We investigate the influence of the heteroatom on the electronic and photophysical properties of four conjugated polymers based on 3,7-didodecyl-2,6-di(thiophen-2-yl)benzo[1,2-b:4,5-b′]difuran (BDF) as the donor and 3,6-di(thiophen-2-yl)-1,4-diketopyrrolo[3,4-c]pyrrole (TDPP) or 3,6-di(2-furanyl)-1,4-diketopyrrolo[3,4-c]pyrrole (FDPP) as the acceptor. The polymers with a furan as the linker showed higher extinction coefficients than their thiophene counterparts. Ultrafast fluorescence decay showed that the exciton relaxation process is affected by the type of linker in these conjugated polymers. Theoretical calculations showed that the polymers with a furan as the linker are more planar than their thiophene analogues. Also, theoretical calculation showed that the polymers with a thiophene as the linker have larger transition dipole moments. The two-photon absorption cross sections (TPACS) of the polymers with a furan as the linker were larger than their thiophene polymer analogues. These results suggest that the polymers with a furan as the linker have higher charge transfer character than their thiophene polymers analogues. The photovoltaics performance for these polymers are correlated with their optical properties. These results suggest that furan-derivatives are good candidates for synthetic exploration for long-range energy transport materials in photovoltaic applications.

New hyperbranched polymers have been investigated to provide new structure–function relationships necessary for electrical and optical applications. We can take advantage of long-range delocalization in these structures for energy storage applications. This is accomplished by obtaining high dielectric constants. In this Article, we demonstrate the need for developing high dielectric hyperbranched polymers by first investigating a ceramic/polymer hybrid system and then studying the design criteria for these hyperbranched systems using detailed optical and electronic characterization techniques. Provided in this contribution are the energy storage results with ion-doped polyaniline (PANI) polymers. An enhancement in the dielectric constant emerged from strong long-range polaron delocalization and the mechanism of a hyperelectronic polarization in these polymer systems. A copper phthalocyanine (CuPc) core was selected to build novel hyperbranched polymers to investigate their energy storage and optical properties. We report the results of these hyperbranched polymers, which exhibited high dielectric constants and low dielectric losses. Detailed structure–function relationships were carried out to probe the polaron delocalization mechanism. An outstanding result of a new hyperbranched polymer showed the greatest energy storage capacity of 7.97 J cm–3. These results provide new insights into the design of new organic macromolecules for energy storage.

We report the fluorescence emission from organic systems selectively excited by entangled pairs of photons. We have demonstrated a linear dependence of this two-photon excited fluorescence on the excitation intensity which is a unique nonclassical feature of two-photon interactions induced by entangled photons. The entangled photon (ETPA) excited fluorescence has been detected in several organic molecules possessing a high entangled photon absorption cross section. The ETPA fluorescence showed a nonmonotonic dependence on the delay between signal and idler beams. The fluorescence signal was detectable within the signal–idler relative delay time interval of ∼100 fs. This time is comparable with the estimated entanglement time, TE, making the ETPA-excited fluorescence in organic materials an ideal ultrafast coincidence detector. These results have widespread impact in applications ranging from spectroscopy to chemical and biological sensing, imaging, and microscopy.

The recent discovery of stable Ag nanoclusters presents new opportunities to understand the detailed electronic and optical properties of the metal core and the ligands using ultrafast spectroscopy. This paper focuses on Ag32 and Ag15 (with thiolate ligands), which are stable in solution. The steady state absorption spectra of Ag nanoclusters show interesting quantum size effects, expected for this size regime. Using a simple structural model for Ag32, TDDFT calculations show absorption at 480 nm and 680 nm that are in reasonable correspondence with experiments. Ag32(SG)19 and Ag15(SG)11 have quantum yields up to 2 orders of magnitude higher than Au nanoclusters of similar sizes, with an emission maximum at 650 nm, identified as the metal–ligand state. The emission from both Ag nanoclusters has a common lifetime of about 130 ps and a common energy transfer rate of KEET ≥ 9.7 × 109 s–1. A “dark state” competing with the emission process was also observed and was found to be directly related to the difference in quantum yield (QY) for the two Ag clusters. Two-photon excited emission was observed for Ag15(SG)11, with a cross-section of 34 GM under 800 nm excitation. Femtosecond transient absorption measurements for Ag32 recorded a possible metal core state at 530 nm, a metal–ligand state at 651 nm, and ground state bleaches at 485 and 600 nm. The ground state bleach signals in the transient spectrum for Ag32 are 100 nm blue-shifted in comparison to Au25. The transient spectrum for Ag15 shows a weak ground state bleach at ∼480 nm and a broad excited state centered at 610 nm. TDDFT calculations indicate that the electronic and optical properties of Ag nanoclusters can be divided into core states and metal–ligand states, and photoexcitation generally involves a ligand to metal core transition. Subsequent relaxation leaves the electron in a core state, but the hole can be either ligand or core-localized. This leads to emission/relaxation that is consistent with the observed photophysics.

