D. F. Kelley

Photoluminescence from nanosize gold clusters

We have observed visible light emission from nanosize gold clusters. Liquid chromatographic analysis of the metal clusters shows that relatively intense photoluminescence occurs only when the size of the metal nanocluster is sufficiently small (<5 nm). The emission is strongly Stokes shifted and is assigned to radiative recombination of Fermi level electrons and sp- or d-band holes. The electron and/or hole states are perturbed by surface states, as indicated by the dependence of the emission spectrum on the nature of the cluster surface. Finally, we found that large, nonemitting gold clusters can also be made luminescent by partial dissolution using KCN.

Proton transfer dynamics in substituted 3‐hydroxyflavones: Solvent polarization effects

The spectroscopy and excited state proton transfer (ESPT) dynamics of 4’‐N,N‐dimethylamino‐3HF (I) and 4’‐N,N‐diethylamino‐3HF (II) have been studied in acetonitrile/benzene solvent mixtures. Solvent composition‐dependent spectral shifts are observed and can be understood in terms of an Onsager cavity model. Analysis of these spectral shifts accurately predicts solvent composition‐dependent excited state equilibrium constants, which are also experimentally determined. The ESPT rates are analyzed within the framework of a transition state theory treatment of solvent polarization‐mediated proton transfer. This treatment is analogous to electron transfer theory. In this treatment, the energetics of the transition state are largely determined by known solvent properties and the solvent‐dependent spectroscopy. This analysis yields solvent‐dependent ESPT activation energies. The corresponding calculated ESPT rates are in excellent agreement with the experimentally determined rates.

Vibrational dynamics of aniline(Ar)1 and aniline(CH4)1 clusters

The first excited electronic state (S1) vibrational dynamics of aniline(Ar)1 and aniline(CH4)1 van der Waals (vdW) clusters have been studied using molecular jet and time resolved emission spectroscopic techniques. The rates of intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) as functions of vibrational energy are reported for both clusters. For vibrational energy in excess of the cluster binding energy, both clusters are observed to dissociate. The dispersed emission spectra of these clusters demonstrate that aniline(Ar)1 dissociates to all energetically accessible bare molecule states and that aniline(CH4)1 dissociates selectively to only the bare molecule vibrationless state. The emission kinetics show that in the aniline(Ar)1 case, the initially excited states have nanosecond lifetimes, and intermediate cluster states have very short lifetimes. In contrast, the initially excited aniline(CH4)1 states and other intermediate vibrationally excited cluster states ...

Excited‐state proton transfer in 1‐naphthol/ammonia clusters

Excited‐state proton transfer dynamics are reported for the 1‐naphthol(NH3)n cluster system for n=3 and 4. Picosecond time‐ and mass‐resolved pump (S1←S0)–probe (I←S1) experiments demonstrate the following results: (1) excited‐state proton transfer occurs for n=3 and 4 clusters only; (2) for n=5 clusters the proton is transferred in the ground state and for n=2 clusters no proton transfer can be observed; (3) the proton transfer time in the n=3 cluster at the 000 transition is ca. 60 ps; (4) this time is reduced to ca. 40 ps and ca. 10 ps for 800 and 1400 cm−1 of vibrational energy in S1, respectively; (5) for the n=4 clusters these times are approximately 70, 70, and 30 ps, for 0, 800, and 1400 cm−1 of vibrational energy in S1, respectively; (6) both n=3 and 4 clusters exhibit a second low‐amplitude decay component, which is about an order of magnitude slower than the initial decay; and (7) 1‐naphthol‐d1(ND3)n clusters have a greatly reduced rate constant for the excited‐state proton transfer dynamics. T...

Electron transfer dynamics in MoS2 nanoclusters: Normal and inverted behavior

The photophysics and electron transfer (ET) dynamics of quantum confined MoS2 nanoclusters have been studied using static and time resolved emission spectroscopy. The MoS2 nanoclusters consist of a single S–Mo–S trilayer, having diameters of ∼2.5 or 4.5 nm. Two types of electron acceptors are adsorbed on these nanoclusters: 2,2′‐bipyridine (bpy) and 4,4′,5,5′‐tetramethyl‐2,2′‐bipyridine (TMB). The ET reaction exothermicities may be varied by changing the electron acceptor or by varying the size of the MoS2 nanocluster. TMB is harder to reduce, and thus has a smaller ET driving force than bpy. The smaller nanoclusters have a higher energy conduction band, and thus have a larger ET driving force. In all cases, the ET driving force may be calculated from bulk MoS2 properties and quantum confinement theory. Both ‘‘normal’’ and ‘‘inverted’’ behaviors are observed. A reorganization energy of 0.40 eV is calculated from energy dependent ET rates.

