Draft:Twisted Intramolecular Charge Transfer

• File:Symbol opinion vote.svg Comment: Article was resubmitted without any appreciable revisions, apart from DOI's added to references. WP:NOT issues not addressed. WP:LLM issues not addressed. Submitter clearly isn't listening. We're done here. WeirdNAnnoyed (talk) 23:57, 17 November 2025 (UTC)

• File:Symbol opinion vote.svg Comment: Fails WP:NOTGUIDE. This isn't Angewandte Chemie and we don't publish review articles. This is supposed to be a succinct overview for a general audience. I also suspect some undisclosed AI use. All references are malformed, with inconsistent capitalization, lack of DOIs and PMIDs, and only one has a URL provided. The text isn't too LLM-ish in style, but it does have the meandering quality and tendency to go off on tangents that LLMs do when writing on technical topics (I can't prove any of this, however). Anyway, the excessive depth and detail makes the LLM issue moot. If this could be cut to a single paragraph with 2-3 references it might be a decent article, or just added as a short section (again, one paragraph) in charge-transfer complex. WeirdNAnnoyed (talk) 14:42, 16 November 2025 (UTC)

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File:Intro jablonski diagram v2.tif

A simplified Jablonski diagram representation of the twisted intramolecular charge transfer (TICT). Adapted from Wang and coworkers.

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. 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. TICT has been observed in a large number of fluorescing organic and main group compounds 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.

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. TICT is also notably distinct from twisting phenomenon observed from photoinduced electron transfer (PET) 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.

Qualitative background

Fundamental TICT model

In rationalizing the dual fluorescence behavior of DMABN using TICT (see the discussion below), the following potential energy surface diagram was proposed by Grabowski and coworkers:

File:DMABN jablonski diagram.tif

A Jablonski diagram representation of TICT. Note that the “complex reaction coordinate” includes not only the intramolecular twisting motion but also other coordinates as well, including those for electron transfer, solvent dipolar relaxation, and (potentially) some rehybridization/pyramidalization of the amino group. Adapted from Rettig.

Irradiation of DMABN, existing in the singlet S₀ ground state, photoexcites the molecule to the first singlet excited state, S₁, also termed the “locally excited (LE) state” (historically, this has also been denoted as B*). Along the first excited state potential energy manifold, there are several possibilities that can occur.

• DMABN can relax radiatively from the first excited state (S₁) down to the ground state (S₀) with a rate given by $k_{rad}$ via fluorescence. This is denoted as the “emitting” transition from the LE state down to the ground state (this has also been denoted as the “LE/ICT” transition to encompass systems that deviate from a planar conformation in their ground state). As the minimum of S₁ is (usually) displaced from that of S₀, Franck-Condon relaxation to the ground state further requires relaxation to the 0th vibrational state of S₀.
• DMABN can relax non-radiatively from the first excited state (S₁) down to the ground state (S₀) with a rate given by $k_{nr}^{LE}$ via a number of pathways, including coupling with vibrational modes. Similar to the fluorescence/radiative decay outcome, relaxation to the ground state is followed by vibrational relaxation to the 0th vibrational state of S₀.
• DMABN can also undergo an intramolecular charge transfer reaction such that charge is transferred (or, at the very least, shifted) from the donor to the acceptor. This charge transfer is coupled with rotation of the dimethylamino donor group (or “twisting”) from an in-plane to an out-of-plane, perpendicular conformation.
  • The charge transfer from the LE state to a twisted intramolecular charge transfer (TICT) excited state occurs with a rate constant $k_a$; the rate of the reverse charge transfer is denoted with the constant $k_b$. Note that depending on the system being considered, the intramolecular charge transfer reaction to generate the TICT excited state from the LE state may be reversible, leading to an excited state equilibrium.
  • The degree to which the TICT state is favored depends on the energetic barrier $\Delta E_{TS}^{*}$. This activation barrier includes both an intrinsic barrier as well as an extrinsic barrier resulting from the viscosity of the solvent medium.
  • In the limit of the perpendicular TICT excited state, the donor and acceptor π-orbitals are orthogonal and have zero orbital overlap. This leads to a high degree of charge separation and, by extension, large excited state dipole moments. By contrast, the planar configuration allows for resonance between the donor and acceptor, giving rise to relatively small excited state dipole moments.
  • The TICT excited state can then undergo radiative or non-radiative decay to the ground state. Following the Franck-Condon principle, relaxation to the ground state is followed by vibrational relaxation back to the minimum of the S₀ potential energy surface.

