Anjan Chattopadhyay

Amine functionalized tetraphenylethylene: a novel aggregation-induced emission based fluorescent chemodosimeter for nitrite and nitrate ions

A novel AIE-based fluorescent probe for the detection of trace amounts of nitrite and nitrate ions in water has been developed. The probe, a monoamine of tetraphenylethylene, spontaneously detects nitrites (or nitrates) by a fluorescence “turn-off” method via diazotization followed by formation of a non fluorescent TPE-azodye. The salient features of this method are high sensitivity and selectivity, cost effective synthesis, fast detection process and low detection limit.

Design, synthesis, physicochemical studies, solvation, and DNA damage of quinoline-appended chalcone derivative: comprehensive spectroscopic approach toward drug discovery.

The present study epitomizes the design, synthesis, photophysics, solvation, and interaction with calf-thymus DNA of a potential antitumor, anticancer quinoline-appended chalcone derivative, (E)-3-(anthracen-10-yl)-1-(6,8-dibromo-2-methylquinolin-3-yl)prop-2-en-1-one (ADMQ) using steady state absorption and fluorescence spectroscopy, molecular modeling, molecular docking, Fourier-transform infrared spectroscopy (FTIR), molecular dynamics (MD) simulation, and gel electrophoresis studies. ADMQ shows an unusual photophysical behavior in a variety of solvents of different polarity. The dual emission has been observed along with the formation of twisted intramolecular charge transfer (TICT) excited state. The radiationless deactivation of the TICT state is found to be promoted strongly by hydrogen bonding. Quantum mechanical (DFT, TDDFT, and ZINDO-CI) calculations show that the ADMQ is sort of molecular rotor which undergoes intramolecular twist followed by a complete charge transfer in the optimized excited state. FTIR studies reveals that ADMQ undergoes important structural change from its native structure to a β-hydroxy keto form in water at physiological pH. The concentration-dependent DNA cleavage has been identified in agarose gel DNA electrophoresis experiment and has been further supported by MD simulation. ADMQ forms hydrogen bond with the deoxyribose sugar attached with the nucleobase adenine DA-17 (chain A) and result in significant structural changes which potentially cleave DNA double helix. The compound does not exhibit any deleterious effect or toxicity to the E. coli strain in cytotoxicity studies. The consolidated spectroscopic research described herein can provide enormous information to open up new avenues for designing and synthesizing chalcone derivatives with low systematic toxicity for medicinal chemistry research.

Spectroscopic features of the low-lying electronic states of some sodium–helium and potassium–helium van der Waals systems

Configuration interaction studies on MHe and MHe2 (where M = Na, K) systems have revealed several interesting characteristics in the properties of their low-lying electronic states. Binding energy values of the 12Π1/2, 3/2 states in MHe systems are found to be lower than the values of 12Πu (1/2, 3/2) states in the He–M–He systems by a margin of more than 200 cm−1, indicating better exciplex stabilities of the latter systems. Excited states of the other variety of the linear MHe2 (M–He–He) systems are almost repulsive. The characteristic energy barrier of the first excited spin–orbit state of alkali metal-helium systems is found to be only 15 cm−1 in KHe and 19 cm−1 in He–K–He. For the Na*He and K*He exciplexes, predicted radiative lifetime values of 18.5 ns and 29.8 ns, respectively, are in excellent agreement with the experimental values. The red-tail portions of their emission bands are contributed by M*He2 exciplexes with relatively high radiative lifetimes. The repulsive excited state of 2Σ+1/2 (or 2Σ+g,1/2) symmetry in these van der Waals systems is likely to play an important role in the pumping of the blue side of the ns2S1/2 → np2P3/2 transition, which eventually may give rise to the np2P1/2 → ns2S1/2 lasing transition.

Spectroscopic features of the low-lying singlet states of some N-alkyl retinylnitrone model systems and their involvement in oxaziridine formation

