Synthesis of Functionalized Organosilicon Compounds/Distal Ketones via Ring-Opening Giese Addition of Cycloalkanols under Organophotocatalytic Conditions.

Visible-light-induced organophotocatalyzed ring-opening followed by remote Giese addition of tertiary cycloalkanols with ß-silylmethylene malonates has been developed under mild reaction conditions for the synthesis of organosilicon compounds, bearing a ketone group distally substituted with a silyl group with an additional dialkyl malonate functional handle in moderate to good yields (34-72%). The protocol also worked well with diverse Michael acceptors, such as alkylidene/benzylidene malonates, trifluoro methylidene malonate, benzylidene malononitrile, α-cyano-enone, and α-cyano vinyl sulfone, and delivered desired valuable distally functionalized ketones. To showcase the potential of the method, various synthetic transformations of the obtained product were also demonstrated.

Design and Synthesis of BODIPY-Hetero[5]helicenes as Heavy-Atom-Free Triplet Photosensitizers for Photodynamic Therapy of Cancer.

Designing heavy-atom-free triplet photosensitizers (PSs) is a challenge for the efficient photodynamic therapy (PDT) of cancer. Helicenes are twisted polycyclic aromatic hydrocarbons (PAHs) with an efficient intersystem crossing (ISC) that is proportional to their twisting angle. But their difficult syntheses and weak absorption profile in the visible spectral region restrict their use as heavy-atom-free triplet PSs for PDT. On the other hand, boron-containing PAHs, BODIPYs are highly recognized for their outstanding optical properties. However, planar BODIPY dyes has low ISC and thus they are not very effective as PDT agents. We have designed and synthesized fused compounds containing both BODIPY and hetero[5]helicene structures to develop red-shifted chromophores with efficient ISC. One of the pyrrole units of the BODIPY core was also replaced by a thiazole unit to further enhance the triplet conversion. All the fused compounds have helical structure, and their twisting angles are also increased by substitutions at the boron centre. The helical structures of the BODIPY-hetero[5]helicenes were confirmed by X-ray crystallography and DFT structure optimization. The designed BODIPY-hetero[5]helicenes showed superior optical properties and high ISC with respect to [5]helicene. Interestingly their ISC efficiencies increase proportionally with their twisting angles. This is the first report on the relationship between the twisting angle and the ISC efficiency in twisted BODIPY-based compounds. Theoretical calculations showed that energy gap of the S1 and T1 states decreases in BODIPY-hetero[5]helicene as compared to planar BODIPY. This enhances the ISC rate in BODIPY-hetero[5]helicene, which is responsible for their high generation of singlet oxygen. Finally, their potential applications as PDT agents were investigated, and one BODIPY-hetero[5]helicene showed efficient cancer cell killing upon photo-exposure. This new design strategy will be very useful for the future development of heavy-atom-free PDT agents.

Pore-Confined π-Chromophoric Tetracene as a Visible Light Harvester toward MOF-Based Photocatalytic CO2 Reduction in Water.

Integrating photoactive π-chromophoric guest molecules inside the MOF nanopore can result in the emergence of light-responsive features, which in turn can be utilized for developing photoactive materials with inherent properties of MOF. Herein, we report the confining of π-chromophoric tetracene (TET) molecules inside the nanospace of postmodified Zr-MOF-808 (Zr-MOF) with MBA molecules (MBA = 2-(5'-methyl-[2,2'-bipyridine]-5-yl)acetic acid) for effectively utilizing its light-harvesting properties toward photocatalytic CO2 reduction. The confinement of the TET molecules as a photosensitizer and the covalent grafting of a catalytically active [Re(MBA)(CO)3Cl] complex, postsynthetically, result in a single integrated catalytic system named Zr-MBA-TET-Re-MOF. Photoreduction of CO2 over Zr-MBA-TET-Re-MOF showed the evolution of 805 µmol g-1 CO with 99.9% selectivity after 10 h of continuous visible light irradiation in water without any additional sacrificial electron donor and having the apparent quantum efficiency of 1.3%. In addition, the catalyst demonstrated an appreciable activity even under direct sunlight irradiation in aqueous medium with a maximum production of 362.7 µmol g-1 CO, thereby mimicking artificial photosynthesis. Moreover, electron transfer from TET to the catalytic center was supported by the formation of photoinduced TET radical cation, as inferred from in situ UV-vis spectra, electron paramagnetic resonance (EPR) analysis, and transient absorption (TA) studies. Additionally, the in situ diffuse reflectance infrared Fourier transform (DRIFT) measurements support that the photoreduction of CO2 to CO proceeds via *COOH intermediate formation. The close proximity of the light-harvesting molecule and catalytic center facilitated facile electron transfer from the photosensitizer to the catalyst during the CO2 reduction.