The relevance of the TICT state compared to the LE state can be determined by the following equation:

$E_{LE}-E_{TICT}>0$

The energy of the TICT state can be determined from the following equation:

$E_{TICT}=IP\left(D\right)-EA\left(A\right)+C+E_{solvent}$

Here, $IP(D)$ denotes the ionization potential of the donor, while $EA(A)$ denotes the electron affinity of the acceptor. $C$ denotes the mutual Coulombic attraction of the linked donor and acceptor radical cation/anion pair; a large value of C stabilizes the TICT state. $E_{solvent}$ denotes the polar solvent stabilization factor, which also contributes favorably to the TICT state formation rate. By considering each of these factors, the energy of the TICT excited state can be calculated.

Biradicaloid charge transfer model

Though initially presented for single bonds (e.g. DMABN), TICT is now often rationalized in terms of the theory of biradicaloid states, giving rise to the biradicaloid charge transfer (BCT) model. This model unifies the older model of TICT in DMABN to now include both twisted single and double bonds.

One qualitative approach described by Rettig is to consider the p-orbitals of twisted ethylene and its isoelectronic analog aminoborane. In either case, the energies of the HOMO and LUMO are dictated by those of the atomic orbitals, as there is zero orbital overlap between the p-orbitals. Thus, in twisted ethylene, the “HOMO” and “LUMO” are degenerate, whereas in twisted aminoborane, the HOMO is fully localized on nitrogen, and the LUMO is completely localized on boron. The energy difference between the HOMO and LUMO of aminoborane is denoted by the symbol δ, in analogy to crystal field splitting.

File:Biradicaloid Charge Transfer Model.tif

(A) MO diagrams for twisted ethylene and aminoborane. (B) States that emerge from two electrons populating two orbitals. (C) Energies for the states from (B) as a function of the splitting energy δ. δ_c denotes the critical value of δ at which the "hole-pair" state and the "dot-dot" states cross.

Here, we note that in considering the possible microstates occupied by two electrons in two “molecular” orbitals, six possible microstates emerge: two microstates with two electrons occupying the same orbital, two microstates with two electrons of opposite spin occupying different orbitals, and two microstates with two electrons of same spin occupying different orbitals. These are accounted for by three terms: a triplet “dot-dot” term (³HL), a singlet “dot-dot” term (¹HL), and a singlet “hole-pair” term (¹(H²).

In the limit of twisted ethylene, where δ = 0, the “dot-dot” states are lower in energy than the “hole-pair” state, owing to the favorable electron exchange interaction. In this region, the excited state has “hole-pair,” or closed-shell, character, leading to a phenomenon known as “sudden polarization.” However, as the splitting energy δ increases, the “hole-pair” state becomes more favorable, until the limit of twisted aminoborane is reached, where the “hole-pair” state is lower in energy than the “dot-dot” states. Here, the excited state has open shell character, giving rise to TICT phenomenon. The critical value of δ where the energy of the “hole-pair” state crosses that of the “dot-dot” states is denoted as δ_c and corresponds to a potential energy surface crossing point between S₀ and S₁ at a twisting angle of 90°. The borderline scenario where potential energy surfaces touch at δ = δ_c can yield ultrafast nonradiative decay.

The BCT model is strictly valid only for cases where there are no other low-lying excited states present. However, in larger aromatic systems, the lower-lying states of π symmetry (including both the π and π* molecular orbitals) interact (via configuration interaction) with the biradicaloid states, causing a deviation away from the predictions of the BCT model.

