Synthesis of Green Emitting Multi‐substituted Dibenzodioxins and Related Heteroacenes and Computational Investigation of Substituent Effects on Emission Spectra

Abstract

Push‐pull dibenzodioxins with electron withdrawing and donating groups were prepared in good yields through a short and simple synthesis. Strong green emission above 500 nm occurs in those derivatives where there is maximum charge transfer to the most electron deficit terephthalonitrile ring, from proximal cyclic amino donor groups. Theoretical calculations support experimental findings through evaluation of excited state properties. In molecules with a nitro group, the excited state localizes electron density exclusively onto it and twisted nitro geometry was also found. In effect, electronic charge cannot relocate into the molecular plane, rendering them non‐fluorescent. Also, studies on the fluorescent derivatives show that best emission would occur when the donor moiety contains a saturated cyclic amino ring and the amines are 2°. Overall, our study establishes structure‐property guidelines and limits on dibenzodioxin functionalization towards preparing fluorescent derivatives of the same.

Amino‐Terephthalonitrile‐Based Single Benzene Fluorophores with Large Stokes Shifts and Solvatochromic Behavior

“…[13] We synthesized an SBF containing a cyclic push-pull moiety (Figure 1, 8) which had strong green emission due to uniform excited state charge distribution into the electron poor dicyano subunits. [14] We also reported earlier that small heteroaromatic push-pull systems bearing nitro groups are non-emissive. Being a strong electron-withdrawing group, À NO 2 pulls all the charge towards itself and assumes a twisted geometry in the excited state.…”

“…This TICT state of nitro compounds have been reported by us previously for À NO 2 substituted dibenzodioxins and phenazines. [14] In fact, the nonradiative decay channel due to the twisted nitro group, quenching the fluorescence, has been reported by several theoretical and experimental groups [21b-h] in past, as well.…”

“…To that end, different examples were prepared using the well-known nucleophilic aromatic substitution reaction. [14] Scheme 1A shows the synthesis of compound 11 consisting of a dinitrobenzene as the acceptor and a single diethyl amino group as donor. Scheme 1B depicts the synthesis of SBF compound 13 consisting of a terephthalonitrile (TN) acceptor unit and one ethylamine moiety as the donor group.…”

“…This prevents uniform charge distribution in the molecule, thus rendering the system non-fluorescent. [14][15] However, there are few examples in the literature like the SBF system 7-nitrobenzo[c] [1,2,5]oxadiazol-4amine (NBD-amine) (Figure 1, 9) where the presence of nitro group does not have detrimental effects on the fluorescence emission. [16] This implies that there is a deeper interplay between conformation of molecular subunits and extent of intramolecular charge transfer in heteroacenes, which dictates the outcome of emissive behavior.…”

“…The amino-terephthalonitrile type SBFs (Figure 1, 5, 6, 8) were found to be strongly emissive by us and others. [12,14] We wished to further explore these types of structures and their acyclic variants, with a few crucial questions: 1) How would the absorption, emission and redox behavior be modulated upon changing the arrangement and numbers of different donor and acceptor groups in the system, attached to a single benzene unit? 2) Would the degree of substitution at the donor nitrogen atom influence the emissive property of the SBF?…”