The photo-excitation process and non-radiative decay channels of the model compounds of some N-alkyl retinylnitrones are studied at the CASSCF/6-31G*, CASMP2/6-31G* and PM3/CI level of theories. A relaxed planar geometry of the first excited singlet state is reached after the initial photo-excitation, which is followed by non-radiative decay processes through conical intersection (S0/S1) channels. Their first-excited singlet states (S1) have mixed biradical–ionic nature, and are found to be dominated by configurations arising from HOMO2 → LUMO2, HOMO − 1 → LUMO and HOMO → LUMO excitations. Conical intersection geometries originating from the one-bond-flip and Hula-twist motions in the central part of these molecules are found at higher energies in comparison to their terminally twisted counter parts. In the N-methyl nitrone system, the lowest-energy intersection point arises due to a twist in the terminal part with an out-of-plane CNO-kink (RC–O = 2.12 A, RN–O = 1.38 A) or oxygen-bridge structure. Following the directions of its gradient difference vectors, the probable oxaziridine ground-state geometry (RC–O = 1.38 A, RN–O = 1.44 A, <OCN = 62.3°, <ONC = 57.6°) has been located as a saddle point, which is the only experimentally reported photoproduct of N-methyl retinylnitrone compound under room light. The radiative transition studies on the allowed S0 → S1 transitions at the ground state equilibrium geometry have given transition moment values between 4.5 and 5.0 Debye.

A comprehensive spectroscopic investigation of α-(2-naphthyl)-N-methylnitrone: a computational study on photochemical nitrone–oxaziridine conversion and thermal E–Z isomerization processes

This comprehensive spectroscopic analysis of α-(2-naphthyl)-N-methylnitrone has proposed its photochemical oxaziridine formation and thermal E–Z isomerization mechanisms. The activation energy for the conversion of the unstable non-planar E isomer to the stable planar Z-isomer is found to be 23.7 kcal mol−1 at the CASSCF/6-31G* level of calculation. A transition state with a negative frequency of 350 cm−1 is likely to be responsible for this process. Both CASSCF and ONIOM-based studies have revealed that the nitrone–oxaziridine photochemical conversion involves non-radiative decay channels which include biradicaloid conical intersection (CI) points through Hula-twist and terminal one-bond-flip motions, situated at 35–40 kcal mol−1 below the first excited singlet state (S1). Following the directions of their gradient-difference vectors, the optimized oxaziridine geometries are obtained. The nature of the low-lying singlet–singlet transitions of these α-naphthyl N-methylnitrones is found to be similar to that of the conjugated non-polar polyenes, and differs appreciably from our previously studied long-chain conjugated nitrone systems. The fluorescent S1 state with a radiative lifetime of nanosecond order is populated by the weak upward S0–S1 transition (transition moment: 0.3 Debye) and through the decay of the S2 state, which eventually gets involved in the S0/S1 conical intersections.

A computational investigation of the photochemical oxaziridine and amide conversion process of open-chain conjugated nitrone with electron-withdrawing trifluoromethyl group on nitrogen

AbstractThis computational study investigates the photo-excitation process and subsequent photoproduct formation steps through non-radiative deactivation channels in open-chain conjugated N-substituted nitrone systems (model compounds of corresponding retinylnitrones) having electron-withdrawing groups on nitrogen. Calculations mostly based on CASSCF/6-31G* and CASMP2/6-31G* level of theories on a representative system with N-trifluoromethyl substituent have predicted initial photo-excitation to a planar singlet state. This photochemical path is subsequently followed by a barrierless non-radiative channel towards the lowest-energy conical intersection (CI) geometry having a terminal CNO kink, and situated at 30 kcal/mol below the planar excited state. Following the direction of its gradient difference (GD) vectors, an oxaziridine-type species (R C−O=1.38 Å, R N−O=1.53 Å, < CNO =55.8∘) appears at 3–6 kcal mol −1 below the ground state nitrone system through a transition state (along its reverse direction of minimum-energy path), situated on the reaction pathway. This species with an elongated N-O bond seems to be heading towards an amide geometry. On the other hand, in the opposite GD vector direction a proper oxaziridine geometry has been obtained with a much shorter N-O bond distance (R N−O=1.42 Å). Graphical AbstractCASSCF-based photochemical studies on conjugated nitrones with N-trifluoromethyl group have revealed a non-radiative decay route of the singlet excited state through a terminally twisted conical intersection. This eventually leads to an oxaziridine-type biradical species with an elongated N-O bond and it seems to be heading towards an amide as photoproduct.

Synthesis and photophysical characterization of quasi push–pull dicyanodibenzodioxins and their anti-tumor activity against glioma cell line C6

Dibenzodioxins bearing multiple electron withdrawing groups were synthesized using a simple one-step methodology including examples of molecules possessing electron acceptor groups in both ends. As a consequence internal charge delocalization occurs and the optical spectra are found to be bathochromically shifted compared to similar examples known thus far. A theoretical analysis of the molecular orbitals reveals the origin of the peaks in the dibenzodioxin optical spectra. Select examples exhibit in vitro neuro-cytotoxicity against glioma cell line C6, a finding which enhances existing knowledge about the pharmacologically relevant structural motifs in dibenzodioxins.