New approaches in molecular nanoscopy are greatly desired for interrogation of biological, organic, and inorganic objects with sizes below the diffraction limit. Our current work investigates emergent monolayer-protected gold quantum dots (nanoclusters, NCs) composed of 25 Au atoms by utilizing two-photon-excited fluorescence (TPEF) near-field scanning optical microscopy (NSOM) at single NC concentrations. Here, we demonstrate an approach to synthesize and isolate single NCs on solid glass substrates. Subsequent investigation of the NCs using TPEF NSOM reveals that, even when they are separated by distances of several tens of nanometers, we can excite and interrogate single NCs individually. Interestingly, we observe an enhanced two-photon absorption (TPA) cross section for single Au25 NCs that can be attributed to few-atom local field effects and to local field-induced microscopic cascading, indicating their potential for use in ultrasensitive sensing, disease diagnostics, cancer cell therapy, and molecular computers. Finally, we report room-temperature aperture-based TPEF NSOM imaging of these NCs for the first time at 30 nm point resolution, which is a ∼5-fold improvement compared to the previous best result for the same technique. This report unveils the unique combination of an unusually large TPA cross section and the high photostability of Au NCs to (non-destructively) investigate stable isolated single NCs using TPEF NSOM. This is the first reported optical study of monolayer-protected single quantum clusters, opening some very promising opportunities in spectroscopy of nanosized objects, bioimaging, ultrasensitive sensing, molecular computers, and high-density data storage.

The classical model for DNA groove binding states that groove binding molecules should adopt a crescent shape that closely matches the helical groove of DNA. Here, we present a new design strategy that does not obey this classical model. The DNA-binding mechanism of small organic molecules was investigated by synthesizing and examining a series of novel compounds that bind with DNA. This study has led to the emergence of structure–property relationships for DNA-binding molecules and/or drugs, which reveals that the structure can be designed to either intercalate or groove bind with calf thymus dsDNA by modifying the electron acceptor properties of the central heterocyclic core. This suggests that the electron accepting abilities of the central core play a key role in the DNA-binding mechanism. These small molecules were characterized by steady-state and ultrafast nonlinear spectroscopies. Bioimaging experiments were performed in live cells to evaluate cellular uptake and localization of the novel small molecules. This report paves a new route for the design and development of small organic molecules, such as therapeutics, targeted at DNA as their performance and specificity is dependent on the DNA-binding mechanism.

New light harvesting organic conjugated polymers containing 4,8-bis(2-ethylhexyloxy)benzo[1,2-b;3,4-b′]dithiophene(BDT) donor groups and thiophene with various electron-withdrawing acceptor groups were investigated. Also investigated was poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), which is one of the most efficient photovoltaic conjugated polymers. In this study, the steady state absorption, steady state emission, ultrafast fluorescent decay dynamics, and nonlinear optical properties of these light harvesting conjugated polymers were probed in solution. All of the conjugated polymers investigated have significant absorption over much of the visible spectrum due to small band gaps due to low lying LUMO energies created by the electron-withdrawing groups. Fluorescence upconversion studies on the conjugated polymers showed short decay dynamics for conjugated polymers with strong electron-withdrawing groups. Two-photon absorption spectroscopy showed large two-photon absorption cross sections for the conjugated polymers with strong electron-withdrawing acceptors. Fluorescence anisotropy decay studies showed contributions from both hopping and a coherent energy migration process for some of the polymers. The polymers were investigated for their photovoltaic efficiency and correlated with both the steady-state and time-resolved dynamics of the investigated donor–acceptor polymers.

