ABSTRACT
The nonlinear transmission and the excited-state absorption spectra of three fluorene derivatives exhibiting large two-photon absorptivity were measured by the third harmonic of a picosecond Nd:YAG laser. We analyzed their capability for exhibiting stimulated emission in polar solvents and found that asymmetrical fluorene compounds with a diphenylamino substituent exhibited large Stokes shifts (approximately 8000 cm(-1)), high quantum yields (approximately 0.9-1.0), and no optical gain over their entire fluorescence spectral region. In contrast, a symmetrical fluorene derivative with vinylphenylbenzothiazole substituents in positions 2 and 7 underwent lasing under one-photon excitation by use of picosecond pulsed irradiation.
ABSTRACT
High-order multiphoton excitation processes are becoming a reality for fluorescence imaging and phototherapy treatment because they afford minimization of scattered light losses and a reduction of unwanted linear absorption in the living organism transparency window, making them less susceptible to photodamage, while improving the irradiation penetration depth and spatial resolution. We report the four-photon-excited fluorescence emission of (7-benzothiazol-2-yl-9,-didecylfluoren-2-yl)diphenylamine in hexane and its four-photon absorption cross section sigma4' = 8.1 x 10(-109) cm8 s3 photon(-3) for the transition S0 --> S1 when excited at 1600 nm with a tunable optical parametric generator (OPG) pumped by picosecond laser pulses. When pumped at 1200 nm, three-photon absorption was observed, corresponding to the same transition.
Subject(s)
Fluorenes/radiation effects , Fluorescence Resonance Energy Transfer/methods , Fluorescent Dyes/chemical synthesis , Fluorescent Dyes/radiation effects , Microscopy, Fluorescence, Multiphoton/methods , Thiazoles/radiation effects , Benzothiazoles , Dose-Response Relationship, Radiation , LasersABSTRACT
The quantum yields of the photochemical reactions of 9,9-didecyl-2,7-bis(N,N-diphenylamino)fluorene have been determined in hexane and CH2Cl2 under one-photon (linear) and near-IR two-photon (nonlinear) absorption conditions. The photochemical decomposition proceeds by a first-order reaction and is independent of the type of excitation (one- or two-photon). In hexane solution, the quantum yields of the photoreactions are in the range (2-5) x 10(-4) and increase dramatically to 10(-2) in CH2Cl2. The predominant mechanisms of the photoreactions and the photoproducts products which result were investigated via UV-visible absorption, fluorescence, and excitation anisotropy spectral methods.
