Absorption and Quantum Yield of Single Conjugated Polymer Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) Molecules

Abstract

We simultaneously measured the absorption and emission of single conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) molecules in a poly(methyl methacrylate) (PMMA) matrix using near-critical xenon to enhance the photothermal contrast for direct absorption measurements. We directly measured the number of monomers and the quantum yield of single conjugated polymer molecules. Simultaneous absorption and emission measurements provided new insight into the photophysics of single conjugated polymers under optical excitation: quenching in larger molecules is more efficient than in smaller ones. Photoinduced traps and defects formed under prolonged illumination lead to decrease of both polymer fluorescence and absorption signals with the latter declining slower.

Photobleaching of YOYO-1 in super-resolution single DNA fluorescence imaging

“…The bleaching profile ( Supporting Information File 1 , Figure S1) is uniform in the beam spot indicating a relatively uniform light distribution of the laser after the flat-top beam shaper, consistent with a relatively even photocount distribution in a typical fluorescent image. Note that the actual photons emitted from a molecule is a function of the measured photocounts, the EM gain (fixed at 200× during all measurements), and the photon collection efficiency of the optical pathway [ 47 – 48 ]. The number of photocounts is at the linear response region of the EMCCD under our imaging conditions [ 48 ].…”

“…The two-photon absorption probability can be estimated by comparing the number of photons absorbed by the dye per unit time to the fluorescence lifetime of the dye. The number of photons absorbed can be estimated using the following equation [ 47 ]: Photon absorption = σ P / E ph , where σ is the absorption cross section, P is the laser power density and E ph is the energy of a photon. The photons absorbed by each YOYO-1 molecule at the highest laser power studied (62 W cm −2 ) is calculated to be 24 photons/ms using the energy of a photon at 473 nm (4.2 × 10 −19 J/photon) and calculating the absorption cross section, 1.64 × 10 −16 cm 2 that is calculated from the extinction coefficient, 9.89 × 10 7 cm 2 mol −1 (≈10 5 M −1 cm −1 ) [ 11 ].…”

Nonlinear spectroscopy of plasmonic nanoparticles

“…This scheme is special compared to all other processes that we discuss in this review as pump-and probe-field interact with different materials and as the effect on the probe beam is mainly dispersive. In the first experiments, the nano-object has been a plasmonic nanoparticle, but since then also semiconductor nanocrystals [56], molecules [57] and polymers [58] have been investigated. The surrounding medium can be water or a polymer, or, as shown recently, a liquefied gas under high pressure near its triple point, as in this situation the temperature-induced variation of the refractive index is largest [58].…”

“…In the first experiments, the nano-object has been a plasmonic nanoparticle, but since then also semiconductor nanocrystals [56], molecules [57] and polymers [58] have been investigated. The surrounding medium can be water or a polymer, or, as shown recently, a liquefied gas under high pressure near its triple point, as in this situation the temperature-induced variation of the refractive index is largest [58]. The combination of an infrared pump beam and a visible probe beam leads to an optical resolution defined by the visible wavelength in combination with spectroscopic information of the infrared wavelength [59].…”

Drumming up single-molecule beats

“…Increased responsivity can be achieved with materials that possess a higher thermooptic coefficient (10). Single-molecule detection was achieved with glycerol immersion (11), and substantially higher responsivities have been demonstrated through the use of thermotropic liquid crystals (12,13) and supercritical Xe (14,15) while probing individual particles and polymers, respectively. Another way of detecting the shifted refractive index is to monitor the response of an optical resonator.…”