Energy transfer-mediated white light emission from Nile red-doped 9,10-diphenylanthracene nanoaggregates upon excitation with near UV light.

The low cost, ease of preparation, colour tunability and wide application range garnered huge research interest on organic light emitting diode materials (OLED). The development of white light-emitting organic diode materials is mostly targeted for this. Anthracene derivatives have recently emerged as low-cost and efficient blue light-emitting diodes. However, developing efficient organic diode materials that cover the entire visible spectrum is very challenging. Herein, we demonstrated that Nile red (NR)-doped 9,10-diphenylanthracene (DPA) nanoaggregates provided strong white light emission upon excitation with near UV light. The dual emissions of the DPA nanoaggregates covering the blue and green regions were exploited and combined with the controlled red emission of the properly doped NR dye to cover the full visible spectrum, rendering white light emission with a quantum yield of >0.4. The fluorescence spectra of the DPA nanoaggregates doped with NR at various concentrations were monitored and their CIE coordinates were followed to evaluate the proper doping ratio for equal-energy white-light emission. Concurrent time-resolved emission studies provided mechanistic insights into the energy transfer from the exciton and excimer states of DPA to NR. It was revealed that the energy transfer from the singlet excitonic state of DPA followed the diffusion-assisted resonance energy transfer (RET) model. On the other hand, the excimer state showed negligible diffusion and energy transfer from this state found to follow the single-step Förster resonance energy transfer mechanism. The observation of efficient white light emission in the doped DPA nanoaggregates was proposed to have prospective applications in OLED devices, given the fact that triplet excitons may be exploited for emission through the efficient triplet-triplet annihilation contribution to fluorescence enhancement.

Interplay of exciton-excimer dynamics in 9,10-diphenylanthracene nanoaggregates and thin films revealed by time-resolved spectroscopic studies.

Anthracene and its derivatives are organic semiconducting materials that have prospective applications in organic solar cells and organic light emitting diodes. A thorough understanding of the photophysics and exciton dynamics is imperative for the effective utilization of these materials in optoelectronics. Herein, we have presented the exciton dynamics of a highly emissive anthracene derivative, namely, 9,10-diphenylanthracene (DPA) in nanoaggregate, thin film and crystalline forms. In contrast to the strong blue molecular emission in solution, DPA in the solid form exhibits emission from both the exciton and excimer state, covering the blue and green spectral region. In the well-ordered crystalline state, excimer emission dominates, while in the nanoaggregate form, the relative contribution of the excimer state decreases with the decrease in the size of the nanoparticle. In crystals, thin films and larger size nanoaggregates, favourable intermolecular orbital overlap by the adjacent phenyl substituents of DPA in the dominant α-phase packing allows for the fast relaxation of the exciton to excimer state in a sub-nanosecond timescale, as measured by time-resolved emission studies. Prior to trapping in the excimer state, the diffusion of singlet excitons has been revealed via the exciton-exciton annihilation kinetics measured by the excitation fluence dependent ultrafast emission decay. The singlet exciton diffusion coefficient in the DPA nanoaggregate is noted to be almost an order of magnitude slower than that in the anthracene nanoaggregate previously reported. The near perpendicular orientation of the two phenyl rings at the 9 and 10 position of anthracene causes an elongation in the intermolecular distance of the anthracene cores, which impedes the exciton diffusion rate. However, the ordered molecular packing of the DPA molecules in the thin film and crystal facilitates significantly faster singlet exciton diffusion.

