Excited State Intramolecular Charge Transfer Research Articles

NIR Emission and Acid-Induced Intramolecular Electron Transfer Derived from a SOMO–HOMO Converted Non-Aufbau Electronic Structure

Some organic radicals violate the Aufbau principle and have unique electronic structures in which the energy level of the singly occupied molecular orbital (SOMO) is formally lower than that of the highest occupied molecular orbital (HOMO). We synthesized a novel organic radical with a SOMO–HOMO converted electronic structure, TPA-R•, a novel donor–acceptor hybrid of triphenylamine (an electron donor), and a stable polychlorinated diphenyl(4-pyridyl)methyl radical (an electron acceptor). TPA-R• exhibited fluorescence in the near-infrared region (λmax = 910 nm) in cyclohexane from a polar intramolecular charge-transfer excited state. Cyclic voltammetry, absorption spectroscopy, and density functional theory calculation revealed the inversion of the SOMO and HOMO levels in the electronic structure of TPA-R•. Addition of trifluoromethanesulfonic acid to TPA-R• caused a two-step change. Protonation initially occurred on the diphenylpyridylmethyl radical moiety to form TPA-[RH]•+. This protonation induced larg...

Colorimetric fluorescence probe detecting Hg2+ and OCl– based on intramolecular charge transfer and excited-state intramolecular proton transfer mechanisms

The recognition and detection of Hg2+ and OCl– have been subjected to extensively attention, since these ions can cause extremely adverse effects on living body. Recently, a novel fluorescence probe molecule simultaneously detecting Hg2+ and OCl– ions had been researched in experiment, while a carefully molecular-level theoretical investigation was lacking. In this work, the photochemical and photophysical processes in excited state were researched for title species, the corresponding sensing mechanism was reported detailedly for the first time. The computational data of absorption and emission spectra agreed well with experimental results. The calculated hydrogen bond parameters and the visualized reduced density gradient isosurfaces indicated that the strengthening hydrogen bond in excited state facilitated the occurrence of the excited-state intramolecular proton transfer (ESIPT) reactions. The fluorescence signal that detected Hg2+ ion was also reassigned in our research, which originated from photoisomeric structure S-P instead of enol form S-N. The ESIPT mechanism was quantitatively explained by calculating the reactive Gibbs free energies. Finally, the frontier molecular orbitals and overlap between hole and electron analyses confirmed that the interaction between probe molecule and analytes impeded the excited-state intramolecular charge transfer (ESICT) behavior. Essential comprehending for ESICT and ESIPT properties were helpful in illuminating the probing mechanism, and to design an excellent fluorescence probe molecule.

Studies of the Optical and Sensing Properties of 1,3,5-Triazine-Cored Star-Shaped (D-π)3-A Molecules with Various Amino-Donor-Type Cation Receptors

Abstract We newly designed and synthesized two kinds of 1,3,5-triazine-cored star-shaped (D-π)3-A molecules with 1-aza 15-crown 5-ether receptors (TSM-ACE) and dipicolylamino receptors (TSM-DPA), which possess an intramolecular charge transfer (ICT) nature and exhibit ICT-based optical properties (ICT transition absorption and fluorescence from a ICT excited state). Both TSM-ACE and TSM-DPA display proton-induced optical spectral blue-shifts followed by red-shifts depending on their inherent two-step protonation behavior. Furthermore, TSM-ACE displays optical spectral blue-shifts followed by red-shifts upon stepwise complexation of Mg2+, whereas TSM-DPA displays optical spectral blue-shifts upon the complexation of Zn2+. The cation sensing mechanism can be explained as follows: (i) the initial blue shift is caused by the decreased ICT donor abilities of three amino-donor-type receptors (a decrease of the ICT efficiency) upon binding cations, and (ii) the second red shift is caused by an increased acceptor ability of the 1,3,5-triazine core upon binding cations. Consequently, we revealed that TSM-ACE and TSM-DPA with different cation-recognition functionalities serve as a unique optical sensor capable of visual and distinct detection of not only protons but also different kinds of metal ions.

