Weak Donor-/Strong Acceptor-Linked Anthracenyl π-Conjugates as Solvato(fluoro)chromophore and AEEgens: Contrast between Nitro and Cyano Functionality

Abstract

Steady development on photophysical behaviors for a variety of organic fluorophores inspired us to generate anthracene-based fluorescent molecules with a strong acceptor and a significantly weak donor through a π-spacer. Such molecules are found to have substantial twisted conformational orientations in the solid state and enhanced apolar character because of the attachment of tolyl or mesityl group with an anthracenyl core. Upon exposure to a variety of solvents, strong solvatochromism was noticed for 4-nitro compound (84 nm red shift) in contrast to the cyano analogue (18 nm red shift). Both these probes were highly emissive in apolar solvents while nitro-analogue, in particular, could discriminate the solvents of E T (30) (a measure of microscopic solvent polarity) ranging from 31 to 37. Thus, 4-nitro compounds can be successfully employed to detect and differentiate the apolar solvents. On the contrary, the 2-nitro analogue is almost nonemissive for the same range of solvents perhaps because of favorable excited-state intramolecular proton-transfer process. The fundamental understanding of solvatochromic properties through the formation of twisted intramolecular charge-transfer (TICT) state is experimentally analyzed by synthesizing and studying the π-conjugates linked to only benzene in place of nitro or cyanobenzene, which exhibits no solvatochromism and that helped finding the possible emission, originated from the locally excited state. Moreover, the molecular structures for these compounds are determined by the single-crystal X-ray diffraction studies to examine the change in emission properties with molecular packing and alignment in the aggregated state. The measurement of dihedral angles between the substituents and anthracenyl core was helpful in finding the possible extent of electronic conjugations within the system to decipher both solvatochromism and aggregation enhanced emission (AEE)-behavior. The cyano analogue exhibited prominent AEE-behavior, whereas nitro analogues showed the aggregation-caused quenching effect. The reason behind such dissimilarity in solvatochromism and AEE-behavior between cyano- and nitro-linked anthracenyl π-conjugates are also addressed through experimental outcomes.

Carbazole-Anthranyl π-Conjugates as Small and Stable Aggregation-Induced Emission-Active Fluorogens: Serving as a Reusable and Efficient Platform for Anticounterfeiting Applications with an Acid Key and Multicolor Ink for a Quill Pen

“…The absorption maxima are found at ∼400 nm for both 3a and 3b due to π–π* transitions (Figure S1: for all absorption and emission spectra of 3a and 3b ), and emission maxima are identified around ∼530 nm in tetrahydrofuran (THF) and acetonitrile (MeCN, Figure 1). However, there is not much a significant difference in the absorption spectra by changing the solvent polarity as expected, 19 and the solvatofluorochromic behavior was observed for these compounds (a 23 nm shift for 3a and 51 nm shift for 3b in emission λ max , Figure 1) by varying the solvent polarity of hexane to MeOH.…”

A Facile Molecular Machine: Optically Triggered Counterion Migration by Charge Transfer of Linear Donor‐π‐Acceptor Phosphonium Fluorophores

“…This observation reflects the higher electron‐acceptor ability of the ‐ + PPh 3 versus ‐PPh 2 O group. The values of λ abs for all the studied species 1 – 9 are only slightly dependent on the solvent polarity, as the solvation sphere remains virtually unrelaxed upon fast vertical excitation . Nevertheless, for all the salts, the lowest‐lying absorptions in water are systematically blue‐shifted with respect to DCM solutions (hypsochromic shifts of 90 cm −1 for 1 to 1183 cm −1 for 6 ); this can be termed as negative solvatochromism, as described for other ionic bipolar dyes…”

Fluorescent Chromophores Containing the Nitro Group: Relatively Unexplored Emissive Properties

“…Moreover, as proposed by Niko's groups on the pyrene system, [37] increasing the number of the alkyl groups such as methyl groups are also able to facilitate the σ-π conjugation, enhancing the emission in the case of MÀ NO 2 . [36][37] As a result, among the three compounds, PLQY of TÀ NO 2 is substantially lower than others.…”

“…[25] As an extension of stilbene, four anthracene-based fluorophores shown in Figure 3, which were reported by Khalid Baig et al, exhibit intriguing fluorescence behaviours that may help us gain insights into how nitro group affects the electronic transition of a π-conjugated system. [36] The absorption and emission spectra of these anthracene-based fluorophores are organized in Figure 4 and tabulated in Table 5. As illustrated in Figure 4, the para-position of styryl benzene of MÀ BZ, MÀ CN, and MÀ NO 2 are hydrogen, cyano group, and nitro group, respectively.…”

“…Khalid Baig et al's work in 2018. [36] "T" stands for the toluene group, and "M" stands for the mesityl group. red-shifted absorption and emission peak wavelengths of MÀ BZ as recorded in Table also indicate the dominant locally excited ππ* transition.…”