Rare Example of TICT Based Optical Responses for the Specific Recognition of Cr³⁺ by a 2,2′:6′,2″-Terpyridine Derivative and Demonstration of Multiple Logic Operations

Chemosensor <b>L</b> showed a <i>nonlinear fluorescence</i> response on specific binding to Cr³⁺ ion in the presence of various alkali, alkaline-earth, transition, and lanthanide metal ions. A luminescence band with maxima at 512 nm for <b>L</b> was observed (λ_ext = 330 nm) for a twisted intramolecular charge transfer (TICT) transition following antienergy gap law behavior. However, normal energy gap law behavior prevailed on formation of a new nonluminescent charge transfer complex, M^<i>n</i>+·<b>L</b>. For paramagnetic metal ions, paramagnetism induced luminescence quenching could have also contributed to this. A new <i>switched on</i> fluorescence response at λ_ems of 356 nm (λ_ext of 330 nm) was observed due to Franck–Condon charge transfer (FC-CT) transition only on the formation of a complex, (Cr³⁺)₂·<b>L</b>. Spectral studies revealed a unique dynamic coordination behavior and migration of Cr³⁺ from the terpyridyl fragment to the N_NMe₂ center of <b>L</b> as a function of the varying concentration of another ion (Zn²⁺) and the subtle difference in the binding affinities of the terpyridyl moiety toward Cr³⁺ and Zn²⁺. Further, spectral responses of <b>L</b> toward Zn²⁺, different concentration of Cr³⁺, H⁺ and on subsequent addition of F^– as different ionic inputs could be correlated well for demonstrating various basic and combinatorial circuits

Size Quantization Effects on Interfacial Electron Transfer Dynamics in Ru(II)–Polypyridyl Complex Sensitized ZnO QDs

Quantum-size confinement in semiconductor material offers size based tunability of interband gap energy as well as intraband sublevels. In this work, size quantization of wide bandgap ZnO quantum dots has been explored in the study of interfacial charge separation reaction using a catechol functionalized Ru­(II)–polypyridyl complex as a photosensitizer molecule. Femtosecond time-resolved transient absorption studies have revealed multiple electron injection events based on discrete conduction band states of ZnO QDs. The electron injection rates have been rationalized for quantum confinement effects owing to different sizes of ZnO QDs. Furthermore, the size dependency of the intrinsic lifetime of electrons injected into discrete energy levels of ZnO QDs has been revealed in charge recombination reaction with the Ru­(III)–polypyridyl complex cation. The charge recombination dynamics reveals a competing trend of carrier confinement and carrier leak upon reducing particle size. This study shows the optimization of finite size effects in achieving better interfacial charge separation at the dye/semiconductor interface

[2]Pseudorotaxane Formation with FRET Based Luminescence Response: Demonstration of Boolean Operations through Self-Sorting on Solid Surface

Binary pseudorotaxane formation between an aza crown derivative as host (<b>H</b>) and two different imidazolium derivatives as guests (<b>G</b>_<b>1</b> and <b>G</b>_<b>2</b>) have been studied in detail by NMR (¹H NMR, 2D NOESY), optical (steady state electronic and emission spectroscopy), and mass spectroscopy. Binding stoichiometry (1:1), association constant for the respective [2]­pseudorotaxane formation (<i>K</i>_a^{<b>H.G</b>_<b>1</b>} = (2.61 ± 0.015) × 10³ M^–1 and <i>K</i>_a^{<b>H.G₂</b>} = (1.27 ± 0.16) × 10³ M^–1), and associated thermodynamic parameters are also evaluated based on isothermal titration calorimetric (ITC) studies. FRET based <i>luminescence ON</i> responses are observed on formation of the binary pseudorotaxane (<b>H.G</b>_<b>1</b> and <b>H.G</b>_<b>2</b>) in a nonpolar medium like dichloromethane. The thermodynamic feasibility of such an energy transfer process is also examined. The higher affinity of <b>H</b> and 18-crown-6 toward K⁺, as compared to those toward <b>G</b>_<b>1</b> or <b>G</b>_<b>2</b>, and the reversibility in the host–guest binding process are utilized in demonstrating the self-sorting phenomena with associated changes in luminescence responses that could be correlated for Boolean operators like YES, INHIBIT, OR, and AND gates

Synthesis, Steady-State, and Femtosecond Transient Absorption Studies of Resorcinol Bound Ruthenium(II)- and Osmium(II)-polypyridyl Complexes on Nano-TiO₂ Surface in Water

The synthesis of two new ruthenium­(II)- and osmium­(II)-polypyridyl complexes <b>3</b> and <b>4</b>, respectively, with resorcinol as the enediol anchoring moiety, is described. Steady-state photochemical and electrochemical studies of the two sensitizer dyes confirm strong binding of the dyes to TiO₂ in water. Femtosecond transient absorption studies have been carried out on the dye–TiO₂ systems in water to reveal <120 fs and 1.5 ps electron injection times along with 30% slower back electron transfer time for the ruthenium complex <b>3</b>. However, the corresponding osmium complex <b>4</b> shows strikingly different behavior for which only a <120 fs ultrafast injection is observed. Most remarkably, the back electron transfer is faster as compared to the corresponding catechol analogue of the dye. The origin and the consequences of such profound effects on the ultrafast interfacial dynamics are discussed. This Article on the electron transfer dynamics of the aforesaid systems reinforces the possibility of resorcinol being explored and developed as an extremely efficient binding moiety for use in dye-sensitized solar cells

