The triplet excited state of Bodipy: formation, modulation and application

Abstract

Boron dipyrromethene (Bodipy) is one of the most extensively investigated organic chromophores. Most of the investigations are focused on the singlet excited state of Bodipy, such as fluorescence. In stark contrast, the study of the triplet excited state of Bodipy is limited, but it is an emerging area, since the triplet state of Bodipy is tremendously important for several areas, such as the fundamental photochemistry study, photodynamic therapy (PDT), photocatalysis and triplet-triplet annihilation (TTA) upconversion. The recent developments in the study of the production, modulation and application of the triplet excited state of Bodipy are discussed in this review article. The formation of the triplet state of Bodipy upon photoexcitation, via the well known approach such as the heavy atom effect (including I, Br, Ru, Ir, etc.), and the new methods, such as using a spin converter (e.g. C60), charge recombination, exciton coupling and the doubly substituted excited state, are summarized. All the Bodipy-based triplet photosensitizers show strong absorption of visible or near IR light and the long-lived triplet excited state, which are important for the application of the triplet excited state in PDT or photocatalysis. Moreover, the methods for switching (or modulation) of the triplet excited state of Bodipy were discussed, such as those based on the photo-induced electron transfer (PET), by controlling the competing Förster-resonance-energy-transfer (FRET), or the intermolecular charge transfer (ICT). Controlling the triplet excited state will give functional molecules such as activatable PDT reagents or molecular devices. It is worth noting that switching of the singlet excited state and the triplet state of Bodipy may follow different principles. Application of the triplet excited state of Bodipy in PDT, hydrogen (H2) production, photoredox catalytic organic reactions and TTA upconversion were discussed. The challenges and the opportunities in these areas were briefly discussed.

Visible‐Light Photoactive, Highly Efficient Triplet Sensitizers Based on Iodinated Aza‐BODIPYs: Synthesis, Photophysics and Redox Properties

“…[16,17] An interesting group of organic chromophores that can be utilized ast riplet photosensitizers are aza-BODIPYs (aza-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene). [18,19] Structurally,a za-BODIPY dyes are derivatives of BODIPY (4,4-difluoro-4-bora3a,4a-diaza-s-indacene) dyes obtained by replacing the mesocarbona tom that connects both pyrrole rings with an azabridge. [20][21][22][23][24][25][26][27][28][29] The consequence of the introduction of an azabridge into the structure of the BODIPY chromophores ystem is the bathochromic shift of both absorption and emission bands of around 80 nm.…”

Photophysics, Redox Processes, and Electronic Structures of Ferrocenyl-Containing BODIPYs, aza-BODIPYs, BOPHYs, Transition-Metal Dipyrromethenes and aza-Dipyrromethenes

“…The energies of the most intense low-energy bands in BODIPYs and their analogues can be easily tuned by electron-donating or electronwithdrawing substituents located at α-or β-positions of the BODIPY core as it correlatesto a large extent the HOMO-LUMO energy gap. [39][40][41][42] As discussed above, addition of the ferrocenyl substituents onto the BODIPY core introduces ferrocene-centered molecular orbitals (MOs) to the HOMO-LUMO energy region. Depending on the nature of the organic π-system, the energies of such ferrocene-centered MOs can be higher or lower than the BODIPY-centered occupied π-orbitals.…”

“…[39][40][41][42] The most intense, low-energy transition is these chromophores is typically dominated by the HOMO→LUMO single-electron excitation and has clear π-π* character. The energies of the most intense low-energy bands in BODIPYs and their analogues can be easily tuned by electron-donating or electronwithdrawing substituents located at α-or β-positions of the BODIPY core as it correlatesto a large extent the HOMO-LUMO energy gap.…”

“…Although "all organic" BODIPYs and aza-BODIPYs are known for a long time, their organometallic analogues emerged in this research field only recently. [39][40][41][42] Synthetic pathways for preparation and some selected properties of ferrocenyl-containing BODIPYs and their analogues were recently reviewed, [1,2] while systematic analysis of optical, photophysical, and redox properties as well as their electronic structures have not being targeted so far. In this mini-review, we will provide a systematic analysis of these properties in ferrocenyl-containing BODIPYs, aza-BODIPYs, BOPHYs, and transition-metal complexes of dypyrromethenes and aza-dipyrromethenes (Chart 1).…”

A theoretical and experimental NMR study of BODIPY 493/503: difluoro{2‐[1‐(3,5‐dimethyl‐2H‐pyrrol‐2‐ylidene‐N)ethyl]‐3,5‐dimethyl‐1H‐pyrrolato‐N}boron

“…Compound 1 is bright, green fluorescent dye with similar excitation and emission to fluorescein with several uses in biochemistry, for staining lipids, membranes and other lipophilic compounds, because they exhibit narrow emission bandwidths, they have high extinction coefficient and fluorescence quantum yields and they are photostable. [1] Other applications of these dyes are related to fluorescent probes, [2][3][4] deep-red lighting sources, [5] bioactive CO-releasing molecules [6] and solar cells, [7] amongst many others. [8][9][10] Concerning BODIPY NMR properties, there are several compounds with reported data (Scheme 1) especially related to [11] ; 4, 1 J 11B19F = 33.4 Hz [12] ; 5, 11 B = 0.28, 1 J 11B19F = 28.7 Hz [13] ; 6, 11 B = 0.…”