Computational Investigation on the Photophysical Properties of Halogenated Tetraphenyl BODIPY

Abstract

The electronic structure, transition probabilities and corresponding quantum yields of fluorescence in a family of dihalogen-tetraphenyl-aza-BODIPY were calculated at the Time-Dependent Density Functional and post-Hartree-Fock levels of theory. Excellent agreement between theoretical and experimental spectral-luminescent data was achieved with the HSE06 functional and the 6-311G* basis set. Since the fluorescence can be quenched through nonradiative intersystem spin crossing transitions from the lowest photoactive singlet state to triplet excited states, spin-orbit coupling matrix elements were calculated and applied along with Marcus-Levich-Jortner theory, leading to satisfactory agreement for the lifetimes in comparison with available experimental data. The anomalous dependence of the fluorescence efficiency on the atomic number of the halogen congeners was elucidated and shown to be due to an inversion between the fluorescent and the nearest triplet states in the iodinated compounds. The high rate of fluorescence quenching by intersystem crossings and the probability of collisions in a solvent between oxygen molecules and the molecules studied, shows that these molecules can provide efficient triplet sensitization. The most preferable sites for such interactions were predicted using electrostatic potential mapping at the extreme positive and negative charge points.

First-principles calculations of anharmonic and deuteration effects on the photophysical properties of polyacenes and porphyrinoids

“…Calculating the ISC k constant is generally not a very difficult task. The ISC k value can even be estimated using the Marcus's theory [30,31]. However, calculating IC k is more difficult, since the non-adiabatic coupling interaction matrix element (NACME) between the electronic states of the same spin multiplicity is needed.…”

Stability of Dibromo-Dipyrromethene Complexes Coordinated with B, Zn, and Cd in Solutions of Various Acidities

“…A high accuracy of the calculated spectral-luminescence properties of the isolated halogen-dpm compounds coordinated with BF 2 , Zn, and Cd was achieved in our previous theoretical description and the explanation of the observed photo-induced processes [24,29]. The same TD-DFT settings for optimization in both the ground and fluorescence states, as well as for calculating the long wavelength regions of absorption and emission spectra of the brominated compounds in neutral and HCl acidified forms in the ethanol solvent model were used in the present research (Table 2).…”

“…The next higher bright state is nearly the same for both complexes (508, 507 nm), while Br 2 (CH 3 ) 4 BODIPY exhibits an intensive first band at 528 nm, which determines high fluorescence efficiency vs. the Zn and Cd complexes. The two lowest states show the same symmetry of ED localization on both fragments, forming a dissipative channel of internal conversion between the bright and dark states preferable over emission, leading to very low quantum harvest of fluorescence from the higher state (Figure 4 and [29]). On the other hand, despite the small oscillator strength of the dark state, it contributes an intensity to the emission spectra such that the spectral feature corresponds to the observed very weak emission intensities of the Zn and Cd systems (Figures 2 and 3), with the peak at 515 and 508, respectively, as well as the small hill near 550 nm for both compounds, recognizable on the spectral manifolds.…”

“…The mechanism of losing emission intensity is simpler for the BF 2 -based compounds, which exhibit very efficiency fluorescence, but halogenation, particularly with Br, quenches the process due to increasing nonradiative intersystem spin crossing from the lowest excited photoactive singlet to the triplet system [29]. The calculated longwave absorptions for all the compounds in the neutral solvent are blueshifted about ∆λ abs = 40 nm relative to the measured spectra (Table 1), which is an acceptable accuracy in comparison with the previous TD-DFT results, as previously discussed [24,29].…”

“…Such a configuration allows the ESP minima to be equivalently located only near the halogenated ends for all bromines (−0.59 eV) of both M[Br 2 (CH 3 ) 4 dpm] 2 in the ground state because of the nearly C 2v symmetry of the compounds (Figures 4, S1 and S2, Tables S2-S7). The symmetry is distorted significantly in the excited state [29], providing different ED's on the dpm ends and the nearby ESP minima. The deepest minimum (−0.84 eV and −0.82 eV) was chosen to study the HCl acidification of the excited Zn and Cd complexes, respectively, and one of the minima in ground state.…”