Substituent control of the ultrafast twisted intramolecular charge transfer rate in dimethylaminochalcone derivatives.

The effect of acceptor strength variation on ultrafast twisting relaxation dynamics was investigated by comparing the ultrafast relaxation dynamics in a series of dimethylaminochalcone (DMAC) derivatives. Employing femtosecond resolved transient absorption and fluorescence experiments, the twisted intramolecular charge transfer (TICT) relaxation rate is shown to vary from 2.0 picoseconds in a weak electron accepting system to 420 femtoseconds in a strong electron accepting system. The strength of the acceptor, empirically expressed as Hammett's constant, is shown to exhibit a linear free energy relationship (LFER) with the twisting rate. It is proposed that variation in the charge pulling capacity of the acceptor modifies the torsional barrier along the TICT coordinate in the S1 state, resulting in a tunable TICT relaxation rate, depending on the acceptor strength.

Protonation-induced ultrafast torsional dynamics in 9-anthrylbenzimidazole: a pH activated molecular rotor.

We report the photophysical properties and excited state dynamics of 9-anthrylbenzimidazole (ANBI) which exhibits protonation-induced molecular rotor properties. In contrast to the highly emissive behavior of neutral ANBI, protonation of the benzimidazole group of ANBI induces efficient nonradiative deactivation by ultrafast torsional motion around the bond connecting the anthracene and benzimidazole units, as revealed by ultrafast transient absorption and fluorescence spectroscopy. Contrary to viscosity-independent fluorescence of neutral dyes, protonated ANBI is shown to display linear variation of emission yield and lifetime with solvent viscosity. The protonation-induced molecular rotor properties in the studied system are shown to be driven by enhanced charge transfer and are corroborated by quantum chemical calculations. Potential application as a microviscosity sensor of acidic regions in a heterogeneous environment by these proton-activated molecular rotor properties of ANBI is discussed.

The Role for the DSB Response Pathway in Regulating Chromosome Translocations.

In response to DNA double strand breaks (DSB), mammalian cells activate the DNA Damage Response (DDR), a network of factors that coordinate their detection, signaling and repair. Central to this network is the ATM kinase and its substrates at chromatin surrounding DSBs H2AX, MDC1 and 53BP1. In humans, germline inactivation of ATM causes Ataxia Telangiectasia (A-T), an autosomal recessive syndrome of increased proneness to hematological malignancies driven by clonal chromosomal translocations. Studies of cancers arising in A-T patients and in genetically engineered mouse models (GEMM) deficient for ATM and its substrates have revealed complex, multilayered roles for ATM in translocation suppression and identified functional redundancies between ATM and its substrates in this context. "Programmed" DSBs at antigen receptor loci in developing lymphocytes employ ubiquitous DDR factors for signaling and repair and have been particularly useful for mechanistic studies because they are region-specific and can be monitored in vitro and in vivo. In this context, murine thymocytes deficient for ATM recapitulate the molecular events that lead to transformation in T cells from A-T patients and provide a widely used model to study the mechanisms that suppress RAG recombinase-dependent translocations. Similarly, analyses of the fate of Activation induced Cytidine Deaminase (AID)-dependent DSBs during mature B cell Class Switch Recombination (CSR) have defined the genetic requirements for end-joining and translocation suppression in this setting. Moreover, a unique role for 53BP1 in the promotion of synapsis of distant DSBs has emerged from these studies.

Solvent- and DNA-Controlled Phototriggered Linkage Isomerization in a Ruthenium Sulfoxide Complex Incorporating Dipyrido[3,2-a:2',3'-c]phenazine (dppz).

