Synthesis and Characterization of Novel Thienothiadiazole-Based D−π–A−π–D Fluorophores as Potential NIR Imaging Agents

Abstract

As fluorescence bioimaging has increased in popularity, there have been numerous reports on designing organic fluorophores with desirable properties amenable to perform this task, specifically fluorophores with emission in the near-infrared II (NIR-II) region. One such strategy is to utilize the donor−π−acceptor−π−donor approach (D−π−A−π−D), as this allows for control of the photophysical properties of the resulting fluorophores through modulation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Herein, we illustrate the properties of thienothiadiazole (TTD) as an effective acceptor moiety in the design of NIR emissive fluorophores. TTD is a well-known electrondeficient species, but its use as an acceptor in D−π−A−π−D systems has not been extensively studied. We employed TTD as an acceptor unit in a series of two fluorophores and characterized the photophysical properties through experimental and computational studies. Both fluorophores exhibited emission maxima in the NIR-I that extends into the NIR-II. We also utilized electron paramagnetic resonance (EPR) spectroscopy to rationalize differences in the measured quantum yield values and demonstrated, to our knowledge, the first experimental evidence of radical species on a TTD-based small-molecule fluorophore. Encapsulation of the fluorophores using a surfactant formed polymeric nanoparticles, which were studied by photophysical and morphological techniques. The results of this work illustrate the potential of TTD as an acceptor in the design of NIR-II emissive fluorophores for fluorescence bioimaging applications.

NIR-II emissive donor–acceptor–donor fluorophores for dual fluorescence bioimaging and photothermal therapy applications

“… 74 This is also confirmed upon comparison to our previously synthesized TTD-based fluorophores that utilized an aryl amine carbazole as the donor. 27 We attribute the large Stokes shift values to the changes in bond length between the ground- and excited-state structures. 50 Electron-transfer processes between the alkyl amine donors and the TTD acceptor result in either the shortening and elongation of the specified carbon–carbon bonds as indicated by computational results (Table S5, ESI ‡ ).…”

“…However, when comparing the experimental absorbance properties of DMA-TTDT 2 , Morp-TTDT 2 , and Pip-TTDT 2 to previous TTD-based fluorophores with aryl amine donors, there are noticeable bathochromic shifts that indicate the alkyl amine donors do positively affect the optical properties. 27 The narrow energy gaps can also be attributed to an increase in planarity due to the relatively small size of the peripheral amine donors. The DFT-computed ground state dihedral angles between the thiophene spacers and the amine donors ( θ 2 and θ 3 , Table S3, ESI ‡ ) are found to deviate ∼23–24° from the conjugated backbone.…”

“… 26 Previously, we synthesized a series of TTD-based fluorophores that utilized the aryl amine carbazole as the donor unit, which had emission maxima of 900 nm with the emission bands extending into the NIR-II. 27 We utilized electron paramagnetic resonance spectroscopy (EPR) to rationalize differences in quantum yield values and provided evidence of radical species on the TTD-based small molecule fluorophores. Despite possessing both an excellent acceptor and donor, these studies highlighted the effects of sterics, competing transitions, and efficient π-contribution on NIR-II fluorophore design and application.…”

Designing Thiadiazoloquinoxaline-Based Conjugated Polymers for Efficient Organic Photovoltaics: A DFT/TDDFT Study

“…Ground state geometry optimizations were performed using a density functional theory (DFT) approach at the B3LYP/6-311G(d,p) level of theory. The basis set 6-311G(d,p) performed well enough to capture the frontier molecular orbital energies of the conjugated copolymers [44][45][46][47][48]. For the selenium (Se) atom, the effective core potential basis set, LANLDZ, was used [49].…”

Near-Infrared Organic Photodetectors with Spectral Response over 1200 nm Adopting a Thieno[3,4-c]thiadiazole-Based Acceptor

“…Besides using very strong acceptor end groups, the band gap of SMAs could also be reduced if proquinoidal acceptor units possessing strong electron deficiency are incorporated. , The nonclassical ten-pi-electron thieno­[3,4- c ]­thiadiazole (TTD) meets this end quite well. First reported by Bower et al in 1969, TTD was used as a strong acceptor for constructing functional NIR absorbing/emission small molecules − and polymers − in the following years. Excellent backbone planarity is realized when inserting TTD between thiophene-based units because of TTD’s low steric repulsive 5,5-fused-ring structure and intramolecular S···N weak bonds .…”