Effect of the iodine atom position on the phosphorescence of BODIPY derivatives: a combined computational and experimental study

Abstract

A new BODIPY derivative (o-I-BDP) containing an iodine atom in the ortho position of the meso-linked phenyl group was prepared. Photophysical and electrochemical properties of the molecule were compared to previously reported iodo BODIPY derivatives, as well as to the non-iodinated analog. While in the case of derivatives featuring iodine substituents in the BODIPY core, efficient population of the triplet state is accompanied by a substantial positive shift of the reduction potential compared to pristine BODIPY, o-I-BDP displays phosphorescence and simultaneously maintains the electrochemical properties of unsubstituted BODIPYs. A theoretical investigation was settled to analyze results and rationalize the influence of iodine position on electronic and photophysical properties, with the purpose of preparing a fully organic phosphorescent BODIPY derivative. TD-DFT and spin-orbit coupling calculations shed light on the subtle effects played by the introduction of iodine atom in different positions of BODIPY.

New Insights into the Exploitation of BODIPY Derivatives as Organic Photocatalysts

“…As expected, the photochemical properties of the BODIPY derivatives are strongly related to their chemical structures. [4,18,23,24] In particular, we found that the presence of halogen atoms (I or Cl) at the pyrrolic positions has a major impact on the main photocatalytic features. For instance, comparing the absorption and emission maxima of PC5 and PC1, we observed a red-shift when the halogen's atomic number is higher, whereas the molar absorption coefficients (ɛ) at the absorption maxima are only slightly influenced by the nature of the halogen.…”

“…This trend might be due to the fact that a more electron rich meso-substituent on the BODIPY skeleton could enhance the stability of the corresponding excited state, thus increasing its ability to undergo photoreduction with sodium ascorbate. [4,18,27] Lastly, we investigated the effect of the light source on the reactivity. To this end, we recorded the kinetic profiles of the reaction using PC1 as chromophore and different Kessil lamps (427 nm, 440 nm, 456 nm, 525 nm) as sources of photons (see SI).…”

“…On the other hand, we observed that PC4 , which contains a strong electron‐donating group (EDG, namely ‐OMe), shows an initial photocatalytic activity similar to PC1 , which are both much higher than those of PC3 . This trend might be due to the fact that a more electron rich meso ‐substituent on the BODIPY skeleton could enhance the stability of the corresponding excited state, thus increasing its ability to undergo photo‐reduction with sodium ascorbate [4,18,27] . Lastly, we investigated the effect of the light source on the reactivity.…”

“…The presence of the iodine atoms on the BODIPY skeleton, by means of the heavy atom effect, facilitates the intersystem crossing which drives the conversion of the singlet into the triplet excited state of PC1. [4,[17][18][19][20] The BODIPY photocatalyst is then reduced to the corresponding radical anion by using sodium ascorbate (NaAsc) as electron donor. This electron-rich intermediate induces the reductive cleavage of the CÀ Br bond in 2 a by single electron transfer (SET), which affords the electrophilic radical I a.…”

“…From a mechanistic point of view, the BODIPY‐based photocatalyst directly reaches an electronically singlet excited state upon light absorption (Figure 1a). The presence of the iodine atoms on the BODIPY skeleton, by means of the heavy atom effect, facilitates the intersystem crossing which drives the conversion of the singlet into the triplet excited state of PC1 [4,17–20] …”