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.

The synthesis of a series of four new compounds containing fluorenyl chromophores is presented, along with the results of spectroscopic and photochemical studies aimed at understanding the two-photon absorption properties and energetics of their electronically excited states. The molecular structures of the compounds were systematically varied to allow comparison of molecules possessing high and low molecular symmetry, short and long alkyl chains, and a fluorenyl conjugated π-system. Solvent-dependent absorption and emission were investigated along with π-conjugation length. Preliminary measurements of two-photon absorption (2PA) using a two-photon fluorescence method indicate that these chromophores exhibit high two-photon absorptivity. A symmetrical molecule (3), possessing a relatively large π-conjugated system, flanked on either side by electron-withdrawing groups (benzothiazole), exhibited a peak 2PA cross section (δ) of 6000 × 10 -50 cm 4 s photon -1 molecule -1 at 600 nm. Excitation anisotropy studies revealed the position of the S 0 f S 1 and S 0 f S 2 electronic transitions. Consistent with quantum mechanical selection rules, the two-photon allowed transition (S 0 f S 2 ) was dominant.

A series of linear, symmetrical, diphenylaminofluorene-based materials are reported. The series investigated was model 9,9-didecyl-2,7-bis(N,N-diphenylamino)fluorene (1), oligomer 9,9-didecyl-N,N-bis(9,9-didecyl-7-N,N-diphenylaminofluoren-2-yl)-N, N-diphenyl-fluorene-2,7-diamine (2), and poly(9,9-didecyl-2,7-diphenylaminofluorene) (3). Structural characterization and photophysical properties, including linear absorption, quantum yields, single photon fluorescence, and two-photon absorption (2PA) spectra, were studied in polar and nonpolar solvents. 2PA spectra were determined by two independent methods:  a nonlinear transmission method employing a femtosecond white-light continuum (WLC) and a two-photon fluorescence (2PF) method. Polymer 3, a low-molecular-weight polymer with fourteen fluorene units, exhibited a very large two-photon absorption cross-section of 17 200 and 6800 × 10-50 cm4 s photon-1 molecule-1 as estimated from the WLC and 2PF methods, respectively.