A new tris-heteroleptic complex [Ru(bpy)(dppz)(OSO)](ClO4), [1](ClO4) (bpy = 2,2'-bipyridine, dppz = dipyrido[3,2-a:2',3'-c]phenazine and OSO = 2-methylsulfinylbenzoate), was synthesized and characterized to control the photochromic Ru-S → Ru-O linkage isomerization. Details isomerization kinetics studied by UV-visible absorption spectroscopy and cyclic voltammetry revealed that efficient photochromic S → O isomerization and thermal O → S reversal take place in solvents like propylene carbonate (PC), methanol, and dichloromethane. Strikingly, photoisomerization of [1](ClO4) is arrested in water although is active in the analogous compound [Ru(bpy)2(OSO)](ClO4). Effective excited state deactivation through dark 3MLCT state involving dppz ligand of [1](ClO4) switches off photochromism in aqueous medium. Interestingly, the photochromism is activated in aqueous solution in the presence of DNA which shields the dppz localized dark state through intercalation. Ultrafast transient absorption spectroscopic measurement sheds light on the differential behavior of photochromism in aqueous and nonaqueous solvents.

Comparative photophysics and ultrafast dynamics of dimethylaminochalcone and a structurally rigid derivative: experimental identification of TICT coordinate.

Photophysical parameters and ultrafast dynamics of dimethylaminochalone (DMAC-A) have been compared with a structurally bridged analogue (namely, c-DMAC) in solvents of varying polarities. Conformational locking of the single bond connecting the donor and acceptor groups in c-DMAC arrests the twisted intramolecular charge transfer (TICT) relaxation and consequently the photophysical parameters such as fluorescence yield, fluorescence lifetime and triplet yield change significantly. The intramolecular charge transfer character of the LE state is found to be unaffected by conformational restriction. Femtosecond transient absorption spectroscopic studies confirmed that in polar solvents, solvation is the only relaxation process in c-DMAC as opposed to the ultrafast TICT of DMAC-A. The solvent polarity effect on the relaxation dynamics has also been discussed.

Solvent sensitive intramolecular charge transfer dynamics in the excited states of 4-N,N-dimethylamino-4'-nitrobiphenyl.

Organic molecules substituted with the nitro group show efficient nonlinear optical (NLO) properties, which are a consequence of the strong intramolecular charge transfer (ICT) character of the molecules because of the strong electron withdrawing nature of the nitro group and rapid responsiveness because of highly movable π-electrons. Dynamics of the ICT process in the excited states of a push-pull biphenyl derivative, namely, 4-N,N-dimethylamino-4'-nitrobiphenyl (DNBP), an efficient NLO material, has been investigated using ultrafast transient absorption spectroscopy. The experimental results have been corroborated with DFT and TDDFT calculations. In solvents of large polarity, e.g. acetonitrile, the ultrafast ICT process of DNBP is associated with the barrierless twisting of the N,N-dimethylaniline (DMA) group with respect to the nitrobenzene moiety to populate the twisted ICT (or TICT) state, and the rate of this process is solely governed by the viscosity of the medium. In solvents of moderate polarity, e.g. ethyl acetate, the rate of the twisting process is significantly slowed down and the LE and TICT states remain in equilibrium because of a low energy barrier for interconversion between these two states. By further lowering the polarity of the solvent, e.g. in dioxane, the twisting process is completely retarded. In nonpolar solvents, e.g. cyclohexane, a reverse twisting motion towards the planar geometry (i.e. the PICT process) has been evident in the excited state dynamics. In this solvent, the S1 state undergoes an ultrafast intersystem crossing to the triplet state because of its close proximity with the T2 state.

Effect of Donor-Acceptor Coupling on TICT Dynamics in the Excited States of Two Dimethylamine Substituted Chalcones.

