Two-dimensional electronic coherence spectroscopy (ECS) is an important method to study the coupling between distinct optical modes of a material system. Such studies often involve excitation using a sequence of phased ultrashort laser pulses. In conventional approaches, the delays between pulse temporal envelopes must be precisely monitored or maintained. Here, we introduce a new experimental scheme for phase-selective nonlinear ECS, which combines acousto-optic phase modulation with ultrashort laser excitation to produce intensity modulated nonlinear fluorescence signals. We isolate specific nonlinear signal contributions by synchronous detection, with respect to appropriately constructed references. Our method effectively decouples the relative temporal phases from the pulse envelopes of a collinear train of four sequential pulses. We thus achieve a robust and high signal-to-noise scheme for phase-selective ECS to investigate the resonant nonlinear optical response of photoluminescent systems. We demonstrate the validity of our method using a model quantum three-level system-atomic Rb vapor. Moreover, we show how our measurements determine the resonant complex-valued third-order susceptibility.

By applying a phase-modulation fluorescence approach to 2D electronic spectroscopy, we studied the conformation-dependent exciton coupling of a porphyrin dimer embedded in a phospholipid bilayer membrane. Our measurements specify the relative angle and separation between interacting electronic transition dipole moments and thus provide a detailed characterization of dimer conformation. Phase-modulation 2D fluorescence spectroscopy (PM-2D FS) produces 2D spectra with distinct optical features, similar to those obtained using 2D photon-echo spectroscopy. Specifically, we studied magnesium meso tetraphenylporphyrin dimers, which form in the amphiphilic regions of 1,2-distearoyl-sn-glycero-3-phosphocholine liposomes. Comparison between experimental and simulated spectra show that although a wide range of dimer conformations can be inferred by either the linear absorption spectrum or the 2D spectrum alone, consideration of both types of spectra constrain the possible structures to a "T-shaped" geometry. These experiments establish the PM-2D FS method as an effective approach to elucidate chromophore dimer conformation.fluorescence-detected 2D photon echo | nonlinear spectroscopy | supramolecular conformation | excitonically coupled dimer T he ability to determine three-dimensional structures of macromolecules and macromolecular complexes plays a central role in the fields of molecular biology and material science. Methods to extract structural information from experimental observations such as X-ray crystallography, NMR, and optical spectroscopy are routinely applied to a diverse array of problems, ranging from investigations of biological structure-function relationships to the chemical basis of molecular recognition.In recent years, two-dimensional optical methods have become well established to reveal incisive information about noncrystalline macromolecular systems-information that is not readily obtainable by conventional linear spectroscopic techniques. Twodimensional optical spectroscopy probes the nanometer-scale couplings between vibrational or electronic transition dipole moments of neighboring chemical groups, similar to the way NMR detects the angstrom-scale couplings between adjacent nuclear spins in molecules (1). For example, 2D IR spectroscopy probes the couplings between local molecular vibrational modes and has been used to study the structure and dynamics of mixtures of molecular liquids (2), aqueous solutions of proteins (3), and DNA (4). Similarly, 2D electronic spectroscopy (2D ES) probes correlations of electronic transitions and has been used to study the mechanisms of energy transfer in multichromophore complexes. Such experiments have investigated the details of femtosecond energy transfer in photosynthetic protein-pigment arrays (5-8), conjugated polymers (9), and semiconductors (10, 11).Following the examples established by 2D NMR and 2D IR, 2D ES holds promise as a general approach for the structural analysis of noncrystalline macromolecular systems, albeit for the nanometer length scales ...

Two-dimensional fluorescence spectroscopy (2D FS) is applied to determine the conformation and femtosecond electronic population transfer in a dimer of magnesium meso tetraphenylporphyrin. The dimers are prepared by self-assembly of the monomer within the amphiphilic regions of 1,2-distearoyl-sn-glycero-3-phosphocholine liposomes. A theoretical framework to describe 2D FS experiments is presented, and a direct comparison is made between the observables of this measurement and those of 2D electronic spectroscopy (2D ES). The sensitivity of the method to varying dimer conformation is explored. A global multivariable fitting analysis of linear and 2D FS data indicates that the dimer adopts a "bent T-shaped" conformation. Moreover, the manifold of singly excited excitons undergoes rapid electronic dephasing and downhill population transfer on the time scale of ∼95 fs. The open conformation of the dimer suggests that its self-assembly is favored by an increase in entropy of the local membrane environment.

Polyethylenimine (PEI) polymers have become increasingly utilized for a myriad of applications including self-decontaminating materials and nonviral gene transfection. While the bulk properties of PEIs have been studied in detail, their surfacespecific structure/behavior remain unexplored. Here, we report the effects of relative humidity on the surface structure of linear polyethylenimine (LPEI), as investigated by vibrational sum frequency generation (VSFG) spectroscopy. Results show that the surface structure of (as prepared) anhydrate LPEI is highly dependent on relative humidity. As the relative humidity is varied from 0% to 75%, surface spectra of LPEI in the C−H and N−H stretching regions reveal multiple crystalline and amorphous states, including the gel-phase amorphous state that has previously only been observed in appreciable quantities above LPEI's upper critical solution temperature (64°C). Utilizing DFT calculations, we have assigned large characteristic frequency shifts (∼50 cm −1 ) of LPEI anhydrate crystalline methylene modes to the Bohlmann effect, which is the delocalization of the nitrogen lone electron pair causing a weakening of the C−H bonds of methylene moieties adjacent to the amine functionality. Similar frequency shifts (∼20 cm −1 ) observed in the hydrated crystalline forms are likely due to intermolecular interactions mediated by hydrogen bonding within the LPEI/water matrix. ■ INTRODUCTIONPolyethylenimines (PEIs) have found widespread utility in numerous fields including chemical and biological sensing, 1 carbon dioxide capture, 2 and electrical energy storage. 3 Recently, some of the more interesting and impactful applications are based around PEI's biocompatibility, where it is being investigated for its use as a nonviral vector for gene transfection, 4−8 enzyme or protein stabilization, 9,10 and cell culture. 11 Additionally, due to the generally high concentration of reactive amine moieties present in PEI materials, they are also of interest for reactive/self-decontaminating technologies for protection against nerve-type chemical warfare agents (CWAs), wherein amine groups hydrolyze CWAs to produce less lethal or nonlethal byproducts. Although surface reorganization of various polymeric materials due to environmental adsorbates (water and organics) has been established, 12−16 the characterization of self-decontaminating materials' environmentally mediated surface reorganization and its effects on surface functionality and reactivity is largely absent. To these ends, developing a surface-specific, molecular level understanding of the behavior of polyethylenimine films in varied environmental conditions will provide valuable insights into the molecular-level details that mediate the functionality of these materials for numerous applications.In the present study, we employ linear polyethylenimine (LPEI) as a model system for understanding the effects of relative humidity, and therefore hydrogen bonding (amine− amine, water−amine, and water−water hydrogen bonding), on the surface stru...

The kinetics of bio-molecular conformational transitions can be studied by two-dimensional (2D) magnetic resonance and optical spectroscopic methods. Here we apply polarization-modulated Fourier imaging correlation spectroscopy (PM-FICS) to demonstrate a new approach to 2D optical spectroscopy. PM-FICS enables measurements of conformational fluctuations of fluorescently labeled macromolecules on a broad range of time scales (10 -3 -10 2 s). We examine the optical switching pathways of DsRed, a tetrameric complex of fluorescent protein subunits. An analysis of PM-FICS coordinate trajectories, in terms of 2D spectra and joint probability distributions, provides detailed information about the transition pathways between distinct dipole-coupled DsRed conformations.