An intramolecular charge transfer and excited state intramolecular proton transfer based fluorescent probe for highly selective detection and imaging of formaldehyde in living cells.

Formaldehyde (FA), as a reactive carbonyl species, is endogenously generated in various biological processes. Abnormal levels of FA could lead to various cellular dysfunction and pathological conditions. Here, we develop a new activatable fluorescent probe for highly selective visualization of FA in living cells. Our probe (Naph-1) is designed using a naphthalene derivative as the fluorophore and hydrazone as a recognition site for FA. Naph-1 is essentially nonemissive. After reacting with FA, the amine moiety is converted into a Schiff base with electron-withdrawing ability and the fluorescence is simultaneously turned on due to synergetic intramolecular charge transfer and favoured excited state intramolecular proton transfer effects. Naph-1 exhibits a large Stokes shift upon reaction with FA. Furthermore, it possesses high selectivity and superior sensitivity toward FA with an estimated limit of detection of 0.35 μM. Moreover, Naph-1 is also successfully applied to image both endogenous and exogenous formaldehyde in living cells. These features demonstrate that Naph-1 holds great potential in the detection and imaging of formaldehyde in biological systems.

Intramolecular charge transfer and solvation dynamics of push-pull dyes with different π-conjugated linkers.

The solvation-dependent excited state dynamics of two push-pull fluorophores with donor-π-acceptor (D-π-A) structures were investigated using steady-state and ultrafast transient absorption (TA) spectroscopy, backed by theoretical calculations. Identical D and A groups were present in both dyes, which differed only in the structure of their central π-conjugated linkers. Dye 1 features a p-phenylenediethynyl linker, while dye 2 contains a 2,5-diethynylthiophene linker. From the steady-state spectra, no appreciable shifts in absorption bands were observed, whereas large red-shifts in emission were seen with increasing solvent polarity, which indicated that the excited states were more polar than the ground state. Theoretical calculations support charge transfer from the triphenylamine (TPA) donor to the pentafluorosulfanyl (SF5) acceptor viaπ-conjugated linkers to form an intramolecular charge transfer (ICT) state. TA spectra revealed that a solvation-stabilized conformationally relaxed intramolecular charge transfer (ICT') state was formed in polar solvents, but only an ICT state was observed in nonpolar solvent. The SE band was quenched within 1 ps in high-polarity solvent, which corresponds to the low fluorescence quantum yield. It can be concluded that the dye with the p-phenylenediethynyl π-linker (i.e., dye 1) exhibits a larger degree of ICT than the thiophene analogue (i.e., dye 2). These findings demonstrate how solvation can fine-tune the photophysical properties of push-pull dyes, and this study highlights the importance of π-conjugated linkers in the excited state ICT process.

Nitrostyrene-Modified 2-(2-Hydroxyphenyl)benzothiazole: Enol-Emission Solvatochromism by ESICT-ESIPT and Aggregation-Induced Emission Enhancement.

Excited-state intramolecular charge transfer (ESICT) and excited-state intramolecular proton transfer (ESIPT) are two competitive reactions that occur in the excited states of organic dyes that contain intramolecular hydrogen-bonds and electron acceptors and donors. Determining the mechanisms of these processes is key to understanding their multiple emission features, as the manner in which these processes interact can be modulated by modifying the dye structure. In addition, donor-π-acceptor (D-π-A) molecules often suffer from aggregation-induced quenching. Herein, we report the synthesis of three nitrophenyl-modified 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives, HBT-s-NO2 , HBT-d-NO2 , and HBT-t-NO2 , which have C-C, C=C, and C≡C bonds between their HBT and nitrophenyl moieties, respectively. Compared with the enol emissions from HBT-s-NO2 and HBT-t-NO2 , that from HBT-d-NO2 exhibits outstanding solvatochromism owing to consecutive ESICT-ESIPT. In addition, X-ray diffraction reveals that despite the highly planar and polar nature of HBT-d-NO2 , which is strongly H-aggregated, it exhibits highly efficient fluorescence. Hence, this study provides new insight into the design of ESICT/ESIPT-coupled systems and for engendering planar dipolar molecules with excellent emission properties in the solid state.

