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In chemistry, twisted intramolecular charge transfer, often abbreviated as TICT, is a type of intramolecular charge-transfer band that involves “twisting,” or rotation, of the donor and acceptor portions of the molecule with respect to one another.^[2] TICT is most commonly associated with systems with an electron donor and an electron acceptor linked by a single bond, wherein the donor and acceptor portions of the molecule become perpendicularly configured following photoexcitation.^[3] TICT has been observed in a large number of fluorescing organic and main group compounds^[4] and has been utilized strategically in a wide range of applications, including as sensors, probes, dyes and bioimaging stains, organic light-emitting diodes, nonlinear optics, and solar energy conversion.^[2]

Though similar, TICT is not to be confused with twisted intramolecular charge shuttling (TICS), where the donor and acceptor portions of the molecule are able to reversibly swap roles via photoexcitation and charge transfer.^[5] TICT is also notably distinct from twisting phenomenon observed from photoinduced electron transfer (PET)^[6] and are largely differentiated by the degree of orbital mixing from the donor and acceptor. Specifically, compounds that exhibit TICT contain excited states with significant orbital mixing from both donor and acceptor, while compounds that exhibit PET have largely localized excited states centralized on either the donor or the acceptor.^[1]

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One of the first systems investigated in the context of TICT is 4-(N,N-dimethylamino)-benzonitrile (DMABN). DMABN was observed to display two different fluorescence bands that were both temperature- and solvent-dependent. In nonpolar solvents, only one fluorescence band (often denoted as the B fluorescence band, or F_B), the transition from the locally excited (LE) state to the ground state, was observed. In polar solvents, a different fluorescence band (often denoted as the A fluorescence band, of F_A) at lower energies grows in.^[7][8] The nature of this second transition was of particular interest and subsequently prompted significant scientific debate. As a result, a variety of explanations have emerged to rationalize the dual fluorescence in DMABN.^[4]

In their original reports,^[7][8] Lippert and coworkers showed that the FA band exhibited a strong solvatochromic shift, which was analyzed to determine that the F_A-emitting excited state (later determined to be the twisted excited state, TES) was highly polarized.^[4] Moreover, the ratio of the intensities of the F_A and F_B bands depended heavily on the temperature.

On the basis of the observed solvatochromic behavior, the large excited state dipole moment, and the temperature-dependence, Lippert and colleagues hypothesized that in polar solvents, orientational relaxation of the solvent coordination sphere led to the F_A-emitting singlet excited state becoming the lowest energy excited state. Thus, it was hypothesized that emission from the F_B-emitting excited state (without solvent reorganization) and from the F_A-emitting excited state (with polar solvent reorganization imparting stability) led to the observation of the two fluorescence bands.^[7]

While initially convincing, the emergence of new experimental data led to the formulation of a plethora of alternative hypotheses.^[4]

First introduced by Grabowski and coworkers,^[9] TICT implicates that the electron donating amino group twists such that the plane formed from the C-N-C of the amino group is out of plane (and perhaps even perpendicular to) the plane formed by the aromatic ring. Investigations from Grabowski and coworkers showed that positioning methyl groups ortho to the electron donating amino group led to the sole observation of the F_A band.^[10] The F_A-emitting excited state was thus assigned to the charge transfer twisted excited state (TES) with a highly twisted amino group, while the F_B-emitting excited state corresponded to the approximately coplanar, locally excited (LE) state.^[9] Thus, by extension, the F_A band is assigned to the TICT transition, while the F_B band corresponds to the locally excited intramolecular charge transfer (LE/ICT) transition. Note that in the fully twisted, orthogonal excited state conformation, a fully charge-separated excited state conformation can occur, owing to the zero orbital overlap between the donor and acceptor orbitals.

It was later determined that this excited state phenomenon, which couples charge transfer and charge separation with rotational twisting of the electron-donating group, was generalizable to other donor-acceptor molecules beyond DMABN, at which point the term twisted intramolecular charge transfer was coined.^[4][11]

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This hypothesis suggested that the dual fluorescence bands observed in DMABN were emitted from an excited state dimer (or aggregates) formed between multiple DMABN molecules.^[12] While such excimers do form in high concentration regimes, this hypothesis was ultimately rejected after finding that the intensity ratio of both the F_A and F_B fluorescence bands remained independent of the concentration of DMABN.^[9]

A number of hypotheses have emerged surrounding the implication that solvent interactions play a major role in the fluorescence emission of DMABN. These include hypotheses implicating the role of proton transfer to the nitrile group of DMABN,^[13] hydrogen bonding to the amino group, a “water clustering” mechanism,^[14] and the formation of excited state complexes between DMABN and solvent. However, contradicting evidence against each of these hypotheses has led to the disfavoring of solute-solvent interactions being the main reason for dual emission.^[4]

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As a result of the symmetry forbidden nature of the transition, TICT transitions are often non-radiative and decay via vibrational modes (although it should be noted that TICT transitions are not usually the sole nonradiative decay pathway in molecules).^[1] Such non-radiative transitions often compete with alternative radiative pathways, including fluorescence and phosphorescence.

