Ultrafast Aggregation-Induced Tunable Emission Enhancement in a Benzothiadiazole-Based Fluorescent Metal–Organic Framework Linker

Abstract

Aggregation-induced emission enhancement (AIEE) is a process recently exploited in solid-state materials and organic luminophores, and it is explained by tight-molecular packaging. However, solution-phase AIEE and its formation mechanism have not been widely explored. This work investigated AIEE phenomena in two donor–acceptor–donor-type benzodiazole-based molecules (the organic building block in metal–organic frameworks) with an acetylene and phenyl π-conjugated backbone tapered with a carboxylic acid group at either end. This was done using time-resolved electronic and vibrational spectroscopy in conjunction with time-dependent density functional theory (TD-DFT) calculations. Fluorescence up-conversion spectroscopy and time-correlated single-photon counting conclusively showed an intramolecular charge transfer-driven aggregate emission enhancement. This is shown by a red spectral shift of the emission spectra as well as an increase in the fluorescence lifetime from 746 ps at 1.0 × 10–11 to 2.48 ns at 2.0 × 10–3 M. The TD-DFT calculations showed that a restricted intramolecular rotation mechanism is responsible for the enhanced emission. The femtosecond infrared (IR) transient absorption results directly revealed the structural dynamics of aggregate formation, as evident from the evolution of the CC vibrational marker mode of the acetylene unit upon photoexcitation. Moreover, the IR data clearly indicated that the aggregation process occurred over a time scale of 10 ps, which is consistent with the fluorescence up-conversion results. Interestingly, time-resolved results and DFT calculations clearly demonstrated that both acetylene bonds and the sulfur atom are the key requirements to achieve such a controllable aggregation-induced fluorescence enhancement. The finding of the work not only shows how slight changes in the chemical structure of fluorescent chromophores could make a tremendous change in their optical behavior but also prompts a surge of research into a profound understanding of the mechanistic origins of this phenomenon. This may lead to the discovery of new chemical strategies that aim to synthesize novel chromophores with excellent optical properties for light-harvesting applications.

Tunable Photoinduced Charge Transfer at the Interface between Benzoselenadiazole-Based MOF Linkers and Thermally Activated Delayed Fluorescence Chromophore

“…The emission band of the TADF molecule at 620 nm was also observed with a much lower intensity than in the SDZ–TADF system. These features, such as the presence of new emission bands and considerable PL quenching, could be related to intermolecular electron transfer from the donor molecule (Ac-SDZ) to the acceptor molecule (TADF), and locally excited-state formation. ,, This observation provides another piece of evidence of the charge transfer process in this system. In other words, based on the absorption and emission spectroscopic measurements, we can assign the peak at 500 nm to the locally excited-state formation in Ac-SDZ caused by the intermolecular charge transfer phenomenon between Ac-SDZ and the TADF molecule. , The formation of this locally excited-state band in Ac-SDZ (500 nm) is a consequence of the new charge density distribution after the charge transfer to the TADF molecule.…”

“…The smaller SDZ molecule structure makes it more prone to coalesce in solution due to its stronger intermolecular interactions, like π−π stacking and H bondings. 29,33,34,42 Once the steady-state optical properties of the isolated molecules were analyzed, and the TADF and Ac-SDZ molecules were found to be prone to forming charge transfer states, the next step was to study them as donor−acceptor (D− A) systems (SDZ−TADF and Ac-SDZ−TADF) in DCM solutions. UV−vis absorption and fluorescence spectra were measured under the same conditions for both systems to understand the effect of minor structural modification on the excited-state dynamics at the D−A interface.…”

“…This feature in the absorption spectrum is dominated by aggregate formation rather than solvent polarity. The smaller SDZ molecule structure makes it more prone to coalesce in solution due to its stronger intermolecular interactions, like π–π stacking and H bondings. ,,, …”

“…One interesting family of chromophores that involves the control of different excited states is thermally activated delayed fluorescence (TADF) molecules, which have attracted substantial attention as luminescent materials due to their unique ability to harvest often detrimental triplet states as emissive singlet states through the reverse intersystem crossing process. − This phenomenon prolongs the fluorescence lifetime of TADF materials from nanoseconds to microseconds, , making them excellent candidates for several applications as luminescence sensors, − organic light-emitting devices (OLEDs), ,,, and more recently, as X-ray imaging materials. − However, due to the intricate interplay between the triplet and singlet excited states in TADF, it is challenging to control the optical properties of TADF materials by direct structural modifications, but they can serve as excellent energy and/or electron acceptor units. ,,,,,,− Other promising luminescent materials are heterobenzodiazole rings, which have also demonstrated excellent luminescent and photochemical behaviors. − Unlike TADF materials, the structures of these organic linkers can be easily altered to achieve different reaction outcomes upon light excitation, and they can serve as energy/electron donors in a variety of chemical composites. ,, Moreover, this kind of organic linker is designed to be an easy building block of metal–organic frameworks (MOFs) − that can serve not only to improve the optical properties of MOFs but also to favor and increase the efficiency of energy transfer processes at the donor–acceptor interface due to the highly ordered structure of the frameworks. These properties have been exploited in sensing, ,− catalysis, − and light-harvesting applications, i...…”

Interplay of conformational relaxation and hydrogen bond dynamics in the excited states of fluorescent Schiff base anions

“…Fluorogenic molecules have attracted significant attention in recent times because of their importance in the fields of organic electronics and sensing of analytes. [1][2][3][4][5][6][7][8][9][10][11] Excited state dynamics of such molecules are governed by highly efficient nonradiative processes, cessation of which by specific chemical or physical inputs renders the molecules strongly fluorescent. 12 Schiff bases constitute a class of fluorogenic molecules that are attractive from several different perspectives.…”