The photo-optical and electrochemical activity promoted by trifluoromethyl-substituted and ortho-catenated triphenylamine core in poly(ether-imide)s

A new series of five thermally and dimensionally stable aromatic poly(ether-imide)s consisting of an electron donating ortho-catenated triphenylamine (TPA) derivative bearing two trifluoromethyl groups and various electron-accepting phthalimide moieties were synthesized and thoroughly investigated. These polyimides display excellent organosolubility, unexpectedly even in ethanol, which allows their processing in environmentally friendly conditions. The twisting of the backbone structure induced by the ortho-catenated TPA core significantly affects their electronic properties. It was found that the CF3-substituted TPA moiety may facilitate dual intramolecular charge transfer (ICT) transitions, an aspect not highlighted yet for this class of polymers. In addition, we brought evidence for the occurrence of Förster excitation energy transfer (FRET) between the two ICT excited states that modulates the light emission toward the green-yellow spectral range, another novelty encountered at these polymers. The electron-withdrawing CF3 units directly graphthed on the triphenylamine core induced a fine-tuning of HOMO-LUMO energy levels, leading to low LUMO levels and energy band gaps. The novel characteristics endow the developed polymers with a wide application prospect, especially in the field of biology and opto-electronics.

ESIPT‐Based Dual Chemosensor for Sequential Detection of Cd2+/Zn2+ and Nucleoside Triphosphates in Water: Application in Logic Gates

AbstractHere in, novel excited state intramolecular charge transfer (ESIPT)‐modulated fluorophore based on diphenylpyrimidinone DPHQ has been designed and synthesized. DPHQ shows selective 8‐fold ‘turn‐on’ fluorescence response with Cd2+/Zn2+ in buffer (containing 1% CH3CN, pH 7.2) with low detection limits of 55.7/2.7 nM accompanied with a fluorescent colour change from colourless to cyan. DPHQ–Cd2+ ensemble undergo selective fluorescence quenching with nucleoside triphosphates but it is silent to nucleoside di‐and mono‐phosphates. It can detect 58.4 nM concentration of adenosine triphosphate (ATP). The binding mode and sensing mechanism of DPHQ with Cd2+/Zn2+ was supported by the change in optical behaviors, density functional theory (DFT) calculations and Job's plot. DPHQ was successfully used for construction of ‘Set‐Reset’ memorized logic devices and OR, INHIBIT logic gates. As practical applications of DPHQ, we utilized DPHQ for detection of Cd2+/Zn2+ in blood serum and quantification of Zn2+ in tap water samples.

Synthesis and Optical Properties of Donor-Acceptor-Type 1,3,5,9-Tetraarylpyrenes: Controlling Intramolecular Charge-Transfer Pathways by the Change of π-Conjugation Directions for Emission Color Modulations.

In dipolar organic π-conjugated molecules, variable photophysical properties can be realized through efficient excited-state intramolecular charge transfer (ICT), which essentially depends on the π-conjugation patterns. Herein, we report a controllable regioselective strategy for synthesis and optical properties of two donor–acceptor (DA)-type 1,3,5,9-tetraarylpyrenes (i.e., 1,3-A/5,9-D (4b) and 1,3-D/5,9-A (4c)) by covalently integrating two phenyl rings and two p-OMe/CHO-substituted phenyl units into the 2-tert-butylpyrene building block, in which the two phenyl rings substituted at the 1,3-positions act as acceptors for 4b or as donors for 4c and the two p-OMe or p-CHO-substituted phenyl moieties substituted at the K-region of 5,9-positions act as donors for 4b or as acceptors for 4c, respectively. Density functional theory calculations on their frontier molecular orbitals and UV–vis absorption of S0 → S1 transition theoretically predicted that the change of π-conjugation directions in the two DA pyrenes could be realized through a variety of substitution patterns, implying that the dissimilar ground-state and excited-state electronic structures exist in each molecule. Their single-crystal X-ray analysis reveal their highly twisted conformations that are beneficial for inhibiting the π-aggregations, which are strikingly different from the normal 1,3,5,9-tetraphenylpyrenes (4a) and related 1,3,6,8-tetraarylpyrenes. Indeed, experimental investigations on their optical properties demonstrated that the excited-state ICT pathways can be successfully controlled by the change of π-conjugation directions through the variety of substitution positions, resulting in the modulations of emission color from deep-blue to green in solution. Moreover, for the present DA pyrenes, highly fluorescent emissions with moderate-to-high quantum yields both in the thin film and in the doped poly(methyl methacrylate) film were obtained, suggesting them as promising emitting materials for the fabrication of organic light-emitting diodes.

