Formation Of Twisted Intramolecular Charge Transfer State Research Articles

Twisted intramolecular charge‐transfer state of trans‐3‐(N,N‐Dimethylamino)‐4'‐cyanostilbene: The C−C bond twisting

AbstractWhether a twisted intramolecular charge‐transfer (TICT) state is formed is an important issue for understanding the fluorescence properties of a push‐pull organic dye. Here we report a position effect of the donor substituent on the TICT state formation of aminostilbenes: namely, trans‐3‐(N,N‐dimethylamino)‐4′‐cyanostilbene (mDCS) behaves differently from its TICT‐free para isomer DCS and forms a TICT state in acetonitrile (MeCN). The TICT state of mDCS also differs from that of many previously reported aminostilbenes by twisting a C−C bond instead of a C−N bond. In addition, among the ring‐bridged model compounds mDCS‐N, mDCS‐C1, and mDCS‐C2, we observed an unusual electronic effect of the ring‐bridging in mDCS‐C2 that mitigates the impact of the TICT state on the fluorescence properties. Both the C−C bond twisting in mDCS and the ring‐bridging electronic effect in mDCS‐C2 provide new insights into the TICT chemistry of aminostilbenes.

Substituent Effect on AIE mechanism of two coumarin derivatives: uncommon TICT fluorescence in aggregation state.

Two coumarin derivatives, 7-diethylamino-3-(4-nitrophenyl)coumarin (DNC) and 7-hydroxy-3-(4-nitrophenyl)coumarin (HNC), were synthesized via Knoevenagel condensation of salicylaldehyde derivatives with 4-nitrophenylacetonitrile and then cyclization reaction. Both of them were characterized by single-crystal X-ray diffraction. The molecules of DNC are stacked via π-π interaction, while the hydrogen bond interactions instead of π-π interaction were observed in the crystal packing of HNC. Both of DNC and HNC showed solvatochromic properties and aggregation-induced emission (AIE) activities, but the AIE characteristics of them were entirely different. HNC exhibited an AIE phenomenon as the result of the restriction of twisted intramolecular charge transfer (TICT), while DNC emitted peculiar dual fluorescence which was assigned to the emission based on the inhibition of TICT state formation and the emission from the TICT state respectively.

Boosting Photothermal Theranostics via TICT and Molecular Motions for Photohyperthermia Therapy of Muscle-Invasive Bladder Cancer.

The development of photothermal agents with high photothermal conversion efficiency (PCE) can help to reduce drug and laser dosage, but still remains a big challenge. Herein, a novel approach is reported to design photothermal agents with high PCE values by promoting nonradiative heat generation processes through the cooperation of twisted intramolecular charge transfer (TICT) and molecular motions. Within the designed molecule 2DMTT-BBTD, the tetraphenylethenes act as molecular rotors, the long alkyl chain grafted thiophene helps to twist the molecular geometry to facilitate TICT state formation and preserve molecular motions in aggregate, while the strong electron-withdrawing BBTD unit enhances TICT effect. 2DMTT-BBTD exhibits NIR-absorption and a high PCE value of 74.8% under 808nm laser irradiation. Gambogic acid (GA) which surmounts tumor cell thermotolerance by inhibiting heat shock protein 90 (HSP90) expression is coloaded into the nanoparticles, RGD peptide is further introduced to the nanoparticle surface to improve tumor accumulation. The resultant nanoparticles facilitate the effective low-temperature hyperthermia therapy of muscle-invasive bladder cancer (MIBC) with minimal damage to surrounding heathy tissues. This work delivers a new design concept for development of highly efficient photothermal agents, which also provides a safer approach for noninvasive treatment of MIBC and other malignant tumors.

Temperature-Controlled Locally Excited and Twisted Intramolecular Charge-Transfer State-Dependent Fluorescence Switching in Triphenylamine-Benzothiazole Derivatives.

