Twisted Intramolecular Charge Transfer Emission Research Articles

A detailed theoretical investigation on intramolecular charge transfer mechanism of primary, secondary, and tertiary p-amino substituted benzaldehyde

This article reports a detailed theoretical investigation on dual fluorescence properties of p-amino substituted benzaldehyde molecules by taking specifically p-N,N-dimethylaminobenzaldehyde (3° PABA), p-N-methylaminobenzaldehyde (2° PABA), and p-aminobenzaldehyde (1° PABA) molecules. The calculations are performed in gas phase as well as in butanol (BuOH) and dichloromethane (DCM) solvents. Twisted intramolecular charge transfer emission property is particularly looked at by scanning the potential energy curves as a function of -NR2 (R = H/CH3) rotation (twisting) angle. The results suggest that there are dual emission possibilities for 3° and 2° PABA molecules. Experimental validation is available for the former but not for the latter. 1° PABA, on the other hand, does not show any possibility of dual fluorescence except in BuOH. The theoretical absorption and emission spectra are also calculated and compared with experiment where it is available, and they are in close correlation. Excited state molecular dynamics simulations show that for 3° PABA, the molecule reaches to the 90° twisting angle and stays there with a probability higher than the same probability for 2° PABA. 1° PABA, on the other hand, had showed variations in the twisting angle from 0° to above 100°, but no evidence of this molecule to have considerable lifetime at 90° twisting angle.

2,3-Diphenylquinoxalin-6-yl based dyes: Synthesis and studies of photophysical, aggregation-induced emission enhancement and electrochemical properties

Herein, 2,3-diphenylquinoxalin-6-yl based derivatives were synthesized, characterized by various spectroscopic method and studied for photophysical, aggregation-induced emission enhancement (AIEE) and electrochemical properties. Existence of intramolecular charge transfer transition (ICT) at 372−379 nm in absorption spectra of molecules confer weak push-pull interaction within it. Broad range of emission from blue to orange with λem = 412−592 nm in dyes arose from different energy state emission viz., locally excited (LE), Twisted intramolecular charge transfer (TICT) and excimer emission. Switching in emission state of molecules from excimer to TICT emission were observed by changing the polarity of solvent due to stabilization of TICT in polar media. Dyes have good solid state emission and AIEE activity desirable for its solid/aggregate state application in OLED's. Good Stokes shift (2183−9703 cm−1), thermal stability and comparable HOMO (−5.65 to −6.02 eV) energy levels with reported hole transporting materials confer desirable role of molecules in various optoelectronic applications. Further, DFT calculation of dyes, designate comparable experimental and theoretical photophysical and electrochemical properties with respect to structure of dyes.

Tuning the Emission of a Nonconventional Aggregation-Induced Emission Polymer via Silicon-Bridged Twisted Intramolecular Charge Transfer for Targeted Delivery and Visualized Drug Release.

The design of tunable luminescent biomaterials with large Stokes shifts is usually pursued by a twisted intramolecular charge transfer (TICT) effect with switchable emission colors in response to various external stimuli. However, such a strategy is usually realized in conjugated molecules containing benzene or its derivatives and consequently suffers from poor biocompatibility. In this work, a hyperbranched polysiloxane (HBPSi)-based non-conjugated fluorescent polymer with TICT and aggregation-induced emission (AIE) features is developed, and its luminescent properties, fluorescence mechanism, and potential applications are investigated. Initially, the non-conjugated HBPSi exhibits remarkable AIE characteristics due to the formation of through-space conjugation. With the introduction of the sulfur atom, a non-conjugated D-A type AIE material, HBPSi-Cys, that exhibits a dual-state emission with a large Stokes shift of 213 nm, is obtained. The correlation of the lower-energy emission band with solvent polarity suggests the existence of the TICT state. TICT and AIE characteristics direct different properties of HBPSi-Cys, with TICT regulating solvatochromic emission wavelengths and AIE manipulating the emission intensity with a compensation effect. Density functional theory calculations reveal that the non-conjugated D-A structure in HBPSi-Cys was formed across the silicon bridge, with auxochromic sulfhydryl groups and adjacent amide groups as acceptor units and amine and hydroxyl groups as donor units. Additionally, the AIE-active HBPSi could be utilized as a fluorescent probe for the analysis of metal ions. After grafting the AS1411 aptamer to HBPSi-Cys as the recognition motif, HBPSi-Apt possesses excellent targeted bioimaging, drug loading, pH/GSH dual-responsive drug release, and visualized drug delivery performance. This work provides a new way to design functional AIE polymers with tunable optical properties, and the synthesized HBPSi-Cys shows great potential as a smart fluorescent biomaterial.

