Microcavity-controlled single-molecule fluorescence.

We present for the first time cavity-controlled fluorescence spectra and decay curves of single dipole emitters interacting at room temperature with the first longitudinal mode of a Fabry-Perot microcavity offering a lambda/2-spacing between its silver mirrors. The spontaneous emission rate of individual dye molecules was found to be enhanced by the Purcell effect by up to three times compared to the rate in free space, in agreement with theoretical predictions. Moreover, our new microcavity design was found to provide long-term stability and single-molecule sensitivity under ambient conditions for several months without noticeable reduction of the cavity-Q value. We consider this as a significant advance for single-photon sources operating at room temperature.     

Unexpected Photoluminescence of Fluorinated Naphthalene Diimides

Two new amino core‐substituted naphthalene diimides (cNDIs) bearing fluorinated side chains have been synthesised. Steady‐state and time‐resolved fluorescence spectroscopy reveals unprecedented optical properties for the cNDIs with high quantum yields of ～0.8 and fluorescence lifetimes of ～13 ns in a range of solvents. These properties are apparent at the level of single molecules, where the compounds also show exceptional photostability under pulsed‐laser excitation. Photon emission is remarkably consistent with very few long timescale (millisecond or longer) interruptions with molecules regularly undergoing >10⁷ cycles of excitation and emission. Intermittencies owing to triplet‐state formation occur on a sub‐millisecond timescale with a low yield of 1–2 %, indicating that the presence of the fluorine atoms does not lead to a significant triplet yield through the heavy‐atom effect. These properties make the compounds excellent candidates for single‐molecule labelling applications.     

Thinner,Smaller, Faster: IR Techniques To Probe the Functionality of Biological and Biomimetic Systems

New techniques in vibrational spectroscopy are promising for the study of biological samples as they provide exquisite spatial and/or temporal resolution with the benefit of minimal perturbation of the system during observation. In this Minireview we showcase the power of modern infrared techniques when applied to biological and biomimetic systems. Examples will be presented on how conformational changes in peptides can be traced with femtosecond resolution and nanometer sensitivity by 2D IR spectroscopy, and how surface‐enhanced infrared difference absorption spectroscopy can be used to monitor the effect of the membrane potential on a single proton‐transfer step in an integral membrane protein. Vibrational spectra of monolayers of molecules at basically any interface can be recorded with sum‐frequency generation, which is strictly surface‐sensitive. Chemical images are recorded by applying scanning near‐field infrared microscopy at lateral resolutions better than 50 nm.     

Ultrafast fluorescence depolarisation in the yellow fluorescent protein due to its dimerisation.

Transient absorption spectroscopy with sub-100 fs time resolution was performed to investigate the oligomerisation behaviour of eYFP in solution. A single time constant tau(AD)=2.2+/-0.15 ps is sufficient to describe the time-resolved anisotropy decay up to at least 200 ps. The close contact of two protein barrels is deduced as the exclusive aggregation state in solution. From the final anisotropy r(infinity)=0.28+/-0.02, the underlying quaternary structure can be traced back to the somewhat distorted structure of the dimers of wt-GFP. The use of autofluorescent proteins as rulers in F?rster resonance energy transfer (FRET) measurements may demand polarisation-sensitive detection of the fluorescence with high time resolution.     

Synthesis, spectroscopic, and electrochemical properties of three tetrathiafulvalenes attached to perylene

Three donor–acceptor dyads 1–3 comprising of a tetrathiafulvalene (TTF) unit linked with perylene by a simple σ-bond were synthesized and characterized. Spectroscopy and cyclic voltammetry provided an indication that intramolecular charge-transfer interactions in their ground states between TTF and perylene for dyads 1–3 are negligible. Compared with the compound perylene, dyads 1–3 exhibited large fluorescence quenching, which might be ascribed to photo-induced electron transfer interaction between TTF and perylene units in the excited state. Correspondence: Yongjia Shen, Laboratory of Advanced Materials, Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, P.R. China.     

Structure‐Dependent Electronic Nature of Star‐Shaped Oligothiophenes,Probed by Ensemble and Single‐Molecule Spectroscopy

We have investigated the photophysical properties of star‐shaped oligothiophenes with three terthiophene arms (meta to each other, S3 ) or six terthiophene arms (ortho‐, meta‐, and para‐arranged, S6 ) connected to an ethynylbenzene core to elucidate the relationship between their molecular structure and electronic properties by using a combination of ensemble and single‐molecule spectroscopic techniques. We postulate two different conformations for molecules S3 and S6 on the basis of the X‐ray structure of hexakis(5‐hexyl‐2‐thienlyethynyl)benzene and suggest the coexistence of these conformers by using spectroscopic methods. From the steady‐state spectroscopic data of compound S6 , we show that the exciton is delocalized over the core structure, but that the meta‐linkage in compound S3 prevents the electronic communication between the arms. However, in single‐molecule spectroscopic measurements, we observed that some molecules of compound S3 showed long fluorescence lifetimes (about 1.4 ns) in the fluorescence‐intensity trajectories, which indicated that π electrons were delocalized along the meta linker. Based on these observations, we suggest that the delocalized exciton is intensely sensitive towards the dihedral angle between the core and the adjacent thiophene ring, as well as to the substituted position of the terthiophene arms. Our results highlight that the fluorescence lifetimes of compounds S3 and S6 are strongly correlated with the spatial location of their excitons, which is mainly affected by their conformation, that is, whether the innermost thiophene rings are facing each other or not. More interestingly, we observed that the difference between the degrees of ring‐torsional flexibility of compounds S3 and S6 results in their sharply contrasting fluorescence properties, such as a change in fluorescence intensity as a function of temperature.     