Gold nanoclusters have been extensively studied in solution for their unique optical properties. However, many applications of nanoclusters involve the use of the material in the solid state such as films. Au25(SR)18 in polymeric hosts was used as the model for studying the optical properties of nanocluster films. Different film-processing conditions as well as types of polymers were explored to produce a good-quality film that is suitable for optical measurements. The best optical film was made using Au25(C6S)18 and polystyrene. The formation of nanocluster films drastically reduces the intercluster distances to a few nanometers, which were estimated and characterized by optical absorption. The steady-state absorption and emission properties of the nanocluster film maintained their molecular characteristics. The emissions from the nanocluster films are found to be strongly enhanced at 730 nm with a smaller enhancement at 820 nm when the intercluster distance is below 8 nm. The emission enhancement can be attributed to the energy transfer between clusters due to the small intercluster distance. Two-photon Z scan revealed that the two-photon absorption cross sections are in the order of 106 GM, which is an order of magnitude higher than it is in solution. The two-photon absorption enhancement is correlated with strong dipole coupling. These results show that metal nanoclusters can be made into optical quality films, which increase the interaction between clusters and enhances their linear and nonlinear optical responses.Keywords: enhanced two-photon absorption; film; gold cluster; solid-state fluorescence;

Four new low-bandgap electron-accepting polymers—poly(4,10-bis(2-butyloctyl)-2-(2-(2-ethylhexyl)-1,1-dioxido-3-oxo-2,3-dihydrothieno[3,4-d]isothiazol-4-yl)thieno[2′,3′:5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H-dione) (PNSW); poly(4,10-bis(2-butyloctyl)-2-(5-(2-ethylhexyl)-4,6-dioxo-5,6-dihydro-4H-thieno[3,4-c]pyrrol-1-yl)thieno[2′,3′:5,6]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione) (PNTPD); poly(5-(4,10-bis(2-butyloctyl)-5,11-dioxo-4,5,10,11-tetrahydrothieno[2′,3′:5,6]pyrido[3,4-g]thieno[3,2-c]isoquinolin-2-yl)-2,9-bis(2-decyldodecyl)anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10(2H,9H)-tetraone) (PNPDI); and poly(9,9-bis(2-butyloctyl)-9H-fluorene-bis((1,10:5,6)2-(5,6-dihydro-4H-cyclopenta[b]thiophene-4-ylidene)malonitrile)-2-(2,3-dihydrothieno[3,4-b][1,4]dioxine)) (PECN)—containing thieno[2′,3′:5′,6′]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione and fluorenedicyclopentathiophene dimalononitrile, were investigated to probe their structure–function relationships for solar cell applications. PTB7 was also investigated for comparison with the new low-bandgap polymers. The steady-state, ultrafast dynamics and nonlinear optical properties of all the organic polymers were probed. All the polymers showed broad absorption in the visible region, with the absorption of PNPDI and PECN extending into the near-IR region. The polymers had HOMO levels ranging from −5.73 to −5.15 eV and low bandgaps of 1.47–2.45 eV. Fluorescence upconversion studies on the polymers showed long lifetimes of 1.6 and 2.4 ns for PNSW and PNTPD, respectively, while PNPDI and PECN showed very fast decays within 353 and 110 fs. PECN exhibited a very high two-photon absorption cross section. The electronic structure calculations of the repeating units of the polymers indicated the localization of the molecular orbitals in different co-monomers. As the difference between the electron affinities of the co-monomers in the repeating units decreases, the highest occupied and lowest unoccupied molecular orbitals become more distributed. All the measurements suggest that a large difference in the electron affinities of the co-monomers of the polymers contributes to the improvement of the photophysical properties necessary for highly efficient solar cell performance. PECN exhibited excellent photophysical properties, which makes it to be a good candidate for solar cell device applications.

