Control of triplet state generation in heavy atom-free BODIPY–anthracene dyads by media polarity and structural factors

Abstract

A family of heavy atom-free BODIPY-anthracene dyads (BADs) exhibiting triplet excited state formation from charge-transfer states is reported. Four types of BODIPY scaffolds, different in the alkyl substitution pattern, and four anthracene derivatives have been used to access BADs. Fluorescence and intersystem crossing (ISC) in these dyads depend on donor-acceptor couplings and can be accurately controlled by substitution or media polarity. Under conditions that do not allow charge transfer (CT), the dyads exhibit fluorescence with high quantum yields. Formation of charge-transfer states triggers ISC and the formation of long-lived triplet excited states in the dyads. The excited state properties were studied by steady-state techniques and ultrafast pump-probe spectroscopy to determine the parameters of the observed processes. Structural information for various BADs was derived from single crystal X-ray structure determinations alongside DFT molecular geometry optimization, revealing the effects of mutual orientation of subunits on the photophysical properties. The calculations showed that alkyl substituents on the BODIPY destabilize CT states in the dyads, thus controlling the charge transfer between the subunits. The effect of the dyad structure on the ISC efficiency was considered at the M06-2X level of theory, and a correlation between mutual orientation of the subunits and the energy gap between singlet and triplet CT states was studied using a multireference CASSCF method.

Bodipy Derivatives as Triplet Photosensitizers and the Related Intersystem Crossing Mechanisms

“…The same researchers prepared a series of analog Bodipy-derived electron donor/acceptor dyads. The dyads generally showed satisfactory SOCT-ISC (Filatov et al, 2018). Zhang also prepared Bodipy-based electron donor/acceptor dyads, and the SOCT-ISC was observed (Zhang and Feng, 2017;Hu et al, 2019).…”

Efficient Intersystem Crossing in the Tröger's Base Derived From 4‐Amino‐1,8‐naphthalimide and Application as a Potent Photodynamic Therapy Reagent

“…[21] The 1,7methyl groups on the boron dipyrromethene (Bodipy) moiety play as imilar role in Bodipy-derived electron donor/acceptor dyads showingS OCT-ISC. [22][23][24][25][26] We found that the presence of peri-H(C) of electron donor phenothiazine is helpful for conformation restriction. [24,27,28] However,i na ll these molecular structural motifs, rotation about the CÀCo rt he CÀNl inker is not fully restricted, and various conformationsa re still accessible with thermale nergy.D eviation from the orthogonal geometry may be detrimental to the SOCT-ISC efficiency.T hus, fully rigid linkers are desired for linking the electron donora nd acceptor in these dyads, but this kind of linker is difficult to obtain from the point of view of synthesis.…”

Heavy-atom-free BODIPY photosensitizers with intersystem crossing mediated by intramolecular photoinduced electron transfer

“…Phenylene-separated BODIPY-anthracene and pyrene dyads 54 and 55 were reported to undergo PET in polar solvents, however both exhibit very low singlet oxygen quantum yields ( ≈ 0.01), evidencing low efficiency of the triplet state formation. 59,64 The dihedral angle between the subunits in 54 is approximately 53°, suggesting that SOCT-ISC is not operative in this system. Majima and co-workers described structurally similar BODIPYanthracene dyads 56a-b, which do not exhibit PET in dimethyl sulfoxide and methanol.…”