Katherine J. Schafer

Multiphoton-absorbing organic materials for microfabrication, emerging optical applications and non-destructive three-dimensional imaging

Non-resonant two-photon absorption (TPA) can be defined as the simultaneous absorption of two photons, via a virtual state, in a medium. TPA exhibits a quadratic dependence of absorption on the incident light intensity, resulting in highly localized photoexcitation. Recent developments in the design and synthesis of efficient, stable TPA organic materials are discussed. Microfabrication via two-photon induced free radical polymerization of acrylate monomers and cationic polymerization of epoxide monomers was accomplished using commercially available photoinitiators, and also a custom-made compound possessing high two-photon absorptivity. Two-photon facilitated photoisomerization of a fulgide in solution and in a polymer thin film demonstrated two-photon induced photochromism and its application in interferometric image recording, respectively. Greatly enhanced signal-to-noise ratios and resolution were achieved in the non-destructive three-dimensional two-photon fluorescence imaging of a polymer-coated substrate versus conventional single-photon laser scanning confocal microscopic imaging. Multifunctional TPA organic materials and fabrication of functional microstructures are also discussed. Copyright

Two-photon absorption cross-sections of common photoinitiators

Recent interests in and applications of two-photon absorption (2PA) induced photopolymerization have afforded advanced opportunities to perform three-dimensionally resolved polymerization, resulting in intricate microfabrication and imaging. Many of the reported 2PA-induced polymerizations make use of commercially available photoinitiators, and a key parameter to consider is the two-photon absorption cross-section (δ) of the initiator. To date, there has been no comprehensive investigation of two-photon absorptivity of commercial photoinitiators, though a few studies presenting novel photoinitiators for two-photon polymerization have appeared. Herein, we report the 2PA properties of common, commercially available photoinitiators typically utilized in conventional radiation curing science and technologies, and often used in 2PA-based polymerizations. Z-scan and white-light continuum (WLC) pump–probe techniques were utilized to obtain two-photon absorption cross-sections ( δ). The results for most compounds were found to yield good agreement between the two methods. Most of the photoinitiators studied possess low δ, except Irgacure OXE01, indicating a need for the development of new photoinitiators with improved properties optimized for 2PA applications. A compound prepared in our laboratories exhibits high 2PA and was useful as a two-photon free-radical photoinitiator.

Resonant enhancement of two-photon absorption in substituted fluorene molecules

The degenerate and nondegenerate two-photon absorption (2PA) spectra for a symmetric and an asymmetric fluorene derivative were experimentally measured in order to determine the effect of intermediate state resonance enhancement (ISRE) on the 2PA cross section delta. The ability to tune the individual photon energies in the nondegenerate 2PA (ND-2PA) process afforded a quantitative study of the ISRE without modifying the chemical structure of the investigated chromophores. Both molecules exhibited resonant enhancement of the nonlinearity with the asymmetric compound showing as much as a twentyfold increase in delta. Furthermore, the possibility of achieving over a one order of magnitude enhancement of the nonlinearity reveals the potential benefits of utilizing ND-2PA for certain applications. To model ISRE, we have used correlated quantum-chemical methods together with the perturbative sum-over-states (SOS) expression. We find strong qualitative and quantitative correlation between the experimental and theoretical results. Finally, using a simplified three-level model for the SOS expression, we provide intuitive insight into the process of ISRE for ND-2PA.

Three- and four-photon absorption of a multiphoton absorbing fluorescent probe

The utility of multiphoton excitation processes has been the subject of increased attention due to their potential applications in biophotonics, biology, and medicine through three-dimensional fluorescence imaging and photodynamic therapy. Evidence of this are the multiple applications of two-photon absorption (2PA) in fluorescence spectroscopy and 3D imaging over the last several years because of its large effective Stoke¿s Shift and high spatial resolution.[l,2] However, because the irradiation penetration depth of 2PA is limited in medical and biological applications due to the unwanted absorption and scattering when two red photons are used, the scientific communib recently started to explore higher order absorption processes at longer wavelengths such as three- (3PA) and four-photon absorption (IPA) that minimize the scattered light losses, and reduce the unwanted linear absorption in the living organism transparency window.

