One- and Two-Photon Absorption and Emission Properties of a Zn(II) Chemosensor

Action Plots in Action: In-Depth Insights into Photochemical Reactivity

Predicting wavelength-dependent photochemical reactivity is challenging. Herein, we revive the well-established tool of measuring action spectra and adapt the technique to map wavelength-resolved covalent bond formation and cleavage in what we term "photochemical action plots". Underpinned by tunable lasers, which allow excitation of molecules with near-perfect wavelength precision, the photoinduced reactivity of several reaction classes have been mapped in detail. These include photoinduced cycloadditions and bond formation based on photochemically generated o-quinodimethanes and 1,3-dipoles such as nitrile imines as well as radical photoinitiator cleavage. Organized by reaction class, these data demonstrate that UV/vis spectra fail to act as a predictor for photochemical reactivity at a given wavelength in most of the examined reactions, with the photochemical reactivity being strongly red shifted in comparison to the absorption spectrum. We provide an encompassing perspective of the power of photochemical action plots for bond-forming reactions and their emerging applications in the design of wavelength-selective photoresists and photoresponsive soft-matter materials.

Anion-Responsive Molecules That Exhibit Switching of Two-Photon Optical Properties

Two-photon-excited fluorescent probes are important for two-photon microscopy for biomedical studies. In contrast to the many examples of probes for cationic species, such as metal ions, there have been fewer reports on the control of two-photon optical properties by anions because in such systems it is difficult to control the associated π-electronic states. This Minireview summarizes anion-responsive molecules that exhibit changes in two-photon optical properties and describes their molecular design and anion-response mechanisms, which are driven by changes in covalent bonds and noncovalent interactions. Results from a recent study of two-photon systems, where geometries and optical properties are modulated by anion binding, are also discussed.

Switching of Two-Photon Optical Properties by Anion Binding of Pyrrole-Based Boron Diketonates through Conformation Change

Two-photon absorption (TPA) dyes with intense fluorescence can be used to detect small chemical species and as sensors and bioimaging probes for specific analytes. Various TPA dyes responding to a number of external stimuli have been reported. Among them, biologically important anionic species have not been used as agents to control TPA properties because their direct electronic influences on the transition dipole moments of dyes are typically small. In this study, dipyrrolyldiketone BF₂ complexes substituted with π-extended units exhibited efficient TPA properties that could be regulated by conformation changes induced by anion binding. The TPA intensity decreased to 1/5 of the original intensity upon anion binding, which was much larger than that observed for one-photon absorption. Anion detection was achieved by a change in the emission intensity of spatially resolved spots of two-photon-excited fluorescence (TPEF) in the sample. Experimental and theoretical studies were performed to understand the mechanism of the TPA property control and showed that the drastic changes in the transition dipole moments upon conformation changes between the straight and bending forms of the π-electronic systems caused the TPA and TPEF intensities drop.

Two-photon fluorescent probes for metal ions

Two-photon microscopy (TPM) has become an indispensible tool in biology and medicine owing to the capability of imaging the intact tissue for a long period of time. To make it a versatile tool in biology, a variety of two-photon probes for specific applications are needed. In this context, many research groups are developing two-photon probes for various applications. In this Focus Review, we summarize recent results on model studies and selected examples of two-photon probes that can detect intracellular free metal ions in live cells and tissues to provide a guideline for the design of useful two-photon probes for various in vivo imaging applications.

Holstein-Peirls-Hubbard trimer as a model for quadrupolar two-photon absorbing dyes

The linear and nonlinear optical properties of a Donor-Acceptor-Donor system have been investigated by using a two-electron three-point-site model system. Some basic features of electron correlations are included in the model by means of a bi-electronic density matrix. The polarizabilities and second hyperpolarizabilities have been computed with a modified version of the Collective Electronic Oscillators (CEO) method which allowed us to include the electron-phonon coupling. Both singly- and doubly-excited states are taken into account in the computation of (hyper-)polarizabilities. The effects of electron-phonon coupling on the two-photon absorption and on the third harmonic generation in the infrared region are discussed.

Fluorescent chemosensors for Zn(2+)

In the past decade, fluorescent chemosensors for zinc ion (Zn(2+)) have attracted great attention because of the biological significance of zinc combined with the simplicity and high sensitivity of fluorescence assays. Chemosensors can be divided into a fluorophore, a spacer and a receptor unit; the receptor is the central processing unit (CPU) of a chemosensor. This tutorial review will classify zinc chemosensors based on receptor types.

Solvent effect on two-photon absorption and fluorescence of rhodamine dyes

For a series of rhodamine dyes, two-photon absorption (TPA) and two-photon fluorescence (TPF) have been performed in different solvents. Solvent-dependent TPA spectra of these dyes were measured with open aperture z-scan method and compared to their respective single-photon spectra at equivalent energies. In the TPA spectra, relative peak intensities and positions are highly solvent dependent, which could be a result of vibronic couplings that depend on solvent environment. Measured TPA cross-sections of rhodamine dyes are consistently higher in nonpolar solvents. Certain complementary and similarity between TPA and TPF are also elucidated. Finally, a two-photon figure-of-merit is presented for these dyes in different solvents as a function of wavelength.