Controlling the S₁ Energy Profile by Tuning Excited-State Aromaticity

Abstract

The shape of the lowest singlet excited-state (S1) energy profile is of primary importance in photochemistry and related materials science areas. Here we demonstrate a new approach for controlling the shape of the S1 energy profile which relies on tuning the level of excited-state aromaticity (ESA). In a series of fluorescent π-expanded oxepins, the energy decrease accompanying the bent-to-planar conformational change in S1 becomes less pronounced with lower ESA levels. Stabilization energies following from ESA were quantitatively estimated to be 10–20 kcal/mol using photophysical data. Very fast planarization dynamics in S1 was revealed by time-resolved fluorescence spectroscopy. The time constants were estimated to be shorter than 1 ps, regardless of molecular size and level of ESA, indicating barrierless S1 planarization within the oxepin series.

Vibratile Dihydrophenazines with Controllable Luminescence Enabled by Precise Regulation of π-Conjugated Wings

“…In addition, the emission spectra of all these compounds in frozen toluene at 77 K (Figure S7 These values imply that the diminished energy changes of the S 1 states during the planarization process may arise from the decreasing excited state aromaticity (ESA). These results are consistent with those of the recently reported oxepins, 34 which show the inherent relationships between the energy and aromaticity of the S 1 states.…”

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

“…The development of design rules that allow for the identication and synthesis of new singlet ssion chromophores is therefore arguably one of the most important challenges in functional organic materials research today. There have been several proposed design rules including the use of diradicaloid systems, [3][4][5] the assessment of the diradicaloid character of a chromophore, 6,7 and the manipulation of aromaticity to engineer the excited states, [8][9][10][11][12] which has led to a diverse range of structures in the hunt for improved singlet ssion materials. [13][14][15] However, the fact that linear acenes remain the most successful chromophore for use in singlet ssion photovoltaics, despite their well-documented instability, 16,17 demonstrates that more work must be done to understand the underlying design principles.…”

Functional Group Introduction and Aromatic Unit Variation in a Set of π-Conjugated Macrocycles: Revealing the Central Role of Local and Global Aromaticity

“…In this section, we consider excitedstate energies and shielding tensors to examine if these Stokes shifts can be seen as a consequence of excited-state aromaticity (see also Ref. 25 ). For this discussion, one would ideally compute shielding tensors for the S1 state at the relaxed S1 geometry.…”