Synthesis, Characterization, Physico-Chemical and DFT Studies of Potential Organic NLO Materials: Experimental and Theoretical Combined Study

1-(2,4-difluorobenzyl)-2-(2,4-difluorophenyl)-6-methyl-1H-benzo[d]imidazole(1) and 1-(3-(tri fluoromethyl)benzyl)2-(4-(trifluoromethyl)phenyl)-6-methyl-1H-benzo[d]imidazole (2) were synthesized and characterized using FT-IR,1H-NMR,13C-NMR and mass spectral studies. UV-Vis absorption and emission spectral studies were studied in hexane and ethanol medium. In addition to these, density functional theory studies (DFT/B3LYP/6-31G(d,p)) has also been carried out to compute the relevant electronic properties of the molecules and the observed theoretical results of the non-zero dipole moments, polarizability and hyper polarizability values which show that these types of molecules are promising candidates for applications in various non-linear optical (NLO) devices because of their significant values of the molecular hyperpolarizabilities and fine microscopic NLO behaviour. The high NLO responses of the donor-acceptor π-conjugated molecules are related to the intramolecular charge transfer excited state. The results confirmed that the synthesized benzimidazoles can act as a non-linear optical material and changing the substituents in the aldehydic phenyl ring will change the energy gap in the molecule.

Investigation of the Intermolecular Hydrogen Bonding Effects on the Intramolecular Charge Transfer Process of Coumarin 340 in Tetrahydrofuran Solvent

Density functional theory and time-dependent density functional theory methods were carried out to investigate the excited-state intramolecular charge transfer process (ICT) of coumarin 340 (C340) in tetrahydrofuran (THF) solvent for the first time. The plotted electrostatic potential and optimized geometric structures vividly indicated that the intermolecular hydrogen bond can be formed between the N–H group of C340 monomer and the oxygen atom of THF solvent. The analysis of frontier molecular orbitals and the differences in electronegativity confirmed the occurrence of ICT process in hydrogen-bonded complex C340-THF upon photoexcitation. The results of calculated bond lengths and bond energies revealed that the intermolecular hydrogen bond in C340-THF is significantly strengthened in the first excited state. Furthermore, the calculated electronic spectra, non-covalent interactions and IR spectra provided strong evidence for the hydrogen bond reinforce. Based on the theoretical calculations, we demonstrated that the ICT process in C340-THF can be facilitated effectively by the excited-state intermolecular hydrogen bond strengthening.

Synthesis, structure characterization, photoluminescence properties and TD-DFT calculations for two new borates.

Due to their rich structural chemistry and wide variety of applications, borate materials have provided a rich area of research. In a continuation of this research, diethylammonium bis(2-oxidobenzoato-κ2O1,O2)borate, C4H12N+·BO4(C7H4O)2-, (1), and propylammonium bis(2-oxidobenzoato-κ2O1,O2)borate, C3H10N+·BO4(C7H4O)2-, (2), have been synthesized by the reaction of boric acid with salicylic acid under ambient conditions. In both structures, the B atom exhibits a slightly distorted tetrahedral environment formed by the bidentate coordination of two salicylate anions via the O atoms of the central carboxylate and oxide groups. In the crystals of salts (1) and (2), mixed cation-anion layers lying parallel to the (101) plane are formed through N-H...O, C-H...O and C-H...π/N-H...O hydrogen-bonding interactions, resulting, in each case, in a two-dimensional supramolecular architecture in the solid state. The photoluminescence properties of the salts were studied using the as-synthesized samples and reveal that salts (1) and (2) both display a strong blue-light emission, with maxima at 489 and 491 nm, respectively. In DFT/TD-DFT (time-dependent density functional theory) studies, the blue emission appears to be derived from an intramolecular charge transfer (ICT) excited state. In addition, IR and UV-Vis spectroscopies were used to investigate the title salts.

