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Spectral tuning effects on visible chromophores by hydrogen bonds are central to the chemistry of vision and of photosynthesis. A model for large spectral tuning effects by hydrogen bond switching is provided by the 7-hydroxyquinoline·HCOOH complex, which forms two isomers, CTN1 and CTN2, both with an HCOOH...N hydrogen bond but with different (quinoline)C–H...O==C hydrogen bonds. A 180° rotation of the HCOOH moiety around the O–H...N hydrogen bond exchanges the C–H...O hydrogen bonds, rotates the dipole moment of HCOOH, and leads to an ∼850 cm-1 shift of the electronic spectrum. Mass-selected S1←S0 resonant two-photon ionization, UV-UV holeburning, S1→S0 fluorescence spectra, and photoionization efficiency curves of the two 7-hydroxyquinoline·HCOOH isomers were measured in supersonic expansions. Comparison to ab initio calculations allow us to determine the H-bond connectivity and structure of the two isomers and to assign their inter- and intramolecular vibrations. The Franck-Condon factors of the intermolecular shear vibration χ in the S1←S0 spectra indicate that the weak C–H...O hydrogen bond contracts markedly in the CTN1 isomer but expands in the CTN2 isomer. These changes of H-bond lengths agree with the spectral shifts. In contrast, the strong O–H...N hydrogen bond undergoes little change upon S1←S0 excitation. [ABSTRACT FROM AUTHOR]

Excitation of the 7-hydroxyquinoline·(NH3)3 [7HQ·(NH3)3] cluster to the S1 1ππ* state results in an O-H→NH3 hydrogen atom transfer (HAT) reaction. In order to investigate the entrance channel, the vibronic S1↔S0 spectra of the 7HQ·(NH3)3 and the d2-7DQ·(ND3)3 clusters have been studied by resonant two-photon ionization, UV-UV depletion and fluorescence techniques, and by ab initio calculations for the ground and excited states. For both isotopomers, the low-frequency part of the S1←S0 spectra is dominated by ammonia-wire deformation and stretching vibrations. Excitation of overtones or combinations of these modes above a threshold of 200–250 cm-1 for 7HQ·(NH3)3 accelerates the HAT reaction by an order of magnitude or more. The d2-7DQ·(ND3)3 cluster exhibits a more gradual threshold from 300 to 650 cm-1. For both isotopomers, intermolecular vibrational states above the threshold exhibit faster HAT rates than the intramolecular vibrations. The reactivity, isotope effects, and mode selectivity are interpreted in terms of H atom tunneling through a barrier along the O-H→NH3 coordinate. The barrier results from a conical intersection of the optically excited 1ππ* state with an optically dark 1πσ* state. Excitation of the ammonia-wire stretching modes decreases both the quinoline-O-H...NH3 distance and the energetic separation between the 1ππ* and 1πσ* states, thereby increasing the H atom tunneling rate. The intramolecular vibrations change the H bond distance and modulate the 1ππ*↔1πσ* interaction to a much smaller extent. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mass-selected S[sub 1]↔S[sub 0] two color resonant two photon ionization (2C-R2PI) spectra, fluorescence spectra and fluorescence decay times are measured for supersonically cooled 7-hydroxyquinoline (7HQ)·(NH[sub 3])[sub n] clusters with n=4–10. For n=4, the S[sub 1]←S[sub 0] 2C-R2PI spectrum shows a 20 cm[sup -1] broad electronic origin at 27 746 cm[sup -1], followed by an intermolecular vibrational progression with band widths that increase up to ≈45 cm[sup -1]. In contrast, the 2C-R2PI spectra of the mixed 7HQ·(NH[sub 3])[sub 3]H[sub 2]O and 7HQ·(NH[sub 3])[sub 2](H[sub 2]O)[sub 2] clusters exhibit narrow bands of 1–2 cm[sup -1] width. The large band widths of 7HQ·(NH[sub 3])[sub 4] are due to a fast (k>10[sup 12] s[sup -1]) excited state process which is blocked