In chemical systems that exhibit both emission and TICT, two key parameters are considered: the rotation rate/barrier and the driving energy.^[1] Computational analysis of the potential energy surface between the TICT (non-emissive) and the locally-excited/intramolecular charge-transfer (LE/ICT) emissive excited state illustrates that the careful interplay between the rotation barrier and the driving energy affects the degree of contribution from the TICT transition in the excited state, which may affect the intensity of fluorescence observed.^[1][15]

From a synthetic perspective, a number of factors can be tuned to modulate the TICT formation rate and the resulting quantum yield of the fluorophore, including the steric restrictions of the system, the polarity of the solvent environment, and the strength of both the donor and acceptor within a molecule.^[1][2] These factors can be combined in various degrees to tune the fluorescence emission and are discussed below:

Dialkylamino groups are often used as electron donating groups within systems that exhibit intramolecular charge transfer. By cyclizing the alkyl groups of the amine, rotation of the electron donating group with respect to the electron accepting portion of the molecule can be disfavored, thereby suppressing TICT and nonradiative decay of the excited state. A number of examples have been reported wherein cyclization of the alkyl groups of the amine can lead to suppressed TICT and enhanced fluorescence, including in dipolar coumarin derivatives,^[16] rhodamine derivatives,^[17] and in cyanine dyes.^[18]

Cyclization of the alkyl groups, however, can pose significant synthetic challenges. Moreover, the addition of hydrocarbon groups may lead to solubility challenges, particularly in biological environments where water is the solvent.

The steric bulk of the alkyl groups on the amine plays a critical role in the fluorescence quantum yield and the favoring of either the ‘’’TICT’’’ or LE/ICT state. Detailed DFT calculations and experimental work on aziridinyl- and azetidinyl-substituted fluorophores showed that the fluorescence quantum yield increases as a function of the angular displacement of the alkyl substituents away from the perpendicular ‘’’TICT’’’ conformation.^[19] That is, if the ground state conformation of the alkyl substituents on the amine more closely resembles the perpendicular configuration, than the ‘’’TICT’’’ transition is favored, and emission intensity decreases.

Sterically bulky alkyl groups favor the twisted conformation due to minimization of steric clashing with the rest of the fluorophore, thereby favoring ‘’’TICT’’’ and non-radiative decay. In cases where full planarity of the entire fluorophore, including the electron-donating or electron-accepting groups, would lead to significant 1,3-allylic strain, “pre-twisting” of the electron-donating group is observed, which decreases overlap of the donor orbitals with the rest of the fluorophore and favors the ‘’’TICT’’’ transition. Examples of this are reported by Grabowski and coworkers on various DMABN derivatives containing methyl groups on the ortho-position.^[10] Here, 1,3-allylic strain leads to “pretwisting” of the electron-donating amine group relative to the rest of the DMABN molecule, causing emission from the planar LE/ICT excited state to disappear and leaving only one fluorescence signal.

By contrast, alkyl groups that minimize steric contact with the fluorophore scaffold of the molecule favor the LE/ICT state due to the resonance/mesomeric effect, leading to increased fluorescence and emission.^[1] Substitution of the dimethylamine group in model compounds exhibiting TICT with strained amine groups that push the alkyl groups away from the rest of the fluorophore, including 7-azabicyclo[2.2.1]heptane in a sulfur rhodamines^[20] or functionalized azetidinyl groups in a variety of fluorophores,^[21][22][23] led to significant enhancements in fluorescence quantum yields. One limitation in these systems is the utilization of strained rings, which can lead to decreased chemical stability.

The TICT excited state corresponds to a state with complete charge separation between the electron donor and acceptor. Reducing the electron-donating strength of the electron donor or the electron-withdrawing ability of the electron acceptor may be able to disfavor the formation of the charge-separated state, leading to enhanced fluorescence from the LE/ICT state.

