Photophysics and Mechanisms of Intramolecular Electronic Energy Transfer in Bichromophoric Molecular Systems: Solution and Supersonic Jet Studies.

A unified treatment of dipole–dipole excitation transfer in disordered systems is presented for the cases of direct trapping (DT) in two‐component systems and donor–donor transfer (DD) in one‐component systems. Using the two‐particle model proposed by Huber we calculate the configurational average of Gs(t), the probability of finding an initially excited molecule still excited at time t. For the isotropic three‐dimensional case treated by Huber excellent correspondence is found with the previously reported infinite diagrammatic approximation. The anisotropy of the dipole–dipole interaction is included in the averaging procedure. Two regimes of orientational mobility are considered: the dynamic and static limit, rotations being much faster or slower, respectively, than the energy transfer. The following geometrical distributions are investigated: (a) Infinite systems of one, two, and three dimensions which lead to Forster‐like decays. Two orientational distributions are considered for monolayers: dipoles c...

Excitation transfer in disordered two‐dimensional and anisotropic three‐dimensional systems: Effects of spatial geometry on time‐resolved observables

We have built a new apparatus to time resolve ultrafast fluorescence following ultraviolet excitation. A synchronously pumped dye laser produces optical pulses of 1‐ps or 70‐fs full‐width half‐maximum, depending upon dyes and optical configuration. These pulses are amplified at a 8.2‐kHz repetition rate using a copper vapor laser‐pumped multipass amplifier. The resulting amplified laser pulses are frequency doubled to obtain ultrafast pulses in the ultraviolet. This ultraviolet light is used to electronically excite a sample; the resulting fluorescence is time resolved using fluorescence upconversion as the optical gating technique. A minimum 300‐fs full‐width half‐maximum instrument response function is obtained. After a brief introduction, we discuss the principles involved in this method of time resolving fluorescence. We review special considerations for femtosecond laser experimentation, and discuss the construction of our apparatus. Finally, as an example, we show how this system can be used to stud...

Ultrafast emission spectroscopy in the ultraviolet by time‐gated upconversion

We present a subpicosecond time resolved polarization selective transient grating (TG) investigation of pentylcyanobiphenyl (5CB) in its isotropic liquid phase. This system shows complex molecular dynamics with various contributions to the TG signal. With an excitation wavelength λexc=665 nm one induces an anisotropy in the sample from the electronic and nuclear Kerr effects. Exciting with λexc=575 nm generates an excited state grating because of two photon absorption. Solvent relaxation around the excited molecule on a few ps time scale is observed because it spectrally shifts the excited state–excited state transition which is monitored by the TG signal. In addition, radiationless relaxation leads to local heating of the sample in a spatial pattern which mimics the optical interference pattern which established the TG. The heating generates an acoustic standing wave. It is demonstrated that with a single TG setup, polarization selection allows separation and identification of all of the above‐mentioned ...

Analysis of complex molecular dynamics in an organic liquid by polarization selective subpicosecond transient grating experiments

Picosecond transient thermal phase grating spectroscopy: A new approach to the study of vibrational energy relaxation processes in proteins

A detailed experimental examination of the dynamics of energy transport and trapping in two component systems, using rhodamine 6G (R6G) as the donor and malachite green (MG) as the trap in both glycerol and ethanol solvents, is presented. The experiments were performed using fluorescence mixing and ground state recovery techniques providing temporal resolution of ∼50 ps. Samples ranging from high trap‐low donor concentrations (the Forster limit) to the opposite regime of high donor and low trap concentrations, were studied. These results were compared with no adjustable parameters to the recent theoretical work of Loring, Andersen, and Fayer (LAF). The excellent agreement between theory and experiment over the entire donor‐trap concentration range confirms the theoretical results of LAF and yields a comprehensive description of excited‐state dynamics in solution. A variety of dynamic properties are calculated using the LAF theory and the measured parameters associated with R6G‐MG system.

Electronic excited state transport and trapping in disordered systems: Picosecond fluorescence mixing, transient grating, and probe pulse experiments

The previously reported coherent photoacoustic method for measuring weak absorptions is extended to excited-state-excited-state spectroscopy for which conventional acoustic and thermal lensing techniques are greatly reduced in sensitivity. The method involves a picosecond pulse sequence with four pulses. The first pulse populates the excited state. Then, a pair of time coincident pulses cross in the sample, making an optical interference pattern and generating an acoustic diffraction grating. The amplitude of the acoustic grating is proportional to the excited-state-excited-state absorption cross section. The amplitude is measured by the Bragg diffraction of the fourth pulse from the acoustic grating. This method can be readily applied to liquids, glasses, or crystals. Excited-state absorption at 532 nm by the SI state of pentacene in p-terphenyl host crystals is examined at room temperature and 1.4 K. The results are used in addressing the mechanism for acoustic diffraction of a probe beam nearly resonant with a strong and narrow ground-state transition, e.g., pentacene in p-terphenyl at 1.4 K. It is demonstrated that there is a density wave induced spectral shift contribution to the acoustic grating which results in greatly increased diffraction efficiency near resonance.

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A coherent photoacoustic approach to excited-state-excited-state absorption spectroscopy: application to the investigation of a near-resonant contribution to ultrasonic diffraction

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Electronic excited state transport and trapping in disordered systems: Picosecond fluorescence mixing, transient grating, and probe pulse experiments

A coherent photoacoustic approach to excited-state-excited-state absorption spectroscopy: application to the investigation of a near-resonant contribution to ultrasonic diffraction

A coherent photoacoustic approach to excited-state-excited-state absorption spectroscopy: application to the investigation of a near-resonant contribution to ultrasonic diffraction