Internal Charge Transfer State Research Articles

Differential Chromogenic Response towards F− and H2PO4−: Hydrogen Bonding vs Deprotonation

AbstractHighly colored oxidized 3,3/‐di(indolyl)arylmethanes (DIAM) based probes have been synthesized in good yield using modified single‐step synthesis. The internal charge transfer state of the molecules as well as protonation equilibrium of indolyl units are dominated by the electronic nature of substituents (R=−CH3, −NMe2, −NO2 etc) present on the central aryl moiety. However, irrespective to the nature of the substituents, all compounds (HL) show selective chromogenic response towards basic anions, like F− and H2PO4− etc in CH3CN. At low concentration, both analytes induce hypsochromic shifts in absorption maximum owing to the formation of H‐bonded complex (HL….A). However, only F− ion at high concentration results in a large bathochromic shift (▵λ∼90 nm) at λmax, probably due to the formation of deprotonated receptor (L−), attributed by higher stability of H‐bonded [HF2−] dimer. Thus, one can easily discriminate between these ions via color‐changing response. On the other hand, HSO4− being an acidic anion results in protonation of indolyl nitrogen center (H2L+) even at low concentration in CH3CN‐H2O (1 : 1) mixture medium, leading to bathochromic shift of absorption maxima.

Theoretical Investigations on the Excited-State Dynamics of an Al3+ Fluorescence Sensor.

Twisted internal charge transfer (TICT) states are of fundamental importance during the photo-physical processes of dyes and sensors. In this contribution, excited-state dynamics of an Al3+ fluorescence sensor 1-{[(2-hydroxyphenyl)-imino]methyl}naphthalen-2-ol based on the turn-on signal is clarified. Two different dark TICT states are observed by exploring the excited-state potential energy surface. With the twist of the C2-N bond, the two dark states can be reached facilely, which induce the experimentally observed weak fluorescence of the sensor. The sensing mechanism is then uncovered by investigating the electronic coupling between the sensor and analyte. Al3+ is proved to form strong coordination bonds with the sensor, which restricts the motion of the C2-N bond. Consequently, the TICT states are eliminated, which generate the turn-on signal. This sensing mechanism is trustworthy and intrinsically different from the previously proposed one, which would shed some light on the design of turn-on sensors.

PH-stimulus response dye-cucurbituril sensor for amino acids in aqueous solution.

The host-guest inclusion complexes that comprise an inverted cucurbit[7]uril (iQ[7]) and a quinoline derivative, 4-(4-dimethylaminostyryl) quinoline (DSQ) at different pHs were exploited as multiple supramolecular sensors to sense l-α-amino acids. DSQ complexation inside iQ[7] at different pHs leads to increased fluorescence and formation of different-colored iQ[7]-DSQ complexes. The enhanced fluorescence of DSQ after iQ[7] encapsulation may be attributed to limited dimethylamine rotation and the formation of a twisted internal charge transfer (TICT) state. The DSQ@iQ[7] sensors have different affinities for l-α-amino acids at different pHs. Therefore, we propose a pH-stimulus response supramolecular sensor for the discrimination of structurally similar l-α-amino acids in aqueous solution.

Twisted Intramolecular Charge Transfer States in Trinary Star-Shaped Triphenylamine-Based Compounds.

Excited state dynamics of trinary star-shaped dendritic compounds with triphenylamine arms and different cores were studied by means of time-resolved fluorescence and transient absorption. Under optical excitation, nonpolar C3 symmetry molecules form polar excited states localized on one of the molecular substituents. Conformational excited state stabilization of molecules with an electron-accepting core causes a formation of twisted internal charge transfer (TICT) states in polar solvents. A low transition dipole moment from TICT state to the ground state causes very weak fluorescence of those compounds and strong dependence on the solvent polarity. The compound formed from the triphenylamine central core and identical arms also experiences excited state twisting, however, weakly sensitive to the solvent polarity.

Excited-State Decay Pathways of Molecular Rotors: Twisted Intermediate or Conical Intersection?

The fluorescence intensity of molecular rotors containing the dicyanomethylenedihydrofuran (DCDHF) motif increases strongly with solvent viscosity. Single-bond and double-bond rotations have been proposed as pathways of nonradiative decay for this and related molecular rotors. We show here that both are involved in the case of DCDHF rotors: Fluorescence is quenched by rotation around the dicyanomethylene double bond in nonpolar solvents, but in a sufficiently polar environment rotation about a formally single bond leads to a dark internal charge-transfer state.