Nanoparticle and surfactant controlled switching between proton transfer and charge transfer reaction coordinates.

The reaction coordinates of a molecular photo-switch 2-(4'-diethylamino-2'-hydroxyphenyl)-1H-imidazo-[4,5-b]pyridine (DHP) was tuned with a nanoparticle and surfactant. DHP undergoes excited state intramolecular proton transfer (ESIPT) and emits normal and tautomer emissions in N,N-dimethylformamide. Silver nanoparticles suppress the ESIPT and induce twisted intramolecular charge transfer (TICT). Further addition of surfactants alters the process. Interestingly, different surfactants cause different effects. Accordingly, the luminescence characteristics are altered. The anionic surfactant sodium dodecyl sulfate (SDS) restores the ESIPT process by completely detaching the molecule from the nanoparticle. The nonionic surfactant Triton X-100 (TX-100), at lower concentration, enhances the TICT emission and the ESIPT process is also observed due to the release of some fluorophore from the nanoparticle complex. But at higher concentration the fluorophores are released completely and the ESIPT process is restored. The cationic surfactant cetyltrimethyl ammonium bromide (CTAB), at lower concentration, simply restores the ESIPT process by releasing the fluorophore. But at higher CTAB concentration, DHP enters the metalparticle-CTAB aggregate and shows enhanced ESIPT.

Light-Driven Switching between Intramolecular Proton-Transfer and Charge-Transfer States.

A molecular photoswitch, 2-(4'-diethylamino-2'-hydroxyphenyl)-1H-imidazo-[4,5-b]pyridine (DHP), with mutually independent paths of excited-state intramolecular proton transfer (ESIPT) and twisted intramolecular charge transfer (TICT) was developed. Control over these processes was attained by switching the solvents. Depending on the solvent's hydrogen-bond capacity and polarity, either one of the photoprocesses (ESIPT or TICT) or both can be triggered. Accordingly, normal and tautomer emissions, normal and TICT emissions, or triple emission of normal, tautomer, and TICT were obtained from the molecule. The emissions were resolved by fluorescence lifetime. The conclusions were established by synthesizing and studying the methoxy derivative of the molecule.

The suppression of intramolecular charge transfer emission by tautomerism in 2-(4′-amino-2′-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine: Intramolecular proton transfer versus intermolecular proton transfer

2-(4′-Aminophenyl)-1H-imidazo[4,5-c]pyridine (APIP-c) emits twisted intramolecular charge transfer (TICT) emission in methanol. However, when hydroxyl group was introduced in APIP-c, the hydroxy compound 2-(4′-amino-2′-hydroxyphenyl)-1H-imidazo[4,5-c]pyridine (AHPIP-c) does not emit TICT emission in methanol. But it undergoes excited state intramolecular proton transfer (ESIPT) to emit tautomer emission. The cause for the suppression of TICT by ESIPT in AHPIP-c was not clear. Therefore, DFT calculations were performed using polar continuum model with explicit methanol molecule to understand the cause. The vibrational frequency analyses were performed to know the strength of the bond in the ground and excited states. The potential energy curves for both intra- and intermolecular proton transfer processes were constructed in the ground and excited states. Our theoretical studies reveal that the relative strength of the intermolecular hydrogen bond and intramolecular hydrogen bond plays a crucial role in determining the dominating excited state process.

