Cage effects on conformational preference and photophysics in the host-guest complex of benzenediols with 18-Crown-6

Photodissociation dynamics of m- and p-cresol in the S1 state: Interplay between the mode-randomization and H atom tunneling reaction

The H atom tunneling dissociation dynamics of the S1 state of meta- or para-cresol has been investigated by using the picosecond time-resolved pump-probe spectroscopy in a state-specific manner. The S1 state lifetime (mainly due to the H atom tunneling reaction) is found to be mode-dependent whereas it quickly converges and remains constant as the rapid intramolecular vibrational energy redistribution (IVR) starts to participate in the S1 state relaxation with the increase of the S1 internal energy (Eint). The IVR rate and its change with increasing Eint have been reflected in the parent ion transients taken by tuning the total energy (hνpump + hνprobe) just above the adiabatic ionization threshold (so that the dissipation of the initial mode-character could be monitored as a function of the reaction time), indicating that the mode randomization rate into the S1 isoenergetic manifolds exceeds the tunneling rate quite early in terms of Eint for m-cresol (≤∼1200 cm-1) or p-cresol (≤∼800 cm-1) compared to the case of phenol (≤∼1800 cm-1). Though the H atom tunneling dynamics of phenol (S1) seems to be little influenced by the methyl substitution on the either m- or p-position, the IVR rate has been found to be strongly accelerated due to the sharply-increasing (S1) density of states with increasing Eint due to the pivotal role of the low-frequency CH3 torsional mode.

Spherand complexes with Li+ and Na+ ions in the gas phase: encapsulation structure and characteristic unimolecular dissociation

We investigated the complexes of Cram's hexa(p-anisole) spherands (SPR, 1) with Li⁺ and Na⁺ ions (1·Li⁺ and 1·Na⁺) isolated in the gas phase. Despite the small conformational difference between 1·Li⁺ and 1·Na⁺ owing to the rigid framework of 1, ultraviolet photodissociation (UVPD) spectroscopy under cryogenic (∼10 K) conditions yielded clearly distinguishable absorption edges: ∼34 000 and ∼34 500 cm^-1 for 1·Li⁺ and 1·Na⁺, respectively. The spectral assignment and the preorganization characteristics of the host molecule were compared with those of dibenzo-18-crown-6-ether (DB18C6) complexes, which have more flexible frameworks. Furthermore, we revealed the characteristic unimolecular dissociation of the 1·Li⁺ complex using UVPD and collision-induced dissociation (CID); the formation of fragment ions with dibenzofuran moieties was detected. This dissociation pattern was ascribed to the efficient release of dimethyl ether molecule(s) from the 1·Li⁺ complex, which is characteristic of the cyclic skeleton formed with six methoxy groups in the SPR.

S1-State Decay Dynamics of Benzenediols (Catechol, Resorcinol, and Hydroquinone) and Their 1:1 Water Clusters

The S₁-state decaying rates of the three different benzenediols, catechol, resorcinol, and hydroquinone, and their 1:1 water clusters have been state-specifically measured using the picosecond time-resolved parent ion transients obtained by the pump (excitation) and probe (ionization) scheme. The S₁ lifetime of catechol is found to be short, giving τ ∼ 5.9 ps at the zero-point level. This is ascribed to the H-atom detachment from the free OH moiety of the molecule. Consistent with a previous report (J. Phys. Chem. Lett. 2013, 4, 3819-3823), the S₁ lifetime gets lengthened with low-frequency vibrational mode excitations, giving τ ∼ 9.0 ps for the 116 cm^-1 band. The S₁ lifetimes at the additional vibronic modes of catechol are newly measured, showing the nonnegligible mode-dependent fluctuations of the tunneling rate. When catechol is complexed with water, the S₁ lifetime is enormously increased to τ ∼ 1.80 ns at the zero-point level while it shows an unusual dip at the intermolecular stretching mode excitation (τ ∼ 1.03 ns at 146 cm^-1). Otherwise, it is shortened monotonically with increasing the internal energy, giving τ ∼ 0.67 ns for the 856 cm^-1 band. Two different asymmetric or symmetric conformers of resorcinol give the respective S₁ lifetimes of 4.5 or 6.3 ns at their zero-point levels according to the estimation from our transients taken within the temporal window of 0-2.7 ns. When resorcinol is 1:1 complexed with H₂O, the S₁ decaying rate is slightly accelerated for both conformers. The S₁ lifetimes of trans and cis forms of hydroquinone are measured to be more or less same, giving τ ∼ 2.8 ns at the zero-point level. When H₂O is complexed with hydroquinone, the S₁ decaying process is facilitated for both conformers, slightly more efficiently for the cis conformer.

