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

The conformational preference and modification of photophysics of benzenediols, namely hydroquinone (HQ), resorcinol (RE) and catechol (CA), upon host-guest complex formation with 18-Crown-6 (18C6) have been investigated, under supersonically jet-cooled conditions. Laser induced fluorescence (LIF) and UV-UV hole-burning spectra indicate the presence of two conformers for HQ and RE and one conformer for CA. On the other hand, the number of isomers is reduced to one in the 18C6·HQ and 18C6·RE complexes, while the 18C6·CA complex has three stable isomers. The IR spectra of the OH stretching vibration reveal that the two OH groups are H-bonded in 18C6·CA and 18C6·RE. In 18C6·RE, RE adopts the highest energy conformation in the bare form. In 18C6·HQ, the H-bonding of one OH group affects the orientation of the other OH group. The complex formation changes the photophysics of the S1 state of the benzenediols in a different manner. In our previous work, we reported a remarkable S1 lifetime elongation in 18C6·CA complexes; the S1 lifetime of CA is elongated more than 1000 times longer (8 ps → 10.3 ns) in 18C6·CA (F. Morishima et al., J. Phys. Chem. B, 2015, 119, 2557-2565), which we called the "cage effect". In 18C6·RE, the increase of S1 lifetime is moderate: 4.0 ns (monomer) → 10.5 ns (complex). On the other hand, the S1 lifetime of HQ is shortened in 18C6·HQ: 2.6 ns (monomer) → 0.54 ns (complex). Density functional theory (DFT) calculations suggest that these behaviors are related to the S1 ((1)ππ*)-(1)πσ* energy gap, the character of the S2 state and the symmetry of benzenediol. These experimental results clearly show the potential ability of 18C6 to control the conformation and modification of the electronic structure of guest species.

Ultraviolet Photodissociation Spectroscopy of the Cold K⁺·Calix[4]arene Complex in the Gas Phase.

The cooling of ionic species in the gas phase greatly simplifies the UV spectrum, which is of special importance when studying the electronic and geometric structures of large systems, such as biorelated molecules and host-guest complexes. Many efforts have been devoted to achieving ion cooling with a cold, quadrupole Paul ion trap (QIT), but one problem was the insufficient cooling of ions (up to ∼30 K) in the QIT. In this study, we construct a mass spectrometer for the ultraviolet photodissociation (UVPD) spectroscopy of gas-phase cold ions. The instrument consists of an electrospray ion source, a QIT cooled with a He cryostat, and a time-of-flight mass spectrometer. With great care given to the cooling condition, we can achieve ∼10 K for the vibrational temperature of ions in the QIT, which is estimated from UVPD spectra of the benzo-18-crown-6 (B18C6) complex with a potassium ion, K(+)·B18C6. Using this setup, we measure a UVPD spectrum of cold calix[4]arene (C4A) complex with potassium ion, K(+)·C4A. The spectrum shows a very weak band and a strong one at 36018 and 36156 cm(-1), respectively, accompanied by many sharp vibronic bands in the 36000-36600 cm(-1) region. In the geometry optimization of the K(+)·C4A complex, we obtain three stable isomers: one endo and two exo forms. On the basis of the total energy and UV spectral patterns predicted by density functional theory calculations, we attribute the structure of the K(+)·C4A complex to the endo isomer (C2 symmetry), in which the K(+) ion is located inside the cup of C4A. The vibronic bands of K(+)·C4A at 36 018 and 36 156 cm(-1) are assigned to the S1(A)-S0(A) and S2(B)-S0(A) transitions of the endo isomer, respectively.

UV photodissociation spectroscopy of cryogenically cooled gas phase host-guest complex ions of crown ethers.

The geometric and electronic structures of cold host-guest complex ions of crown ethers (CEs) in the gas phase have been investigated by ultraviolet (UV) fragmentation spectroscopy. As host CEs, we chose 15-crown-5 (15C5), 18-crown-6 (18C6), 24-crown-8 (24C8), and dibenzo-24-crown-8 (DB24C8), and as guests protonated-aniline (aniline·H(+)) and protonated-dibenzylamine (dBAM·H(+)) were chosen. The ions generated by an electrospray ionization (ESI) source were cooled in a quadrupole ion-trap (QIT) using a cryogenic cooler, and UV spectra were obtained by UV photodissociation (UVPD) spectroscopy. UV spectroscopy was complemented by quantum chemical calculations of the most probable complex structures. The UV spectrum of aniline·H(+)·CEs is very sensitive to the symmetry of CEs; aniline·H(+)·18C6 shows a sharp electronic spectrum similar to aniline·H(+), while aniline·H(+)·15C5 shows a very broad structure with poor Franck-Condon factors. In addition, a remarkable cage effect in the fragmentation process after UV excitation was observed in both complex ions. In aniline·H(+)·CE complexes, the cage effect completely removed the dissociation channels of the aniline·H(+) moiety. A large difference in the fragmentation yield between dBAM·H(+)·18C6 and dBAM·H(+)·24C8 was observed due to a large barrier for releasing dBAM·H(+) from the axis of rotaxane in the latter complex.