Triphenylamine–benzothiazole derivatives, N-(4-(benzo[d]thiazol-2-yl)phenyl)-N-phenylbenzenamine (1), N-(4-(benzo[d]thiazol-2-yl)-3-methoxyphenyl)-N-phenylbenzenamine (2), and 2-(benzo[d]thiazol-2-yl)-5-(diphenylamino)phenol (3), showed unusual temperature-controlled locally excited (LE) and twisted intramolecular charge-transfer (TICT) state fluorescence switching in polar solvents. The detailed photophysical studies (absorption, fluorescence, lifetime, and quantum yield) in various solvents confirmed polarity-dependent LE and TICT state formation and fluorescence tuning. 1 and 2 exhibited strong fluorescence with short lifetime in nonpolar solvents compared to polar solvents. 1, 2, and 3 in dimethylformamide (DMF) at room temperature showed low-energy weak TICT state fluorescence, whereas high-energy strong LE state fluorescence was observed at −196 °C. Interestingly, further increasing the temperature from 20 to 100 °C, the DMF solution of 1 and 2 exhibited rare fluorescence enhancement with a slight blue shift of λmax via activating more vibrational bands of the TICT state. Thus, 1 and 2 showed weak TICT state fluorescence at room temperature, strong LE state fluorescence at −196 °C, and activation of TICT state at 100 °C. Moreover, molecular conformation and aggregation in the solid state influenced strongly on the fluorescence properties of 1, 2, and 3. Solid-state fluorescence and pH-responsive imidazole nitrogen have been exploited for demonstrating halochromism-induced fluorescence switching. Computational studies provided further insights into the fluorescence tuning and switching. The present studies provide understanding and opportunity to make use of D–A organic molecules in the LE and TICT states for achieving fluorescence switching and tuning.

Gigantic Relevance of Twisted Intramolecular Charge Transfer for Organic Dyes Used in Solar Cells

Within this work, we emphasis on the importance of twisted intramolecular charge transfer (TICT) process in organic dyes based on triphenyl amine moiety to achieve high performance in dye-sensitized solar cells. Through the comparison between two recent made dyes, L1 and L1Fc, on different semiconductors (TiO2, and ZrO2), we could spectrally and dynamically detect for the first time the formation of TICT state for L1 on ZrO2 after localized charge transfer (LCT) state population, and an electron injection process from TICT state on TiO2. However, for the excited L1Fc dye, the ultrafast electron transfer from ferrocene (Fc) moiety to the L1 unit quenched the formation of TICT state in L1Fc on semiconductors, leading instead to an electron injection process from the LCT state. The electron injection from TICT state in L1 associated with structural rearrangements on TiO2 leads to slow recombination process and an efficiency improvement of about 325%, compared to solar cells based on L1Fc dye, in which TICT s...

Dual emission from 2-(4′-N,N-dimethylaminophenyl)pyridoimidazole–nanoparticle composite: Effect of β-cyclodextrin

The interactions of the silver nanoparticles with 2-(4′-N,N-dimethylaminophenyl)benzimidazole and its nitrogen substituted analogues, 2-(4′-N,N-dimethylaminophenyl)pyridoimidazoles are investigated by absorption, steady-state and time resolved fluorescence, field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) techniques. The surface plasmon resonance band, the FESEM and the TEM images of the particles suggest that the fluorophores can stabilize the nanoparticles even in the absence of any other stabilizing agent. On the other hand, in the absence of fluorophores the nanoparticles are unstable and coagulate. In contrary to the earlier literature reports that interactions of nanoparticles with intramolecular charge transfer (ICT) or twisted intramolecular charge transfer (TICT) species quenches their fluorescence, to the best of our knowledge, the first ever formation of TICT state by interactions of nanoparticles with the fluorophores is observed. The formation of TICT state in 2-(4′-N,N-dimethylaminophenyl)pyridoimidazoles results in dual emission. The TICT emissions from the nanoparticle-fluorophore complexes are weak. But the emissions become prominent upon complexation with β-cyclodextrin.