when replacing one or more NH[sub 3] molecules by H[sub 2]O in the cluster. For the n=5–10 clusters, the 2C-R2PI spectra display two broad absorption bands peaking at 25 000 and 27 000 cm[sup -1]. The latter is characteristic of the 7-quinolinate (7Q[sup -]) anion, implying that ground state proton transfer from 7HQ to the ammonia cluster occurs for n>=5. Excitation at 27 000 cm[sup -1] leads to fluorescence from the 7Q[sup -*] anion clusters at 22 500 cm[sup -1] with τ[sub fl]=14–27 ns, and also to fluorescence emission from 7KQ[sup *] clusters at 18 370 cm[sup -1] with τ[sub fl]≈7 ns, signaling excited state anion→keto proton transfer. The 25 000 cm[sup -1] absorption band is characteristic of ground state 7KQ, implying that the n>=5 clusters also undergo ground state enol→keto tautomerization. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mass- and isomer-selected S[SUB1]←S[SUB0] resonant two-photon ionization and S[SUB1]→S[SUB0] fluorescence spectra were measured for the 7-hydroxyquinoline·(NH[SUB3])[SUB2] [7HQ·(NH[SUB3])[SUB2]] and d[SUB2]-7-hydroxyquinoline ·(ND[SUB3])[SUB2] clusters cooled in supersonic expansions. UV/UV hole burning measurements prove that a single cluster isomer is formed. Ab initio self-consistent field and density functional calculations predict that the most stable cluster form has an ''ammonia wire'' hydrogen bonded to the -OH and N groups of the cis-7HQ rotamer. The experimental S[SUB0] and S[SUB1] frequencies are in very good agreement with the calculated normal mode frequencies for both the normal and deuterated ammonia-wire clusters. S[SUB1]←S[SUB0] excitation leads to contractions of the -O-H...N and NH[SUB3]...NH[SUB3] hydrogen bonds, as well as smaller displacements for the NH[SUB3]...N(quinoline) stretch and the in plane rotation (or bend) of the ammonia dimer relative to 7HQ. The coupling of these modes to the S[SUB1]←S[SUB0] electronic excitation indicates that hydrogen bond contractions in the excited state are important and may be prerequisite for the S[SUB1] state proton transfer processes that occur in the larger 7HQ· (NH[SUB3])[SUBn] (n≥4) clusters. The calculated electron density differences upon S[SUB1]←S[SUB0] excitation show large π-electron flows on the 7HQ moiety. However, the σ-electronic rearrangements that directly drive the hydrogen bond rearrangements are one to two orders of magnitude smaller. [ABSTRACT FROM AUTHOR]

The supersonically cooled 7-hydroxyquinoline·(H[sub 2]O)[sub 3] cluster was investigated by mass- and isomer-selected S[sub 1]←S[sub 0] resonant two-photon ionization and S[sub 1]→S[sub 0] fluorescence spectroscopy. UV(ultraviolet)/UV-holeburning measurements prove that a single cluster isomer is formed, although different tautomers (7-keto- and 7-hydroxyquinoline), rotamers (cis and trans) and isomers (cyclic and water-wire) are possible. Ab initio calculations of structures and vibrations of different tautomers and isomers of this cluster predict that the cis-enol-7-hydroxyquinoline·(H[sub 2]O)[sub 3] "water-wire" cluster is the most stable species. The experimental S[sub 0] and S[sub 1] inter- and intramolecular vibrational frequencies are in good agreement with the calculated harmonic frequencies. S[sub 1]←S[sub 0] excitation leads to contraction of all four hydrogen bonds along the hydrogen bonded water wire, inducing intense intermolecular stretching vibrations of the O-H...O and HO-H...N(quinoline) hydrogen bonds which terminate the water-wire. There are no spectroscopic signs of proton transfer. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mass- and isomer-selected S[sub 1]←S[sub 0] resonant two-photon ionization and S[sub 1]→S[sub 0] fluorescence spectra were obtained for the supersonically cooled 7-hydroxyquinoline·(H[sub 2]O)[sub 2] cluster. UV/UV-holeburning measurements show that >98% of the spectrum is due to a single "water-chain" cluster isomer, although two different tautomers (7-keto- and 7-hydroxyquinoline), two different rotamers (cis- and trans-hydroxy), and two torsional conformers of the chain are possible. Ab initio calculations of structures and vibrations of five different tautomers/ rotamers/ conformers of this cluster are reported. These predict that the cis-7-hydroxyquinoline·(H[sub 2]O)[sub 2] "up/down" water-chain form is the most stable cluster. The experimentally observed S[sub 0] and S[sub 1] state vibrational frequencies agree well with those calculated for this isomer. We find no evidence for either the trans-rotamer or the keto tautomer clusters. S[sub 1]←S[sub 0] excitation leads to contraction of all three hydrogen-bonds along the hydrogen-bonded water chain, inducing intermolecular stretching vibrations, but no proton transfer. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mass- and isomer-selected S[sub 1]←S[sub 0] resonant two-photon ionization and S[sub 1]→S[sub 0] fluorescence spectra were obtained for the supersonically cooled 7-hydroxyquinoline·(NH[sub 3])[sub n] clusters with n=2-16. For n=2 and 3, the absorption and emission spectra exhibit discrete and narrow bands, characteristic of nonreactive hydrogen bonded ammonia-chain clusters. For n>=4, the S[sub 1]←S[sub 0] R2PI spectra are completely broadened, with an onset at approx. 355 nm. For n=4-7, a weak fluorescence emission is observed, Stokes-shifted by approx. 185 nm, with a maximum at approx. 540 nm, which shows discrete structure on a broad background. From comparison to fluorescence emission observed in bulk solution, we conclude that S[sub 1] state enol→keto tautomerization occurs. For the n>=7 or 8 clusters, the fluorescence emission spectra become completely unstructured and shift to the blue, peaking at 435-450 nm. This emission indicates the occurrence of either S[sub 1]-state proton transfer to the ammonia solvent cluster and formation of the 7-HQ anion, or of the ground-state intracluster enol→keto tautomerization reaction. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Excitation of the 7-hydroxyquinoline·(NH3)3 [7HQ·(NH3)3] cluster to the S1 1ππ* state results in an O-H→NH3 hydrogen atom transfer (HAT) reaction. In order to investigate the entrance channel, the vibronic S1↔S0 spectra of the 7HQ·(NH3)3 and the d2-7DQ·(ND3)3 clusters have been studied by resonant two-photon ionization, UV-UV depletion and fluorescence techniques, and by ab initio calculations for the ground and excited states. For both isotopomers, the low-frequency part of the S1←S0 spectra is dominated by ammonia-wire deformation and stretching vibrations. Excitation of overtones or combinations of these modes above a threshold of 200–250 cm-1 for 7HQ·(NH3)3 accelerates the HAT reaction by an order of magnitude or more. The d2-7DQ·(ND3)3 cluster exhibits a more gradual threshold from 300 to 650 cm-1. For both isotopomers, intermolecular vibrational states above the threshold exhibit faster HAT rates than the intramolecular vibrations. The reactivity, isotope effects, and mode selectivity are interpreted in terms of H atom tunneling through a barrier along the O-H→NH3 coordinate. The barrier results from a conical intersection of the optically excited 1ππ* state with an optically dark 1πσ* state. Excitation of the ammonia-wire stretching modes decreases both the quinoline-O-H...NH3 distance and the energetic separation between the 1ππ* and 1πσ* states, thereby increasing the H atom tunneling rate. The intramolecular vibrations change the H bond distance and modulate the 1ππ*↔1πσ* interaction to a much smaller extent. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]