The qualitative backgrounds for TICT and BCT are discussed in greater detail in the following references.

Notable systems that display TICT

Several classes of molecules and fluorophores have emerged that readily display TICT-active modes. Often, these molecules contain an electron donor and an electron acceptor as two separate moieties. In these systems, the twist that occurs in TICT is exemplified by a rotation about the connection (i.e. single bond, π-conjugated chain, etc.) between the donor and the acceptor.

4-(N,N-dimethylamino)-benzonitrile

One of the first systems investigated in the context of TICT is 4-(N,N-dimethylamino)-benzonitrile (DMABN). DMABN was observed to display temperature- and solvent-dependent fluorescence. 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. 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.

Polarity-induced inversion of excited states

In their original reports, Lippert and coworkers showed that the F_A band exhibited a strong solvatochromic shift, which was analyzed to determine that the F_A-emitting excited state (later determined to be the TICT excited state) was highly polarized. 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.

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

Twisted Intramolecular Charge Transfer

File:DMABN.tif

DMABN undergoes a TICT transition in its excited state, which is responsible for its dual fluorescence emission. Adapted from Grabowski and coworkers.

First introduced by Grabowski and coworkers, 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. TICT was originally supported by experimental work from Grabowski and coworkers and has since been widely accepted as the main mechanism for rationalizing the dual fluorescence in DMABN. The F_A-emitting excited state was thus assigned to the TICT excited state with a highly twisted amino group, while the F_B-emitting excited state corresponded to the approximately coplanar, locally excited (LE) state.

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.

Donor and acceptor rehybridization models

In the ground state of DMABN, the resonance effect leads to planarization of the donor amino group. However, Zachariasse and coworkers hypothesized that the small energy gap between the LE and TICT states and the favoring of the twisted, charge separated excited state originates from rehybridization of the amino group, causing it to lose its planarity and reform a pyramidal geometry. This “rehybridization” of the amino group into a pyramidal geometry is assumed to arise from a pseudo Jahn-Teller effect, which couples the LE and TICT excited states via the wagging, or inversion, vibrational mode of the amino group. This hypothesis is often referred to as the wagging intramolecular charge transfer (WICT) model.

The donor rehybridization model was subsequently restructured into a separate model known as the planar intramolecular charge transfer (PICT) model. Under this scheme, which bears resemblance to Lippert’s original explanation, planarization of the molecule, which causes DMABN to adopt a highly dipolar quinone-type structure, leads to the F_A-emitting state becoming lower in energy than the F_B-emitting excited state.

Separate from the donor rehybridization model are hypotheses that invoke a structural change in the electron accepting unit, including the rehybridization by intramolecular charge transfer (RICT) model. In DMABN, excited states wherein the cyano group adopts a bent configuration were initially calculated to have similar energies to the TICT excited state. Further computational work on 4’-dimethylaminophenyl acetylene, an isoelectronic compound analogous to DMABN that substitutes the cyano group for an ethynyl group, also showed the presence of two highly polar states – one adopting a TICT excited state conformation and the other with a bent cyano (or ethynyl) group. However, 4’-dimethylaminophenyl acetylene was shown to not exhibit dual fluorescence. Moreover, the RICT hypothesis could not rationalize the observed emission in a variety of other analogous compounds.

Emission from excited state dimers

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. 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.

Formation of an excited state complex with solvent

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, hydrogen bonding to the amino group, a “water clustering” mechanism, 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.

Systems with aromatic donors and/or polycyclic acceptors

Compared to DMABN, the presence of TICT in large, π-conjugated donor-acceptor systems is more ambiguous. Often, these systems are already twisted in the ground state, require specialized methods to demonstrate dual emission (in analogy to DMABN), and absorb at a charge transfer band as the lowest energy transition. Despite this ambiguity, an enormous body of literature has been devoted to developing and investigating large aromatic systems that exhibit TICT-active behavior. These often include either dialkylamine or aromatic donors, such as pyrroles, indoles, carbazoles, or phenanthridine donors. The electron acceptors are also heavily varied as well but typically include polycyclic aromatic groups such as anthracene, naphthalene, acridine, purine, bispyrazolopyridine, and pyrazoloquinoline.