Combined spectroscopic studies on post-functionalized Au25 cluster as an ATR-FTIR sensor for cations

Recently, significant research activity has been devoted to thiolate-protected gold clusters due to their attractive optical and electronic properties. These properties as well as solubility and stability can be controlled by post-synthetic modification strategies. Herein, the ligand exchange reaction between Au25(2-PET)18 cluster (where 2-PET is 2-phenylethanethiol) and di-thiolated crown ether (t-CE) ligands bearing two chromophores was studied. The post-functionalization aimed to endow the cluster with ion binding properties. The exchange reaction was followed in situ by UV-vis, 1H NMR and HPLC. MALDI mass analysis revealed the incorporation of up to 5 t-CE ligands into the ligand shell. Once functionalized MALDI furthermore showed complexation of sodium ions to the cluster. ATR-FTIR spectroscopic studies using aqueous solutions of K+, Ba2+, Gd3+ and Eu3+ showed noticeable spectral shifts of the C-O stretching band around 1100 cm-1 upon complexation. Further spectral changes point towards a conformational change of the two chromophores that are attached to the crown ether. Density functional theory calculations indicate that the di-thiol ligand bridges two staple units on the cluster. The calculations furthermore reproduce the spectral shift of the C-O stretching vibrations upon complex formation and reveal a conformational change that involves the two chromophores attached to the crown ether. The functionalized clusters have therefore attractive ion sensing properties due to the combination of binding properties, mainly due to the crown ether, and the possibility for signal transduction via an induced conformational change involving chromophore units.

Electronic States and Nonradiative Decay of Cold Gas-Phase Cinnamic Acid Derivatives Studied by Laser Spectroscopy with a Laser-Ablation Technique

We performed UV spectroscopy for p-coumaric acid (pCA), ferulic acid (FA), and caffeic acid (CafA) under jet-cooled gas-phase conditions by using a laser-ablation source. These molecules showed the S₁(¹ππ*)-S₀ absorption in the 31 500-33 500 cm^-1 region. Both pCA and FA exhibited sharp vibronic bands, while CafA showed only a broad feature. The decay time profile of the ¹ππ* state was measured by picosecond pump-probe spectroscopy, and the transient state produced through the nonradiative decay (NRD) from ¹ππ* and its time profile were measured by nanosecond UV-deep UV pump-probe spectroscopy. The transient state was observed for pCA and FA and assigned to the T₁ state, and we concluded that the NRD process of ¹ππ* is S₁(¹ππ*) → ¹nπ* → T₁(³ππ*), similar to those of methyl cinnamate and para-substituted cinnamates such as p-hydroxy and p-methoxy methyl cinnamate. On the other hand, the transient T₁ state was not detected in CafA, and its NRD route is suggested to be S₁(¹ππ*) → ¹πσ* → H atom elimination, similar to those of phenol and catechol. The effect of a hydrogen bond on the electronic state and NRD process was investigated, and it was found that the H-bonding lowers the ¹ππ* energy and suppresses the NRD process for all the species.

Crown ether-appended calix[2]triazolium[2]arene as a macrocyclic receptor for the recognition of the H2PO4−anion

We report the synthesis of a crown ether-appended calix[2]triazolium[2]arene receptor that shows excellent selectivity for H₂PO₄⁻and the binding ability and mode of the receptor to H₂PO₄⁻were demonstrated by¹H NMR studies and DFT calculation.