Two-photon absorption dye based on 2,5-bis(phenylacrylonitrile)thiophene with aggregration enhanced fluorescence

This paper reports the synthesis and characterization of a 2,5-bis(phenylacrylonitrile)thiophene based two-photon dye, designed to show enhancement in fluorescence quantum yield in nanoaggregated form. Strong solvatochromism has been observed and explained by the favoritism of locally excited (LE) or internal charge transfer (ICT) state depending on the solvent polarity. Aqueous dispersions of nanoparticles have been prepared and investigated regarding their optical properties which were correlated to the LE and ICT state and the molecular structure of the aggregates.

Intramolecular Charge-Transfer Excited-State Processes in 4-(N,N-Dimethylamino)benzonitrile: The Role of Twisting and the πσ* State.

The structural processes leading to dual fluorescence of 4-(dimethylamino)benzonitrile in the gas phase and in acetonitrile solvent were investigated using a combination of multireference configuration interaction (MRCI) and the second-order algebraic diagrammatic construction (ADC(2)) methods. Solvent effects were included on the basis of the conductor-like screening model. The MRCI method was used for computing the nonadiabatic interaction between the two lowest excited ππ* states (S2(La, CT) and S1(Lb, LE)) and the corresponding minimum on the crossing seam (MXS) whereas the ADC(2) calculations were dedicated to assessing the role of the πσ* state. The MXS structure was found to have a twisting angle of ∼50°. The branching space does not contain the twisting motion of the dimethylamino group and thus is not directly involved in the deactivation process from S2 to S1. Polar solvent effects are not found to have a significant influence on this situation. Applying Cs symmetry restrictions, the ADC(2) calculations show that CCN bending leads to a strong stabilization and to significant charge transfer (CT). Nevertheless, this structure is not a minimum but converts to the local excitation (LE) structure on releasing the symmetry constraint. These findings suggest that the main role in the dynamics is played by the nonadiabatic interaction of the LE and CT states and that the main source for the dual fluorescence is the twisted internal charge-transfer state in addition to the LE state.

Spectral properties of BADAN in solutions with different polarities

Spectral characteristics (absorption spectra, excitation and emission spectra of solvatochromic dye 6-bromo-acetyl-2-dimethylamino-naphtalene (BADAN) were studied in solvents with different polarities. Comparison of BADAN fluorescence excitation spectra corrected for the primary inner filter effect and absorption spectra of this dye allowed to conclude that the long-wavelength absorption band of BADAN is determined by the superposition of two BADAN tautomeric forms having different conformation (planar and non-planar) and different distribution of electronic density. The analysis of BADAN excitation and emission spectra has shown that in non-polar solvents (hexane) fluoresces only one dye form which absorbs in more short-wave part of the spectrum than other conformer. In polar solvents (acetonitrile, mixture of acetonitrile/water) both forms fluoresce. It is concluded that the fluorescence of the dye in hexane and short-wavelength component of BADAN fluorescence at excitation in short-wavelength region of absorption spectrum are due to the locally excited state of the planar BADAN molecule. Fluorescence of BADAN in acetonitrile at excitation in long-wavelength part of long-wavelength absorption band and long-wavelength component of BADAN fluorescence spectrum in acetonitrile at excitation in short-wavelength region of absorption spectrum are due to non-planar BADAN conformers with internal charge transfer state.

Carbazole-terpyridine donor–acceptor luminophores

With the aim to combine two versatile molecular units that are widely utilized in materials and coordination chemistry, bischromophoric electron donor–acceptor carbazole–terpyridine systems (Cbz–Tpy) have been synthesized and characterized. The connecting bridge between the two moieties is constituted by phenylene (1), methylene phenylene (2) and ethynylene phenylene (3), which allow tuning of the intercomponent electronic interactions. Electrochemical studies evidence that oxidation and reduction processes occur on the Cbz and Tpy units, respectively, suggesting the possibility of internal charge transfer states located at about 3.10 eV, which is confirmed by photophysical investigations. The absorption spectra of 1 and 3 show a tail above 300 nm, indicating that these conjugated systems exhibit low-energy transitions with charge-transfer character, as confirmed by theoretical studies. At 298 K, 1–3 show a complex pattern of fluorescence profiles as a function of solvent (toluene, dichloromethane, acetonitrile), often with double emissions attributed to transitions localized on individual chromophores or associated to internal charge-transfer processes. The variability of the spectral position affords multiple colours, including white (e.g.3 in acetonitrile). Definitive rationalization and assignment of transitions of 1–3 is obtained through singlet and triplet luminescence spectra at 77 K and by means of DFT and TD-DFT methods using the hybrid functional PBE0 and the long range corrected functional CAM-B3LYP with the polarizable continuum model. This work opens the route to versatile materials based on the Tpy–Cbz motif exhibiting luminescence all across the visible spectral region that can be controlled through electronic conjugation, solvent polarity, temperature, functionalization and cation binding.