A new optical strategy to determine the binding modes (intercalation vs groove binding) of small fluorescent organic molecules with calf thymus DNA was developed using two-photon absorption (TPA) spectroscopy. Two-photon excited emission was utilized to investigate a series of fluorescent nuclear dyes. The results show that TPA cross-sections are able to differentiate the fine details between the DNA binding modes. Groove binding molecules exhibit an enhanced TPA cross-section due to the DNA electric field induced enhancement of the transition dipole moment, while intercalative binding molecules exhibit a decrease in the TPA cross-section. Remarkably, the TPA cross-section of 4,6-bis(4-(4-methylpiperazin-1-yl)phenyl) pyrimidine is significantly enhanced (13.6-fold) upon binding with DNA. The sensitivity of our TPA methodology is compared to circular dichroism spectroscopy. TPA demonstrates superior sensitivity by more than an order of magnitude at low DNA concentrations. This methodology can be utilized to probe DNA interactions with other external molecules such as proteins, enzymes, and drugs.

A series of ladder-type thienoacenes based on benzo[1,2-b:4,5-b′]dithiophene (BDT) have been synthesized and characterized. They were shown to be p-type semiconductors with wide band gaps and able to support multiple stable cationic states. As the conjugation lengthens, these oligomers become more emissive, showing high quantum yields. They were shown to be good two-photon absorbers, exhibiting high two-photon absorption coefficients.

We report the process of singlet exciton fission with high-yield upon photoexcitation of a quinoidal thiophene molecule. Efficient ultrafast triplet photogeneration and its yield are determined by photoinduced triplet–triplet absorption, flash photolysis triplet lifetime measurements, as well as by femtosecond time-resolved transient absorption and fluorescence methods. These experiments show that optically excited quinoidal bithiophene molecule undergoes ultrafast formation of the triplet-like state with the lifetime ∼57 μs. CASPT2 and RAS-SF calculations have been performed to support the experimental findings. To date, high singlet fission rates have been reported for crystalline and polycrystalline materials, whereas for covalently linked dimers and small oligomers it was found to be relatively small. In this contribution, we show an unprecedented quantum yield of intramolecular singlet exciton fission of ∼180% for a quinoidal bithiophene system.

The development of new sensitive methods for the detailed collection of conformational and morphological information about amyloids is crucial for the elucidation of critical questions regarding aggregation processes in neurodegenerative diseases. The combined approach of two-photon and time-resolved fluorescence spectroscopy described in this report interrogates the early conformational dynamics seen in soluble oligomers of amyloid-β(1–42). Concentration-dependent aggregation studies using two-photon absorption show enhanced sensitivity toward conformational changes taking place in the secondary structure of the amyloid peptide as aggregation proceeds. Fluorescence lifetimes and changes in anisotropy values indicate Förster-type energy transfer occurring as a function of aggregation state. The sensitivity of our two-photon methodology is compared to that of circular dichroism (CD) spectroscopy, and the results indicate that the two-photon absorption cross-section method exhibits superior sensitivity. A theoretical model is developed that, together with electronic structure calculations, explains the change in cross section as a function of aggregation in terms of interacting transition dipoles for aggregates showing stacked or parallel structures. This suggests that the two-photon method provides a sensitive alternative to CD spectroscopy while avoiding many of the inherent challenges particular to CD data collection. The implication of this finding is significant, as it indicates that a two-photon-based technique used in conjunction with time-resolved fluorescence might be able to reveal answers to conformational questions about amyloid-β(1–42) that are presently inaccessible with other techniques.

In this study, we present the synthesis of a carbazole-bridged porphyrin dimer system that possesses a 90° change in orientation between porphyrin units and the single-product four dimer macrocycle, which is formed upon self-assembly of the dimer via imidazolyl-to-zinc complementary coordination. Subsequent characterization of the two-photon absorption and ultrafast emission lifetimes of these systems indicates a very strong coupling between constituent dimers in the assembled macrocycle structure. These interactions lead to a red-shifted two-photon response and a full order of magnitude increase in the two-photon absorption (TPA) cross section per building block with respect to the lone dimer. Excitonic coupling through the slipped cofacial arrangement created by imidazolyl-to-zinc interaction has been shown to play a critical role in the observed TPA enhancement. This points to the use of our small light-harvesting mimic for nonlinear optical applications in which aggregation effects have often stymied development.