Tuning Interfacial Charge Separation by Molecular Twist: A New Insight into Coumarin-Sensitized TiO2 Films

The 7-aminocoumarin class of dyes exhibits significant molecular twisting in their electronic excited state, which has important consequences on the reaction mechanism involving the intramolecular charge-transfer (ICT) state. In this work, the relationship between the molecular structure of coumarin dye and charge separation on TiO2 surface has been explored using 7-(dialkylamino)coumarin-3-carboxylic acid dyes C343 and D1421. The alkylamino group in the form of tethered constraint geometry of the julolidyl ring of coumarin C343 and rotation free geometry of 7-N,N′-diethylamino moiety of coumarin D1421 compel planar and twisted ICT (TICT) excited states in polar solvents, respectively. The femtosecond transient absorption studies show that both the charge-transfer states participate in directional electron injection from coumarin dye into the conduction band of the TiO2 semiconductor substrate. The TICT state formation in the excited state minimizes the donor and acceptor orbital overlap and causes slow b...

Photophysics of a Coumarin in Different Solvents: Use of Different Solvatochromic Models

This study reported the photophysics of 7-(diethylamino)coumarin-3-carboxylic acid N-succinimidyl ester (7-DCCAE) in different neat solvents of varying polarity using steady-state absorption, fluorescence emission and picosecond time-resolved spectroscopy. In nonpolar solvents, the dye molecule predominantly exists in nonpolar structure and exhibits very low value of nonradiative decay rate constant (k(nr)), demonstrating the emission takes place from S(1) -LE to S(0) ground state. The fluorescence quantum yields, lifetime values of 7-DCCAE in different solvents are rationalized on the basis of intramolecular charge transfer (ICT) followed by twisted intramolecular charge transfer state formation (TICT) as well as specific solute-solvent interactions. Several solvatochromic models (such as Lippert, Dimroth, Kamlet-Taft, Catalán 3P and Catalán 4P models) were used to analyze the solvatochromic shift of 7-DCCAE in different solvents. The different empirical models show that the observed results are better correlate for nonchlorinated solvents and provide statistically significant best-fit result. A comparison was done between comparatively new solvatochromic model (Catalán 3P and Catalán 4P model) with Kamlet-Taft model. The ground state structure of the said molecule was optimized by using Density Functional Theory (DFT).

Photoionization and Time-Dependent Stokes Shift of Coumarin 307 in Soft Matter: Solvation and Radical-Ion Pair Recombination Dynamics

Photoionization, fluorescence time-dependent Stokes shift (TDSS), and rotational dynamics of coumarin 307 (C307) have been investigated in soft matter system such as micelles using time-resolved transient absorption and fluorescence spectroscopy. Photoionization of C307 leads to the formation of coumarin radical cation and hydrated electron, which were characterized by their respective transient absorption. The photoionization yields are significantly higher in anionic sodium dodecyl sulfate (SDS) micelle than in cationic cetyltrimethylammonium bromide (CTAB) and neutral Triton X-100 (TX-100) micelles, indicating the influence of micellar surface charge on the efficient separation of radical cation-hydrated electron pair. The CTAB micelle favors the recombination of radical cation and hydrated electron leading to the formation of triplet state of C307, which causes a decrease in the photoionization yield. C307 exhibits TDSS in all micelles; the time evolution and the magnitude of the TDSS depend on nature of the micelle. In TX-100 micelles, the decay of the TDSS exhibits ultraslow component (165 ns) and is affected by the presence of electron scavengers. The ultraslow component in TX-100 micelle originates from the recombination of radical cation-hydrated electron, which results in the formation of twisted intramolecular charge transfer (TICT) state; such formation of TICT state was not observed in SDS and CTAB micelles. To the best of our knowledge, this is the first report where the radical-ion pair recombination dynamics is probed using TDSS in combination with time-resolved transient absorption studies. The activation energy for the solvent relaxation and radical-ion pair (solvent separated) recombination process was found to be 6.1 and 3.0 kcal mol(-1), respectively. Temperature effect on TDSS in TX-100 micelles confirmed the increase in the water hydration, and size of the micelle influences the relative contribution of the solvation and radical-ion pair recombination dynamics toward the total TDSS. We propose that TDSS observed in neutral micelles and reported in other biomolecules such as proteins by the 7-amino coumarin probe is not only due to the solvation dynamics alone but also due to the radical-ion pair recombination dynamics.

