Strategies to Enhance the Photosensitization: Polymerization and the Donor-Acceptor Even-Odd Effect.

A particular challenge in the design of organic photosensitizers (PSs) with donor-acceptor (D-A) structures is that it is based on trial and error rather than specific rules. Now these challenges are addressed by proposing two efficient strategies to enhance the photosensitization efficiency: polymerization-facilitated photosensitization and the D-A even-odd effect. Conjugated polymers have been found to exhibit a higher 1 O2 generation efficiency than their small molecular counterparts. Furthermore, PSs with A-D-A structures show enhanced photosensitization efficiency over those with D-A-D structures. Theoretical calculations suggest an enhanced intersystem crossing (ISC) efficiency by these strategies. Both in vitro and in vivo experiments demonstrate that the resulting materials can be used as photosensitizers in image-guided photodynamic anticancer therapy. These guidelines are applicable to other polymers and small molecules to lead to the development of new PSs.

Nucleic-acid-base photofunctional cocrystal for information security and antimicrobial applications.

Cocrystal engineering is an efficient and simple strategy to construct functional materials, especially for the exploitation of novel and multifunctional materials. Herein, we report two kinds of nucleic-acid-base cocrystal systems that imitate the strong hydrogen bond interactions constructed in the form of complementary base pairing. The two cocrystals studied exhibit different colors of phosphorescence from their monomeric counterparts and show the feature of rare high-temperature phosphorescence. Mechanistic studies reveal that the strong hydrogen bond network stabilizes the triplet state and suppresses non-radiative transitions, resulting in phosphorescence even at 425 K. Moreover, the isolation effects of the hydrogen bond network regulate the interactions between the phosphor groups, realizing the manipulation from aggregation to single-molecule phosphorescence. Benefiting from the long-lived triplet state with a high quantum yield, the generation of reactive oxygen species by energy transfer is also available to utilize for some applications such as in photodynamic therapy and broad-spectrum microbicidal effects. In vitro experiments show that the cocrystals efficiently kill bacteria on a tooth surface and significantly help prevent dental caries. This work not only provides deep insight into the relationship of the structure-properties of cocrystal systems, but also facilitates the design of multifunctional cocrystal materials and enriches their potential applications.

"Bright" and "dark" excited states of an alternating at oligomer characterized by femtosecond broadband spectroscopy.

The nature and dynamics of DNA excited states is of pivotal importance in determining both DNA ultraviolet photostability and its vulnerability toward photodamage. The complexity regarding the involvement of "bright" and "dark" excited states, their molecular origin, and the roles played by these states in the course of electronic energy relaxation constitute an active and contentious area in current research of DNA excited states. As a case study, we report here a combined broadband femtosecond time-resolved fluorescence (TRF) and transient absorption (TA) study on a self-complementary d(AT)(10) oligomer and a reference system of an equal molar mixture of the constituent bases represented by adenosine and thymidine (A+T). Comparison of the spectral character and temporal evolution of the TRF and TA data for 267 nm excited d(AT)(10) and A+T provides evidence for a base-localized excitation feature for an early (< approximately 50 fs) "bright" S(LE) state and its ensuing evolution within approximately 3 ps into a approximately 72 ps "dark" S(E) exciplex in d(AT)(10). Combined analysis of the d(AT)(10) TRF and TA results suggests the presence of a weakly fluorescent transient S(G) state that acts as a gateway to mediate the excitation transfer and energy elimination. A distinct base conformation-dependent model involving an ultrafast approximately 0.3 ps conversion of the S(LE) to S(G) that then evolves by approximately 3 ps into the S(E) has been proposed to account for the collective deactivation character of d(AT)(10). This presents a novel excited-state picture that can unify the seemingly conflicting time-resolved results reported previously for related AT DNAs. The direct spectral and dynamical data provided here contributes important photophysical parameters for the description of the excited states of AT oligomers. The possible connection between the energy transfer giving the S(E) and the photostability vs photodamage of A/T DNAs is briefly discussed.