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=== Alkyl cyclization ===

=== 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,<ref name=”jones”>Jones, G. I.; Jackson, W. R.; Choi, C. Y.; Bergmark, W. R. Solvent effects on emission yield and lifetime for coumarin laser dyes. Requirements for a rotatory decay mechanism. ”J. Phys. Chem.” ”’1985”’, ”89 (2)”, 294–300.</ref> rhodamine derivatives,<ref>Karstens, T.; Kobs, K. Rhodamine B and rhodamine 101 as reference substances for fluorescence quantum yield measurements. ”J. Phys. Chem.” ”’1980”’, ”84 (14)”, 1871–1872.</ref> and in cyanine dyes.<ref>Michie, M. S.; Götz, R.; Franke, C.; Bowler, M.; Kumari, N.; Magidson, V.; Levitus, M.; Loncarek, J.; Sauer, M.; Schnermann, M. J. Cyanine conformational restraint in the far-red range. ”J. Am. Chem. Soc.” ”’2017”’, ”139 (36)”, 12406–12409.</ref>

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,<ref name=”jones”>Jones, G. I.; Jackson, W. R.; Choi, C. Y.; Bergmark, W. R. Solvent effects on emission yield and lifetime for coumarin laser dyes. Requirements for a rotatory decay mechanism. ”J. Phys. Chem.” ”’1985”’, ”89 (2)”, 294–300.</ref> rhodamine derivatives,<ref>Karstens, T.; Kobs, K. Rhodamine B and rhodamine 101 as reference substances for fluorescence quantum yield measurements. ”J. Phys. Chem.” ”’1980”’, ”84 (14)”, 1871–1872.</ref> and in cyanine dyes.<ref>Michie, M. S.; Götz, R.; Franke, C.; Bowler, M.; Kumari, N.; Magidson, V.; Levitus, M.; Loncarek, J.; Sauer, M.; Schnermann, M. J. Cyanine conformational restraint in the far-red range. ”J. Am. Chem. Soc.” ”’2017”’, ”139 (36)”, 12406–12409.</ref>

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.

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.jpg|thumb|Alkyl cyclization in various fluorophores suppresses TICT and improves quantum yields (denoted by Φ). Data from Jones II and coworkers,<ref name=”jones”/>, Karstens and coworkers,<ref name=”karstens”/>, and Michie and coworkers <ref name=”michie”/> ]]

=== Modification of steric hindrance ===

=== Modification of steric hindrance ===

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=== Modulating bridge length ===

=== 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.<ref>Ye, S.; Zhang, H.; Fei, J.; Wolstenholme, C. H.; Zhang, X. A general strategy to control viscosity sensitivity of molecular rotor-based fluorophores. ”Angew. Chem. Int. Ed.” ”’2021”’, ”60 (3)”, 1339–1346.</ref> 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.

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.<ref>Ye, S.; Zhang, H.; Fei, J.; Wolstenholme, C. H.; Zhang, X. A general strategy to control viscosity sensitivity of molecular rotor-based fluorophores. ”Angew. Chem. Int. Ed.” ”’2021”’, ”60 (3)”, 1339–1346.</ref> 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 ==

== Applications ==

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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.^[1] 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.^[2] TICT has been observed in a large number of fluorescing organic and main group compounds^[3] and has been utilized strategically in a wide range of applications, including sensing, organic light-emitting diodes, nonlinear optics, dyes, bioimaging, and solar energy conversion.^[1]

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.^[4] TICT is also notably distinct from twisting phenomenon observed from photoinduced electron transfer (PET)^[5] 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.^[6]

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One of the first systems that displayed notable TICT is 4-(N,N-dimethylamino)-benzonitrile (DMABN). Lippert and coworkers observed that DMABN displayed temperature-dependent dual fluorescence bands in polar solvents; notably, swapping to nonpolar solvents yielded only one fluorescence band.^[7][8] This dual emission subsequently prompted significant scientific debate, and as a result, a variety of explanations have emerged to rationalize the dual fluorescence in DMABN.^[3]

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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).^[6] 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.^[6] 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.^[6][9]

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.^[6][1] 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,^[10] rhodamine derivatives,^[11] and in cyanine dyes.^[12]

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.^[13] 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.^[14] 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.^[6] 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^[15] or functionalized azetidinyl groups in a variety of fluorophores,^[16][17][18] 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.^[10] 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^[19] or sulfone^[20] 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.^[21]

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.^[22] 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 TICT and its effect on fluorescence emission for a variety of applications.

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