Proton-Coupled Electron Transfer in a Hydrogen-Bonded Charge-Transfer Complex

A proton-coupled electron transfer (PCET) reaction in a hydrogen-bonded charge-transfer (CT) complex of 4-([2,2′-bipyridin]-4-yl)­phenol (bpy-phenol) with a F^– ion has been investigated by ultrafast time-resolved transient absorption spectroscopy. The phenolic receptor molecule, bpy-phenol, binds to the F^– ion through a hydrogen bond and senses the F^– ion via the Stokes-shifted CT band. Upon photoexcitation, CT from the phenol residue to the bpy residue promotes proton transfer from the phenol radical cation (ArOH^•+) to the fluoride ion at ultrafast time scales of <150 fs (instrument response function limited) and 3 ps, separately. The fast and slow proton-transfer times are linked to two different types of hydrogen-bonding networks between the phenol residue and fluoride ion. Crystalline water in the fluoride salt hydrates mediates the proton-transfer reaction. This work demonstrates the participation of a hydrogen-bonded water bridge within a PCET reaction in a water-restricted environment

Hydrogen Bond and Ligand Dissociation Dynamics in Fluoride Sensing of Re(I)–Polypyridyl Complex

Hydrogen bonding interaction plays an essential role in the early phases of molecular recognition and colorimetric sensing of various anions in aprotic media. In this work, the host–guest interaction between <i>fac</i>-[Re­(CO)₃Cl­(L)] with L = 4-([2,2′-bipyridin]-4-yl)­phenol and fluoride ions is investigated for the hydrogen bond dynamics and the changing local coordination environment. The stoichiometric studies using ¹H NMR and ESI-MS spectroscopies have shown that proton transfer in the H-bonded phenol–fluoride complex activates the dissociation of the CO ligand in the Re­(I) center. The phenol-to-phenolate conversion during formation of HF₂^– ion induces nucleophilic lability of the CO ligand which is probed by intraligand charge transfer (ILCT) and ligand-to-metal charge transfer (LMCT) transitions in transient absorption spectroscopy. After photoexcitation, phenol–phenoxide conversion rapidly equilibrates in 280 fs time scale and the ensuing excited state [Re^II(bpy•^–phenolate¯) (CO)₃Cl]* undergoes CO dissociation in the ultrafast time scale of ∼3 ps. A concerted mechanism of hydrogen cleavage and coordination change is established in anion sensing studies of the rhenium complex

A Taco Complex Derived from a Bis-Crown Ether Capable of Executing Molecular Logic Operation through Reversible Complexation

As learned from natural systems, self-assembly and self-sorting help in interconnecting different molecular logic gates and thus achieve high-level logic functions. In this context, demonstration of important logic operations using changes in optical responses due to the formation of molecular assemblies is even more desirable for the construction of a molecular computer. Synthesis of an appropriate divalent as well as a luminescent crown ether based host <b>1</b> and paraquat derivatives, <b>2</b>(PF₆)₂ and <b>3</b>(PF₆)₂, as guests helped in demonstrating a reversible [3]­(taco complex) (<b>1</b>·{<b>2</b>(PF₆)₂}₂ or <b>1</b>·{<b>3</b>(PF₆)₂}₂) formation in nonpolar solvent. Detailed ¹H NMR studies revealed that two paraquat units were bound cooperatively by the two crown units in <b>1</b>. Because of preorganization, the flexible host molecule <b>1</b> adopts a folded conformation, where each of two paraquat units remain sandwiched between the two aromatic units of each folded crown ether moiety in <b>1</b>. Disassembly of the “taco” complex in the presence of KPF₆ and reassembly on subsequent addition of DB18C6 was initially demonstrated by ¹H NMR spectral studies, which were subsequently corroborated through luminescence spectral studies. Further, luminescence spectral responses as output signals with appropriate and two independent molecular inputs could be correlated to demonstrate basic logic operation like OR and YES gates, while the results of the three molecular inputs could be utilized to demonstrate important logic operation like an INHIBIT gate

Charge Delocalization in the Cascade Band Structure CdS/CdSe and CdS/CdTe Core–Shell Sensitized with Re(I)–Polypyridyl Complex

