A Naphtho-p-quinodimethane Exhibiting Baird’s (Anti)Aromaticity, Broken Symmetry, and Attractive Photoluminescence

Abstract

We report a novel reductive desulfurization reaction involving π-acidic naphthalene diimides (NDI) 1 using thionating agents such as Lawesson's reagent. Along with the expected thionated NDI derivatives 2-6, new heterocyclic naphtho-p-quinodimethane compounds 7 depicting broken/reduced symmetry were successfully isolated and fully characterized. Empirical studies and theoretical modeling suggest that 7 was formed via a six-membered ring oxathiaphosphenine intermediate rather than the usual four-membered ring oxathiaphosphetane of 2-6. Aside from the reduced symmetry in 7 as confirmed by single-crystal XRD analysis, we established that the ground state UV-vis absorption of 7 is red-shifted in comparison to the parent NDI 1. This result was expected in the case of thionated polycyclic diimides. However, unusual low energy transitions originate from Baird 4nπ aromaticity of compounds 7 in lieu of the intrinsic Hückel (4n + 2)π aromaticity as encountered in NDI 1. Moreover, complementary theoretical modeling results also corroborate this change in aromaticity of 7. Consequently, photophysical investigations show that, compared to parent NDI 1, 7 can easily access and emit from its T state with a phosphorescence (7a)* lifetime of τ = 395 μs at 77 K indicative of the formation of the corresponding "aromatic triplet" species according to the Baird's rule of aromaticity.

Quinoidization of π‐Expanded Aromatic Diimides: Photophysics, Aromaticity, and Stability of the Novel Quinoidal Acenes

“…For the reaction depicted in Scheme , we hypothesize that the reaction pathway leading to QDM 2 (and 3 ) could also produce the thermodynamic regio‐isomers, where the thiocarbonyl groups are outside the fjord of the π‐expansion: a comprehensive mechanistic rationale is depicted in the Supporting Information (Figure S18). Nonetheless, on the basis of our previously reported mechanistic rationale (for QDM 1 ) and additional computational data, it was found that the pathway to form the doubly de‐aromatized phosphinine intermediate(s) that would lead to the regio‐isomers of 2 (and 3 ) is kinetically not favored.…”

“…We recently demonstrated that the fusion of a p ‐QDM unit to a phenylene ring can generate a new naphthoquinodimethane chromophore 1 (Figure ) which exhibits unusual ground and excited state aromaticity and photophysical properties . Our previously reported quinoidal chromophore can also be employed as a light‐harvesting triplet sensitizer for photonic research such as triplet–triplet annihilation photon upconversion .…”

Prediction of Spin Density, Baird‐Antiaromaticity, and Singlet–Triplet Energy Gap in Triplet‐State Polybenzenoid Systems from Simple Structural Motifs

“…For example, several reports demonstrate that aromatic and antiaromatic character drive reactivity in excited states and therefore can be used to describe and predict photochemical reactions [24–30] . Additionally, new functional materials have been designed by using the concept of excited‐state aromaticity and antiaromaticity as a strategy to modulate the singlet–triplet (S–T) energy gap, [20, 31, 32] which is relevant to organic electronics, [32–34] photoswitching, [35, 36] photoluminescence, [37] and singlet‐fission [33, 38–40] capable materials. One possible avenue to modulate the S–T gap is through the use of compounds with “adaptive aromaticity”, which are aromatic in both the ground state and the excited state, such as metallopentalenes [41, 42] or aromatic chameleons [43–45] …”

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

“…We had previously employed QDM as a Baird‐type heavy‐atom‐free aromatic triplet chromophore to sensitize polyaromatic hydrocarbons and achieved triplet–triplet annihilation based photon upconversion (TTA‐UC) (16–18). Dipyrrolonaphthyridine‐dione ( DPND ), the precursor of I 2 – DPND, was first reported by Gryko and Kozankiewicz (19) and later utilized by Wang, Zhu and Fu to achieve singlet fission (SF) in the solid state (20).…”