ChemInform Abstract: Triazinylaniline Derivatives as Fluorescence Probes. Part 1. Absorption and Fluorescence in Organic Solvents and in Aqueous Media in Relation to Twisted Intramolecular Charge-Transfer State Formation, Hydrogen Bonding, and Protic Equi

The UV absorption and fluorescence characteristics of new optical probes based on a 1,3,5-triazinylaniline chromophore and of general structure p-Et2NC6H4C3N3(Cl)NR1R2 are reported. Increase in alkanol solvent polarity strongly quenches the fluorescence by a process attributed to TlCT (twisted intramolecular charge-transfer) state formation with co-operative solvent relaxation (log kNR/s–1 8.5–10.5). In aqueous media no changes in fluorescence band position or intensity are observed in the pH range 3.5 to 11 but below pH 3.5 protonation of the anilino nitrogen (pKaca. 2.4) eliminates both the λ= 360 nm absorption band and the weak λ= 460 nm fluorescence band. Free and hydrogen-bonded species, with respect to the anilino nitrogen centre, are present in aqueous dioxane solutions.The triazinylaniline derivatives are prototypes of promising sensitive probes for organized media and as models of proteins labelled with the triazinylaniline fluorophore.

Theoretical study on photochemical behavior of trans-2-[4′-(dimethylamino)styryl]benzothiazole

Photochemistry of trans-2-[4′-(dimethylamino)styryl]benzothiazole ( t-DMASBT) has been examined theoretically. Ground state calculations are performed by density functional theory (DFT) method. For excited-state calculations restricted configuration interaction singles (RCIS) combined with time dependent DFT (TDDFT) approach was used. Theoretical calculations predicted the existence of two ground state conformers and the photophysics of both the conformers are nearly identical. With torsional rotation of dimethylamino group, the energies of S 1 and S 2 states increase, but that of S 3 state decreases. In contrary to an earlier report [J. Photochem. Photobiol. A: Chem. 195 (2008) 368–377], the calculation predicted at the perpendicular geometry the HOMO is localized on dimethylamino and, HOMO and LUMO are decoupled. Excitation from HOMO to LUMO at perpendicular geometry results in twisted intramolecular charge transfer (TICT) state. Avoided crossing of S 3 and S 1 states causes a barrier for formation of TICT state from locally excited state. Potential energy surface of isomerization constructed by TDDFT method suggests that the photoisomers are formed through phantom state in a nonradiative way, not by a radiative way from cis and trans isomers as proposed earlier by Saha et al. [J. Photochem. Photobiol. A: Chem. 199 (2008) 179–187].

Contrasting Solvent Polarity Effect on the Photophysical Properties of Two Newly Synthesized Aminostyryl Dyes in the Lower and in the Higher Solvent Polarity Regions

Solvent polarity effect on the photophysical properties of two newly synthesized aminostyryl-thiazoloquinoxaline dyes, one with a flexible diphenylamino group, namely, N,N-diphenyl-4-[2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl]aniline (TQ1), and the other with a rigid julolidinylamino group, namely, (9-[2-(thiazolo[4,5-b]quinoxalin-2-yl)vinyl]julolidine) (TQ2), have been investigated in different aprotic solvents and solvent mixtures. From the polarity dependent changes in the absorption and fluorescence spectral properties, it is indicated that the fluorescent states of the dyes are of intramolecular charge transfer (ICT) character. For both the dyes, the photophysical properties like fluorescence quantum yields (Phi(f)), fluorescence lifetimes (tau(f)), radiative rate constants (k(f) = Phi(f)/tau(f)), and nonradiative rate constants (k(nr) = 1/tau(f) - Phi(f)/tau(f)) show clearly contrasting solvent polarity effects in the lower and in the higher solvent polarity region, causing an interesting reversal in the properties below and above an intermediate solvent polarity. It is inferred that the domination of the cis-trans isomerization in the lower solvent polarity region and that of the twisted intramolecular charge transfer (TICT) state formation in the higher solvent polarity region are responsible for the observed contrasting solvent polarity effects on the photophysical properties of the two dyes. As both isomerization and TICT state formation causes an enhancement in the nonradiative decay rate of the excited dyes and both the processes become less significant at the intermediate solvent polarity region, the two dyes show their largest Phi(f) and tau(f) values at intermediate solvent polarities. Suitable mechanistic schemes have been proposed and qualitative potential energy diagrams have been presented to explain the observed results with the changes in the polarity of the solvents used.