Charge-carrier dynamics of CdS quantum dot (QD) and CdS/CdSe type-I and CdS/CdTe type-II core–shell nanocrystals (NCs) sensitized with a Re­(I)–polypyridyl complex have been carried with special emphasis on studies on carrier delocalization and the role of Re-complex as a hole acceptor and sensitizer molecule. Our investigation confirmed photoexcited hole transfer from CdS and CdS/CdSe to the Re-complex, while no hole transfer was observed in the CdS/CdTe–Re-complex system. This was rationalized by the evaluation of the relative energy levels, which revealed that such hole migration was not energetically favorable due to low-lying highest occupied molecular orbital (HOMO) of the Re-complex as compared with the valence band (VB) of CdTe shell; however, luminescence quenching from upper excited states of Re-complex was observed in the presence of all three QD and core–shell systems, which has been attributed to electron injection from hot state (energetically higher than the LUMO state) of the Re-complex to the conduction band (CB) of the QDs. Transient absorption (λ_pump = 400 nm, λ_probe = 450–750 nm) spectra recorded for Re-complex-sensitized CdS and CdS/CdSe composite in the femtosecond time domain revealed a broad transient absorption band in the 580–750 nm region with a peak around 595 nm, and this was attributed to the cation radical formation for Re-complex, either by capturing photoexcited hole from the NCs or by injecting electron to the CB of the NCs. As anticipated, no such spectrum was observed for the CdS/CdTe–Re-complex composite system after 400 nm excitation. Electron injection from photoexcited Re-complex to CdS QD and CdS/CdSe core–shell was found to be <100 fs, while the hole transfer from photoexcited CdS QD and CdS/CdSe core–shell to Re-complex took place within the time scale of 900 fs and 2.5 ps, respectively

Role of Metal Ion in Specific Recognition of Pyrophosphate Ion under Physiological Conditions and Hydrolysis of the Phosphoester Linkage by Alkaline Phosphatase

Complexes synthesized from Zn­(II), Cu­(II), and Cd­(II), using a dipicolyl amine derivative (<b>L</b>), showed unique specificity toward pyrophosphate ion (PPi or P₄O₇^4–) among all other common anionic analytes, including different biologically significant phosphate ion (PO₄^3–, H₂PO₄^2–) or phosphate-ion-based nucleotides, such as AMP, ADP, ATP, and CTP. However, the relative affinities of PPi toward these three metal complexes were found to vary and follow the order <i>K</i>_a^{<b>L.Zn</b>–PPi} > are given in units of _a^{<b>L.Cu</b>–PPi} ≥ <i>K</i>_a^{<b>L.Cd</b>–PPi}. Luminescence responses of the receptor <b>L</b> were substantial on binding to Zn²⁺ and Cd²⁺, while relatively a much smaller luminescence response was observed in the presence of Cu²⁺. Luminescence responses of <b>L.M</b>–PPi (<b>M</b> is Zn²⁺, Cd²⁺, and Cu²⁺) were further modified on binding to the PPi ion. This could be utilized for quantitative detection of PPi in physiological condition as well as for developing a real time “turn-on” (for <b>L.Zn</b> and <b>L.Cu</b>) and “turn-off” (for <b>L.Cd</b>) fluorescence assay for evaluating the enzymatic activity of alkaline phosphatase (ALP). Experimental results revealed how the subtle differences in the binding affinities between PPi and M in <b>L.M</b> (<b>M</b> is Zn²⁺, Cd²⁺, and Cu²⁺), could influence the cleavage of the phosphoester linkage in PPi by ALP. The DFT calculations further revealed that the hydrolytic cleavage of the metal ion coordinated phosphoester bond is kinetically faster than that for free PPi and thus, rationalized the observed difference in the cleavage of the phosphoester bond by an important mammalian enzyme such as ALP in the presence of different metal complexes

Ultrafast Electron Injection, Hole Transfer, and Charge Recombination Dynamics in CdSe QD Super-Sensitized Re(I)–Polypyridyl Complexes with Catechol and Resorcinol Moiety: Effect of Coupling

Ultrafast charge-transfer dynamics have been demonstrated in CdSe quantum dots (QD) using two Re­(I)–polypyridyl complexes having pendent catechol (<b>Re1,2</b>) and resorcinol (<b>Re1,3</b>) as the sensitizer molecules. The energy level diagram of CdSe QD and <b>Re1,2</b> and <b>Re1,3</b> sensitizer reveals that photoexcited hole of CdSe QD can be transferred to both <b>Re1,2</b> and <b>Re1,3</b> molecule, and photoexcited <b>Re1,2</b> and <b>Re1,3</b> can inject electron in the conduction band, which has been confirmed by steady-state and time-resolved photoluminescence studies with selective photoexcitation. Femtosecond transient absorption studies have been carried out to monitor charge-transfer dynamics in early time scale. Transient absorption spectra show formation of cation radicals for both <b>Re1,2</b> and <b>Re1,3</b> in the 550–650 nm region with a peak at 590 nm region and broad absorption in the 650–1000 nm region, which can be attributed to photoexcited electron in the conduction band of CdSe QD. Charge recombination was determined by monitoring the decay of cation radicals as well as decay of an electron and found to be slower in the <b>Re1,3</b>/CdSe system as compared to that of the <b>Re1,2</b>/CdSe system, which is due to weaker electronic coupling in the former system