Significant effect of coupling between the electron donor and acceptor groups in intramolecular charge transfer (ICT) dynamics has been demonstrated by comparing the photophysical properties of two isomeric N,N-dimethylaminochalcone derivatives (namely, DMAC-A and DMAC-B). In the case of the DMAC-B molecule, the distance between the donor (N,N-dimethylaniline or DMA) and the acceptor (carbonyl) groups is larger by one ethylene unit as compared to that in the case of DMAC-A. The excited singlet (S1) states of both the isomers have strong ICT character but their photophysical properties are remarkably different. In polar solvents, fluorescence quantum yields (and the lifetimes of the S1 state) of DMAC-A are more than 2 orders of magnitude lower (and shorter) than those of DMAC-B. Remarkable differences in the photophysical properties of these two isomers arise due to occurrence of the ultrafast twisting of the DMA group (or the TICT process) during the course of deactivation of the S1 state of the DMAC-A molecule, but not in the case of DMAC-B. In the later case, because of the presence of a large energy barrier along the twisting coordinate(s), TICT is not a feasible process, and hence, the S1 state of DMAC-B has the planar ICT structure. In the DMAC-A molecule, the strength of coupling between the donor and acceptor groups is relatively stronger because of a shorter distance between these groups. Femtosecond transient absorption spectroscopic measurements and DFT/TDDFT calculations have been adopted to establish the above aspects of the relaxation dynamics of the S1 states of these two isomeric chalcones.

Ultrafast twisting dynamics of thioflavin-T: spectroscopy of the twisted intramolecular charge-transfer state.

Understanding the excited-state properties of thioflavin-T (ThT) has been of immense importance, because of its efficient amyloid-sensing ability related to neurodegenerative disorders. The excited-state dynamics of ThT is studied by using sub-pico- and nanosecond time-resolved transient absorption techniques as well as density functional theory (DFT)/time-dependent DFT calculations. Barrierless twisting around the central C-C bond between two aromatic moieties is the dominant process that contributes to the ultrafast dynamics of the S1 state. The spectroscopic properties of the intramolecular charge-transfer state are characterized for the first time. The energetics of the S0 and S1 states has also been correlated with the experimentally observed spectroscopic parameters and structural dynamics. A longer-lived transient state populated with a very low yield has been characterized as the triplet state.

Probing excited state charge transfer dynamics in a heteroleptic ruthenium complex.

Dynamics of metal to ligand charge transfer in the excited states of ruthenium polypyridyl complexes, which have shown promise as materials for artificial solar energy harvesting, has been of immense interest recently. Mixed ligand complexes are especially important for broader absorption in the visible region. Dynamics of ultrafast vibrational energy relaxation and inter-ligand charge transfer processes in the excited states of a heteroleptic ruthenium complex, [Ru(bpy)2(pap)](ClO4)2 (where bpy is 2,2'-bipyridine and pap is 2-(phenylazo)pyridine) have been investigated using femtosecond to nanosecond time-resolved transient absorption spectroscopic techniques. A good agreement between the TA spectrum of the lowest excited (3)MLCT state of [Ru(bpy)2(pap)](ClO4)2 complex and the anion radical spectrum of the pap ligand, which has been generated using the pulse radiolysis technique, confirmed the charge localization at the pap ligand. While the lifetime of the inter-ligand charge transfer from the bpy to the pap ligand in the (3)MLCT state is about 2.5 ps, vibrational cooling of the pap-localized(3)MLCT state occurs over a much longer time scale with a lifetime of about 35 ps. Ultrafast charge localization dynamics observed here may have important consequences in artificial solar energy harvesting systems, which employ heteroleptic ruthenium complexes.

Selective recognition and extraction of iodide from pure water by a tripodal selenoimidazol(ium)-based chalcogen bonding receptor.

A selenium-based tripodal chalcogen bond (ChB) donor TPI-3Se is demonstrated for the recognition and extraction of I- from 100% water medium. NMR and ITC studies with the halides reveal that the ChB donor selectively binds with the large, weakly hydrated I-. Interestingly, I- crystallizes out selectively in the presence of other halides supporting the superiority of the selective recognition of I-. The X-ray structure of the ChB-iodide complex manifests both the µ1 and µ2 coordinated interactions, which is rare in the C-Se···I chalcogen bonding. Furthermore, to validate the selective I- binding potency of TPI-3Se in pure water, comparisons are made with its hydrogen and halogen bond donor analogs. The computational analysis also provides the mode of I- recognition by TPI-3Se. Importantly, this receptor is capable of extracting I- from pure water through selenium sigma-hole and I- interaction with a high degree of efficiency (∼70%).