In the past 20 years, researchers studying nanomaterials have uncovered many new and interesting properties not found in bulk materials. Extensive research has focused on metal nanoparticles (>3 nm) because of their potential applications, such as in molecular electronics, image markers, and catalysts. In particular, the discovery of metal nanoclusters (<3nm) has greatly expanded the horizon of nanomaterial research. These nanosystems exhibit molecular-like characteristics as their size approaches the Fermi-wavelength of an electron. The relationships between size and physical properties for nanomaterials are intriguing, because for metal nanosystems in this size regime both size and shape determine electronic properties. Remarkably, changes in the optical properties of nanomaterials have provided tremendous insight into the electronic structure of nanoclusters. The success of synthesizing monolayer protected clusters (MPCs) in the condensed phase has allowed scientists to probe the metal core directly. Au MPCs have become the “gold” standard in nanocluster science, thanks to the rigorous structural characterization already accomplished. The use of ultrafast laser spectroscopy on MPCs in solution provides the benefit of directly studying the chemical dynamics of metal nanoclusters (core), and their nonlinear optical properties.In this Account, we investigate the optical properties of MPCs in the visible region using ultrafast spectroscopy. Based on fluorescence up-conversion spectroscopy, we propose an emission mechanism for these nanoclusters. These clusters behave differently from nanoparticles in terms of emission lifetimes as well as two-photon cross sections. Through further investigation of the transient (excited state) absorption, we have found many unique phenomena of nanoclusters, such as quantum confinement effects and vibrational breathing modes. In summary, based on the differences in the optical properties, the distinction between nanoclusters and nanoparticles appears at a size near 2.2 nm. This is consistent with simulations from a free-electron model proposed for MPCs. The use of ultrafast techniques on these nanoclusters can answer many of the fundamental questions about the nature of these exciting nanomaterials and their applications.

Silsesquioxanes (SQs) are of considerable interest for hybrid electronic and photonic materials. However, to date, their photophysical properties have not been studied extensively, thus their potential remains conjecture. Here we describe the first known efforts to map structure–photophysical properties as a function of cage symmetry and size by comparing identically functionalized systems. Our focus here is on the solution photophysical properties of the title stilbenevinyl-SQs, which were characterized using single photon absorption, two-photon absorption, fluorescence emission, and fluorescence lifetime kinetics. We offer here the first detailed photophysical study of the larger pure T10 and T12 silsesquioxanes and show photophysical properties that differ as a function of size, especially in their fluorescence behavior, indicating that cage size and/or symmetry can strongly affect photophysical properties. We also find that they offer excitation-dependent emission (evidence of rare “red-edge” effects). The T10 stilbenevinyl-SQ offers up to a 10-fold increase in two-photon absorption cross section per chromophore over a free chromophore, signifying increased electronic coupling. The SQ cage compounds show “rise times” of 700–1000 fs and low anisotropy (∼0.1) in fluorescence lifetime kinetic studies. These results indicate excited state energy transfer, unobserved for the free chromophores and unexpected for systems with “inert” silica cores and for 3-D hybrid molecular species. These findings provide the first detailed photophysical study of chromophore-functionalized T10 and T12 silsesquioxanes and show that SQs may be considered a separate class of compounds/materials with anticipated novel properties of value in developing new components for electronic and photonic applications.

We utilize quantum entangled photons to carry out nonlinear optical spectroscopy in organic molecules with an extremely small number of photons. For the first time, fluorescence is reported as a result of entangled photon absorption in organic nonlinear optical molecules. Selectivity of the entangled photon absorption process is also observed and a theoretical model of this process is provided. Through these experiments and theoretical modeling it is found that while some molecules may not have strong classical nonlinear optical properties due to their excitation pathways; these same excitation pathways may enhance the entangled photon processes. It is found that the opposite is also true. Some materials with weak classical nonlinear optical effects may exhibit strong non-classical nonlinear optical effects. Our entangled photon fluorescence results provide the first steps in realizing and demonstrating the viability of entangled two-photon microscopy, remote sensing, and optical communications.Keywords: entangled two-photon excited fluorescence; excitation pathway; nonlinear optical; remote sensing;

A series of novel oligothiophene-perylene bisimide hybrid (DOTPBI) dendrimers up to the second generation (G0, G1, and G2) were investigated. Optical measurements such as nonlinear optical and time-resolved spectroscopy, including two-photon absorption, fluorescence upconversion, and excited state transient absorption were carried out. Results of these measurements revealed the ability of these molecules to undergo intramolecular fluorescence resonance energy transfer (FRET) from the dendritic oligothiophenes (DOT) to the perylene bismide (PBI) moiety. The delocalization length and the photoinduced electron transfer (PET) rate were investigated as a function of dendrimer generation. A fast energy transfer process from the DOT dendron to the PBI core was observed. For the case of the G2 dendrimer, with relatively large thiophene dendrons attached to the bay area of the perylene bisimide, the PBI core is highly twisted and its ability to self-assemble into π–π stacked aggregates is destroyed. As a result, among the three generations studied, G1, which has the best two-photon cross section and the most efficient energy transfer, is the best light harvesting material.