Anomalous cage effect of the excited state dynamics of catechol in the 18-crown-6-catechol host-guest complex.

We determined the number of isomers and their structures for the 18-crown-6 (18C6)-catechol host-guest complex, and examined the effect of the complex formation on the S1 ((1)ππ*) dynamics of catechol under a supersonically cooled gas phase condition and in cyclohexane solution at room temperature. In the gas phase experiment, UV-UV hole-burning spectra of the 18C6-catechol 1:1 complex indicate that there are three stable isomers. For bare catechol, it has been reported that two adjacent OH groups have an intramolecular hydrogen (H) bond. The IR-UV double resonance spectra show two types of isomers in the 18C6-catechol 1:1 complex; one of the three 18C6-catechol 1:1 isomers has the intramolecular H-bond between the two OH groups, while in the other two isomers the intramolecular H-bond is broken and the two OH groups are H-bonded to oxygen atoms of 18C6. The complex formation with 18C6 substantially elongates the S1 lifetime from 7 ps for bare catechol and 2.0 ns for the catechol-H2O complex to 10.3 ns for the 18C6-catechol 1:1 complex. Density functional theory calculations of the 18C6-catechol 1:1 complex suggest that this elongation is attributed to a larger energy gap between the S1 ((1)ππ*) and (1)πσ* states than that of bare catechol or the catechol-H2O complex. In cyclohexane solution, the enhancement of the fluorescence intensity of catechol was found by adding 18C6, due to the formation of the 18C6-catechol complex in solution, and the complex has a longer S1 lifetime than that of catechol monomer. From the concentration dependence of the fluorescence intensity, we estimated the equilibrium constant K for the 18C6 + catechol ⇄ 18C6-catechol reaction. The obtained value (log K = 2.3) in cyclohexane is comparable to those for alkali metal ions or other molecular ions, indicating that 18C6 efficiently captures catechol in solution. Therefore, 18C6 can be used as a sensitive sensor of catechol derivatives in solution with its high ability of fluorescence enhancement.

Structure and hydrogen-bonding ability of estrogens studied in the gas phase.

The structures of estrogens (estrone(E1), ß-estradiol(E2), and estriol(E3)) and their 1:1 hydrogen-bonded (hydrated) clusters with water formed in supersonic jets have been investigated by various laser spectroscopic methods and quantum chemical calculations. In the S1-S0 electronic spectra, all three species exhibit the band origin in the 35,050-35,200 cm(-1) region. By use of ultraviolet-ultraviolet hole-burning (UV-UV HB) spectroscopy, two conformers, four conformers, and eight conformers, arising from different orientation of OH group(s) in the A-ring and D-ring, are identified for estrone, ß-estradiol, and estriol, respectively. The infrared-ultraviolet double-resonance (IR-UV DR) spectra in the OH stretching vibration are observed to discriminate different conformers of the D-ring OH for ß-estradiol and estriol, and it is suggested that in estriol only the intramolecular hydrogen bonded conformer exists in the jet. For the 1:1 hydrated cluster of estrogens, the S1-S0 electronic transition energies are quite different depending on whether the water molecule is bound to A-ring OH or D-ring OH. It is found that the water molecule prefers to form an H-bond to the A-ring OH for estrone and ß-estradiol due to the higher acidity of phenolic OH than that of the alcoholic OH. On the other hand, in estriol the water molecule prefers to be bound to the D-ring OH due to the formation of a stable ring-structure H-bonding network with two OH groups. Thus, the substitution of one hydroxyl group to the D-ring drastically changes the hydrogen-bonding preference of estrogens.

Laser spectroscopic study of ß-estradiol and its monohydrated clusters in a supersonic jet.

The structures of 17ß-estradiol (estradiol) and its 1:1 cluster with water have been investigated in supersonic jets. The S(1)-S(0) electronic spectrum of estradiol monomer shows four strong sharp bands in the 35050-35200 cm(-1) region. Ultraviolet-ultraviolet hole-burning (UV-UV HB) and infrared-ultraviolet double-resonance (IR-UV DR) spectra of these bands indicate that they are due to four different conformers of estradiol originating from the different orientation of the OH groups in the A- and D-rings. The addition of water vapor to the sample gas generates four new bands in the 34700-34800 cm(-1) region, which are assigned to the estradiol-H(2)O 1:1 cluster with the A-ring (phenyl ring) OH acting as a hydrogen(H)-bond donor. In addition, we found very weak bands near the origin bands of bare estradiol upon the addition of water vapor. These bands are assigned to the isomers of estradiol-H(2)O 1:1 cluster having an H-bond at the D-ring OH. We determine the conformation of bare estradiol and the structures of its monohydrated clusters with the aid of density functional theory calculation and discuss the relationship between the stability of hydrated clusters and the conformation of estradiol.