Highly selective and sensitive turn-on fluorescent sensor for detection of Al3+ based on quinoline-base Schiff base

A new aluminum ion fluorescent probe (4-(diethylamino)-2-hydroxybenzylidene)isoquinoline-1-carbohydrazide (HL1) has been conveniently synthesized and characterized. HL1 exhibited a highly selective and pronounced enhancement for Al3+ in the fluorescence emission over other common cations by forming a 2:1 complex, with a recognition mechanism based on excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT). The strong fluorescent emission can be observed even at ppm level concentration of the probe in the presence of Al3+ with 41 fold intensity enhancement at 545 nm. HL1 displays good linear relationship with Al3+ in the low concentration and the limit of detection is 8.08 × 10−8 mol/L. Similar molecules with different substituents on salicylaldehyde phenyl ring were synthesized for studying the structure-activity relationship. Density-functional theory (DFT) calculations are in agreement with the proposed mechanism. It is confirmed that HL1 could be used to detect Al3+ ions in real sample by fluorescence spectrometry and Al3+ ions in cells by bioimaging.

Boronate-Based Fluorescence Probes for the Detection of Hydrogen Peroxide.

In this work, we synthesized a series of boronate ester fluorescence probes (E)‐4,4,5,5‐tetramethyl‐2‐(4‐styrylphenyl)‐1,3,2‐dioxaborolane (STBPin), (E)‐N,N‐dimethyl‐4‐(4‐(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)styryl)aniline (DSTBPin), (E)‐4‐(4‐(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)styryl)benzonitrile (CSTBPin), (E)‐2‐(4‐(4‐methoxystyryl)phenyl)‐4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolane (MSTBPin), (E)‐N,N‐dimethyl‐4‐(4‐(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)styryl)naphthalen‐1‐amine (NDSTBPin), and N,N‐dimethyl‐4‐(2‐(4‐(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)phenyl)oxazol‐5‐yl)aniline (DAPOX‐BPin) for the detection of hydrogen peroxide (H2O2). DSTBPin and MSTBPin displayed an “Off–On” fluorescence response towards H2O2, owing to the loss of the intramolecular charge transfer (ICT) excited state. Whereas, CSTBPin displayed a decrease in fluorescence intensity in the presence of H2O2 owing to the introduction of an ICT excited state. STBPin, on the other hand, produced a small fluorescence decrease, indicating the importance of an electron‐withdrawing or electron‐donating group in these systems. Unfortunately, the longer wavelength probe, NDSTBPin, displayed a decrease in fluorescence intensity. Oxazole‐based probe DAPOX‐BPin produced a “turn‐on” response. Regrettably, DAPOX‐BPin required large concentrations of H2O2 (>3 mm) to produce noticeable changes in fluorescence intensity and, therefore, no change in fluorescence was observed in the cell imaging experiments.

Ingenious modification of molecular structure effectively regulates excited-state intramolecular proton and charge transfer: a theoretical study based on 3-hydroxyflavone.

Harnessing ingenious modification of molecular structure to regulate excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) characteristics holds great promise in fluorescence sensing and imaging. Based on the 3-hydroxyflavone (3HF) molecule, 2-(2-benzo[b]furanyl)-3-hydroxychromone (3HB) and 2-(6-diethylamino-benzo[b]furan-2-yl)-3-hydroxychromone (3HBN) were designed by the extension of the furan heterocycle and the introduction of a diethylamino group. The analysis of important hydrogen bond length, frontier molecular orbitals, infrared spectra, and potential curves have cross-validated our results. The results indicate that proper site furan heterocycle extension and diethylamino donor group substitution not only shift the absorption and emission spectra to the red but also effectively modulate the excited-state dynamic behaviors. Strengthened ICT characteristics from 3HF to 3HB and to 3HBN make the occurrence of ESIPT increasingly difficult due to the higher energy barriers, which indicates that the ESIPT and ICT processes are competitive mechanisms. We envision that our work would open new windows for improving molecular properties and developing more fluorescent probes and organic radiation scintillators.