Abnormal Enhancement of the Photoisomerization Process in a trans-Nitroalkoxystilbene Dimer Sequestered in β-Cyclodextrin Cavities

We report on the synthesis and the photophysical properties of a trans-nitroalkoxystilbene dimer (DPNS). The fluorescence quantum yield (Φ(f)), the Stokes shift, and the quantum yield for the trans-to-cis photoisomerization (Φ(t→c)) are strongly dependent on the nature of the solvent. Upon increasing solvent polarity, Φ(f) increases together with the decrease of Φ(t→c). This solvent-induced reverse behavior mainly stems from the progressive stabilization of a highly polar twisted internal charge transfer state (TICT) at excited singlet level which opens a competing channel to photoisomerization. In the presence of hydroxylic substrates (i.e., alcohols or water), fluorescence of DPNS is strongly quenched due to a hydrogen bonding interaction at excited state. The efficiency of the process is clearly correlated to the H-bond donor ability of the quencher. In aqueous solution, the major formation of a 2:1 host-guest complex with β-cyclodextrins (β-CD) prevents the quenching by H(2)O and leads to a 50-fold increase of the fluorescence signal together with a strong band blue-shift with respect to that of the free chromophore. This latter effect was rationalized in terms of a severe reduction of the solvent-induced stabilization of the TICT state. As a consequence, the trans-to-cis photoisomerization reaction is reactivated and leads to a paradoxical 14-fold increase of Φ(t→c) even though DPNS is sequestered in β-CD cavities.

Exciton diffusion, annihilation and their role in the charge carrier generation in fluorene based copolymers

Exciton diffusion, annihilation and their role in the charge carrier generation in fluorene based copolymers F8BT and PSF-BT have been investigated by means of femtosecond transient absorption combined with photoluminescence and photoconductivity measurements. No indications of exciton–exciton annihilation were observed in polymer solutions. Exciton–exciton annihilation was found to be much more efficient in F8BT film than in PSF-BT film. Annihilation facilitates charge carrier generation in F8BF films, while it only speeds up formation of the internal charge transfer states in PSF-BT films.

Synthesis, Characterization, and Spectroscopic Investigation of Benzoxazole Conjugated Schiff Bases

Two Schiff bases were synthesized by reaction of 2-(4'-aminophenyl)benzoxazole derivatives with 4-N,N-diethylaminobenzaldehyde. UV-visible (UV-vis) and steady-state fluorescence in solution were applied in order to characterize its photophysical behavior. The Schiff bases present absorption in the UV region with fluorescence emission in the blue-green region, with a large Stokes' shift. The UV-vis data indicates that each dye behaves as two different chromophores in solution in the ground state. The fluorescence emission spectra of the dye 5a show that an intramolecular proton transfer (ESIPT) mechanism takes place in the excited state, whereas a twisted internal charge transfer (TICT) state is observed for the dye 5b. Theoretical calculations were performed in order to study the conformation and polarity of the molecules at their ground and excited electronic states. Using density functional theory (DFT) methods at theoretical levels BLYP/Aug-SV(P) for geometry optimizations and B3LYP/6-311++G(2d,p) for single-point energy evaluations, the calculations indicate that the lowest energy conformations are in all cases nonplanar and that the dipole moments of the excited state relaxed structures are much larger than those of the ground state structures, which corroborates the experimental UV-vis absorption results.

PH-Responsive chromogenic-sensing molecule based on bis(indolyl)methene for the highly selective recognition of aspartate and glutamate

Bis(indolyl)methene displays high selectivity and sensitivity for aspartate and glutamate in water-containing medium based on the proton transfer signaling mode. The presence of acid can easily induce proton transfer to the basic H-bond acceptor moiety, which modulates the internal charge transfer state of the bis(indolyl)methene skeleton and gives rise to dramatic change in color. The detection limits for aspartate and glutamate were 0.80 ppm and 1.12 ppm, respectively.

