Metallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer.

We report on the discovery and detailed exploration of the unconventional photo-switching mechanism in metallofullerenes, in which the energy of the photon absorbed by the carbon cage π-system is transformed to mechanical motion of the endohedral cluster accompanied by accumulation of spin density on the metal atoms. Comprehensive photophysical and electron paramagnetic resonance (EPR) studies augmented by theoretical modelling are performed to address the phenomenon of the light-induced photo-switching and triplet state spin dynamics in a series of Y x Sc3-x N@C80 (x = 0-3) nitride clusterfullerenes. Variable temperature and time-resolved photoluminescence studies revealed a strong dependence of their photophysical properties on the number of Sc atoms in the cluster. All molecules in the series exhibit temperature-dependent luminescence assigned to the near-infrared thermally-activated delayed fluorescence (TADF) and phosphorescence. The emission wavelengths and Stokes shift increase systematically with the number of Sc atoms in the endohedral cluster, whereas the triplet state lifetime and S1-T1 gap decrease in this row. For Sc3N@C80, we also applied photoelectron spectroscopy to obtain the triplet state energy as well as the electron affinity. Spin distribution and dynamics in the triplet states are then studied by light-induced pulsed EPR and ENDOR spectroscopies. The spin-lattice relaxation times and triplet state lifetimes are determined from the temporal evolution of the electron spin echo after the laser pulse. Well resolved ENDOR spectra of triplets with a rich structure caused by the hyperfine and quadrupolar interactions with 14N, 45Sc, and 89Y nuclear spins are obtained. The systematic increase of the metal contribution to the triplet spin density from Y3N to Sc3N found in the ENDOR study points to a substantial fullerene-to-metal charge transfer in the excited state. These experimental results are rationalized with the help of ground-state and time-dependent DFT calculations, which revealed a substantial variation of the endohedral cluster position in the photoexcited states driven by the predisposition of Sc atoms to maximize their spin population.

PAH/PAH(CF3 )n Donor/Acceptor Charge-Transfer Complexes in Solution and in Solid-State Co-Crystals.

A solution, solid-state, and computational study is reported of polycyclic aromatic hydrocarbon PAH/PAH(CF3 )n donor/acceptor (D/A) charge-transfer complexes that involve six PAH(CF3 )n acceptors with known gas-phase electron affinities that range from 2.11(2) to 2.805(15) eV and four PAH donors, including seven CT co-crystal X-ray structures that exhibit hexagonal arrays of mixed π-stacks with 1/1, 1/2, or 2/1 D/A stoichiometries (PAH=anthracene, azulene, coronene, perylene, pyrene, triphenylene; n=5, 6). These are the first D/A CT complexes with PAH(CF3 )n acceptors to be studied in detail. The nine D/A combinations were chosen to allow several structural and electronic comparisons to be made, providing new insights about controlling D/A interactions and the structures of CT co-crystals. The comparisons include, among others, CT complexes of the same PAH(CF3 )n acceptor with four PAH donors and CT complexes of the same donor with four PAH(CF3 )n acceptors. All nine CT complexes exhibit charge-transfer bands in solution with λmax between 467 and 600 nm. A plot of E(λmax ) versus [IE(donor)-EA(acceptor)] for the nine CT complexes studied is linear with a slope of 0.72±0.03 eV eV-1 . This plot is the first of its kind for CT complexes with structurally related donors and acceptors for which precise experimental gas-phase IEs and EAs are known. It demonstrates that conclusions based on the common assumption that the slope of a CT E(λmax ) versus [IE-EA] plot is unity may be incorrect in at least some cases and should be reconsidered.

Photoelectron spectroscopy of solvated dicarboxylate and alkali metal ion clusters, M+[O2C(CH2)2CO2]2- [H2O]n (M = Na, K; n = 1-6).