Spectral properties of several fluorene derivatives with potential as two-photon fluorescent dyes

Investigations of the absorption, steady-state fluorescence, excitation and excitation anisotropy properties of several fluorene derivatives, (7-benzothiazol-2-yl-9,9-didecylfluoren-2-yl)-diphenylamine, 9,9-didecyl-2,7-bis-(N,N-diphenylamino)fluorene and {4-[2-(7-diphenylamino-9,9-diethylfluoren-2-yl)vinyl]phenyl}phosphoric acid diethyl ester, in liquid solutions have been conducted. Spectral characteristics of these compounds, including fluorescence quantum yields, were measured in acetonitrile, methylene chloride, tetrahydrofuran and hexane at room temperature. Excitation anisotropy spectra provided a means to determine the nature of the short wavelength absorption bands as an electronic transition into a higher excited singlet state. It was found that excitation spectra in the short wavelength region do not correspond to the absorption bands that are correlated with the wavelength dependence of the fluorescence quantum yields. Major reasons of such spectral behavior are discussed.

Photostability of a series of two-photon absorbing fluorene derivatives

Abstract The photochemical stability of a series of two-photon absorbing (TPA) fluorene derivatives was investigated in air- and N2-saturated acetonitrile (ACN) at room temperature. The quantum yields of the photoreactions,Φ, were determined at various concentrations of the fluorene derivatives, oxygen concentration of the solvent, and irradiation wavelength. The absorption and fluorescence spectra of the photoproducts, corresponding to different excitation conditions, were analyzed. Photooxidation and electron transfer processes are proposed as photobleaching mechanisms for the fluorene derivatives in ACN. The relatively low photochemical quantum yields (Φ∼10−4) make the derivatives particularly promising for linear and nonlinear optical applications.

Steady-State Spectroscopic and Fluorescence Lifetime Measurements of New Two-Photon Absorbing Fluorene Derivatives

Steady-state excitation anisotropy, lifetimes, and time-resolved emission spectra of new 2-photon absorbing fluorene derivatives were measured in aprotic solvents at room temperature. Excitation anisotropy spectra in viscous silicon oil allowed the determination of the spectral position of three electronic transitions S0 → S1, S0 → S2, S0 → S3 (Si, i = 1, 2, 3 are the singlet electronic states) and the angles (≅ 30°) between absorption S0 → S1 and emission S1 → S0 dipole moments for the first electronic transition. Solvate relaxation processes in the first excited state of the investigated fluorene molecules affect the lifetimes of these states, τ1, so that experimental values of τ1 do not correspond to those calculated by Strickler and Berg theory. The influence of the molecular concentration on the fluorescence quantum yields and τ1 have been investigated.

One- and two-photon fluorescence anisotropy of selected fluorene derivatives.

The steady-state excitation anisotropy spectra of fluorene derivatives were measured in viscous solvents, under the one- and two-photon excitation, over a broad spectral range (UV–Visible). The orientation of their absorption transition moments for the first, S0→S1, and second, S0→S2, excited states were determined. It was shown experimentally that a decrease in the angle between S0→S1and S0→S2 transitions corresponded to an increased value of two-photon absorption (2PA) cross section for these molecules. Two-photon excitation anisotropy was nearly constant over the spectral region investigated (in contrast to one-photon excitation anisotropy spectra) and can be roughly explained by a simple model of 2PA based on the single intermediate state approximation. For comparison, the same trend in two-photon excitation anisotropy was observed for Rhodamine B inglycerol.

One- and two-photon photostability of 9,9-didecyl-2,7-bis(N,N-diphenylamino)fluorene

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) × 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.

Photophysical characterization of 2,9-bis(7-benzothiazole-9,9'-didecylfluoren-2-yl)perylene diimide: a new standard for steady-state fluorescence anisotropy

Abstract The absorption, fluorescence excitation and emission spectra have been obtained in solution for 2,9-bis(7-benzothiazole-9,9-didecylfluoren-2-yl)perylene diimide. Efficient resonance energy transfer from the fluorenyl group to the perylene ring center was observed. Interestingly, fluorescence emission was detected from the second excited electronic state of the perylene ring system. Fluorescence excitation anisotropy spectra obtained at room temperature exhibited a parallel orientation of the main absorption and emission band transition moments for the perylene-based dye in CH 2 Cl 2 . The value of excitation fluorescence anisotropy for the perylene dye in solution approached the theoretical maximum limit ( r ≈0.4), and indicated that the rotational correlation time exceeded the lifetime of the first excited state. These results provide the basis for using this unique compound as an anisotropy reference standard.