The generation of singlet oxygen (O) has been established as the principal mechanism of photodynamic therapy (PDT). Various dyes, metal nanoparticles, and clusters have been shown to sensitize O. However, metal nanoclusters are even more promising candidates as photosensitizers for this purpose. By understanding the optical properties that lead to efficient O generation, one can fully realize their potential as PDT photosensitizers. Three different metal nanoclusters, Au, Ag, and Au, are investigated for their O generation efficiency. The Au showed a O generation rate that is 2 orders of magnitude higher than that for Au and Ag, and several orders of magnitude higher than nanoparticles (>5 nm) due to Au's high absorption cross section-to-volume ratio. The effectiveness of PDT in live cells with nanoclusters was demonstrated by two-photon excitation compared to one-photon excitation. The implication of these results points toward new efficient two-photon O sensitizers for photodynamic therapy.

A series of ladder-type thienoacenes based on benzo[1,2-b:4,5-b']dithiophene (BDT) have been synthesized and characterized. They were shown to be p-type semiconductors with wide band gaps and able to support multiple stable cationic states. As the conjugation lengthens, these oligomers become more emissive, showing high quantum yields. They were shown to be good two-photon absorbers, exhibiting high two-photon absorption coefficients.

A simulation-led strategy enacts two-electron transfer between an intramolecular singlet fission chromophore (tetracyanomethylene quinoidal bithiopehene with β,β'-solubilizing groups) and multielectron acceptor (anthraquinone). The thermodynamic plausibility of multielectron transfer from a double-triplet state and the absorption spectra of electron transfer (ET) products were predicted using quantum chemical simulations. These predictions are consistent with experimental observations of reduced lifetimes in time-resolved fluorescence spectroscopy, changes in transmission profile, and appearance of new absorption bands in transient absorption spectroscopy, all of which support multi-ET in the QOT2/AQ mixture. The analysis suggests 2ET is favored over 1ET by a 2.5:1 ratio.

Gold nanoclusters have been extensively studied in solution for their unique optical properties. However, many applications of nanoclusters involve the use of the material in the solid state such as films. Au25(SR)18 in polymeric hosts was used as the model for studying the optical properties of nanocluster films. Different film-processing conditions as well as types of polymers were explored to produce a good-quality film that is suitable for optical measurements. The best optical film was made using Au25(C6S)18 and polystyrene. The formation of nanocluster films drastically reduces the intercluster distances to a few nanometers, which were estimated and characterized by optical absorption. The steady-state absorption and emission properties of the nanocluster film maintained their molecular characteristics. The emissions from the nanocluster films are found to be strongly enhanced at 730 nm with a smaller enhancement at 820 nm when the intercluster distance is below 8 nm. The emission enhancement can be attributed to the energy transfer between clusters due to the small intercluster distance. Two-photon Z scan revealed that the two-photon absorption cross sections are in the order of 10(6) GM, which is an order of magnitude higher than it is in solution. The two-photon absorption enhancement is correlated with strong dipole coupling. These results show that metal nanoclusters can be made into optical quality films, which increase the interaction between clusters and enhances their linear and nonlinear optical responses.

Using nanoclusters as building blocks for supracrystals can offer unique optical and electronic properties of metal nanoclusters for designing new materials. The advantage of building nanocluster crystals through molecular linkers is that the linker can be used as a functional group for the nanoclusters (e.g., dye−nanocluster systems). In this study, we employed the ligand exchange reaction to synthesize chromophore−Au 25 nanocluster oligomers and investigated their linear and nonlinear optical properties. The chromophore−Au 25 nanocluster oligomers product mixture was separated into four bands by polyacrylamide gel electrophoresis and characterized by matrix-assisted laser desorption ionization mass spectrometry and scanning transmission electron microscopy imaging. The linear optical properties of the systems were investigated by steady state UV−vis absorption and fluorescence spectroscopy. The chromophore−Au 25 nanocluster oligomers showed increased oscillator strength and transition dipole moment compared to single Au 25 nanoclusters. Energy transfer from the chromophore 4,4′-thiodibenzenethiol (TBT) to the metal cluster was observed in the chromophore−Au 25 nanocluster dimer system. The excited state and fluorescence dynamics were investigated by transient absorption spectroscopy, time-resolved fluorescence upconversion, and time-correlated single photon counting. The chromophore−Au 25 nanocluster oligomers have a long-lived surface state due to the contribution of energy transfer by two nanocluster cores. The two-photon absorption cross sections of the chromophore−Au 25 nanocluster oligomers showed an increasing enhancement trend with increasing oligomer length. An enhancement factor of up to 68 times was found compared to single Au 25 nanoclusters. Finally, we performed a structure−property correlation analysis to explain the observed optical properties of these systems.