Photophysical Properties of Coumarin‐7 Dye: Role of Twisted Intramolecular Charge Transfer State in High Polarity Protic Solvents

Photophysical studies on coumarin-7 (C7) dye in different protic solvents reveal interesting changes in the properties of the dye on increasing the solvent polarity (Deltaf; Lippert-Mataga solvent polarity parameter) beyond a critical value. Up to Deltaf approximately 0.31, the photophysical properties of the dye follow good linear correlations with Deltaf. For Deltaf > approximately 0.31, however, the photophysical properties, especially the fluorescence quantum yields (Phi(f)), fluorescence lifetimes (tau(f)) and nonradiative rate constants (k(nr)), undergo large deviations from the above linearity, suggesting an unusual enhancement in the nonradiative decay rate for the excited dye in these high polarity protic solvents. The effect of temperature on the tau(f) values of the dye has also been investigated to reveal the mechanistic details of the deexcitation mechanism for the excited dye. Studies have also been carried out in deuterated solvents to understand the role of solute-solvent hydrogen bonding interactions on the photophysical properties of the dye. Observed results suggest that the fluorescence of the dye originates from the planar intramolecular charge transfer (ICT) state in all the solvents studied and the deviations in the properties in high polarity solvents (Deltaf > approximately 0.31) arise due to the participation of a new deexcitation channel associated with the formation of a nonfluorescent twisted intramolecular charge transfer (TICT) state of the dye. Comparing present results with those of a homologous dye coumarin 30 (C30; Photochem. Photobiol., 2004, 80, 104), it is indicated that unlike in C30, the TICT state of the C7 dye does not experience any extra stability in protic solvents compared to that in aprotic solvents. This has been attributed to the presence of intramolecular hydrogen bonding between the NH group (in the 3-benzimidazole substituent) of the C7 dye and its carbonyl group, which renders an extra stability to the planar ICT state, making the TICT state formation relatively difficult. Qualitative potential energy diagrams have been proposed to rationalize the differences observed in the results with C7 and C30 dyes in high polarity protic solvents.

Fluorescence Properties of DMABN in Aqueous Micellar Solutions of Surfactants

Fluorescence spectroscopic properties of p-N,N-dimethylaminobenzonitrile (DMABN) in various kinds of surfactant micellar solutions were investigated. The results showed that the characteristics of the palisade layers of different surfactant micelles affected the twisted intramolecular charge transfer (TICT) properties of DMABN. In the ionic micellar solution, the electric field of the charged head-groups in the palisade layer was found to be a main affecting factor that promoting the formation of TICT state. The larger of the counterion dissociation degree, the stronger of Ia/Ib in the micellar solution. In the nonionic micellar solution, the effect of hydrogen bonding became distinct due to a large amount of water wrapped in the shell of the poly(ethylene oxide) (PEO) chains. Besides, the terminal hydroxyl groups of the short PEO chains were also able to hydrogen-bond with DMABN. In the view of photo-physics of DMABN, the anionic micelles with large dissociation degree of the counterions were suitable to as DMABN dispersed carriers for their TICT properties, for example, SDS or SDSO.

Photoinduced Single- versus Double-Bond Torsion in Donor−Acceptor−Substituted trans-Stilbenes