Dynamics of solvent controlled excited state intramolecular proton transfer coupled charge transfer reactions.

Coupled intramolecular proton and charge transfer reactions play an important role in many biological and chemical reactions. In this article, we report the relaxation dynamics of the excited states of a donor substituted 1,3-diketone (DMADK, ) using steady state and ultrafast transient absorption and fluorescence spectroscopic techniques. Dramatic dependence of the fluorescence quantum yield, nonradiative rate as well as the excited state relaxation pathways on solvent polarity reveals the solvent controlled excited state intramolecular proton transfer (ESIPT) directed intramolecular charge transfer (ICT) dynamics. The molecules in the ground state coexist in two possible cis-enol (Enol-A and Enol-B) forms. Time-dependent density functional theory (TDDFT) calculations reveal solvent polarity controlled thermodynamic stabilization of one of the tautomeric structures in the S1 state, dictating the direction of proton transfer and subsequent structural relaxation. In low and medium polarity solvents, the S1 state of Enol-B (Enol-B*) undergoes ultrafast ESIPT leading to the population of Enol-A*, followed by the ICT process. In polar solvents, the ESIPT process is reversed to populate Enol-B*, which undergoes a twisted intramolecular charge transfer (TICT) process via twisting of the N,N-dimethylaniline group. This work demonstrates that the strength of electronic coupling between the donor and acceptor group determines the structure of the ICT state.

Superiority of the Supramolecular Halogen Bond Receptor over Its H-Bond Analogue toward the Efficient Extraction of Perrhenate from Water.

A halogen bond-based water-soluble tetrapodal iodoimidazolium receptor, (L-I)(4Br), exhibited a high degree of efficiency (∼96%) in extracting ReO4- from 100% aqueous medium within a wide range of concentrations and of pH values along with excellent reusability. The solid-state X-ray diffraction study showed the trapping of ReO4- by (L-I)(4Br) via the Re-O····I halogen bonding interaction. XPS studies also suggested the interaction between I and ReO4- through polarization of the electron density of I atoms by ReO4-. (L-I)(4Br) is found to be capable of retaining its high extraction efficiency in the presence of competing anions such as F-, Cl-, I-, SO42-, H2PO4-, CO32-, NO3-, BF4-, ClO4-, Cr2O72-, and a mixture of these anions. Interestingly, (L-I)(4Br) was found to be superior in ReO4- extraction as compared to its hydrogen-bond donor analogue, (L-H)(4Br), as confirmed by a series of control experiments and theoretical calculations. Our synthesized dipodal and tripodal halogen bond donor receptors and their H-analogues validated the superiority of these classes of supramolecular halogen bond donor receptors over their hydrogen-bond analogues. (L-I)(4Br) also showed superior practical applicability in terms of the removal of ReO4- at anion concentrations as low as ∼100 ppm, which was a major shortcoming of (L-H)(4Br).

Modulated Ultrathin NiCo-LDH Nanosheet-Decorated Zr3+-Rich Defective NH2-UiO-66 Nanostructure for Efficient Photocatalytic Hydrogen Evolution.