Single carbon nanotube optical spectroscopy.

This Minireview discusses novel insights into the electronic structure of carbon nanotubes obtained using single-molecule fluorescence spectroscopy. Fluorescence spectra from single nanotubes are well described by a single, Lorentzian lineshape. Nanotubes with identical structures fluoresce with different energies due to local electronic perturbations. Carbon nanotube fluorescence unexpectedly does not-show any intensity or spectral fluctuations at 300 K The lack of intensity blinking or bleaching demonstrates that carbon nanotubes have the potential to provide a stable, single-molecule infrared photon source, allowing for the exciting possibility of applications in quantum optics and biophotonics.     

A perylene diimide rotaxane: synthesis, structure and electrochemically driven de-threading

The first example of a [2]-rotaxane in which a perylene diimide acts as a recognition site has been synthesised and characterised. The interlocked nature of the compound has been verified by both NMR studies and an X-ray structure determination. Electrochemical investigations confirm that the nature of the redox processes associated with the perylene diimide are modified by the complexation process and that it is possible to mono-reduce the [2]-rotaxane to give a radical anion based rotaxane. Further reduction of the compound leads to de-threading of the macrocycle from the reduced PTCDI recognition site. Our synthetic strategies confirm the potential of PTCDI-based rotaxanes as viable targets for the preparation of complex interlocked species.     

Fluorescence Spectroscopy of Single Molecules under Ambient Conditions: Methodology and Technology

This review presents an overview of the fluorescence detection and spectroscopy of single molecules (SMS) in liquids and on surfaces under ambient conditions. The various techniques of SMS, such as confocal epifluorescence detection and wide‐field imaging are presented and discussed, together with the different methods of data analysis such as fluorescence correlation spectroscopy and burst‐by‐burst analysis. Selected applications of the various techniques in physics, chemistry, and biology are described.     

A 1,3‐Indandione‐Functionalized Tetraphenylethene: Aggregation‐Induced Emission,Solvatochromism, Mechanochromism,and Potential Application as a Multiresponsive Fluorescent Probe

A tetraphenylethene (TPE) derivative substituted with the electron‐acceptor 1,3‐indandione (IND) group was designed and prepared. The targeted IND‐TPE reserves the intrinsic aggregation‐induced emission (AIE) property of the TPE moiety. Meanwhile, owing to the decorated IND moiety, IND‐TPE demonstrates intramolecular charge‐transfer process and pronounced solvatochromic behavior. When the solvent is changed from apolar toluene to highly polar acetonitrile, the emission peak redshifts from 543 to 597 nm. IND‐TPE solid samples show an evident mechanochromic process. Grinding of the as‐prepared powder sample induces a redshift of emission from green (peak at 515 nm) to orange (peak at 570 nm). The mechanochromic process is reversible in multiple grinding–thermal annealing and grinding–solvent‐fuming cycles, and the emission of the solid sample switches between orange (ground) and yellow (thermal/solvent‐fuming‐treated) colors. The mechanochromism is ascribed to the phase transition between amorphous and crystalline states. IND‐TPE undergoes a hydrolysis reaction in basic aqueous solution, thus the red‐orange emission can be quenched by OH^? or other species that can induce the generation of sufficient OH^?. Accordingly, IND‐TPE has been used to discriminatively detect arginine and lysine from other amino acids, due to their basic nature. The experimental data are satisfactory. Moreover, the hydrolyzation product of IND‐TPE is weakly emissive in the resultant mixture but becomes highly blue‐emissive after the illumination for a period by UV light. Thus IND‐TPE can be used as a dual‐responsive fluorescent probe, which may extend the application of TPE‐based molecular probes in chemical and biological categories.     

Geminate Recombination as a Photoprotection Mechanism for Fluorescent Dyes

Despite common presumption due to fast photodestruction pathways through higher excited states, we show that further improvement of photostability is still achievable with diffusion‐limited photoprotection formulas. Single‐molecule fluorescence spectroscopy reveals that thiolate ions effectively quench triplet states of dyes by photoinduced electron transfer. Interestingly, this reaction rarely yields a radical anion of the dye, but direct return to the ground state is promoted by an almost instantaneous back electron transfer (geminate recombination). This type of mechanism is not detected for commonly used reductants such as ascorbic acid and trolox. The mechanism avoids the formation of radical cations and improves the photostability of single fluorophores. We find that a combination of β‐mercaptoethanol and classical reducing and oxidizing systems yields the best results for several dyes including Atto532 and Alexa568.