Multicomponent synthesis of chromophores – The one-pot approach to functional π-systems

Multicomponent reactions, conducted in a domino, sequential or consecutive fashion, have not only considerably enhanced synthetic efficiency as one-pot methodology, but they have also become an enabling tool for interdisciplinary research. The highly diversity-oriented nature of the synthetic concept allows accessing huge structural and functional space. Already some decades ago this has been recognized for life sciences, in particular, lead finding and exploration in pharma and agricultural chemistry. The quest for novel functional materials has also opened the field for diversity-oriented syntheses of functional π-systems, i.e. dyes for photonic and electronic applications based on their electronic properties. This review summarizes recent developments in MCR syntheses of functional chromophores highlighting syntheses following either the framework forming scaffold approach by establishing connectivity between chromophores or the chromogenic chromophore approach by de novo formation of chromophore of interest. Both approaches warrant rapid access to molecular functional π-systems, i.e. chromophores, fluorophores, and electrophores for various applications.

Photoinduced host-to-guest electron transfer in a self-assembled coordination cage

Light–powered host–guest charge transfer (HGCT) is shown for a coordination cage based on electron-rich phenothiazines, containing an anthraquinone acceptor as guest. Transient absorption spectroscopy and spectroelectrochemistry data is presented.

Pairing multicomponent stators with aromatic rotators for new emissive molecular rotors

We demonstrate here that the Ugi-Sonogashira protocol can be successfully used to obtain five new molecular rotors 10a-e with strong emission. They have been synthesized by combining multicomponent Ugi stators and several aromatic rotary components: phenylene, p-xylene, naphthalene and anthracene. The synthesized conjugated rotors are highly fluorescent (Φf = 0.39 to Φf = 0.10), and changes in their emission were observed upon variations of the surrounding media. Particularly, we found that they are sensitive to aggregation (THF/water) or high viscosity (methanol/glycerol) conditions. This work paves the way to develop new emissive rotors with exciting photophysical properties.

Preservation of the donor-acceptor character of a carbazole-phenalenone dyad upon adsorption on Pt(111)

Donor-acceptor molecules are a subject of great attention due to their immense potential in molecular electronics and photovoltaics. Despite numerous extensive studies demonstrating their functionality in solution, the donor-acceptor character is usually lost upon adsorption on a conducting substrate. Here the concept of breaking the conjugation between the donor and acceptor unit by insertion of a bridge is used. Furthermore, the bridge introduces a kink into the dyad and thus, reduces the possibility of hybridization with the substrate. A donor-bridge-acceptor dyad composed of carbazole and phenalenone units joined through a flexible bridge is synthesized and deposited on a Pt(111) surface. Its electronic properties are investigated with a combination of low temperature scanning tunneling microscope measurements and density functional theory simulations. Two preferential adsorption configurations are identified, in which individual molecules form strong bonds to the substrate and to a Pt adatom. Differential conductance measurements and atomistic simulations evidence the preservation of a reduced donor-acceptor character upon adsorption of the molecule, where this reduction is ascribed to the strong molecule-metal hybridization. Our results highlight the changes in donor-acceptor character of the dyad induced by the substrate and provide guidelines for the use of donor-bridge-acceptor molecules as functional units in solid-state devices.

Diversity-Oriented Synthesis of Highly Fluorescent Fused Isoquinolines for Specific Subcellular Localization

A multicomponent diversity-oriented synthesis of new highly emissive tetracyclic isoquinolines that target specific organelles is described. The title compounds were prepared via a three-step protocol starting with an Ugi four-component reaction, followed by either an intramolecular alkyne hydroarylation and subsequent alkene isomerization or through a Pomeranz-Fritsch-type cyclization with a final intramolecular Heck reaction. Subcellular localization studies of these compounds using green channel confocal microscopy revealed remarkable and distinctive distribution patterns in live cells, showing an unprecedented high selectivity and imaging contrast. The differentiated organelle visualization-including localizers for mitochondria, lysosomes, Golgi apparatus, endoplasmic reticulum, and plasma membrane-was achieved by varying the nature of the tetracyclic system and substituent pattern, changing the original four-component set in the starting Ugi reaction.

Unimolecular Exciplexes by Ugi Four-Component Reaction

Exciplex or excited complex emission is an excited state process, arising from considerable charge transfer of an excited energy donor to an acceptor, which can be identified by the occurrence of a redshifted emission band that is absent in the individual constituents. Particularly interesting are exciplexes that are formed by intramolecular excited state interaction, which are inherently concentration independent. Based upon our previous experience in the Ugi-4CR syntheses of donor-acceptor conjugates capable of photo-induced intramolecular electron transfer (PIET), that is, generation of light-induced charge separation, we now disclose the diversity-oriented approach on unimolecular exciplex emitters and their reference systems by Ugi-4CR. The photophysics is studied by absorption and emission spectroscopy and accompanied by density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations.

Near-infrared (NIR) surface-enhanced Raman spectroscopy (SERS) study of novel functional phenothiazines for potential use in dye sensitized solar cells (DSSC)

Phenothiazines are of potential use as dye sensitizers in Grätzel-type dye sensitized solar cells (DSSC). Plasmonic nanoparticles like gold nanoparticles can enhance the power conversion efficiency of these solar cells. In this work near-infrared surface-enhanced Raman spectroscopy (NIR-SERS) is used to investigate the interaction between six novel phenothiazine-merocyanine dyes containing the three different functional groups rhodanine, 1,3-indanedione and cyanoacylic acid with plasmonic nanomaterials, to decide if the incorporation of plasmonic nanoparticles could enhance the efficiency of a Grätzel-type solar cell. The studies were carried out in the solution state using spherical and rod-shaped gold nanostructures. With KCl induced agglomerated spherical gold nanoparticles, forming SERS hot spots, the results showed low detection limits between 0.1 μmol L⁻¹ for rhodanine containing phenothiazine dyes, because of the formation of Au–S bonds and 3 μmol L⁻¹ for cyanoacrylic acid containing dyes, which formed H-aggregates in the watery dispersion. Results with gold nanorods showed similar trends in the SERS measurements with lower limits of detection, because of a shielding effect from the strongly-bound surfactant. Additional fluorescence studies were carried out to determine if the incorporation of nanostructures leads to fluorescence quenching. Overall we conclude that the addition of gold nanoparticles to rhodanine and 1,3-indanedione containing phenothiazine merocyanine dyes could enhance their performance in Grätzel-type solar cells, because of their strong interactions with plasmonic nanoparticles.