We report detailed photophysical studies on the two-photon fluorescence processes of the solvatochromic fluorophore 4-DMN as a conjugate of the calmodulin (CaM) and the associated CaM-binding peptide M13. Strong two-photon fluorescence enhancement has been observed which is associated with calcium binding. It is found that the two-photon absorption cross-section is strongly dependent on the local environment surrounding the 4-DMN fluorophore in the CaM conjugates, providing sensitivity between sites of fluorophore attachment. Utilizing time-resolved measurements, the emission dynamics of 4-DMN under various environmental (solvent) conditions are analyzed. In addition, anisotropy measurements reveal that the 4-DMN–S38C–CaM system has restricted rotation in the calcium-bound calmodulin. To establish the utility for cellular imaging, two-photon fluorescence microscopy studies were also carried out with the 4-DMN-modified M13 peptide in cells. Together, these studies provide strong evidence that 4-DMN is a useful probe in two-photon imaging, with advantageous properties for cellular experiments.

Two new soluble tri-tert-butyl zinc(II) phthalocyanines, 1 and 2, bearing dendritic oligothienylene-ethynylene (DOT) groups as one of the peripheral substituents, have been prepared. The conjugated DOT moieties were introduced to cover the spectral window between 380 and 550 nm, where the ZnPc does not exhibit a strong absorption, in order to improve light harvesting. For their preparation, a convergent approach has been used starting from the corresponding iodoPc as precursor. Further transformation of the iodo groups by a Pd-catalyzed Sonogashira reaction with the appropriate DOT-functionalized terminal alkyne allowed the easy preparation of extended π-conjugated compounds 1 and 2. The compounds have been characterized by standard spectroscopic methods, and their photophysical behaviors have been established by using ultrafast time-resolved techniques. Femtosecond upconversion measurements showed an ultrafast energy transfer from the DOT to zinc phthalocyanine in a time scale of 300 fs. As the number of thiophene groups increases in the dyads, the extent of ultrafast energy transfer was found to increase. Compounds 1 and 2 have been tested as donor components in bulk heterojunction (BHJ) solar cells. Their efficiencies are compared with RuPc analogues previously reported by us.

Metal nanoclusters have interesting steady state fluorescence emission, two-photon excited emission and ultrafast dynamics. A new subclass of fluorescent silver nanoclusters (Ag NCs) are NanoCluster Beacons. NanoCluster Beacons consist of a weakly emissive Ag NC templated on a single stranded DNA (“Ag NC on ssDNA”) that becomes highly fluorescent when a DNA enhancer sequence is brought in proximity to the Ag NC by DNA base pairing (“Ag NC on dsDNA”). Steady state fluorescence was observed at 540 nm for both Ag NC on ssDNA and dsDNA; emission at 650 nm is observed for Ag NC on dsDNA. The emission at 550 nm is eight times weaker than that at 650 nm. Fluorescence up-conversion was used to study the dynamics of the emission. Bi-exponential fluorescence decay was recorded at 550 nm with lifetimes of 1 ps and 17 ps. The emission at 650 nm was not observed at the time scale investigated but has been reported to have a lifetime of 3.48 ns. Two-photon excited fluorescence was detected for Ag NC on dsDNA at 630 nm when excited at 800 nm. The two-photon absorption cross-section was calculated to be ∼3000 GM. Femtosecond transient absorption experiments were performed to investigate the excited state dynamics of DNA–Ag NC. An excited state unique to Ag NC on dsDNA was identified at ∼580 nm as an excited state bleach that related directly to the emission at 650 nm based on the excitation spectrum. Based on the optical results, a simple four level system is used to describe the emission mechanism for Ag NC on dsDNA.