File:Aromatic donors and acceptors.tif

Aromatic donors and polycyclic aromatic acceptors are shown in a variety of systems that exhibit TICT-active modes and charge transfer transitions.

Biaryl systems

Compared to the other classes of molecules that exhibit TICT, biaryl systems are unique in that they tend to have relatively high symmetry and tend to be nonpolar. The most thoroughly investigated biaryl compound is 9,9’-bianthryl, which exhibits a ground state conformation with one anthracene moiety orthogonal (i.e. twisted) relative to the other. Schneider and Lippert demonstrated that 9,9’-bianthryl, as well as analogous symmetrically substituted bianthryls, exhibit a highly polar intramolecular charge transfer excited state, leading to charge transfer fluorescence. Other biaryl systems, including 3,3’-biperylenyl and 1,1’-bipyrenyl, also demonstrated charge transfer fluorescence emission, but the degree of charge separation determined for these compounds was much smaller than a full, completely charge-separated TICT.

This notion of charge separation in TICT motivated the development of oligomeric aryl systems, such as oligoanthracenes. It was suggested that these systems could operate as molecular bistable memory units, or mnemons. As a result, a variety of oligoanthracene molecules were synthesized. While charge transfer emission was detected, charges were estimated to only be separated across two nearby anthracene subunits and not across the full molecule, leading to incomplete charge separation.

File:Biaryls.tif

Selected examples of biaryl systems that exhibit intramolecular charge transfer emission.

Coumarins

File:Coumarin-derivatives.tif

Selected examples of coumarin derivatives from Jones II and coworkers (A, B) and Grimm and coworkers (C) known to take advantage of (or suppress) TICT to improve fluorescence.

Several aminocoumarins (that is, coumarins derivatized with an amine group) emit an intense fluorescence feature with large charge transfer character. Initially, it was hypothesized that the TICT state in coumarin derivatives was also emissive; however, experimentation using rigidly planar coumarins demonstrated that the emissive state was the planar LE state.

Coumarins have been used extensively as the basis for a wide range of fluorescent molecules that take advantage of the TICT phenomenon. For instance, Lavis and coworkers synthesized an azetidinyl coumarin that could be used for fluorescence imaging of cells.

Ionic compounds

In ionic compounds, charge transfer leads not to charge separation but rather to charge shift. This often gives rise to properties that are different than charge neutral compounds that display TICT. A large number of ionic compounds have been studied where twisting-accompanied charge shifts were proposed. These include protonated DMABN derivatives (such as dimethylamino-pyridines and pyrimidines), biphenyl derivatives, stilbazolium derivatives (referred to as DASPMI), and triphenylmethane (TPM) dyes.

File:Ionic compounds with charge transfer fluorescence.tif

Selected examples of ionic systems that display charge transfer/shift fluorescence with concomitant "twisting", or rotation, of a functional group.

Rhodamine dyes are another common class of TICT-active fluroescent ionic systems. Classic examples of rhodamine dyes include Rhodamine B and Rhodamine 101, which notably exhibits a temperature-independent fluorescence quantum yield of ~1. Rhodamines are often used as frameworks to derivatize off of, allowing for the generation of libraries of fluorescent dyes and imaging agents.

Rhodamine dyes were also the subject of a number of studies by Rettig and coworkers regarding the factors that contribute to TICT formation rate and fluorescence quenching rates. It was determined that the degree of pretwisting (i.e. the magnitude of steric hindrance) and the electron donating strength of the amino groups contributed positively to TICT formation, thereby promoting nonradiative decay and fluorescence quenching. Moreover, studies using amino-rhodamine dyes in acidic media demonstrated that not all nonradiative decay pathways are equal; in rhodamine dyes (as well as in triphenylmethane dyes), nonradiative decay from vibrational modes originating from inner flexible bonds contributes more to TICT formation than the “twisting” of the electron donating amino groups. Notably, this trend is not generalizable across all fluorophores; studies into cyanine dyes connected with nonaromatic bridges showed the opposite trend.