We present results of combined experimental photoelectron spectroscopy and theoretical modeling studies of solvated dicarboxylate species (-O2C(CH2)2CO2-) in complex with Na+ and K+ metal cations. These ternary clusters serve as simple models for the investigation of aqueous ion/solute specific effects that play an important role in biological systems. The experimental characterization of these systems was performed in the presence of up to six solvating waters. In both Na+ and K+ cases, we observe the presence of one major broad band that gradually shifts to higher electron binding energy (EBE) with an increasing number of waters. In the Na+ case further detailed analysis of experimental spectra was performed using ab initio calculations. In particular, we have identified the structures of the lowest energy clusters whose EBE values match well the major band in the experimental spectra. Our results show that evolution of an aqueous solvation shell emphasizes the coordination of the negatively charged carboxylate groups accompanied by simultaneous interaction with metal cations. Calculations also indicate that in the solvation range investigated experimentally (up to 6 waters), Na+ retains direct contact with the dicarboxylate species, i.e. a contact ion-pair (CIP) complex. Preliminary modeling studies show evidence of an alternative solvent separated ion-pair complex once the solvation range approaches 8 waters, however its energy still remains above that of (∼7-8 kcal/ mol-1) the CIP complex. At a higher number of waters (n = 3 for Na+ and n = 5 for K+), the experimental spectra also show the development of a weak low energy band. Its origin cannot be precisely identified. Our calculations in the Na+ case point out the existence of a quaternary complex consisting of Na+, H2O, OH- and a singly protonated dicarboxylate anion (HO2C(CH)2CO2-). Such a complex appears to be stabilized in the solvation range corresponding to the appearance of the low EBE band and does match its peak, even though the energy of such a complex is fairly high compared to the ternary structure.

Experimental and DFT Studies of the Electron-Withdrawing Ability of Perfluoroalkyl (RF ) Groups: Electron Affinities of PAH(RF )n Increase Significantly with Increasing RF Chain Length.

Two series of aromatic compounds with perfluoroalkyl (RF ) groups of increasing length, 1,3,5,7-naphthalene(RF )4 and 1,3,5,7,9-corannulene(RF )5 , have been prepared and their electronic properties studied by low-temperature photoelectron spectroscopy (PES) (for gas-phase electron affinity measurements). These and many related compounds were also studied by DFT calculations. The data demonstrate unambiguously that the electron-withdrawing ability of RF substituents increases significantly and uniformly from CF3 to C2 F5 to n-C3 F7 to n-C4 F9 .

How Anion Chaotrope Changes the Local Structure of Water: Insights from Photoelectron Spectroscopy and Theoretical Modeling of SCN(-) Water Clusters.

The behavior of charged solute molecules in aqueous solutions is often classified using the concept of kosmotropes ("structure makers") and chaotropes ("structure breakers"). There is a growing consensus that the key to kosmotropic/chaotropic behaviors lies in the local solvent region, but the exact microscopic basis for such differentiation is not well-understood. This issue is examined in this work by analyzing size selective solvation of a well-known chaotrope, a negatively charged SCN(-) molecule. Combining experimental photoelectron spectroscopy measurements with theoretical modeling, we examine evolution of solvation structure up to eight waters. We observe that SCN(-) indeed fits the description of weakly hydrated ion, and its solvation is heavily driven by stabilization of water-water interaction network. However, the impact on water structure is more subtle than that associated with "structure breaker". In particular, we observe that the solvation structure of SCN(-) preserves the "packing" structure of the water network but changes local directionality of hydrogen bonds in the local solvent region. The resulting effect is closer to that of "structure weakener", where solute can be readily accommodated into the native water network, at the cost of compromising its stability due to constraints on hydrogen bonding directionality.

Fullerene cyanation does not always increase electron affinity: an experimental and theoretical study.

The electron affinities of C70 derivatives with trifluoromethyl, methyl and cyano groups were studied experimentally and theoretically using low-temperature photoelectron spectroscopy (LT PES) and density functional theory (DFT). The electronic effects of these functional groups were determined and found to be highly dependent on the addition patterns. Substitution of CF3 for CN for the same addition pattern increases the experimental electron affinity by 70 meV per substitution. The synthesis of a new fullerene derivative, C70(CF3)10(CN)2, is reported for the first time.

