Joel M. Hales

Design of Polymethine Dyes with Large Third-Order Optical Nonlinearities and Loss Figures of Merit

Dying by Design To make optical-switching applications a reality, losses from scattering and other absorption processes have to be minimized. Hales et al. (p. 1485, published online 18 February; see the Perspective by Haque and Nelson) present a strategy to explore the refraction and absorption properties of a group of cyanine dyes for designing materials that have properties corresponding to technologically interesting telecommunications windows. The optical properties of the cyanine molecule was controlled by adding heavy chalcogen atoms (selenium) into the end groups of the molecular structure. While producing a series of molecules meeting criteria for feasible application, the work also demonstrates a route to improve the performance of nonlinear optical materials. Nonlinear optical materials are designed and characterized for potential applications in all-optical switching. All-optical switching applications require materials with large third-order nonlinearities and low nonlinear optical losses. We present a design approach that involves enhancing the real part of the third-order polarizability (γ) of cyanine-like molecules through incorporation of polarizable chalcogen atoms into terminal groups, while controlling the molecular length to obtain favorable one- and two-photon absorption resonances that lead to suitably low optical loss and appreciable dispersion enhancement of the real part of γ. We implemented this strategy in a soluble bis(selenopyrylium) heptamethine dye that exhibits a real part of γ that is exceptionally large throughout the wavelength range used for telecommunications, and an imaginary part of γ, a measure of nonlinear loss, that is smaller by two orders of magnitude. This combination is critical in enabling low-power, high-contrast optical switching.

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.

Kinetically Controlled Photoinduced Electron Transfer Switching in Cu(I)-Responsive Fluorescent Probes

Copper(I)-responsive fluorescent probes based on photoinduced electron transfer (PET) switching consistently display incomplete recovery of emission upon Cu(I) binding compared to the corresponding isolated fluorophores, raising the question of whether Cu(I) might engage in adverse quenching pathways. To address this question, we performed detailed photophysical studies on a series of Cu(I)-responsive fluorescent probes that are based on a 16-membered thiazacrown receptor ([16]aneNS(3)) tethered to 1,3,5-triarylpyrazoline-fluorophores. The fluorescence enhancement upon Cu(I) binding, which is mainly governed by changes in the photoinduced electron transfer (PET) driving force between the ligand and fluorophore, was systematically optimized by increasing the electron withdrawing character of the 1-aryl-ring, yielding a maximum 29-fold fluorescence enhancement upon saturation with Cu(I) in methanol and a greater than 500-fold enhancement upon protonation with trifluoroacetic acid. Time-resolved fluorescence decay data for the Cu(I)-saturated probe indicated the presence of three distinct emissive species in methanol. Contrary to the notion that Cu(I) might engage in reductive electron transfer quenching, femtosecond time-resolved pump-probe experiments provided no evidence for formation of a transient Cu(II) species upon photoexcitation. Variable temperature (1)H NMR experiments revealed a dynamic equilibrium between the tetradentate NS(3)-coordinated Cu(I) complex and a ternary complex involving coordination of a solvent molecule, an observation that was further supported by quantum chemical calculations. The combined photophysical, electrochemical, and solution chemistry experiments demonstrate that electron transfer from Cu(I) does not compete with radiative deactivation of the excited fluorophore, and, hence, that the Cu(I)-induced fluorescence switching is kinetically controlled.

Metalloporphyrin polymer with temporally agile, broadband nonlinear absorption for optical limiting in the near infrared.

A lead bis(ethynyl)porphyrin polymer possesses strong nonlinear absorption with unprecedented spectral/temporal coverage as a result of broad, overlapping two-photon and excited-state absorption bands with favorable excited-state dynamics. Consequently, this material exhibits effective optical limiting over a range of about 500 nm in the near infrared (ca. 1050 - 1600 nm) and for laser pulsewidths spanning from 75 fs to 40 ns. Introduction of the material in a waveguide device geometry results in a strong optical limiting response.

