Quantum chain reaction of tethered diarylcyclopropenones in the solid state and their distance-dependence in solution reveal a Dexter S2-S2 energy-transfer mechanism.

When promoted to their second singlet excited state (S2) in benzene, alkyl-linked dimers of diarylcyclopropenone undergo a photodecarbonylation reaction with quantum yields varying from Φ = 0.7 to 1.14. Quantum yields greater than 1.0 in solution rely on an adiabatic reaction along the S2 energy surface where the immediately formed excited-state product transfers energy to the unreacted molecule in the dimer to generate a second excited state. By determination of the quantum yields of decarbonylation for the linked diarylcyclopropenones with linkers of various lengths it was shown that S2 → S2 energy transfer is limited to distances shorter than ca. 6 Å. Notably, the quantum chain reaction occurs with similar efficiency for all the linked diarylcyclopropenones dimers in the solid state.

Cruciforms' polarized emission confirms disjoint molecular orbitals and excited states.

Steady-state and time-resolved polarized spectroscopy studies reveal that electronic excitation to the third excited state of 1,4-distyryl-2,5-bis(arylethynyl)benzene cruciforms results in fluorescence emission that is shifted an angle of ca. 60°. This result is consistent with quantum chemical calculations of the lowest electronic excited states and their transition dipole moments. The shift originates from the disjointed nature of the occupied molecular orbitals being localized on the different branches of the cruciforms.

Reaction mechanism in crystalline solids: kinetics and conformational dynamics of the Norrish type II biradicals from α-adamantyl-p-methoxyacetophenone.

In an effort to determine the details of the solid-state reaction mechanism and diastereoselectivity in the Norrish type II and Yang cyclization of crystalline α-adamantyl-p-methoxyacetophenone, we determined its solid-state quantum yields and transient kinetics using nanocrystalline suspensions. The transient spectroscopy measurements were complemented with solid-state NMR spectroscopy spin-lattice relaxation experiments using isotopically labeled samples and with the analysis of variable-temperature anisotropic displacement parameters from single-crystal X-ray diffraction to determine the rate of interconversion of biradical conformers by rotation of the globular adamantyl group. Our experimental findings include a solid-state quantum yield for reaction that is 3 times greater than that in solution, a Norrish type II hydrogen-transfer reaction that is about 8 times faster in crystals than in solution, and a biradical decay that occurs on the same time scale as conformational exchange, which helps to explain the diastereoselectivity observed in the solid state.

Excited state kinetics in crystalline solids: self-quenching in nanocrystals of 4,4'-disubstituted benzophenone triplets occurs by a reductive quenching mechanism.

We report an efficient triplet state self-quenching mechanism in crystals of eight benzophenones, which included the parent structure (1), six 4,4'-disubstituted compounds with NH(2) (2), NMe(2) (3), OH (4), OMe (5), COOH (6), and COOMe (7), and benzophenone-3,3',4,4'-tetracarboxylic dianhydride (8). Self-quenching effects were determined by measuring their triplet-triplet lifetimes and spectra using femtosecond and nanosecond transient absorption measurements with nanocrystalline suspensions. When possible, triplet lifetimes were confirmed by measuring the phosphorescence lifetimes and with the help of diffusion-limited quenching with iodide ions. We were surprised to discover that the triplet lifetimes of substituted benzophenones in crystals vary over 9 orders of magnitude from ca. 62 ps to 1 ms. In contrast to nanocrystalline suspensions, the lifetimes in solution only vary over 3 orders of magnitude (1-1000 µs). Analysis of the rate constants of quenching show that the more electron-rich benzophenones are the most efficiently deactivated such that there is an excellent correlation, ρ = -2.85, between the triplet quenching rate constants and the Hammet σ(+) values for the 4,4' substituents. Several crystal structures indicate the existence of near-neighbor arrangements that deviate from the proposed ideal for "n-type" quenching, suggesting that charge transfer quenching is mediated by a relatively loose arrangement.

Stable radicals during photodecarbonylations of trityl-alkyl ketones enable solid state reactions through primary and secondary radical centers.

The solid state photoexcitation of several triphenylmethyl-alkyl ketones resulted in the loss of CO and the exclusive formation of radical-radical combination products. Differences in reactivity suggest a stepwise mechanism with the unprecedented formation of primary and secondary radicals in some of the radical pair intermediates in the solid state.

Oxyallyl exposed: an open-shell singlet with picosecond lifetimes in solution but persistent in crystals of a cyclobutanedione precursor.

Photoinduced decarbonylation of 2,4-bis(spirocyclohexyl)-1,3-cyclobutanedione 1 in the crystalline solid state resulted in formation of a deep blue transient with λ(max) = 550 nm and a half-life of 42 min at 298 K, identified as kinetically stabilized oxyallyl. Support for an open-shell singlet species was obtained by spectroscopic analysis and (4/4) CASSCF calculations with the 6-31+G(d) basis set and multireference MP2 corrections. The electronic spectrum of the singlet biradical, confirmed by femtosecond pump-probe studies in solution, was matched by coupled cluster calculations with single and double corrections.