The negative ion photoelectron spectrum of cyclopropane-1,2,3-trione radical anion, (CO)3(•-)--a joint experimental and computational study.

Negative ion photoelectron (NIPE) spectra of the radical anion of cyclopropane-1,2,3-trione, (CO)3(•-), have been obtained at 20 K, using both 355 and 266 nm lasers for electron photodetachment. The spectra show broadened bands, due to the short lifetimes of both the singlet and triplet states of neutral (CO)3 and, to a lesser extent, to the vibrational progressions that accompany the photodetachment process. The smaller intensity of the band with the lower electron binding energy suggests that the singlet is the ground state of (CO)3. From the NIPE spectra, the electron affinity (EA) and the singlet-triplet energy gap of (CO)3 are estimated to be, respectively, EA = 3.1 ± 0.1 eV and ΔEST = -14 ± 3 kcal/mol. High-level, (U)CCSD(T)/aug-cc-pVQZ//(U)CCSD(T)/aug-cc-pVTZ, calculations give EA = 3.04 eV for the (1)A1' ground state of (CO)3 and ΔEST = -13.8 kcal/mol for the energy gap between the (1)A1' and (3)A2 states, in excellent agreement with values from the NIPE spectra. In addition, simulations of the vibrational structures for formation of these states of (CO)3 from the (2)A2″ state of (CO)3(•-) provide a good fit to the shapes of broad bands in the 266 nm NIPE spectrum. The NIPE spectrum of (CO)3(•-) and the analysis of the spectrum by high-quality electronic structure calculations demonstrate that NIPES can not only access and provide information about transition structures but NIPES can also access and provide information about hilltops on potential energy surfaces.

Examining the amine functionalization in dicarboxylates: photoelectron spectroscopy and theoretical studies of aspartate and glutamate.

Aspartate (Asp(2-)) and glutamate (Glu(2-)), two doubly charged conjugate bases of the corresponding amino acids, were investigated using low-temperature negative ion photoelectron spectroscopy (NIPES) and ab initio calculations. The effect of amine functionalization was studied by a direct comparison to the parent dicarboxylate species ((-)CO2-(CH2)n-CO2(-), DCn(2-)), succinate (DC2(2-)) and propionate (DC3(2-)). Experimentally, the addition of the amine group for the n = 2 case (DC2(2-), Asp(2-)) significantly improves the stability of the resultant Asp(2-) dianionic species, albeit that NIPES shows only a small increase in adiabatic electron detachment energy (ADE) (+0.05 eV). In contrast, for n = 3 (DC3(2-), Glu(2-)), a much larger ADE increase is observed (+0.15 eV). Similar results are obtained through ab initio calculations. The latter indicates that increased stability of Asp(2-) can be attributed to the lowering of the energy of the singlet dianion state due to hydrogen bonding effects. The effect of the amino group on the doublet monoanion state is more complicated and results in the weakening of the binding of the adjacent carboxylate group due to electronic structure resonance effects. This conclusion is confirmed by the analysis of NIPES results that show enhanced production of near-zero kinetic energy electrons observed experimentally for amine-functionalized species.

Poly(trifluoromethyl)azulenes: structures and acceptor properties.

Six new poly(trifluoromethyl)azulenes prepared in a single high-temperature reaction exhibit strong electron accepting properties in the gas phase and in solution and demonstrate the propensity to form regular π-stacked columns in donor-acceptor crystals when mixed with pyrene as a donor.

C20H4(C4F8)3: a fluorine-containing annulated corannulene that is a better electron acceptor than C60.

At sixes and sevens: The reaction of corannulene with 35 equivalents of 1,4-C4F8I2 is an efficient and a relatively selective process that yields two main products in which six H atoms are substituted with three C4F8 moieties that form six- and seven-membered rings. Low-temperature photoelectron spectroscopy showed the electron affinity of the major isomer (shown) exceeds that of C60 (2.74±0.02 and 2.689±0.008 eV, respectively).