Defect engineering through modification of their surface linkage is found to be an effective pathway to escalate the solar energy conversion efficiency of metal-organic frameworks (MOFs). Herein, defect engineering using controlled decarboxylation on the NH2-UiO-66 surface and integration of ultrathin NiCo-LDH nanosheets synergizes the hydrogen evolution reaction (HER) under a broad visible light regime. Diversified analytical methods including positron annihilation lifetime spectroscopy were employed to investigate the role of Zr3+-rich defects by analyzing the annihilation characteristics of positrons in NH2-UiO-66, which provides a deep insight into the effects of structural defects on the electronic properties. The progressively tuned photophysical properties of the NiCo-LDH@NH2-UiO-66-D-heterostructured nanocatalyst led to an impressive rate of HER (∼2458 µmol h-1 g-1), with an apparent quantum yield of ∼6.02%. The ultrathin NiCo-LDH nanosheet structure was found to be highly favored toward electrostatic self-assembly in the heterostructure for efficient charge separation. Coordination of Zr3+ on the surface of the NiCo-LDH nanosheet support through NH2-UiO-66 was confirmed by X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy techniques. Femtosecond transient absorption spectroscopy studies unveiled a photoexcited charge migration process from MOF to NiCo-LDH which favorably occurred on a picosecond time scale to boost the catalytic activity of the composite system. Furthermore, the experimental finding and HER activity are validated by density functional theory studies and evaluation of the free energy pathway which reveals the strong hydrogen binding over the surface and infers the anchoring effect of the ultrathin layered double hydroxide (LDH) in the vicinity of the Zr cluster with a strong host-guest interaction. This work provided a novel insight into efficient photocatalysis via defect engineering at the linker modulation in MOFs.

An Isothiazolanthrone-Based Self-Assembling Anticancer Color-Changing Dye for Concurrent Imaging and Monitoring of Cell Viability.

We report the photophysical properties, self-assembly and biological evaluation of an isothiazolanthrone-based dye, 7-amino-6H-anthra[9,1-cd]isothiazol-6-one (AAT), which reveals anticancer properties and can be potentially used as dye for monitoring cell viability. The solvent-dependent photophysical studies suggest that the emission of AAT is sensitive to environment polarity due to which interesting changes in the colored emission may be observed owing to the charge transfer (CT) processes. AAT also self-assembles to tree-like branched morphologies and produce, a greenish emission inside the cells when imaged after short interval (15 mins) of incubation while a red fluorescence could be noted after 24 h. Interestingly, AAT also produce differential emission inside mouse normal cells as compared to its cancer cell lines since it possess anticancer activity. The experimental observations were also validated theoretically via computational modeling.

Ultrafast dynamics of the excited states of 1-(p-nitrophenyl)-2-(hydroxymethyl)pyrrolidine.

The dynamics of the excited states of 1-(p-nitrophenyl)-2-(hydroxymethyl)pyrrolidine (p-NPP) has been investigated using the subpicosecond transient absorption spectroscopic technique in different kinds of solvents. Following photoexcitation using 400 nm light, conformational relaxation via twisting of the nitro group, internal conversion (IC) and the intersystem crossing (ISC) processes have been established to be the three major relaxation pathways responsible for the ultrafast deactivation of the excited singlet (S(1)) state. Although the nitro-twisting process has been observed in all kinds of solvents, the relative probability of the occurrence of the other two processes has been found to be extremely sensitive to solvent polarity, because of alteration of the relative energies of the S(1) and the triplet (T(n)) states. In the solvents of lower polarity, the ISC is predominant over the IC process, because of near isoenergeticity of the S(1)(ππ*) and T(3)(nπ*) states. On the other hand, in the solvents of very large polarity, the energy of the S(1)(ππ*) state becomes lower than those of both the T(3)(nπ*) and T(2)(nπ*/ππ*) states, but those of the T(1)(ππ*) state and the IC process to the ground electronic (S(0)) state are predominant over the ISC, and hence the triplet yield is nearly negligible. However, in the solvents of medium polarity, the S(1) and T(2) states become isoenergetic and the deactivation of the S(1) state is directed to both the IC and ISC channels. In the solvents of low and medium polarity, following the ISC process, the excited states undergo IC, vibrational relaxation, and solvation in the triplet manifold. On the other hand, following the IC process in the Franck-Condon region of the S(0) state, the vibrationally hot molecules with the twisted nitro group subsequently undergo the reverse nitro-twisting process via dissipation of the excess vibrational energy to the solvent or vibrational cooling.