File:Rhodamine dyes v2.tif

Selected examples of rhodamines and their derivatives that exhibit TICT. The amino-rhodamine in (C) was reported by Rettig and coworkers.

Boron-dipyrromethene systems

Recent work has utilized boron-dipyrromethene (BODIPY) moieties as electron acceptor groups. Tang and coworkers have taken advantage of cross-coupling methods to tether a variety of electron-donating fragments to the meso-, 3-, and 5-positions of the BODIPY framework, generating a library of BODIPY-based luminogens that utilize both TICT as well as aggregation-induced emission.

File:BODIPY derivative synthesis.tif

A synthetic route to generate a BODIPY-derived fluorescent molecule with Φ = 99% (EtOH) from Prasad and coworkers.

Spiro systems

File:Lactone-rhodamines.tif

Lactone forms of rhodamine B and rhodamine 101 that exhibit fluorescence from a charge transfer state.

Spiro compounds with a separated donor and acceptor enforce a nearly zero orbital overlap of the donor and acceptor orbitals. Despite this, a charge transfer transition remains possible, in analogy to TICT. Classic examples of these spiro compounds include the lactone forms of Rhodamine B and Rhodamine 101, which both exhibit charge transfer fluorescence with forbidden character (i.e. weak intensity) but whose excited states have a large dipole moment. The lactones and spiro forms of other dyes have also exhibited charge transfer fluorescence emission.

Factors that affect TICT and fluorescence

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). 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. 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.

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.

Alkyl cyclization

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, rhodamine derivatives, and in cyanine dyes.

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.

File:Alkyl cyclization examples.tif

Alkyl cyclization in various fluorophores suppresses TICT and improves quantum yields (denoted by Φ, solvent denoted in parentheses). Data from Jones II and coworkers,, Karstens and coworkers,, and Michie and coworkers.

Modification of steric hindrance

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. 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. 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. 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 or functionalized azetidinyl groups in a variety of fluorophores, 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.

File:Steric hindrance.tif

Modification of the electron donating group from a dimethylamino group to an azetidine group leads to an increase in quantum yields (denoted by Φ, solvent denoted in parentheses). Data from Grimm and coworkers.

Modulating donor and acceptor strength

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.

File:Donor acceptor strength.tif

As the donor and acceptor strength of the electron donating and electron withdrawing groups, respectively, increase, the quantum yield (denoted by Φ, all taken in acetonitrile) of the fluorophore decreases. Data from Jones II and coworkers.

A structure-property relationship relating donor/acceptor strength with TICT formation rate and fluorescence quantum yield has been investigated by Jones II and coworkers. 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 or sulfone 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.

Solvent polarity

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.

Modulating bridge length

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. 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.

Applications

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

Sensors

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. Similarly, temperature-sensitive and temperature-insensitive dyes have been made using rhodamine derivatives. The fluorescence emission of TICT-based molecular rotors has also been shown to be dependent on the viscosity of the solvent environment.

Probes

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, small molecules, and in the characterization and labeling of proteins.

Bioimaging

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 and two-photon microscopy on brain, liver, and kidney tissues from mice. 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.

File:Bioimaging horizontal.tif

(A) Two-photon imaging of mouse tissues from the brain, liver, and kidney enabled by fluorophores that suppress TICT. Data from Singha and coworkers. (B) In the presence of a target RNA strand, the fluorophore shown in the figure becomes locked in the planar conformation and fluoresces intensely. Data from Jaffrey and coworkers.

Aggregation-induced emission luminogens

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 and BODIPY derivatives. 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.

Optoelectronic materials

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. In the realm of nonlinear optics, highly twisted donor-acceptor systems based on TICT, known as TICTOID structures, were both theoretically and experimentally demonstrated to have exceptionally large second-order hyperpolarizabilities and electrooptical responses.