Proton transfer dynamics and cluster ion fragmentation in phenol/ammonia clusters

Excited‐state proton transfer dynamics are reported for the phenol(NH3)n cluster system. Excited‐state proton transfer is shown to occur for this system by isotopic substitution of the hydroxyl proton and ammonia protons. The observed dynamics slow from 80 to 600 ps upon cluster deuteration. The effects of cluster vibrational energy and ammonia concentration in the expansion gas are also studied. The results of these experiments are compared with a previous study [Syage and Steadman, J. Chem. Phys. 95, 2497 (1991)] which used a single excitation energy and varied ionization energy. Our experiments indicate that a significant amount of cluster ion fragmentation occurs for this system and caution must be exercised in assigning the decays observed in a given ion mass channel to a particular parent cluster. Due to the difficulties of associating a given decay curve with a cluster of particular mass, a previously proposed model of proton transfer in isolated clusters [Hineman et al., J. Chem. Phys. 97, 3341 (1...

Interligand electron transfer and transition state dynamics in Ru(II)trisbipyridine

The interligand electron transfer (ILET) dynamics in the excited metal‐to‐ligand charge transfer state of RuIItrisbipyridine have been studied using picosecond time‐resolved absorption polarization spectroscopy. The ILET dynamics have been studied in several solvents with widely varying relaxation times, and over the temperature range of −85 °C to room temperature. The solvents studied include acetonitrile, propanol, ethylene glycol, and glycerol. The results show that ILET is relatively fast in acetonitrile (∼47 ps), slower and nonexponential in propanol and ethylene glycol (∼30–200 ps), and fast in glycerol (∼30 ps). The glycerol results are roughly temperature independent. These results can be understood in terms of a model in which photoexcitation places the system near the ILET transition state and solvent relaxation competes with ILET in the nascent distribution. Following solvent relaxation a slower, steady state (probably transition state theory) rate is obtained.The interligand electron transfer (ILET) dynamics in the excited metal‐to‐ligand charge transfer state of RuIItrisbipyridine have been studied using picosecond time‐resolved absorption polarization spectroscopy. The ILET dynamics have been studied in several solvents with widely varying relaxation times, and over the temperature range of −85 °C to room temperature. The solvents studied include acetonitrile, propanol, ethylene glycol, and glycerol. The results show that ILET is relatively fast in acetonitrile (∼47 ps), slower and nonexponential in propanol and ethylene glycol (∼30–200 ps), and fast in glycerol (∼30 ps). The glycerol results are roughly temperature independent. These results can be understood in terms of a model in which photoexcitation places the system near the ILET transition state and solvent relaxation competes with ILET in the nascent distribution. Following solvent relaxation a slower, steady state (probably transition state theory) rate is obtained.

Vibrational dynamics of aniline (N2)1 clusters in their first excited singlet state

The first excited singlet state S1 vibrational dynamics of aniline(N2)1 clusters are studied and compared to previous results on aniline(CH4)1 and aniline(Ar)1. Intramolecular vibrational energy redistribution (IVR) and vibrational predissociation (VP) rates fall between the two extremes of the CH4 (fast IVR, slow VP) and Ar (slow IVR, fast VP) cluster results as is predicted by a serial IVR/VP model using Fermi’s golden rule to describe IVR processes and a restricted Rice–Ramsperger–Kassel–Marcus (RRKM) theory to describe unimolecular VP rates. The density of states is the most important factor determining the rates. Two product states, 00 and 10b1, of bare aniline and one intermediate state ∼(00) in the overall IVR/VP process are observed and time resolved measurements are obtained for the 000 and ∼(000) transitions. The results are modeled with the serial mechanism described above.

Excited state intermolecular proton transfer in matrix isolated β-naphthol/ammonia complexes

The spectroscopy and proton transfer dynamics of matrix isolated β‐naphthol⋅(NH3)n, n=1,2...complexes have been studied. The complexes are formed by annealing of β‐naphthol/NH3/argon matrices. The annealing studies indicate that the n=3, and probably n=4, complexes undergo excited state intermolecular proton transfer (ESPT), and this assignment is confirmed by comparison of experimental and simulated spectra. Time resolved emission studies indicate that the ESPT time is about 20 ps. These results are discussed in terms of simple tunneling theories.

Excited state vibrational dynamics of 4‐ethylaniline (X)1 clusters (X=Ar, N2, and CH4)

Intracluster vibrational redistribution (IVR) and vibrational predissociation (VP) dynamics of 4‐ethylaniline (Ar)1, (N2)1, and (CH4)1 clusters have been studied by time‐correlated single photon counting, mass‐resolved excitation spectroscopy, and dispersed emission spectroscopy. The 4‐ethylaniline molecule has a low frequency ethyl group torsion vibrational mode, which is similar in energy to the van der Waals modes of the clusters. This mode, because of its low energy (∼35 cm−1), plays a role in the vibrational dynamics of the clusters studied. The cluster dissociation rates and product state distributions can be modeled by a serial IVR/VP mechanism for which the VP step is treated by the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The resulting agreement between the calculated and experimental rates and product state intensities indicates that a statistical distribution of energy among all low frequency modes exists for 4‐ethylaniline/polyatomic solvent clusters in which kIVR≫kVP. For 4‐ethylaniline (...