5-Bromoindole: luminescence properties and its application

Introduction

Bromoindoles are compounds with high potential for applications in various domains, especially in the electrochemical industry as electrocatalysts, anode materials in batteries, anticorrosion coating, and fast response potentiometric sensors. These compounds have also attracted great attention in pharmacology, mainly because of their ability to develop antifungal and antibacterial agents, but also as candidates for anti-oxidant, direct oxidation/reduction of biomolecules, and other biological activities. Due to their presence in water and soil as persistent organic pollutants, these organobromine compounds have also been the focus of considerable research work to determine their toxicity effects on the environment [1].

The aromatic and heterocyclic properties of 5-bromoindole (Figure 1) determine its broad applicability and applicability in the field of fine chemical intermediates. The bromine atom at position 5 can be replaced by other active groups under specific conditions, exhibiting certain pharmacological activity and color characteristics. It has important application value in pharmaceutical intermediates, dyes, and other fields.[2]

Luminescence properties of 5-Bromoindole in PVA films

Chavez et al. studied spectral properties of 5-Bromoindole (5-BrI) in poly (vinyl alcohol) (PVA) film. The fluorescence of 5- BrI in PVA films is similar to indole in PVA film but strongly quenched compared to unsubstituted indole. The quantum yield of 5-BrI is only 0.0034 comparing to 0.26 of indole. In contrast, phosphorescence of 5-BrI in PVA is comparable to indole phosphorescence in magnitude and lifetime but its emission shifted towards longer wavelengths by 50 nm for a maximum at 530 nm. The phosphorescence of 5-BrI in PVA can be effectively excited at longer wavelengths (450–500 nm). The maximum of phosphorescence excitation spectrum of the 5-BrI in PVA is at about 480 nm. This direct excitation to the triplet state results in high positive phosphorescence anisotropy of 0.2. Although the S0 → T1 absorption transition is only about 800 fold weaker from the main 280 nm UV absorption band yielding measurable trace of absorption for 5-Bromoindole in PVA film in the visible/blue range. We believe this is a first report of singlet-triplet absorption (S0 → T1) at room temperature, not involving external heavy atoms or oxygen presence. The comparison of phosphorescence of 5-Bromoindole in PVA film to Rhodamine 6G in ethanol fluorescence allows us to estimate the phosphorescence quantum yield of 5-Bromoindole in PVA to be ~0.02. They measured independently fluorescence and phosphorescence lifetimes and quantum yields as well as phosphorescence excitation spectrum across all wavelengths from UV to the visible range. These measurements were used to determine rate constants for all processes involved the in 5-Bromoindole in PVA excited state deactivation.[3]

Electrosyntheses and Characterization of Poly(5-bromoindole)

The electropolymerization of 5-bromoindole was more difficult in neutral solvent because of the electron-withdrawing group substitution (bromine) on indole ring when comparedwith that of indole. Moreover, there were very little reports on the electropolymerization of 5-bromoindole. The interest in this monomer was due to its further modification by other functional groups, through the reactions of bromine atom on the 5-position of indole. Therefore, it is very necessary to investigate the electrochemical polymerization of 5-bromoindole in BFEE.

In this paper, high quality poly(5-bromoindole) (PBrI) films can be easily prepared by anodic oxidation of 5-bromoindole monomer directly in pure BFEE. The electrochemical properties, morphology, and thermal stability of as-prepared poly(5-bromoindole) films were studied. Its structure and polymerization mechanism were determined by FTIR, 1 H-NMR and UV-visible and fluorescent spectra. The oxidation potential of 5-bromoindole in this medium was measured to be only 0.97 V vs. saturated calomel electrode, which was lower than that determined in acetonitrile 0.1 mol/L Bu4NBF4 (1.08 V). PBrI films obtained from this medium showed good electrochemical behavior and good thermal stability. Structural studies showed that the polymerization of 5-bromoindole ring occurred at 2,3 position. As-formed poly(5-bromoindole) films were thoroughly soluble in strong polar solvent dimethylsulfoxide and partly soluble in tetrahydrofuran. Fluorescent spectral studies indicated that poly(5-bromoindole) was a good blue-light emitter. The excitation and emission spectra of poly(5-bromoindole) showed a significant shift to longer wavelength compared with that of the monomer, consistent with the greater extent of electron delocalization.[4]

References

[1] MANUEL AV. RIBEIRO DA SILVA, JOANA ITA. CABRAL. Experimental thermochemical study of 5-bromoindole and 5-bromoindoline[J]. The Journal of Chemical Thermodynamics,2009,41(1):84-89.

[2] SHU Y Z. Recent natural products based drug development: A pharmaceutical industry perspective[J]. Psychiatric Services, 1998, 61(8): 1053-1071.

[3] CHAVEZ J, KIMBALL J, CERESA L, et al. Luminescence properties of 5-Bromoindole in PVA films at room temperature: Direct triplet state excitation[J]. Journal of Luminescence: An Interdisciplinary Journal of Research on Excited State Processes in Condensed Matter,2021,230. 

[4] XU JK, HOU J, PU SZ, et al. Electrosyntheses and characterization of poly(5-bromoindole) in boron trifluoride diethyl etherate[J]. Journal of Applied Polymer Science,2006,101(1):539-547.

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