Study on TICT emission of TPE-BODIPY derivatives mediated by methyl group on BODIPY

A series of tetraphenylethene-borondipyrromethene (TPE-BODIPY) derivatives with different numbers of methyl groups on BODIPY cores were designed and synthesized. The intramolecular charge transfer behaviors from donor TPE to accepter BODIPY were investigated. Aggregation-induced emission (AIE), theoretical calculation, single crystal structure and solvent effect all proved that the introduction of methyl group to BODIPY can inhibit the intramolecular charge transfer behavior. Red organic luminescent material 8-TPEBODIPY (TPB) with no methyl groups showing twisted intramolecular charge transfer (TICT) emission peak both in solution and in film state, presents obviously AIE and solvatochromic properties and a large Stokes shifts over 100 nm, which can be employed as a fluorescent sensor for the detection of water in THF.

Modulation of Twisted Intramolecular Charge Transfer Emission of 2‐(4′‐N,N‐Dimethylaminophenyl)imidazopyridines in Aqueous Cucurbit[7]uril+

AbstractThe twisted intramolecular charge transfer (TICT) of 2‐(4′‐N,N‐dimethylaminophenyl)imidazopyridines (DMAPIPs) is examined in aqueous cucurbit[7]uril (CB7). The fluorescence spectral changes show the formation of a 1:1 inclusion complex between CB7 and DMAPIPs. Using the fluorescence data the association constants for DMAPIP‐CB7 complexes are also estimated. High stabilization energy obtained from DFT calculation corroborates the stability of DMAPIP‐CB7 complexes. The studies infer that imidazopyridine moiety of the DMAPIPs present inside the CB7 cavity and the dimethylamino phenyl ring is present outside in bulk water. The TICT emission is observed from both the DMAPIPs in CB7 environment too. The biexponential decay illustrated the dual emission of the DMAPIPs in aqueous CB7.

Solid‐State Emission of the Anthracene‐o‐Carborane Dyad from the Twisted‐Intramolecular Charge Transfer in the Crystalline State

AbstractThe emission process of the o‐carborane dyad with anthracene originating from the twisted intramolecular charge transfer (TICT) state in the crystalline state is described. The anthracene‐o‐carborane dyad was synthesized and its optical properties were investigated. Initially, the dyad had aggregation‐ and crystallization‐induced emission enhancement (AIEE and CIEE) properties via the intramolecular charge transfer (ICT) state. Interestingly, the dyad presented the dual‐emissions assigned to both locally excited (LE) and ICT states in solution. From the mechanistic studies and computer calculations, it was indicated that the emission band from the ICT should be attributable to the TICT emission. Surprisingly, even in the crystalline state, the TICT emission was observed. It was proposed from that the compact sphere shape of o‐carborane would allow for rotation even in the condensed state.

Solid-State Emission of the Anthracene-o-Carborane Dyad from the Twisted-Intramolecular Charge Transfer in the Crystalline State.

The emission process of the o-carborane dyad with anthracene originating from the twisted intramolecular charge transfer (TICT) state in the crystalline state is described. The anthracene-o-carborane dyad was synthesized and its optical properties were investigated. Initially, the dyad had aggregation- and crystallization-induced emission enhancement (AIEE and CIEE) properties via the intramolecular charge transfer (ICT) state. Interestingly, the dyad presented the dual-emissions assigned to both locally excited (LE) and ICT states in solution. From the mechanistic studies and computer calculations, it was indicated that the emission band from the ICT should be attributable to the TICT emission. Surprisingly, even in the crystalline state, the TICT emission was observed. It was proposed from that the compact sphere shape of o-carborane would allow for rotation even in the condensed state.

Ratiometric Fluorescent Probe for Vicinal Dithiol-Containing Proteins in Living Cells Designed via Modulating the Intramolecular Charge Transfer-Twisted Intramolecular Charge Transfer Conversion Process.

Vicinal dithiol-containing proteins (VDPs) play a significant role in maintaining the cellular redox homeostasis and are implicated in many diseases. To provide new chemical tools for VDPs imaging, we report here a ratiometric fluorescent probe CAsH2 for VDPs using 7-diethylaminiocoumarin as the fluorescent reporter and cyclic 1,3,2-dithiarsenolane as the specific ligand. CAsH2 shows peculiar dual fluorescence emission from the excited intramolecular charge transfer (ICT) and twisted intramolecular charge transfer (TICT) states in aqueous media. However, upon selective binding of protein vicinal dithiols to the trivalent arsenical of CAsH2, the probe was brought from the polar water media into the hydrophobic protein domain, causing the excited state ICT to TICT conversion to be restricted; as a result, an increase from the ICT emission band and a decrease from the TICT emission band were observed simultaneously. The designed probe shows high selectivity toward VDPs over other proteins and biological thiols. Preliminary experiments show that CAsH2 can be used for the ratiometric imaging of endogenous VDPs in living cells. So far as we know, this is a rare example of the ratiometric fluorescent probe designed via modulating the ICT-TICT conversion process, which provides a new way to construct various protein-specific ratiometric fluorescent probes.

