Twisted Intramolecular Charge Transfer (TICT) Controlled by Dimerization: An Overlooked Piece of the TICT Puzzle

Abstract

Organic dyes have shown high efficiencies in solar cells, which is mainly attributed to the push–pull strategy present in such dyes upon attaching to the semiconductor surfaces. We deeply studied the fundamental photophysical properties of cyanoacrylic dyes, mostly the L1 dye, and found unique emission properties that depend on many factors such as the solvent polarity and the concentration of the dye and could present a complete emission picture about this family of dyes. The L1 dye shows an intramolecular charge transfer (ICT) emission state at low concentrations (approximately nanomolar scale) and shows a twisted intramolecular charge transfer (TICT) emission state in specific solvents upon increasing the concentration to the micromolar scale. Moreover, the associated emission lifetimes of the ICT and TICT states of the L1 dye depend on solvent basicity, highlighting the role of hydrogen bond formation on controlling such states. Density functional theory calculations are performed to gain insight into the photophysical properties of the dye and revealed that H-bonding between the carboxylic groups triggers the dimerization at low concentrations. Using femtosecond transient absorption, we assigned the rate of TICT formation to be in the range (160–650 fs) −1 , depending on the size of the studied cyanoacrylic dye. Therefore, we add herein a new dimension for controlling the formation of the TICT state, in addition to the solvent polarity and acceptor strength parameters. These findings are not limited to the studied dyes, and we expect that numerous organic carboxylic acids dyes show similar properties.

Fluorescence Modulation by Ultrafast Chromophore Twisting Events: Developing a Powerful Toolset for Fluorescent-Protein-Based Imaging

“…To capture the molecular twisting events of these embedded chromophores on their intrinsic time scales, suitable ultrafast techniques need to be carefully selected and implemented. Structural motions often lead to a change of charge distribution and dipole moment, particularly for the donor–acceptor compounds. , The specific two-ring system of the GFP chromophore (Figure ) represents a great example wherein the charge in the P-ring could be transferred to the I-ring following photoexcitation and the twisting motions along the methine bridge likely form a twisted intramolecular charge transfer (TICT) state with a reduced energy gap between the ground (S 0 ) and excited (S 1 ) states. ,, These twisting/torsional motions could significantly decrease the transition oscillator strength between S 1 and S 0 (forming a dark state), while a hot ground state (HGS) can quickly form out of the TICT state through a conical intersection (CI). , The conformational events can also lead to clear frequency changes of characteristic excited-state vibrational modes because of their high sensitivity to nonequilibrium structural changes with generally larger electric polarizabilities than their ground-state counterparts at thermal equilibrium, , which present an engaging and effective experimental opportunity to monitor and elucidate the underlying reaction coordinates of the photoexcited chromophore.…”

“…Structural motions often lead to a change of charge distribution and dipole moment, particularly for the donor− acceptor compounds. 19,38 The specific two-ring system of the GFP chromophore (Figure 1) represents a great example wherein the charge in the P-ring could be transferred to the Iring following photoexcitation and the twisting motions along the methine bridge likely form a twisted intramolecular charge transfer (TICT) state with a reduced energy gap between the ground (S 0 ) and excited (S 1 ) states. 27,30,33 These twisting/ torsional motions could significantly decrease the transition oscillator strength between S 1 and S 0 (forming a dark state), while a hot ground state (HGS) can quickly form out of the TICT state through a conical intersection (CI).…”

Thieno[3,4‐c][1,2,5]Thiadiazole‐Based Organic Conjugated Molecules with Visible and Near‐Infrared Dual Emissions for Luminescent Anti‐Counterfeiting Applications

“…In contrast, organic conjugated materials with both visible and NIR emissions are superior candidates for high‐level anti‐counterfeiting, due to their unique advantages of simple and large‐scale preparation, flexibility, tunable structure, low cost, and toxicity. Most importantly, organic conjugated materials bearing with donor‐acceptor (D‐A) structure tend to form charge transfer (CT) state, thus making it possible to achieve visible and NIR dual emissions under single wavelength excitation, [ 17 ] which would provide a simple but effective strategy to establish vis‐NIR dual‐emission anti‐counterfeiting technology. Recently, some CT‐featured organic conjugated molecules have exhibited dual emissions for luminescent anti‐counterfeiting via well molecule design, but most of their emission located in the visible region without extending to NIR region.…”

“…Recently, some CT‐featured organic conjugated molecules have exhibited dual emissions for luminescent anti‐counterfeiting via well molecule design, but most of their emission located in the visible region without extending to NIR region. [ 17c,d ] As reported, the twisted intramolecular charge transfer (TICT) state in D‐A‐typed organic luminophores could be the potential to achieve NIR emission and construct vis‐NIR dual emission together with the emission of locally‐excited (LE) state under single excitation by reasonably regulation. However, the TICT emission is extremely sensitive to microenvironment around molecules and is easily affected by concentration quenching, solvents’ polarity, and structural restriction in the condensed state.…”

Photostable fluorescent probes based on multifunctional group substituted naphthalimide dyes for imaging of lipid droplets in live cells

“…However, the twisted intramolecular charge transfer (TICT) acts as one of the major non-radiative de-excitation pathways, and leads to low photoluminescence quantum yields. 6,7 Rigid dialkylamino donor moieties were used to inhibit TICT. 8 Recently, Lavis and Xu reported that the four-membered azetidine ring effectively suppresses TICT formation, leading to improved photoluminescence quantum yields and superior photostability in a range of fluorophores, in conjunction with enlarged Stokes shifts.…”