A Revisit to the Orthogonal Bodipy Dimers: Experimental Evidence for the Symmetry Breaking Charge Transfer-Induced Intersystem Crossing

Abstract

A series of Bodipy dimers with orthogonal conformation were prepared. The photophysical properties were studied with steady-state and time-resolved transient spectroscopies. We found the triplet-state quantum yield is highly dependent on the solvent polarity in the orthogonally linked symmetric Bodipy dimers, and the intersystem crossing (ISC) is efficient in solvents with moderate polarity. The photoinduced symmetry-breaking charge transfer (SBCT) in polar solvents was confirmed by femtosecond transient absorption spectroscopy, with the charge separation (CS) kinetics on the order of a few picoseconds and the charge recombination (CR) process occurring on the nanosecond time scale in dichloromethane. These observations are supported by the calculation of the charge separated state (CSS) energy levels, which are high in nonpolar solvents, and lower in polar solvents, thus the CR-induced ISC has the largest driven force in solvents with moderate polarity. These results clarify the mechanism of SOCT-ISC in the orthogonally symmetric Bodipy dimers. The acquired information, relating molecular structure and ISC property, will be useful for devising new strategies to induce ISC in heavy atom-free organic chromophores.

BODIPY‐Pyrene and Perylene Dyads as Heavy‐Atom‐Free Singlet Oxygen Sensitizers

“…Similarly, BODIPY dyads containing quinone, [15] methylpyridinium or acridinium ion [16] (structures 3 and 4, Fig. 2) subunits as electron donors and BODIPY dimers [17] demonstrated such properties. Recently, similar properties were observed for N,C-chelate organoboron dyes.…”

“…Data for BPyrD1: C 25 H 15 BF 2 N 2 (M =392.20 g/mol): monoclinic, space group P2 1 /c (no. 14), a = 17.082(17) Å, b = 8.600(12) Å, c = 13.632(14) Å, β = 112.854(15)°, V = 1845(4) Å3 , Z = 4, T = 100.01 K, μ(MoK α ) = 0.097 mm -1 , D calc = 1.412 g/cm3 , 19468 reflections measured (5.176° ≤ 2θ ≤ 59.246°), 5178 unique (R int = 0.0330, R sigma = 0.0328) which were used in all calculations. The final R 1 was 0.0413 (I > 2σ(I)) and wR 2 was 0.1142 (all data).Crystal Data for BPyrD3: C 29 H 23 BF 2 N 2 (M =448.30 g/mol): orthorhombic, space group P2 1 2 1 2 1 (no.…”

Bodipy Derivatives as Triplet Photosensitizers and the Related Intersystem Crossing Mechanisms

“…Later, it was proposed that charge transfers between the two units are involved, and that the charge recombination induced the ISC. This was based on an observation of the charge transfer state with the femtosecond transient absorption spectroscopy (Epelde-Elezcano et al, 2017;Liu et al, 2018). It was proposed that the spin-orbital charge-transfer ISC (SOCT-ISC) was responsible for the ISC.…”

“…1.70 eV) . It should be pointed out that some Bodipy dimers do not show any ISC (Liu et al, 2018), although the so-called symmetry-breaking charge transfer (SBCT) still occurs (Whited et al, 2012).…”

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

“…Although many BODIPY compounds were shown to exhibit PeT when combined with appropriate electron donors or acceptors, 20 observations of efficient formation of long-lived triplet states in such systems are scarce. Among those are BODIPYs with appended transition metal complexes, 21 dyads containing fullerene, 22 quinone 23 or methylpyridinium 24 as electron acceptors and BODIPY dimers. 25 For instance, recently reported BODIPY dyads bearing an extended acridinium cation as an electron donor showed triplet excited state yield of up to 15%.…”