A structure-property relationship relating donor/acceptor strength with TICT formation rate and fluorescence quantum yield has been investigated by Jones II and coworkers.^[16] By changing the identity of the electron-donating amino group on coumarin derivatives from a diethylamino group to a dimethylamino and, eventually, a primary amino group (thereby increasing the donating ability of the electron donor), the quantum yield of the resulting fluorophore increased significantly. In a similar fashion, increasing the strength of the electron acceptor by adding on a trifluoromethyl group led to a significant decrease in the quantum yield.

Another strategy that has emerged is to utilize inductive effects via the installation of electron-withdrawing groups, including quaternary piperazine^[24] or sulfone^[25] groups, close to the electron-donating amines. Derivatizing amines with electron-withdrawing groups led to significant enhancements in the fluorescence quantum yield. Notably, modification of the electron donor with this method leads to a hypsochromic shift in the absorbance and emission spectra.

Due to its charge-separated nature, the TICT state is often stabilized by dipole-dipole interactions with polar solvent molecules, leading to lower fluorescence intensity in polar media. Polar media such as water and ethanol also often have hydrogen bond-mediated non-radiative decay pathways through which fluorescence can be quenched. Accordingly, fluorescent molecules tend to emit stronger in nonpolar solvents.^[26]

The connection, or bridge, between the electron donor and acceptor can also affect the rate of TICT. In a series of rotor-based fluorophores, Zhang and coworkers modulated the length of the bridge between an electron-donating amino group and various electron acceptors and determined that longer π-conjugated bridges tended to suppress TICT formation, leading to smaller changes in the fluorescence intensity.^[27] It should be noted that in these systems, the twist originates from rotation about the C–C bond in the bridge, not in the dimethylamino group.

A plethora of molecular systems have been engineered to take advantage of a pretwisted configuration, TICT, and its effect on fluorescence emission for a variety of applications. A few examples are introduced below.

Due to the sensitivity of TICT and fluorophores to the surrounding environment as well as the high resolution of fluorescence detectors, fluorescent molecules are often used to sense and detect minute changes in the environment. For instance, the heat sensitivity of TICT in rhodamines and their derivatives has been exploited to develop sensors to measure the temperature in the local environment and construct temperature maps and calibration curves.^[28][29] Similarly, temperature-sensitive and temperature-insensitive dyes have been made using rhodamine derivatives.^[30] The fluorescence emission of ‘’’TICT’’’-based molecular rotors has also been shown to be dependent on the viscosity of the solvent environment.^[27]

Molecules that exhibit TICT are used to probe the presence of specific chemical species. Often, the chemical target (if present) will react with the probe to “turn off” TICT, significantly decreasing nonradiative decay and increasing the fluorescence intensity. Examples of this strategy have been demonstrated for the detection of metal ions,^[31][32] small molecules, and in the characterization and labeling of proteins.^[33]

Fluorophores that suppress TICT, such as those based on the acedan and naphthalimide framework, have been used for imaging within live biological samples. Previous work toward this aim include fluorescence imaging in HeLa cells^[21] and two-photon microscopy on brain, liver, and kidney tissues from mice.^[26] The “turning on” of fluorescence by deactivation of TICT modes has also been applied toward the development of RNA tags, such as Spinach from Jaffrey and coworkers.^[1][34]

Aggregation-induced emission luminogens (AIEgens) are a class of fluorophores that exhibit poor emission in polar solvents but become highly luminous in nonpolar solvents or when clustered. In molecules that exhibit TICT in polar solvents, aggregation into molecular clusters can lead to significant restriction on the “twisting” of TICT, recovering fluorescence. Aggregation-induced emission has been observed in systems known to exhibit TICT, including in various TICT-active fluorophores^[2] and BODIPY derivatives.^[35] Notably, as the polarity of the solvent increases, the fluorescence of AIEgens is observed to decrease until a critical threshold is reached, at which point the fluorescence “turns back on”, owing to aggregation-induced restriction of TICT modes. AIEgens have potential applications in both the optoelectronic and bioimaging fields.^[36][37]

Molecules that exhibit pretwisted configurations have been considered for applications in the broad field of optoelectronics, including as organic light-emitting diodes (OLEDs) or in nonlinear optics applications. An example of an application of a pretwisted molecule comes from Uoyama and coworkers, who demonstrate that the low singlet-triplet energy gaps in highly pretwisted molecular systems, owing to the absence of electron-exchange interactions between the singlet and triplet states, can enable facile intersystem crossing and optimal spin populations, leading to high fluorescence efficiency.^[38] In the realm of nonlinear optics, highly twisted donor-acceptor systems based on TICT, known as TICTOID structures, were both theoretically^[39] and experimentally demonstrated to have exceptionally large second-order hyperpolarizabilities and electrooptical responses.^[40]