Bond orders in metalloporphyrins

Estimation of Singlet Oxygen Quantum Yield Using Novel Green-Absorbing Baird-type Aromatic Photosensitizers†

We report two new organic green-absorbing singlet oxygen (¹ O₂ ) photosensitizers: Quinoidal naphthyl thioamide (QDM) and bis-iodol-dipyrrolonaphthyridine-dione (I₂ -DPND), with triplet energies of 40.8 and 47.5 kcal mol^-1 (at 77 K in a glassy matrix) , respectively. The UV-vis absorption and emission characteristics of QDM and I₂ -DPND are similar to other commercially available organic ¹ O₂ photosensitizers such as Rose Bengal, which was used as standard/reference to estimate the ¹ O₂ quantum yield (Φ_∆ ) of the chromophores under study. Using 9,10-diphenylanthracene (DPA) as an ¹ O₂ quencher, we estimated the Φ_∆ ≈ 67-85% for QDM and Φ_∆ ≈ 25-32% for I₂ -DPND. The discrepancy in the Φ_∆ values could be explained by the apparent photo-decomposition of the later dye. Nevertheless, the high Φ_∆ value for QDM is unprecedented, as this chromophore exhibits relatively low structural complexity and could further be derivatized to create novel photodynamic agents.

Excited state character of Cibalackrot-type compounds interpreted in terms of Hückel-aromaticity: a rationale for singlet fission chromophore design

The exact energies of the lowest singlet and triplet excited states in organic chromophores are crucial to their performance in optoelectronic devices. The possibility of utilizing singlet fission to enhance the performance of photovoltaic devices has resulted in a wide demand for tuneable, stable organic chromophores with wide S1-T1 energy gaps (>1 eV). Cibalackrot-type compounds were recently considered to have favorably positioned excited state energies for singlet fission, and they were found to have a degree of aromaticity in the lowest triplet excited state (T1). This work reports on a revised and deepened theoretical analysis taking into account the excited state Hückel-aromatic (instead of Baird-aromatic) as well as diradical characters, with the aim to design new organic chromophores based on this scaffold in a rational way starting from qualitative theory. We demonstrate that the substituent strategy can effectively adjust the spin distribution on the chromophore and thereby manipulate the excited state energy levels. Additionally, the improved understanding of the aromatic characters enables us to demonstrate a feasible design strategy to vary the excited state energy levels by tuning the number and nature of Hückel-aromatic units in the excited state. Finally, our study elucidates the complications and pitfalls of the excited state aromaticity and antiaromaticity concepts, highlighting that quantitative results from quantum chemical calculations of various aromaticity indices must be linked with qualitative theoretical analysis of the character of the excited states.