Porphyrin Dimer Carbocations with Strong Near Infrared Absorption and Third-Order Optical Nonlinearity†

Materials with large ultrafast third-order nonlinearities at near-infrared (NIR) wavelengths are required for all-optical signal processing. Telecommunications systems operate in this spectral region, and all-optical switching and wavelength conversion devices will lead to faster signal processing than can be achieved by current electronic methods, thus providing greater bandwidths and data-transfer rates. Dyes with large optical nonlinearities will be used in a new generation of devices requiring modest drive powers and short path lengths, and in ultrafast image-correlation security systems. Herein we present a family of porphyrin dimer carbocations 1a–c which show intense NIR absorption maxima at 1100–1340 nm and large ultrafast third-order nonlinearities at 1550 nm. It is well known that carbocations tend to be more electronically delocalized than neutral conjugated hydrocarbons, and many common dyes, such as cyanines and rhodamines, can be regarded as resonance-stabilized carbocations. Cyanines are interesting in that they have large negative third-order nonlinearities at energies below the S0–S1 gap. Furthermore, the optical nonlinearity of cyanine dyes scales with a high-order power of the conjugation length, suggesting that extended cyanines could provide extremely large nonlinearities. However, extended cyanines can undergo a symmetry-breaking process which reduces electronic delocalization and nonlinearity. Hales et al. recently reported that extended bis-(dioxaborine) polymethine 2 sustains delocalization at lengths of approximately 14 conjugated bonds, beyond that of conventional cyanines. The nature of the terminal groups evidently plays a key role in achieving efficient electronic delocalization in long chromophores. The idea behind this work is to synthesize a cyaninelike dye with two-dimensionally delocalized porphyrin macrocycles as terminals (Figure 1), thus accessing greater

Experiment and analysis of two-photon absorption spectroscopy using a white-light continuum probe

We present an experimental technique along with the method of data analysis to give nondegenerate two-photon absorption (2PA) spectra. We use a femtosecond pump pulse and a white-light continuum (WLC) probe to rapidly generate the 2PA spectra of a variety of materials. In order to analyze data taken with this method, the spectral and temporal characteristics of the WLC must be known, along with the linear dispersion of the sample. This allows determination of the temporal walk-off of the pump and probe pulses as a function of frequency caused by group-velocity mismatch. Data correction can then be performed to obtain the nonlinear losses. We derive an analytical formula for the normalized nonlinear transmittance that is valid under quite general experimental parameters. We verify this on ZnS and use it for the determination of 2PA spectra of some organic compounds in solution. We also compare some of the data on organics with two-photon fluorescence data and find good agreement.

Dispersion of nonlinear refraction and two-photon absorption using a white-light continuum Z-scan

We use a white-light continuum (WLC) Z-scan technique to measure the degenerate two-photon absorption spectrum and associated dispersion of the nonlinear refraction in ZnSe. The spectral components of the WLC source are separated by using a narrow band variable filter to minimize nondegenerate nonlinearities. We observe a change in sign of the ultrafast nonlinear refractive index around 0.7 of the bandgap energy as predicted by theory.

Excited-state absorption dynamics in polymethine dyes detected by polarization-resolved pump–probe measurements

Polarization-resolved excitation-probe measurements are performed for a new series of polymethine dyes in several solvents and a polyurethane acrylate elastopolymer. We describe our experimental studies and give an analysis of the nature of the rotational motions of excited molecules and orientation of the excited-state transitions relative to transitions from the ground state.

Fluorene-based fluorescent probes with high two-photon action cross-sections for biological multiphoton imaging applications

Two-photon fluorescence microscopy is a powerful tool for the study of dynamic cellular processes and live-cell imaging. Many commercially available fluorescent probes have been used in multiphoton-based imaging studies despite exhibiting relatively low two-photon absorption cross-section values in the tunability range of ultrafast Ti:sapphire lasers commonly used in multiphoton microscopy imaging. Furthermore, available fluorophores may be plagued with low fluorescence quantum yield and/or photoinstability (i.e., photobleaching) on exposure to the high peak power and photon density provided by the ultrafast laser source. To address the demand for better performing dyes, we prepare fluorophores tailored for multiphoton imaging. These fluorophores are based on the fluorene ring system, known to exhibit high fluorescence quantum yield (>0.7) and high photostability. Furthermore, an amine-reactive fluorescent probe for the covalent attachment onto amine-containing biomolecules is also prepared. Epi-fluorescence and two-photon fluorescence microscopy images of H9c2 rat cardiomyoblasts stained with an efficient two-photon absorbing fluorene fluorophore is demonstrated. Additionally, single-photon spectral characteristics of the amine-reactive fluorophore, as well as the two-photon absorption cross sections of its model adduct in solution, and spectral characterization of a bovine serum albumin (BSA) as a model bioconjugate are presented.