Phenothiazines are of potential use as dye sensitizers in Grätzel-type dye sensitized solar cells (DSSC).

Phenothiazine electrophores immobilized on periodic mesoporous organosilicas by ion exchange

Two different phenothiazines carrying quaternary ammonium groups in the side chain have been synthesized and fully characterized.

Ultrafast IR spectroscopy of photo-induced electron transfer in self-assembled donor-acceptor coordination cages

Photo-induced processes in self-assembled coordination cages were studied by femtosecond infrared pump-probe spectroscopy. Densely packed, interpenetrated double cages were constructed from eight bis-monodentate redoxactive ligands bound to four Pd(ii) nodes. Two types of ligands consisting of electron rich phenothiazine (PTZ) or electron deficient anthraquinone (ANQ) chromophores were used to assemble either homo-octameric or mixed-ligand cages. Upon photoexcitation the homo-octameric acceptor cage undergoes intersystem crossing to a long-lived triplet state, similar to the free acceptor ligand. Excitation of the free donor ligand leads to a fluorescent state with intramolecular charge transfer character. This fluorescence is completely quenched in the homo-octameric donor double cage due to a ligand-to-metal charge transfer followed by back electron transfer on a ps timescale. Only for the mixed-ligand cage irradiation produces a charge separated state with an oxidized PTZ radical cation and a reduced ANQ radical anion as proven by their vibrational fingerprints in the transient IR spectra. In dichloromethane the lifetime of this charge separated state extends from tens of ps to >1.5 ns which is attributed to the broad distribution of mixed-ligand cages with different stoichiometry and/or stereo configurations.

Thiophene-forming one-pot synthesis of three thienyl-bridged oligophenothiazines and their electronic properties

The pseudo five-component Sonogashira-Glaser cyclization synthesis of symmetrically 2,5-diaryl-substituted thiophenes is excellently suited to access thienyl-bridged oligophenothiazines in a one-pot fashion. Three thienyl-bridged systems were intensively studied by UV-vis and fluorescence spectroscopy as well as by cyclic voltammetry. The oxidation proceeds with lower oxidation potentials and consistently reversible oxidations can be identified. The Stokes shifts are large and substantial fluorescence quantum yields can be measured. Computational chemistry indicates lowest energy conformers with sigmoidal and helical structure, similar to oligophenothiazines. TD-DFT and even semiempirical ZINDO calculations reproduce the trends of longest wavelengths absorption bands and allow the assignment of these transitions to possess largely charge-transfer character from the adjacent phenothiazinyl moieties to the central thienyl unit.

Multicomponent syntheses of functional chromophores

Multicomponent reactions are a valuable tool for the synthesis of functional π-electron systems. Two different approaches can be taken into account for accessing the target structures. In the more conventional scaffold approach an already existing chromophore is coupled with other components to give a complex functional π-system. Here, electronically monotonous components can also be introduced, which may exert synergistic electronic effects within the novel compound. The more demanding chromophore concept generates a complete π-electron system and a scaffold concurrently. The latter approach is particularly stimulating for methodologists since π-systems might be accessible from simple starting materials. This review encompasses the advances in the preparation of functional π-electron systems via multicomponent processes during the past few years, based both on the scaffold and chromophore concepts. Besides the synthetic strategies the most important properties, i.e. redox potentials, absorption and emission maxima or fluorescence quantum yields, of the synthesized molecules are highlighted.

Thioxanthone in apolar solvents: ultrafast internal conversion precedes fast intersystem crossing

The photophysics of thioxanthone dissolved in cyclohexane was studied by femtosecond fluorescence and transient absorption spectroscopy.

The Ugi four-component reaction as a concise modular synthetic tool for photo-induced electron transfer donor-anthraquinone dyads

Phenothiazinyl and carbazolyl-donor moieties can be covalently coupled to an anthraquinone acceptor unit through an Ugi four-component reaction in a rapid, highly convergent fashion and with moderate to good yields. These novel donor–acceptor dyads are electronically decoupled in the electronic ground state according to UV–vis spectroscopy and cyclic voltammetry. However, in the excited state the inherent donor luminescence is efficiently quenched. Previously performed femtosecond spectroscopic measurements account for a rapid exergonic depopulation of the excited singlet states into a charge-separated state. Calculations of the Gibbs energy of photo-induced electron transfer from readily available UV–vis spectroscopic and cyclovoltammetric data applying the Weller approximation enables a quick evaluation of these novel donor–acceptor dyads. In addition, the X-ray structure of a phenothiazinyl–anthraquinone dyad supports short donor–acceptor distances by an intramolecular π-stacking conformation, an important assumption also implied in the calculations of the Gibbs energies according to the Weller approximation.

SERS spectroscopic evidence for the integrity of surface-deposited self-assembled coordination cages

A series of self-assembled coordination cages was examined by Raman spectroscopy in solution and Surface Enhanced Raman Scattering (SERS) on a nanostructured Au surface.