Shih-Sheng Sun

Recent progress in organic sensitizers for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) are fabricated using low-cost materials and a simple fabrication process; these advantages make them attractive candidates for research on next generation solar cells. In this type of solar cell, dye-sensitized metal oxide electrodes play an important role for achieving high performance since the porous metal oxide films provide large specific surface area for dye loading and the dye molecule possesses broad absorption covering the visible region or even part of the near-infrared (NIR). Recently, metal-free sensitizers have made great progress and become the most potential alternatives. This review mainly focuses on recent progress in metal-free sensitizers for applications in DSSCs. Besides, we also briefly report DSSCs with near-infrared (NIR) organic sensitizers, which provide the possibility to extend the absorption threshold of the sensitizers in the NIR region. Finally, special consideration has been paid to panchromatic engineering, co-sensitization, a key technique to achieve whole light absorption for improving the performance of DSSCs.

High‐Performance Dipolar Organic Dyes with an Electron‐Deficient Diphenylquinoxaline Moiety in the π‐Conjugation Framework for Dye‐Sensitized Solar Cells

We report here the synthesis and electrochemical and photophysical properties of a series of easily prepared dipolar organic dyes and their application in dye-sensitized solar cells (DSSCs). For the six organic dyes, the molecular structures comprised a triphenylamine group as an electron donor, a cyanoacrylic acid as an electron acceptor, and an electron-deficient diphenylquinoxaline moiety integrated in the π-conjugated spacer between the electron donor and acceptor moieties. The incorporation of the electron-deficient diphenylquinoxaline moiety effectively reduces the energy gap of the dyes and broadly extends the spectral coverage. DSSCs based on dye 6 produced the best overall cell performance of 7.35 %, which translates to approximately 79 % of the intrinsic efficiency of the DSSCs based on the standard N719 dye under identical experimental conditions. The high performance of DSSCs based on dye 6 among the six dyes explored is attributed to the combined effects of high dye loading on a TiO(2) surface, rapid dye regeneration, and effective retardation of charge recombination.

Structurally Simple Dipolar Organic Dyes Featuring 1,3-Cyclohexadiene Conjugated Unit for Dye-Sensitized Solar Cells

A series of structurally simple dipolar light-harvesting organic dyes featuring 1,3-cyclohexadiene in the aromatic pi framework for dye-sensitized solar cells has been synthesized and characterized. The highest conversion efficiency of the DSSCs based on these dyes can reach up to 4.4%.

Synthesis, Optical, and Mesomorphic Properties of Self-Assembled Organogels Featuring Phenylethynyl Framework with Elaborated Long-Chain Pyridine-2,6-Dicarboxamides†

A series of new organogelators with pi-conjugated phenylethynyl framework featuring long-chain carboxamides have been synthesized. These organgelators have shown great ability to gel a variety of organic solvents to form stable organogels with the minimum gelation concentration as low as 0.1 wt %. Gelation is completely thermoreversible, and it occurs due to the aggregation of the organogelators resulting in the formation of a fibrous network via a combination of intermolecular hydrogen bonding, pi-pi stacking, and van der Waals interactions that is observed for the xerogels by TEM. The influence of sol-to-gel transition has been explored in detail by variable-temperature 1H NMR, UV-vis absorption, and fluorescence spectroscopy. Aggregation-induced enhanced emission has been observed in these organogelator molecules with an order of higher fluorescence quantum yields from solution to gels. In addition, some molecules also exhibit unique liquid crystalline properties over a large temperature range as revealed by DSC and POM studies.

Quinoxaline based D–A–D molecules: high contrast reversible solid-state mechano- and thermo-responsive fluorescent materials

A new series of AEE-active D–A–D molecules by integrating a diphenylquinoxaline core with two aromatic chromophores such as anthracene or pyrene has been designed and synthesized. These highly pre-twisted molecules exhibited intriguing photophysical properties. The experimental and theoretical investigations revealed that their twisted conjugated skeleton is the key factor for showcasing the AEE property because of the restricted face-to-face co-planar arrangement and strong intermolecular interactions. Although, formulated compounds differ trivially in their chemical composition, they exhibit distinct stimuli-responsive behaviors in the solid state. The pyrene derivative exhibited high-contrast piezo-fluorochromic behavior with the emission colors reversibly changing from bluish green to yellow upon grinding and annealing. On the other hand, the anthranyl analogue showed a unique on–off switching of solid-state luminescence by heating and solvent fuming. This system provides a rare example of distinct thermal reversible fluorescence switching controlled by changing the molecular packing modes and the origin of this unique property was attributed to the formation of donor–acceptor intermolecular stacking which enables an intermolecular electron transfer quenching process. Based on XRD, DSC, and NMR studies, the observed piezofluorochromism of pyrene or thermochromism of anthracene derivatives is attributed to the result of solid–solid phase transition. The present findings would open the way for the development of novel organic luminescent solids that can be switched on and off by external thermal stimuli.