Protochlorophyllide Excited-State Dynamics in Organic Solvents Studied by Time-Resolved Visible and Mid-Infrared Spectroscopy

Protochlorophyllide (PChlide) is a precursor in the biosynthesis of chlorophyll. Complexed with NADPH to the enzyme protochlorophyllide oxidoreductase (POR), it is reduced to chlorophyllide, a process that occurs via a set of spectroscopically distinct intermediate states and is initiated from the excited state of PChlide. To obtain a better understanding of these catalytic events, we characterized the excited state dynamics of PChlide in the solvents tetrahydrofuran (THF), methanol, and Tris/Triton buffer using ultrafast transient absorption in the visible and mid-infrared spectral regions and time-resolved fluorescence emission experiments. For comparison, we present time-resolved transient absorption measurements of chlorophyll a in THF. From the combined analysis of these experiments, we derive that during the 2-3 ns excited state lifetime an extensive multiphasic quenching of the emission occurs due to solvation of the excited state, which is in agreement with the previously proposed internal charge transfer (ICT) character of the S1 state ( Zhao , G. J. ; Han , K. L. Biophys. J. 2008 , 94 , 38 ). The solvation process in methanol occurs in conjunction with a strengthening of a hydrogen bond to the Pchlide keto carbonyl group. We demonstrate that the internal conversion from the S2 to S1 excited states is remarkably slow and stretches out on to the 700 fs time scale, causing a rise of blue-shifted signals in the transient absorption and a gain of emission in the time-resolved fluorescence. A triplet state is populated on the nanosecond time scale with a maximal yield of approximately 23%. The consequences of these observations for the catalytic pathway and the role of the triplet and ICT state in activation of the enzyme are discussed.

Theoretical Study on the Photochemical Behavior of 4-Dimethylamino-4′-cyanodiphenylacetylene

The photochemical behavior of 4-dimethylamino-4'-cyanodiphenylacetylene (DCDPA) has been examined by ab initio complete active space self-consistent field (CASSCF) and its second-order multireference Moller-Plesset perturbation (MRMP2) methods. DCDPA is mainly excited into an internal charge-transfer (ICT) state where the charge transfers from the dimethylanilino group into the benzonitrile part. In S(1), there are two stable geometries. One is an ICT state where DCDPA is skeletally relaxed from the stable geometry in S(0) upon electronic excitation. The other is a diradical state where DCDPA takes a trans-bent form and the acetylenic C identical withC triple bond turns to be an ethylenic C horizontal lineC double bond, as already confirmed in the case of parent diphenylacetylene. On the way to the globally stable trans-bent form in S(1), a conical intersection between the charge-transfer and the diradical state is located. The photochemical behavior of DCDPA in polar solvent has been also examined by CASSCF combined with the polarized continuum model. Thereby, it was found that a twisted ICT (TICT) state where the dimethylanilino group is perpendicularly twisted against the remaining part is formed in polar solvent.

Thioflavin T as a Molecular Rotor: Fluorescent Properties of Thioflavin T in Solvents with Different Viscosity

The effect of solvent viscosity on thioflavin T (ThT) fluorescent properties is analyzed to understand the molecular mechanisms of the characteristic increase in ThT fluorescence intensity accompanying its incorporation into the amyloid-like fibrils. To this end, the dependencies of the ThT quantum yield and fluorescence lifetime on temperature and glycerol content in the water-glycerol mixtures are studied. It has been found that fluorescent properties of ThT are typical for the specific class of fluorophores known as molecular rotors. It has been established that the low ThT fluorescence intensity in the solvents with low viscosity is caused by the nonradiative deactivation of the excited state associated with the torsional motion of the ThT benzthiazole and aminobenzene rings relative to each other, which results in the transition of ThT molecule to nonfluorescent twisted internal charge transfer (TICT) state. The rate of this process is determined by the solvent viscosity, whereas the emission does occur from the nonequilibrium locally excited (LE) state. High polarization degree of the ThT fluorescence (P = 0.45) observed for glycerol solutions of different viscosity confirms the nonequilibrium character of the emission from the LE state and testifies that rotational correlation time of the whole molecule is considerably greater than the time required to accomplish transition to the nonfluorescent TICT state. Torsional movements of the ThT fragments take place in the same temporal interval as solvent relaxation, which leads to nonexponential fluorescence decay of the dye in viscous solvents. This photophysical model successfully explains the fluorescent properties of ThT in solvents with different viscosities. The model is confirmed by the results of the quantum-chemical calculations, which showed that energy minimum for the ground state of ThT corresponds to conformation with torsional angle phi = 37 degrees between the benzthiazole and aminobenzene rings and in the excited-state twisted conformation of ThT with phi = 90 degrees has minimal energy. These data support the idea that the reason for the characteristic increase in the ThT fluorescence intensity accompanying its incorporation into the amyloid fibrils is determined by the rigidity of the dye environment, which prevents the rotation of the benzthiazole ring relative to the aminobenzene ring in the excited state.

