Fluorescence-Encoded Infrared Vibrational Spectroscopy with Single-Molecule Sensitivity.

Single-molecule methods have revolutionized molecular science, but techniques possessing the structural sensitivity required for chemical problems-e.g. vibrational spectroscopy-remain difficult to apply in solution. Here, we describe how coupling infrared-vibrational absorption to a fluorescent electronic transition (fluorescence-encoded infrared (FEIR) spectroscopy) can achieve single-molecule sensitivity in solution with conventional far-field optics. Using the fluorophore Coumarin 6, we illustrate the principles by which FEIR spectroscopy measures vibrational spectra and relaxation and introduce FEIR correlation spectroscopy, a vibrational analogue of fluorescence correlation spectroscopy, to demonstrate single-molecule sensitivity. With further improvements, FEIR spectroscopy could become a powerful tool for single-molecule vibrational investigations in the solution or condensed phase.

Fluorescence-Encoded Infrared Spectroscopy: Ultrafast Vibrational Spectroscopy on Small Ensembles of Molecules in Solution.

Fluorescence-encoded infrared (FEIR) spectroscopy is an ultrafast technique that uses a visible pulse to up-convert information about IR-driven vibrations into a fluorescent electronic population. Here we present an updated experimental approach to FEIR that achieves high sensitivity through confocal microscopy, high repetition rate excitation, and single-photon counting. We demonstrate the sensitivity of our experiment by measuring ultrafast vibrational transients and Fourier transform spectra of increasingly dilute solutions of a coumarin dye. We collect high-quality data at 40 µM (∼2 orders of magnitude below the limit for conventional IR) and make measurements down to the 10-100 nM range (∼5 orders of magnitude) before background signals become overwhelming. At 10 nM we measure the average number of molecules in the focal volume to be ∼20 using fluorescence correlation spectroscopy. This level of sensitivity opens up the possibility of performing fluctuation correlation vibrational spectroscopy or-with further improvement-single-molecule measurements.

Single-stage MHz mid-IR OPA using LiGaS2and a fiber laser pump source.

We present the design and characterization of a tunable mid-IR optical parametric amplifier (OPA) with a 1 MHz 1033 nm fiber laser pump source. The OPA generates >10 nJ/pulse tunable from 3-7.5 µm with 85-165 cm-1 bandwidth and 140-540 fs pulse durations. The design utilizes a single stage of collinear, Type I amplification in lithium gallium sulfide (LGS) seeded by the near-IR portion of a super-continuum (SC) generated in yttrium aluminum garnet (YAG). The resulting mid-IR idler beam is isolated as the output. This single-stage OPA is a simple and convenient source of high-repetition-rate, tunable mid-IR radiation for high-throughput ultrafast infrared spectroscopy.

Resolving ultrafast exciton migration in organic solids at the nanoscale.

Effectiveness of molecular-based light harvesting relies on transport of excitons to charge-transfer sites. Measuring exciton migration, however, has been challenging because of the mismatch between nanoscale migration lengths and the diffraction limit. Instead of using bulk substrate quenching methods, here we define quenching boundaries all-optically with sub-diffraction resolution, thus characterizing spatiotemporal exciton migration on its native nanometre and picosecond scales. By transforming stimulated emission depletion microscopy into a time-resolved ultrafast approach, we measure a 16-nm migration length in poly(2,5-di(hexyloxy)cyanoterephthalylidene) conjugated polymer films. Combined with Monte Carlo exciton hopping simulations, we show that migration in these films is essentially diffusive because intrinsic chromophore energetic disorder is comparable to chromophore inhomogeneous broadening. Our approach will enable previously unattainable correlation of local material structure to exciton migration character, applicable not only to photovoltaic or display-destined organic semiconductors but also to explaining the quintessential exciton migration exhibited in photosynthesis.