The electronic absorption and fluorescence spectra, quantum yields for fluorescence (Phi(f)) and trans --> cis photoisomerization (Phi(tc)), and fluorescence lifetimes of trans-4-(N-arylamino)-4'-cyanostilbenes (2H, 2Me, 2OM, 2CN, and 2Xy with aryl = phenyl, 4-methylphenyl, 4-methoxyphenyl, 4-cyanophenyl, and 2,5-dimethylphenyl, respectively), trans-4-(N-methyl-N-phenylamino)-4'-cyanostilbene (2MP), trans-4-(N,N-diphenylamino)-4'-cyanostilbene (2PP), trans-4-(N-methyl-N-phenylamino)-4'-nitrostilbene (3MP), and three ring-bridged analogues 2OMB, 2MPB, and 3MPB are reported. Whereas fluorescence and torsion of the central double bond account for the excited decay of the majority of these donor-acceptor substituted stilbenes in both nonpolar and polar solvents (i.e., Phi(f) + 2Phi(tc) approximately 1), exceptions are observed for 2OM, 3MP, and 3MPB in solvents more polar than THF and for 2Me and 2MP in acetonitrile as a result of the formation of a weakly fluorescent and isomerization-free twisted intramolecular charge transfer (TICT) state (i.e., Phi(f) + 2Phi(tc) << 1). The TICT state for 2OM, 2Me, and 2MP results from the torsion of the stilbenyl-anilino C-N single bond, but the torsion of the styryl-anilino C-C bond is more likely to be responsible for the TICT state formation of 3MP and 3MPB. In conjunction with the behavior of aminostilbenes 1, a guideline based on the values of Phi(f) and Phi(tc) for judging the importance of a TICT state for trans-stilbenes is provided. Accordingly, the TICT state formation is unimportant for the excited decay of trans-4-(N,N-dimethylamino)-4'-cyanostilbene (DCS). In contrast, our results support the previously proposed TICT state for trans-4-(N,N-dimethylamino)-4'-nitrostilbene (DNS).

Intramolecular Charge-Transfer State Formation of 4-(N,N-Dimethylamino)benzonitrile in Acetonitrile Solution: RISM-SCF Study

Intramolecular charge-transfer (ICT) state formation of 4-(N,N-dimethylamino)benzonitrile in acetonitrile solution is studied by the reference interaction site model self-consistent field (RISM-SCF) method. Geometry optimizations are performed for each electronic state in solution with the complete-active-space SCF wave functions. Dynamic electron correlation effects are taken into account by using the multiconfigurational quasidegenerate perturbation theory. Two-dimensional free energy surfaces are constructed as the function of the twisting and wagging angles of the dimethylamino group for the ground and locally excited (LE) states. The calculated absorption and fluorescence energies are in good agreement with experiments. The validity of the twisted ICT (TICT) model is confirmed in explaining the dual fluorescence, and the possibility of the planar ICT model is ruled out. To examine the mechanism of the TICT state formation, a "crossing" seam between the LE and charge-transfer (CT) state surfaces is determined. The inversion of two electronic states occurs at a relatively small twisting angle. The effect of solvent reorganization is also examined. It is concluded that the intramolecular twisting coordinate is more important than the solvent fluctuation for the TICT state formation, because the energy difference between the two states is minimally dependent on the solvent configuration.

Theoretical Study of Solvent Effects on the Intramolecular Charge Transfer of a Hemicyanine Dye

4-[2-(4-Dimethylaminophenyl)ethenyl]-1-methylpyridinium iodide (HR) is a solvatochromic dye and is widely used in Langmuir−Blodgett (LB) film preparations. Quantum chemical calculations combined with a cylindrical solvation model are used to investigate the possibility of TICT (twisted intramolecular charge transfer) state formation by rotating part of the molecule around several different chemical bonds. The results account for most of the experimental observations and partially confirm the assumption that the structure of HR is a compromise between benzenoid and quinoid resonance forms. It is revealed that the TICT state can be formed by rotating the dimethylamino group to the perpendicular position in polar solvents. But this probability should be very small due to the relatively high energy barrier to the rotation in the excited state. Rotations of the pyridyl ring and the aniline ring enhance the charge transfer in the first excited state. There is a small energy barrier to the rotation of the pyridyl ring in the first excited state. In polar solvents, there is no barrier to rotation of the aniline ring in the excited state. The high-energy barrier to rotation around the central double bond is the cause of the low photoisomerization yield of HR.