Fluorescence Phenomena in Nerve-Labeling Styryl-Type Dyes.

Several classes of diversely substituted styryl type dyes have been synthesized with the goal of extending their expected fluorescent properties as much towards red as possible given the constraint that they maintain drug-like properties and retain high affinity binding to their biological target. We report on the synthesis, optical properties of a series of styryl dyes ( d1-d14 ), and the anomalous photophysical behavior of several of these Donor-Acceptor pairs separated by long conjugated π-systems ( d7-d10 ). We further describe an unusual dual emission behavior with two distinct ground state conformers which could be individually excited to locally excited (LE) and twisted intramolecular charge transfer (TICT) excited state in push-pull dye systems ( d7 , d9 and d10 ). Additionally, unexpected emission behavior in dye systems d7 and d8 wherein the amino- derivative d7 displayed a dual emission in polar medium while the N,N-dimethyl derivative d8 and other methylated derivatives d12-d14 showed only LE emission but did not show any TICT emission. Based on photophysical and nerve binding studies, we down selected compounds that exhibited the most robust fluorescent staining of nerve tissue sections. These dyes ( d7 , d9 , and d10 ) were subsequently selected for in-vivo fluorescence imaging studies in rodents using the small animal multispectral imaging instrument and the dual-mode laparoscopic instrument developed in-house.

Fluorescent chemosensors of carbohydrate triols exhibiting TICT emissions.

4-4'-Disubstituted biphenyl boronic acids (BBAs) are push-pull fluorophores with "turn-on" fluorescence properties. Upon carbohydrate triol binding, BBA boronate esters can participate in photon-induced electron transfer, giving a twisted intramolecular charge transfer (TICT) complex. The resulting TICT emissions distinguish between carbohydrates that bind to boron as a diol versus triol unit, thereby revealing stereochemical information about the carbohydrate.

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.

A twisted-intramolecular-charge-transfer (TICT) based ratiometric fluorescent thermometer with a mega-Stokes shift and a positive temperature coefficient.

The fluorescence intensity of N,N-dimethyl-4-((2-methylquinolin-6-yl)ethynyl)aniline exhibits an unusual intensification with increasing temperature, by activating more vibrational bands and leading to stronger TICT emissions upon heating in dimethyl sulfoxide. Based on the different temperature dependence at various wavelengths, as shown in the TICT fluorescence spectrum, this dye can be employed to ratiometrically detect temperature.

Inclusion complexation of sulfapyridine with α- and β-cyclodextrins: Spectral and molecular modeling study

The inclusion complexes of sulfapyridine (SFP) with α-CD and β-CD were investigated by absorption, fluorescence, time-resolved fluorescence, FTIR, DSC, XRD, 1H NMR, SEM, TEM and molecular modeling methods. The normal fluorescence takes place from locally excited (LE) state while twisted intramolecular charge transfer (TICT) is responsible for highly Stokes shifted fluorescence. The enhancement of TICT emission in both CDs suggesting that the inclusion process plays the major role in this emission. The spectral shifts revealed that part of pyridine ring of SFP is entrapped in the CDs cavities. TEM images confirmed round shaped nanoparticles with the average size about 20–50nm were observed in SFP with α-CD and β-CD inclusion complexes. PM3 calculations have suggested that the large stabilization of excited singlet state of SFP with twisted conformation occurring at the amide SN bond between the electron donor group (aniline ring) and the electron acceptor group (pyridine ring).