Efficient utilization of higher-lying excited states to trigger charge-transfer events.

Several new fullerene-heptamethine conjugates, which absorb as far as into the infrared spectrum as 800 nm, have been synthesized and fully characterized by physicochemical means. In terms of optical and electrochemical characteristics, appreciable electronic coupling between both electroactive species is deduced. The latter also reflect the excited-state features. To this end, time-resolved, transient absorption measurements revealed that photoexcitation is followed by a sequence of charge-transfer events which evolve from higher singlet excited states (i.e., S(2)--fast charge transfer) and the lowest singlet excited state of the heptamethine cyanine (i.e., S(1)--slow charge transfer), as the electron donor, to either a covalently linked C(60) or C(70), as the electron acceptor. Finally, charge transfer from photoexcited C(60)/C(70) completes the charge-transfer sequence. The slow internal conversion within the light-harvesting heptamethine cyanine and the strong electronic coupling between the individual constituents are particularly beneficial to this process.

Dendritic porphyrin-fullerene conjugates: efficient light-harvesting and charge-transfer events.

A novel dendritic C(60)-H(2)P-(ZnP)(3) (P=porphyrin) conjugate gives rise to the successful mimicry of the primary events in photosynthesis, that is, light harvesting, unidirectional energy transfer, charge transfer, and charge-shift reactions. Owing, however, to the flexibility of the linkers that connect the C(60), H(2)P, and ZnP units, the outcome depends strongly on the rigidity/viscosity of the environment. In an agar matrix or Triton X-100, time-resolved transient absorption spectroscopic analysis and fluorescence-lifetime measurements confirm the following sequence. Initially, light harvesting is seen by the peripheral C(60)-H(2)P- *(ZnP)(3) conjugate. Once photoexcited, a unidirectional energy transfer funnels the singlet excited-state energy to H(2)P to form C(60)-*(H(2)P)-(ZnP)(3), which powers an intramolecular charge transfer that oxidizes the photoexcited H(2)P and reduces the adjacent C(60) species. In the correspondingly formed (C(60))(*-)-(H(2)P)(*+)-(ZnP)(3) conjugate, an intramolecular charge-shift reaction generates (C(60))(*-)-H(2)P-(ZnP)(3) (.+), in which the radical cation resides on one of the three ZnP moieties, and for which lifetimes of up to 460 ns are found. On the other hand, investigations in organic media (i.e., toluene, THF, and benzonitrile) reveal a short cut, that is, the peripheral ZnP unit reacts directly with C(60) to form (C(60))(*-)-H(2)P-(ZnP)(3) (*+). Substantial configurational rearrangements- placing ZnP and C(60) in proximity to each other-are, however, necessary to ensure the required through space interactions (i.e., close approach). Consequently, the lifetime of (C(60))(*-)-H(2)P-(ZnP)(3) (*+) is as short as 100 ps in benzonitrile.

Photonic amplification by a singlet-state quantum chain reaction in the photodecarbonylation of crystalline diarylcyclopropenones.

The photochemical decarbonylation of diphenylcyclopropenone (DPCP) to diphenylacetylene (DPA) proceeds with remarkable efficiency both in solution and in the crystalline solid state. It had been previously shown that excitation to the second electronic excited state (S(2)) of DPCP in solution proceeds within ca. 200 fs by an adiabatic ring-opening pathway to yield the S(2) state of DPA, which has a lifetime of ca. 8 ps before undergoing internal conversion to S(1) (Takeuchi, S.; Tahara, T. J. Chem. Phys. 2004, 120, 4768). More recently, we showed that reactions by excitation to S(2) in crystalline solids proceed by a quantum chain process where the excited photoproducts transfer energy to neighboring molecules of unreacted starting material, which are able to propagate the chain. Quantum yields in crystalline suspensions revealed values of Phi(DPCP) = 3.3 +/- 0.3. To explore the generality of this reaction, and recognizing its potential as a photonic amplification system, we have synthesized nine crystalline diarylcyclopropenone derivatives with phenyl, biphenyl, naphthyl, and anthryl substituents. To quantify the efficiency of the quantum chain in the crystalline state, we determined the quantum yields of reaction for all of these compounds both in solution and in nanocrystalline suspensions. While the quantum yields of decarbonylation in solution vary from Phi = 0.0 to 1.0, seven of the nine new structures display quantum yields of reaction in the solid that are above 1. The chemical amplification that results from efficient energy transfer in the solid state, analyzed in terms of the quantum yields determined in the solid state and in solution (Phi(cryst)/Phi(soln)), reveals quantum chain amplification factors that range from 3.2 to 11.0. The remarkable mechanical response of the solid-to-solid reaction previously documented with macroscopic crystals, where large single-crystalline specimens turn into fine powders, was investigated at the nanometer scale. Experiments with dry crystals of DPCP analyzed by atomic force microscopy showed the formation of DPA in the form of isolated crystalline specimens ca. 35 nm in size.