Molecular logic devices (half-subtractor, comparator, complementary output circuit) by controlling photoinduced charge transfer processes

The fluorescence properties of a naphthalene derivative with a methoxy and a pyridyl substituent were investigated, including the influence of protons, electron-donating amines, and dihydrogenphosphate anions. The naphthalene derivative shows strong fluorescence with a maximum at 353 nm, which can be quenched by amines through a diffusion-controlled photoinduced electron transfer. Protonation of the pyridyl residue leads to the observation of a red-shifted broad emission at 470 nm, which has been assigned to an internal charge transfer state. The interpretation of the fluorescence behaviour in the presence or absence of the chemical inputs leads to the realisation of unimolecular logic circuits, which are able to perform advanced arithmetic operations such as subtraction (XOR/INH combination) and comparison (XNOR/INH combination). Furthermore, a complementary output circuit (INH/IMP combination) was implemented. The implicated logic gates can be reconfigured, either by application of different input sets or by varying the fluorescence output observation wavelength. The system works in an all-fluorescence output mode, which leads to a superposition of the gates. This is of particular advantage as it allows reading out without any time lag, which is a commonly encountered problem in silicon circuitry based on electrical signals.

The Geometry of the Excited Charge Transfer States: Flattening or Twisting?

Numerous molecules built of an electron acceptor and electron donor units linked together by single bond exhibit a peculiar behavior in their low-lying electronically excited states. N,N-diethylaminoacetophenone and 4-acetyl-4 0 -dimethylamino-biphenyl, having acetophenone as the acceptor and difierentiated by the donors groups N(CH3)2 and dimethylaniline, respectively, were selected as the subject of study. To recognize the excited state relaxation paths the stationary and time-resolved spectroscopy in absorption and emission was applied. Experimental results indicate that after excitation in polar solvents N,N-diethylaminoacetophenone reduces to minimum the overlap between …-electron systems of the donor and acceptor groups relaxing to the twisted internal charge-transfer state; contrary to that, ∞attening of the 4-acetyl-4 0 -dimethylamino-biphenyl skeleton generates increase in the overlap of the …-electron systems.

Bimodal fluorescence signaling based on control of the excited-state conformational twisting and the ground-state protonation processes

Amino-based fluoroionophores 1 and 2 can selectively sense alkaline earth metal ions in MeCN under both neutral and acidic conditions by different signaling mechanisms. The fluoroionophoric behavior for the neutral probes is characterized by an ‘off–on’ photoinduced electron transfer (PET)-like fluorescence intensity response due to a switching from a twisted internal charge transfer (TICT) to a planar internal charge transfer (PICT) state. For the protonated probes (i.e., 1/H + and 2/H +), the fluorescing species is the localized stilbene fluorophores, but dual fluorescence is induced upon metal-ion recognition through a deprotonation process.

Temporal dependence of the dipole moment of long-wavelength forms of n-dimethylaminobenzonitrile

The temporal dependences μe(t) of the dipole moment of n-dimethylaminobenzonitrile excited to different states by quanta with different energies are evaluated based on the experimental correlation functions of the shift of instantaneous spectra. It is found that, upon excitation into the maximum of absorption, the relaxation changes of the spectra are accompanied by an increase in the dipole moment from 9 D in the Franck-Condon state to 16 D in the stationary state. At the same time, the excitation of luminescence at the red absorption edge by radiation at 403 nm leads to a decrease in the range of variation of the dipole moment. In this case, the initial value of the dipole moment amounts to 14 D and, during the lifetime of the excited state, it increases to 17 D. This indicates that the nature of the radiation excited at the wavelength 403 nm is directly related to internal charge transfer states of n-dimethylaminobenzonitrile.