For molecules in condensed phase, three general restrictions limit the efficient utilization of light energies; first, molecules in higher excited states usually very rapidly relax to lower electronic states (Kasha's rule) and some portion of the absorbed photon energy is diminished in this relaxation, second, a large number of the molecules excited in assemblies undergo fast annihilation and only a small number of excited state molecules can remain, leading to the loss of the number of photons absorbed in the system. In addition, the electronic state accessible through the one- photon absorption is limited by the optical selection rule and we cannot access various dark electronic excited states of molecules. In the present article, we introduce several examples beyond these three restrictions in the photochemical responses of molecules and molecular assemblies through the multiple excitation and multiphoton processes.

Super-resolution imaging techniques that break the diffraction barrier have recently been developed. In several techniques, fluorescent probes with optimal fluorescence switching properties play an essential role. Single-molecule localization microscopy (SLM) enables us to construct super-resolution images by repeating detection and high precision localization of individual probes. During data acquisition, most of the probes should be switched off, and only a small subset of them are switched on, detected and localized at any given time. For this purpose, various methods have been developed in the light of photochemistry, of which some utilize intermolecular reaction with external additives such as reductants or oxidants to convert probes into a dark state such as triplet state or radical anion. Here, we introduce the principle of representative super-resolution techniques and roles of fluorescent probes. Then we report our-developed first-in-class of spontaneously blinking fluorophores and its application to super-resolution imaging in fixed and live cells.

Photochromism changes a molecule into its isomer reversibly by photoirradiation. We have long been interested in photochromism and a number of functional switches have been created in our laboratory. In this Topic article we describe (1) the reversible switching of fluorescent properties of a diarylethene controlled by the photoirradiation as well as by the pH change of the media, and (2) the reversible switching of solution-dissolution phenomena by photoirradiation. These photon-working switching systems are driven by the photoirradiation to the relevant photochromic compounds which are interacting with other chemical species so that the complexes can be regarded as molecularly integrated multi-responsive photochromic systems.

Water-soluble Sb and P porphyrins are reviewed from the standpoints of preparation, bio-affinity, and photosensitized inactivation. Water-solubilization of Sb and P porphyrins was easily achieved by introduction of alkyl (A type), polyoxaalkyl (E type), alkyl- pyridinium (P type), and glucosyl groups (G type) on the axial ligands. The A and E types of porpyrins was applied to photo-inactivation of Saccharomyces cerevisiae (yeast) in the relation to photodynamic therapy. Moreover, bio-affinity of P and G types of porphyrins to human serum albumin (HSA) and concanavalin A were examined.

Förster resonance energy transfer (FRET) has been extensively applied as “molecular ruler” to investigate the structure of macromolecules and interaction between them. Although FRET efficiency depends on both the distance and mutual orientation between the donor and acceptor, only limited examples of orientation-dependent FRET have been reported. Here, we developed a new orientation-dependent FRET system by incorporating a donor (pyrene) and an acceptor (perylene) into DNA via D-threoninol. Distance and orientation between dyes can be controlled by changing the number of intervening base pairs. FRET efficiencies were not only dependent on distance, but drastically on the relative orientation between donor and acceptor. Theoretical calculation of FRET efficiency assuming that the DNA duplex is a rigid cylinder with AT tract geometry fairly coincided with the experimental results. We believe this system is applicable not only as molecular ruler but as helical ruler (molecular protractor).

Organic solid materials that exhibit tunable photoluminescence in the solid state under macroscopic mechanical stimuli have potential applications in sensors and imaging devices. So far, no rational designs have been proposed that impart these mechano-responsive luminescent properties to π-conjugated compounds. We have demonstrated an effective strategy for mechanoresponsive luminescent materials by imparting amphiphilic and dipolar characteristics to a luminescent π-conjugated molecule. The oligo(p-phenylenevinylene) luminophore with a didodecylamino group at one end and a tri(ethylene glycol) ester group at the other end self-assembled into segregated solid structures by separately aggregating its hydrophobic and hydrophilic moieties. The segregated structures force the dipolar π-systems to align in the same direction, thereby generating a conflict between the side-chain aggregation and dipolar stabilization. As a result, these metastable solid structures can be transformed through mechanical stimulation to a more stable structure, from a π−π stacked aggregate to a liquid crystal and further to a crystalline phase with variable luminescence. We also showed the control of the liquid crystalline-crystalline phase transition by the complexation with lithium salt, and the application of such a property to an inkjet-imaging device with a concept of “molecular decoder”.

Transient photocurrent measurements are a useful tool to monitor carrier transport properties for thin films of organic compounds. This topic will be firstly explained a way to analyze the carrier transport properties from the transient photocurrent data. Then, the analysis for the carrier transport properties of the dendrimers composed of p-phenylenevinylene with generation numbers of three and four as a dendron and diphenylamino surface group was carried out. It was found that the carrier transport properties were related to a geometric structure of the dendrimer molecule.