Weak interactions but potent effect: tunable mechanoluminescence by adjusting intermolecular C-H···π interactions.

A new mechanoluminescent material (4-(diphenylamino)phenyl)(4-(diphenylphosphanyl)phenyl)methanone (CDpP), which displays tunable mechanoluminescent emission colors, has been designed and successfully synthesized. CDpP shows two distinct mechanoluminescent colors (blue and green) in different crystalline states. Single-crystal analyses and femtosecond transient emission studies reveal that the striking tunable mechanoluminescence properties of CDpP mainly originate from the different C-H···π interactions in the crystal structures. CDpP crystals with more C-H···π interactions show blue mechanoluminescence (ML), and the emission is attributed to the locally excited (LE)-state because the twisting process for the excited state is restricted by C-H···π interactions. Conversely, CDpP crystals with fewer C-H···π interactions display green ML, in which the red-shifted emission band originates from the twisted intramolecular charge transfer (TICT) excited state because the diphenylamine moiety is relatively free to rotate. The manipulation of weak intermolecular interactions in the crystalline state is a useful and reliable strategy for the tuning of the ML emission wavelengths.

Narrow Band Gap Conjugated Polyelectrolytes.

Two essential structural elements define a class of materials called conjugated polyelectrolytes (CPEs). The first is a polymer framework with an electronically delocalized, π-conjugated structure. This component allows one to adjust desirable optical and electronic properties, for example the range of wavelengths absorbed, emission quantum yields, electron affinity, and ionization potential. The second defining feature is the presence of ionic functionalities, which are usually linked via tethers that can modulate the distance of the charged groups relative to the backbone. These ionic groups render CPEs distinct relative to their neutral conjugated polymer counterparts. Solubility in polar solvents, including aqueous media, is an immediately obvious difference. This feature has enabled the development of optically amplified biosensor protocols and the fabrication of multilayer organic semiconductor devices through deposition techniques using solvents with orthogonal properties. Important but less obvious potential advantages must also be considered. For example, CPE layers have been used to introduce interfacial dipoles and thus modify the effective work function of adjacent electrodes. One can thereby modulate the barriers for charge injection into semiconductor layers and improve the device efficiencies of organic light-emitting diodes and solar cells. With a hydrophobic backbone and hydrophilic ionic sites, CPEs can also be used as dispersants for insoluble materials. Narrow band gap CPEs (NBGCPEs) have been studied only recently. They contain backbones that comprise electron-rich and electron-poor fragments, a combination that leads to intramolecular charge transfer excited states and enables facile oxidation and reduction. One particularly interesting combination is NBGCPEs with anionic sulfonate side groups, for which spontaneous self-doping in aqueous media is observed. That no such doping is observed with cationic NBGCPEs indicates that the interplay between electrostatic forces and the redox chemistry of the organic semiconducting chain is essential for stabilizing the polaronic states and increasing the conductivity of the bulk. Capitalizing upon the properties of NBGCPEs has resulted in a range of new applications. When doped, they can be introduced as interlayers in organic and perovskite solar cells. Single-walled carbon nanotubes can be n- or p-doped with NBGCPEs, depending on whether the same backbone contains attached cationic or anionic side groups, respectively. The resulting dispersions can be used to fabricate flexible thermoelectric devices in which the n- and p-semiconductor legs are nearly identical in terms of chemical composition. Electrostatic interactions with negatively charged cell walls, in combination with the long-wavelength absorption and high photothermal efficiencies, have been used to create effective agents for photothermal killing of bacteria. Additionally, recent results have shown that cationic NBGCPEs can effectively n-dope graphene and that this doping is temperature-dependent. The preferential charge carriers can therefore be chosen to be electrons or holes depending on the applied temperature.

