the chemiluminescence of 5-amino-2,3-dihydro-1,4-phthalazinedione, 1 (also so-called luminol) has been intensively investigated and widely used for forensic, environmental, and biomedical applications, including immunoassays (Western blot), the monitoring of metabolic pathways, and the detection of free radicals and trace amounts of reactive oxygen species. Chemiluminescence of 1 is even more effective in the presence of various metal ions and numerous applications combining 1 with metal ions, complexes, or nanoparticles, respectively, and oxidants have been realized. However, the detailed chemiluminescence mechanism and its modulation by substituents or the presence of various additives are still indefinable.

Fig. 1. Substituted 6,8-dialkyl luminol derivatives and CL enhancement of these substrates in the absence (open boxes) and presence (red boxes) of the catalyst/oxidant system.

Thus, studies of new luminol derivatives can provide structure–activity data which may promote a deeper understanding of the fundamental photochemical processes and their application to demanding analytical problems. Because of a lack of robust and modular syntheses, few reports on substituted luminol derivatives can be found in literature; however, the demonstration of significant chemiluminescence modulation induced by electron-rich substituents has fueled our tenancy into the assessment of new luminol derivatives containing alkyl substituents. Introduction of two substituents vicinal to the amino and one carboxamide group at C6 and C8 might influence to having a pronounced effect on the structure of the excited states. Simple alkyl substituents are stable and lack the presence of polar sites amenable to strong solvent interactions (Fig. 1). Such steric modulation can easily be applied to other photosystems and is complementary to electronic perturbations by heteroatomic electron donating group substituents or chromophore extensions by benzannulation.

Chemiluminescence of the luminol derivatives 2-8 (vs. parent luminol, 1) in the presence of metals as promotors and aqueous H₂O₂ as crucial oxidant has been investigated in detail. The results in this work show a surprising 20-fold chemiluminescence enhancement (Fig. 1) induced by simple alkyl substituents in positions 6 and 8 of the luminol skeleton (derivatives 2-5). The mechanistic rationalization of the unprecedented chemiluminescence improvement from these alkyl luminols has been investigated by theoretical calculations, being interpreted as a “steric gearing” effect on surface crossings (Fig. 2). This effect appears to be a direct consequence of vicinal steric gearing, improving the transition efficiency from an intermediary endoperoxide to the excited phthalate.

Fig. 2

This concept of a mostly steric modulation complements those involving strong electronic variations of the luminophore or the extension of the conjugation length. The application of such rather subtle structural manipulation of photoactive molecules holds great potential for the design and amplification of various photophysical processes and eventually chemiluminescence properties. This might give rise to improvements of the current luminescence tools efficiency for the detection of small molecules or imaging.

Publication

Steric Enhancement of the Chemiluminescence of Luminols.
Griesbeck AG, Díaz-Miara Y, Fichtler R, Jacobi von Wangelin A, Pérez-Ruiz R, Sampedro D
Chemistry. 2015 Jul 6

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