Twisted intramolecular charge transfer effects on fast violet B and fast blue RR: Effect of HP-α- and HP-β-cyclodextrins

Absorption, steady state and time resolved fluorescence spectra of fast blue RR (FBRR) and fast violet B (FVB) were studied in hydroxypropyl-α-cyclodextrin (HP-α-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD). Dual emission observed in both CDs suggested that twisted intramolecular charge transfer (TICT) is present in both molecules. The normal Stokes shifted bands originated from the locally excited state and the large Stokes shifted bands were due to the emission from a TICT state. The ratio of the TICT to the normal emission increased with HP-β-CD concentration, while it was constant upon addition of HP-α-CD. This behavior was in accordance with CD dependent decay times of the TICT and normal emissions, indicating the formation of different 1:1 FVB/CD and FBRR/CD inclusion complexes. The benzoyl moiety of FVB/FBRR was deeply encapsulated in the HP-β-CD cavity, whereas in HP-α-CD, it was exposed to the water phase. Semiempirical quantum optimization was also carried out to assign the encapsulation of the FBRR and FVB.

Selective Formation of Twisted Intramolecular Charge Transfer and Excimer Emissions on 2,7-bis(4-Diethylaminophenyl)-fluorenone by Choice of Solvent

We designed and synthesized a donor-acceptor-donor dye consisting of a 2,7-disubstituted fluorenone with diethylaminophenyl moieties present as strong electron donating groups. Switching between twisted intramolecular charge transfer (TICT) emission and excimer emission was achieved, with no ground state changes, by simply changing the solvent used. In a nonpolar solvent, excimer emission was observed; with increasing polarity, the emission gradually disappeared, and the TICT emission appeared.

Solid States Fluorescence Study of <i>p</i>-Dimethylaminobenzonitrile by Co-grinding with Cyclodextrins

The molecular status of a freeze-dried sample or a ground mixture of p-dimethylaminobenzonitrile (DMABN) with alpha-, beta-, or gamma-cyclodextrins (CDs) was examined using solid-state fluorescence measurements. A twisted intramolecular charge transfer (TICT) emission of DMABN crystals was shown at 475 nm. Emission peaks of freeze-dried samples were observed at 450, 380, and 393 nm in alpha-CD/DMABN, beta-CD/DMABN, and gamma-CD/DMABN systems, respectively. It was speculated that DMABN molecules existed as a twisted form in the cavity of alpha-CD, and as a plane structure in that of beta-CD or gamma-CD. On the other hand, fluorescence emission peaks of ground mixtures of DMABN with alpha-, beta-, or gamma-CD were observed at around 450 nm. When DMABN was ground together with microcrystalline cellulose, which cannot form an inclusion complex, only TICT emission was detected. These results suggest that the observed shift in the fluorescence peak could be due to inclusion phenomena. When the ground mixtures were crystallized under humid conditions, fluorescence emission peaks were observed at 450 nm in alpha-CD and of around 400 nm in beta- and gamma-CD systems. It is concluded that the conformation of the DMABN molecules in a crystalline CD/DMABN inclusion complex change depending on the size of the CD cavity.

Spectral Properties and Solubilization Location of 2-Ethylhexyl 4-(N,N-Dimethylamino) benzoate in Micelles

Dual fluorescence and UV absorption of 2-ethylhexyl 4-(N,N-dimethylamino)benzoate (EHDMAB) were investigated in cationic, non-ionic and anionic micelles. When EHDMAB was solubilized in different micelles, the UV absorption of EHDMAB was enhanced. Twisted intramolecular charge transfer (TICT) emission with longer wavelength was observed in ionic micelles, whereas TICT emission with shorter wavelength was obtained in non-ionic micelles. In particular, dual fluorescence of EHDMAB was significantly quenched by the positively charged pyridinium ions arranged in the Stern layer of cationic micelles. UV radiation absorbed mainly decays via TICT emission and radiationless deactivation. The dimethylamino group of EHDMAB experiences different polar environments in ionic and non-ionic micelles according to the polarity dependence of TICT emission of EHDMAB in organic solvents. In terms of the molecular structures and sizes of EHDMAB and surfactants, each individual EHDMAB molecule should be buried in micelles with its dimethylamino group toward the polar head groups of different micelles and with its 2-ethylhexyl chain toward the hydrophobic micellar core. Dynamic fluorescence quenching measurements of EHDMAB provide further support for the location of EHDMAB in different micelles.