Influence of the Attachment of Chromophores to a Polymer Chain on Their Twisted Intramolecular Charge-Transfer State in Dilute Solution

Fluorescence from the initially excited singlet state (LE) and the twisted intramolecular charge-transfer (TICT) state of 4-(N,N-dimethylamino)benzoate (DMAB), 4-(N,N-diethylamino)benzoate (DEAB), 4-(N-pyrrolidino)benzoate (PYR), 4-(N-piperidino)benzoate (PIP), 4-(N-morpholino)benzoate (MOR), and 4-(N-2,6-dimethylmorpholino)benzoate (26DMM) were compared with the free and polystyrene-bound chromophores. The ground-state twisting of the donor with respect to the acceptor can have a large influence on the charge distribution. The polymeric chain can force a less planar geometry and cause a bathochromic shift and a broadening of the absorption band in the electronic absorption spectra. In the area of 350 nm the emission is assigned to the locally excited state (with a planar geometry) and the emission with in the area of 450−520 nm to the TICT state (with a perpendicular conformation). Experimental results show (i) no polymeric chain effect in emission distribution for the DMAB molecule, (ii) a significant red edge effect (REE) for the PIP probe, (iii) a slight red edge effect for 26DMM, (iv) a specific blue edge effect (BEE) for PYR probe, and (v) a combination of REE and BEE for MOR. Adaptation of Grabowski's TICT hypothesis allows one to calculate the activation energy of TICT state formation, the backreaction TICT → LE, and the thermally activated TICT fluorescence. The polymeric chain essentially does not change the activation energy (E1) for the thermally activated TICT state formation, indicating that the polymeric chain does not change the microscopic viscosity in the range affecting measurable changes in the TICT−LE equilibrium. The polymeric chain increases the gap between the lowest forbidden ν0 vibrational state and less forbidden ν1 vibrational state by 195−225 cm-1 (0.6−2.4 kJ mol-1). Experimental results suggest that the polymer chain affects only the vibrational relaxation of tested molecules without any significant effect on the thermodynamics of the TICT and LE states equilibrium.

Solvent induced fluorescence of 9-chloro-10,10′-bis(dichloromethyleno)-(9′H)-10,10′-dihydro-9,9′-bianthryl (CDDB): twisted intramolecular charge transfer process

Solvent polarity induced fluorescence of 9-chloro-10,10′-bis(dichloromethyleno)-(9′H)-10,10′-dihydro-9,9′-bianthryl (CDDB) is studied in a series of homologous alcohols from methanol to pentanol and compared with the previously reported data for CDDB microcrystal and CDDB in cyclohexane. In microcrystal, the locally excited (LE) state and in non-polar solvents, intramolecular charge transfer (ICT) state are mainly responsible for fluorescence. In polar solvents, it has been proposed from the computed decomposed spectra that polarity dependent twisted intramolecular charge transfer (TICT) state also gives fluorescence along with ICT state fluorescence. The process has been discussed as per a proposed scheme. Formation of TICT state has been further confirmed from the measurement of equilibrium constants between ICT and TICT states in homologous alcohols and by analysing the kinetics of radiative and non-radiative pathways of individual ICT and TICT components. The formation mechanisms of ICT and TICT states are discussed on the basis of solvent induced change in molecular forms and charge separation due to rotation of the two moieties of CDDB along the C 9-C 9′ bond.

Dual fluorescence of ester derivatives of dialkylaniline in polymer matrices: a time-resolved fluorescence study using picosecond dye laser pulses as excitation source

Using time-resolved fluorescence (TRF) measurements, it is shown that, for p-N,N -dimethylamino-benzoic acid ethylester (DMABEE) and p-N,N -diethylamino-benzoic acid ethylester (DEABEE) in polymethacrylate (poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), poly(butyl methacrylate) (PBMA) and poly(hexyl methacrylate) (PHMA)) matrices, the rate of formation and lifetime of the twisted intramolecular charge transfer (TICT) state are determined by the opposing roles of the polarity and the free volume of the polymer sites. The TRF spectra increase with increasing time after excitation, and the most intense spectra are observed in PHMA, which is the least polar and least viscous in this series of polymers. The role of the angle of pretwist of the donor substituent in TICT state formation is confirmed.