I-Ren Lee

Organic Optical Limiter with a Strong Nonlinear Absorptive Response

Molecules with weak ground-state absorption that form strongly absorbing excited states can be used in optical limiters, which can protect sensors or human eyes from optical damage. Phthalocyanine complexes bearing heavy atoms or paramagnetic groups or in solvents containing heavy atoms show optical limiting enhanced by excited triplet-state absorption. A nonhomogeneous distribution of indium tetra(tert-butyl)phthalocyanine chloride along the beam path substantially enhances the excited-state absorption, yielding an optical limiter with a linear transmittance of 0.70 that can attenuate 8-nanosecond, 532-nanometer laser pulses by factors of up to 540.

Two-photon absorption and broadband optical limiting with bis-donor stilbenes

Large two-photon absorptivities are reported for symmetrical bis-donor stilbene derivatives with dialkylamino or diphenylamino groups. These molecules exhibit strong optical limiting of nanosecond pulses over a broad spectral range in the visible. Relative to bis(di-n-butylamino)stilbene, bis(diphenylamino)stilbene exhibits a 90-nm red shift of its optical limiting band but only a minimal shift of ~13 nm of its lowest one-photon electronic absorption band. Mixtures of these compounds offer an unprecedented combination of broad optical limiting bandwidth and high linear transparency.

Ultrafast dynamics of gas-phase excited-state intramolecular proton transfer in 1-hydroxy-2-acetonaphthone

Abstract The excited-state dynamics of 1-hydroxy-2-acetonaphthone (HAN) was studied in the gas phase using femtosecond time-resolved multiphoton ionization spectroscopy. Following femtosecond excitation to its S 1 state, HAN was found to exhibit a biexponential decay behavior which can be consistently interpreted in terms of rapid excited-state intramolecular proton transfer (ESIPT) followed by other slower decay channels. The results revealed that ESIPT in HAN (S 1 ) occurs in ∼60–85 ps in the energy range studied and suggested the existence of an energy barrier to tautomerization.

Direct Structural Determination of Conformations of Photoswitchable Molecules by Laser Desorption-Electron Diffraction

Anfractuous paths: Electron diffraction reveals the involvement of multiple structures in the complex photochemistry of photoswitchable nitro-substituted 1,3,3-trimethylindolinobenzospiropyran. The spiropyran-to-merocyanine isomerization due to ring opening produces primarily the cis–trans–cis structure (see picture; red O, blue N, yellow C), while competing nonradiative pathways lead to other structures, namely the closed forms in their triplet and singlet ground states.

Two-Photon Absorbing Organic Chromophores for Optical Limiting

Strong optical limiting and large two-photon absorptivities are reported for a class of bisdonor diphenylpolyene derivatives with varying polyene bridge lengths. These molecules exhibit strong optical limiting using nanosecond pulses over a broad spectral range. Bis(diphenylamino)stilbene exhibits a 90 nm red shift of its optical limiting band, and only a minimal shift of about 13 nm of its lowest one-photon electronic absorption band relative to bis(di-n-butylamino)stilbene. This suggests a potential for broadband optical limiting with high transparency using mixtures of such compounds. Pulse width dependent nonlinear transmission measurements suggest that two-photon pumped excited-state absorption contributes significantly to the limiting of nanosecond pulses.

Structural Dynamics of Free Amino Acids in Diffraction

The amino acid tryptophan exhibits complex structural changes both in the ground state, owing to multiple conformations, and upon excitation, because of the involvement of nonradiative pathways. The first report of structural dynamics using combined ultrafast electron diffraction and laser desorption methods is presented.

Direct Observation of the Primary Bond-Twisting Dynamics of Stilbene Anion Radical

The anion radicals of stilbenes in the collisionless isolated phase were synthesized by electron attachment, and their dynamics were observed in real time on the femtosecond time scale. The observed coherent vibrational motion (approximately 42 cm(-1)) and the primary bond-twisting dynamics (approximately 650 fs) for the D2 state are on a vastly different time scale from that reported (approximately 10 ns) in solution.

Gas-phase femtosecond transient absorption spectroscopy

A setup for measuring femtosecond transient absorption in the gas phase is reported. The apparatus is based on a 1kHz amplified Ti:sapphire laser system and measures gas-phase transient absorption using a shot-to-shot normalization scheme with background subtraction. We have used this setup to examine the wave packet dynamics of the I2 B state in the vapor phase as a benchmark. The results are consistent with those reported by other groups using indirect transient absorption techniques.