Ultrafast Fluorescence Photoswitch Incorporating Diketopyrrolopyrrole and Benzo[1,3]oxazine

Abstract

With the objective of developing ultrafast fluorescent switch molecules, we have designed and synthesized fluorescence switch molecules incorporating two oxazine photochromes (OX) at the two end of single diketopyrrolopyrrole (DPP) fluorophore giving the shape of the dyad molecule as OX-DPP-OX. For precise characterization, steady-state photophysical properties, acid–base-induced spectroscopic studies and ultrafast transient absorption spectroscopic studies are performed. In acetonitrile (ACN) solution, the benzo­[1,3]­oxazine ring of studied oxazine derivatives in OX-DPP-OX opens up and reduces the fluorescence intensity of DPP by 66% upon addition of 50 equiv of trifluoroacetic acid (CF3COOH, TFA) and addition of an equivalent amount of base, tetrabutylammonium hydroxide ((C4H9)4NOH, TBAOH), closes the oxazine ring, reverting the fluorescence intensity of the DPP unit back to its original intensity. Likewise, upon 330 nm laser excitation, the oxazine ring opens up in less than 135 ps in ACN solution, reducing the DPP fluorescence by 90%. Both the processes, acidochromic effect and 330 nm laser excitation, generate a 3H-indolium cation, p-nitrophenolate (protonated) and p-nitrophenolate anion, respectively. The photogenerated isomer lives for 1.5–1.9 ns in room temperature and reverts to its original conformer with first-order kinetics. This photochromic dyad tolerates thousands of switching cycles with no sign of degradation. However, the Gibbs free energy of the cationic fragment of their photogenerated isomer and DPP fluorophore is exergonic (ΔG 0 < −0.8) and ultrafast intramolecular electron transfer occurs very fast (in 16 ps time) from DPP moiety to 3H-indolium cation. As a result, the photoinduced transformation of the photochromic component within this dyad results in the effective quenching of the DPP emission. The fluorescence of this photoswitchable compound is modulated on a nanosecond time scale with excellent fatigue resistance under femtosecond (fwhm ∼100 fs) photoexcitation. Thus, the choice of OX as a photochromic component and DPP as fluorescence component can ultimately lead to the development of valuable ultrafast photoswitchable fluorescent probes for designing ultrafast switching devices. Such valuable mechanistic insights into their excitation dynamics can guide the design of novel members of this family of photochromic compounds with improved photochemical properties.

Photo‐/Baso‐Chromisms and the Application of a Dual‐Addressable Molecular Switch

“…Multi-addressable molecular switches have attractedalot of attention recently because of their multi-stimuli responsive properties. [1][2][3][4][5][6][7] By integration of the typical molecular switches,s uch as spiropyrans, [8,9] diarylethenes, [10][11][12][13][14] azobenzenes, [15,16] oxazolidines, [17,18] etc. [19,20] into one molecule, aseries of multi-addressable molecular switches have been achieved, which not only expands the stimuli species, buta lso enriches the outputs ignals.…”

“…Multi‐addressable molecular switches have attracted a lot of attention recently because of their multi‐stimuli responsive properties . By integration of the typical molecular switches, such as spiropyrans, diarylethenes, azobenzenes, oxazolidines, etc .…”