Synthesis and electronic properties of ester substituted 1,4-dicyanodibenzodioxins and evaluation of anti-proliferative activity of all isomeric 1,2-, 2,3- and 1,4-dicyanodibenzodioxins against HeLa cell line

1,4-Dicyanodibenzodioxins bearing carboxy methyl ester groups were synthesized using our established one-step SNAr coupling reaction between ortho- and meta-ester substituted catechols and perfluorinated terephthalonitrile. These are the first examples of 1,4-dicyanodibenzodioxins substituted at both the benzene moieties. Optical spectra were similar to the earlier examples reported, with a marginal blue shift for the ester dibenzodioxins. Theoretical analysis of the molecular orbitals reveals modest destabilization of the frontier molecular orbitals of one carboxy methyl ester isomer over the other and overall higher HOMO-LUMO gap for both isomers when compared to the earlier published 1,4-dicyanodibenzodioxins. In vitro cytotoxicity against human cervical cancer HeLa cell line was evaluated for these two compounds and all other previously published dibenzodioxins from our laboratory (1,4-dicyano, 1,2-dicyano and 2,3-dicyano variants). A number of derivatives showed anti-tumor activity in μM ranges and also exhibited no cytotoxicity against normal HEK 293 cell line. Mechanistic investigation of cell death pathways indicated high levels of reactive oxygen species (ROS) in the dibenzodioxin treated tumor cell lines along with cellular nuclear fragmentation, both of which are markers of the apoptotic cell death pathway.

Exploring the isomerization paths of push–pull hexatrienes

This computational study is an attempt to reveal the mechanism of the isomerization processes happening in donor (D)–acceptor (A) hexatriene systems. The photo-excitation of all-trans isomers of these conjugated systems with terminal donor (amine, methoxy) and acceptor (cyano) groups populates the first (S1) and second (S2) singlet excited states which correspond to transitions with reasonably high oscillator strength values. The S1 state of the amine (D), cyano (A)-substituted system forms a twisted and couple of slightly off-planar minima connected by low-energy transition states with configurations equally dominated by HOMO → LUMO and HOMO2 → LUMO2 excitations. Two important low-lying S0/S1 conical intersections have been identified in this system at 7–8 kcal mol−1 and 8–11 kcal mol−1 above the twisted excited state minima. The first one has been identified as the source of a cis–trans–trans isomer while the latter one may be responsible for the trans–cis–trans isomer. In comparison, the presence of a weaker donor group (methoxy) produces a more stable cis–trans–trans isomer from a lower energy S0/S1 conical intersection, situated around 20–23 kcal mol−1 below the vertically excited geometry. Both the isomers have an alternate thermal route of formation from the all-trans isomer through ground state transition states with activation energy values close to 50 kcal mol−1.

Computational Investigation of the Photochemical Reaction Path of Some Synthesized and Experimentally Analyzed Small-Chain Conjugated Nitrones

This combined theoretical and experimental study has revealed the photochemistry of two small open-chain conjugated N-methylnitrone systems with phenyl substitutions at the C-terminal positions. The UV spectra of these synthesized nitrones have shown intense peaks around 330 nm while the new bands formed near 260 nm after their photoirradiation are predicted to be arising from the photoproduct oxaziridine. Photoexcitation of α-styryl N-methylnitrone populates the first excited singlet state which relaxes by 8 kcal/mol from the vertically excited state and subsequently goes toward the lowest-energy conical intersection (CI) geometry (situated 27-30 kcal/mol below) with a terminal CNO-kink. Following the gradient difference vectors of this CI, we have located the oxaziridine structure with its characteristic geometry at roughly 14 kcal/mol above the ground state. This whole process is triggered by a transfer of electronic cloud from oxygen to the conjugated chain side. On the other hand, the photoexcitation of the nonplanar 3,3-diphenylethylene N-methylnitrone has two strong singlet-singlet absorptions with almost 5 D transition moment values. Here the initial S2-S1 relaxation is followed by oxaziridine formation through the terminally twisted CI. However, the initially photoexcited S1 state in this nitrone is found to head toward some other direction with transfer of huge amount of nonbonding electron cloud of oxygen to the π* orbital, creating a stable excited state geometry with an elongated N-O bond which gets involved in a sloped CI with the ground state.