Uncovering Single-Molecule Photophysical Heterogeneity of Bright, Thermally Activated Delayed Fluorescence Emitters Dispersed in Glassy Hosts.

Recently developed all-organic emitters used in display applications achieve high brightness by harvesting triplet populations via thermally activated delayed fluorescence. The photophysical properties of these emitters therefore involve new inherent complexities and are strongly affected by interactions with their host material in the solid state. Ensemble measurements occlude the molecular details of how host-guest interactions determine fundamental properties such as the essential balance of singlet oscillator strength and triplet harvesting. Therefore, using time-resolved fluorescence spectroscopy, we interrogate these emitters at the single-molecule level and compare their properties in two distinct glassy polymer hosts. We find that nonbonding interactions with aromatic moieties in the host appear to mediate the molecular configurations of the emitters, but also promote nonradiative quenching pathways. We also find substantial heterogeneity in the time-resolved photoluminescence of these emitters, which is dominated by static disorder in the polymer. Finally, since singlet-triplet cycling underpins the mechanism for increased brightness, we present the first room-temperature measurement of singlet-triplet equilibration dynamics in this family of emitters. Our observations present a molecular-scale interrogation of host-guest interactions in a disordered film, with implications for highly efficient organic light-emitting devices. Combining a single-molecule experimental technique with an emitter that is sensitive to triplet dynamics, yet read out via fluorescence, should also provide a complementary approach to performing fundamental studies of glassy materials over a large dynamic range of time scales.

Bringing Far-Field Subdiffraction Optical Imaging to Electronically Coupled Optoelectronic Molecular Materials Using Their Endogenous Chromophores.

We demonstrate that subdiffraction resolution can be achieved in fluorescence imaging of functional materials with densely packed, endogenous, electronically coupled chromophores by modifying stimulated emission depletion (STED) microscopy. This class of chromophores is not generally compatible with STED imaging due to strong two-photon absorption cross sections. Yet, we achieve 90 nm resolution and high contrast in images of clusters of conjugated polymer polyphenylenevinylene-derivative nanoparticles by modulating the excitation intensity in the material. This newfound capability has the potential to significantly broaden the range of fluorophores that can be employed in super-resolution fluorescence imaging. Moreover, solution-processed optoelectronics and photosynthetic or other naturally luminescent biomaterials exhibit complex energy and charge transport characteristics and luminescence variations in response to nanoscale heterogeneity in their complex, physical structures. Our discovery will furthermore transform the current understanding of these materials' structure-function relationships that have until now made them notoriously challenging to characterize on their native, subdiffraction scales.

Multiresonant coherent multidimensional spectroscopy of the vibrationally induced decarboxylation of AOT: deuterium oxide reverse micelles.

The multiresonant coherent multidimensional spectroscopy study of D(2)O in AOT micelles reveals two unexpected features in addition to those expected for D(2)O. These features appear when the excitation pulse time ordering defines either fully coherent Liouville pathways or partially coherent pathways where there are intermediate populations. The features shift as the excitation pulse ordering changes between the two sets of pathways. They are attributed to the asymmetric stretch mode of CO(2) that is formed by the infrared excited decarboxylation of AOT. The cross-peaks' intensity is enhanced when the excitation pulse is resonant with the CO(2) P and R branches for the fully coherent pathways, but they are depressed at the P and R branch resonances for the partially coherent pathways. The enhancement is attributed to the simultaneous production of CO(2) during the first excitation pulse when it is resonant with both the CO(2) and the D(2)O. The depression is attributed to the absorption of the last excitation pulse by the CO(2) produced by the previous two excitation pulses. We believe that the decarboxylation occurs because of energy transfer from multiply excited D(2)O molecules. It may cause a Norrish type I α-cleavage in the AOT that is followed by a Hunsdiecker reaction ß-scission of the radical to form the CO(2). This experiment demonstrates the marriage of infrared induced chemistry and multiresonant coherent multidimensional spectroscopy into a single nonlinear process.