Triphenylamine Derived 3-Acetyl and 3-Benzothiazolyl Bis and Tris Coumarins: Synthesis, Photophysical and DFT Assisted Hyperpolarizability Study

Triphenylamine derived bis- and tris-branched donor-pi-acceptor coumarins with acetyl and benzothiazolyl acceptors are studied for their linear and nonlinear optical properties that originate from their photophysical and molecular structure. Plots of solvent polarities versus the Stokes shift, frontier molecular orbital analysis and Generalised Mulliken Hush analysis have established their strong charge transfer character supported by the strong emission solvatochromism of these chromophores. On the basis of excited state intramolecular charge transfer, the first-, second- and third-order polarizability of these dyes are determined by a solvatochromic method and supported by density functional theory calculations using CAM-B3LYP/6-31g(d). Compared to the acetyl group, the benzothiazolyl group is a strong acceptor, and its corresponding derivatives show enhanced absorption, emission maxima and non-linear optical response. Bond length alternation and bond order alternation analysis reveals that these chromophores approach the cyanine-like framework which is responsible for maximum perturbation to produce high nonlinear optical response. Third order nonlinear susceptibility for dyes 1 and 2 is determined by Z-scan measurement. All of these methods are used to determine the nonlinear optical properties, and thermogravimetric analysis suggests that these chromophores are thermally robust and efficient nonlinear optical materials.

Influence of the meso -substituent on strongly red emitting phenanthrene-fused boron–dipyrromethene (BODIPY) fluorophores with a propeller-like conformation

Highly emissive phenanthrene-fused boron–dipyrromethene (PBDP) dyes have been spectroscopically characterized in a series of solvents. The influence of different substituents (−H, −I, −CN, −DMA or a 15C5-crown ether) in the para-position of a phenyl ring attached to the meso-position of the BODIPY core is discussed. This family of dyes has an intense emission at λ ≥ 630 nm, with fluorescence quantum yields between 0.7 and 1.0 in all solvents studied, except in the case of the dimethylamino-substituted derivative, PBDP-DMA, which undergoes excited-state intramolecular charge transfer (CT), leading to broadband dual fluorescence in highly polar solvents. Introduction of a weaker electron donor such as a benzocrown to the meso-position is not able to trigger a second (charge or electron transfer) process and, interestingly, heavy atom (iodine, PBDP-I derivative) substitution at that moiety does also not have a relevant influence on the photophysics, i.e., enhanced intersystem crossing was not observed. Electrochemical studies of PBDP-DMA complement the data reported and stress the fact that the decrease in fluorescence of PBDP-DMA in highly polar solvents is due to an excited-state CT process rather than to a photoinduced electron transfer (PET).

Star-Shaped Asymmetrically Substituted Blue Emitting Carbazoles: Synthesis, Photophyscial, Electrochemical and Theoretical Investigations

A series of star-shaped blue-emitting 1,3,6,8-tetrasubstituted carbazoles containing different functional groups are synthesized and characterized. Electron-transporting N-phenylbenzimidazole is attached to C3 and C6 positions while the chromophore at C1 & C8-positions varied from fluorene to triphenylamine. The effect of the nature of the C1 and C8 chromophores on optical, electrochemical and thermal properties is unravelled. Absorption spectra revealed that the choice of meta− mode of connectivity between the chromophores on carbazole nucleus interrupted the interchromophoric electronic interactions in the ground state. However, the molar extinction coefficients of the dyes increased significantly on the addition of chromophores at C1 and C8 attributable to the increment in the chromophore density. Interestingly, triphenylamine and pyrene containing compounds displayed positive solvatochromism in emission attributable to intramolecular charge transfer and excimer excited states, respectively. Fluorescence and thermal properties are strongly influenced by the nature of chromophores present on C1 & C8. Electrochemical studies revealed low oxidation potential for arylamine containing derivatives and led to high-lying highest occupied molecular orbital in the series. However, the lowest unoccupied molecular orbital is stabilized by pyrene substituent due to its π-acceptor character.