New Chromogenic and Fluorescent Probes for Anion Detection: Formation of a [2 + 2] Supramolecular Complex on Addition of Fluoride with Positive Homotropic Cooperativity

Two new chromogenic and fluorescent probes for anions have been designed, synthesized, and characterized. These probes contain multiple hydrogen bonding donors including hydrazine, hydrazone, and hydroxyl functional groups for potential anion interacting sites. Despite the possible flexible structural framework due to the presence of sp3 carbon linkage, X-ray structure analysis of probe 2 displayed an essentially planar conformation in the solid state owing to strong crystal packing interactions comprising a combination of favorable pi-pi stacking effect and hydrogen bonding to cocrystallized CH3OH molecules. Both probes 1 and 2 display orange color in DMSO solution and show fairly weak fluorescence at room temperature. Binding studies revealed that both probes 1 and 2 show noticeable colorimetric and fluorescent responses only to F-, OAc-, and H2PO4- among the nine anions tested (F-, Cl-, Br-, I-, OAc-, H2PO4-, HSO4-, ClO4-, and NO3-). The general trend of the sensitivity to anions follows the order of F- > OAc- > H2PO4- > Cl- > Br- approximately I- approximately HSO4- approximately ClO4- approximately NO3-. A 1:2 (probe to anion) binding stoichiometry was found for probe 1 with OAc- and H2PO4- and probe 2 with F-, OAc-, and H2PO4-. The binding isotherm of probe 1 to F- was found to be complicated with apparent multiple equilibria occurring in solution. The formation of an aggregated supramolecular complex upon addition of fluoride is proposed to rationalize the observed optical responses and is supported by ESI mass spectrometry and pulsed-field gradient NMR spectroscopy. Data analysis suggests that the binding of probe 1 to F- shows positive homotropic cooperativity.

Photophysics and Photochemistry of Organometallic Rhenium Diimine Complexes

This review describes the photophysics and photochemistry of various diimine rhenium(I) tricarbonyl complexes. The exceptionally diverse photophysical behavior of these complexes is largely dependent on the nature of their lowest excited states. These excited states and the excited-state characteristics can be easily changed by varying the substituents on either the diimine ligands or the ancillary ligands. The prolificacy of the photophysical and photochemical properties of diimine rhenium(I) tricarbonyl complexes allows for a range of important applications such as light- emitting devices, sensors, probes for photo-polymerization, optical switches, nonlinear optical materials, radiopharmaceuticals, carbon dioxide reduction and supramolecular chemistry. We have covered the studies that best characterize the state of the field as far as the most significant fundamental photochemical advances, and the applications of the photochemistry of rhenium complexes.

Recognition, encapsulation, and selective fluorescence sensing of nitrate anion by neutral C3-symmetric tripodal podands bearing amide functionality.

A series of neutral C(3)-symmetric acyclic artificial receptors incorporating amide functionality has been designed, synthesized, and fully characterized. Upon protonation, these conformationally flexible N-bridged tripodal podands 1-5 form in situ cone shape conformation through hydrogen bonding and C-H···π interactions. The protonation-induced interior preorganized cavity is capable of entrapping nitrate anions through the amide N-H bonds to form discrete nitrate complexes (1a-5a), which were fully characterized by NMR, HRESI mass spectra, and single crystal structures. By incorporating suitable fluorophores at each branch of the tripod receptor, the resulting fluorescent receptor 5 selectively recognized nitrate anions by fluorescent quenching in a DMSO solution and displayed one of the highest binding affinities for nitrate anions reported so far in polar media. Receptor 5 represents a unique example of a neutral receptor for the recognition of nitrate anions in polar solvent media by its zwitterionic form. The possible mechanism of proton-induced preorganization of these flexible, acyclic receptors in a convergent cone conformation followed by nitrate complexation has been proposed to rationalize the effective nitrate recognition.

Colorimetric and luminescent sensing of F− anion through strong anion–π interaction inside the π-acidic cavity of a pyridyl-triazine bridged trinuclear Re(I)–tricarbonyl diimine complex

A trinuclear star-shaped organometallic host with a pi-acidic interior cavity has been synthesized and this shows a strong anion-pi interaction with PF6-, as demonstrated in the crystal structure and colorimetric and luminescent responses with F- anion in solution.

A selective colorimetric Hg2+ probe featuring a styryl dithiaazacrown containing platinum (II) terpyridine complex through modulation of the relative strength of ICT and MLCT transitions.

A series of platinum(II) terpyridine complexes featuring an aminostilbene donor-acceptor framework was synthesized. The complex with a dithiaazacrown moiety exhibits a highly sensitive and selective colorimetric response to a Hg(2+) cation through modulation of the relative strength of ICT and MLCT transitions. The results from (1)H NMR titration suggest the existence of a weak Pt(II)···Hg(II) metallophilic interaction at low Hg(2+) concentration.