A series of π-extended cyclic thiophene oligomers of 12, 18, 24, and 30 repeat units have been studied using methods of ultrafast time-resolved absorption, fluorescence upconversion, and three-pulse photon echo. These measurements were conducted in order to examine the structure−function relationships that may affect the coherence between chromophores within the organic macrocycles. Our results indicate that an initial delocalized state can be seen upon excitation of the cyclic thiophenes. Anisotropy measurements show that this delocalized state decays on an ultrafast time scale and is followed by the presence of incoherent hopping. From the use of a phenomenological model, we conclude that our ultrafast anisotropy decay measurements suggest that the system does not reside in the Förster regime and coherence within the system must be considered. Three-pulse photon echo peak shift experiments reveal a clear dependence of initial peak shift with ring size, indicating a weaker coupling to the bath (and stronger intramolecular interactions) as the ring size is increased. Our results suggest that the initial delocalized state increases with ring size to distances (and number of chromophores) comparable to the natural light-harvesting system.

Two-photon active green fluorescent protein -type chromophores were successfully synthesized following investigations directed toward a modified version of zFP538 chromophore, a structural analogue to the GFP-chromophore. A generalized approach for the chromophore synthesis via a well-studied cycloaddition reaction combining an iminoglycine methyl ester and a substituted benzaldehyde was developed allowing for flexibility in the incorporation of functional groups such as donor−acceptor substituents and for additional groups to provide extended conjugation. Steady-state spectroscopy, fluorescence quantum yields, and time-resolved fluorescence lifetimes for synthesized chromophores were extensively investigated for the functionalized chromophores. Time-resolved fluorescence lifetimes were found to be biexponential generally with subpicosecond and picosecond components. The individual effects of substitution position of functional groups and relative bulk size were evaluated and found to be rather significant in changing the fluorescence-decay characteristics in the case of positioning, but ambiguous with respect to relative bulk. The GFP-type chromophores were found to possess modest to low two-photon absorption cross sections with the dimethylamino-substituted analogue possessing the largest value at nearly 40 GM. These molecules show promise as biological markers for application in the study of conformation changes and aggregation of amyloid peptides, known to play an important role in many neurodegenerative diseases.

Detailed here is the use of NIR femtosecond pulsed excitation to drive three-photon absorption (3PA) in a cyano-terminated quinoidal oligothiophene (QOT) dimer to the exclusion of all other fluorescing processes, resulting in 3PA emission bright enough to be visible by eye. Through steady-state, multiphoton, and ultrafast transient spectroscopy, it is shown that despite competing nonlinear optical processes, such as an available two-photon transition (2PA) and excited-state absorption (ESA), emission characteristics remain an I3 process explicitly due to 3PA. Specifically, we will show the viability of a two-photon transition at the pump wavelength and strong ESA; neither occurs to a significant degree in this QOT system when pump energies match both a virtual state near resonant with a viable 2PA state and a higher-lying 3PA energy level. This study provides a clear method for evaluating multiband 3PA emitters, which bypasses much of the ambiguity observed in purely absorption based studies where 2PA, ESA, and 3PA may all contribute to signal and are difficult to distinguish. Additionally, this study introduces a rigid, molecular wire-like thiophene oligomer as a strong nonlinear optical responsive material with gross response changes based on small changes in IR excitation.Keywords: biradical; oligothiophene; three-photon fluorescence; two-photon absorption;

Hyperbranched and dendritic architectures have been targeted for various applications such as sensing, drug delivery, optical limiting, and light harvesting. One interesting development in this area has focused on utilizing the existence of long-range delocalization in hyperbranched structures to achieve high dielectric constants. In this Feature Article, we will review the creation and development of this concept, and we highlight our recent research progress in this aspect. In particular, we discuss (1) synthetic methods for a particular group of hyperbranched polymers; (2) detailed optical and electronic characterization of this group of hyperbranched polymers, revealing the design criteria for achieving a good combination of high dielectric constant and minimum loss in such materials; and (3) the importance and potential applications of these materials.

The lack of characterization regimes available for the rapid single-particle assessment of two-photon (TPA) response in nanomaterials remains a critical barrier to nonlinear optical device development. This is particularly true of nonemissive species whose TPA must often be characterized in the bulk. In this study, self-assembly is used to produce uniform nanoparticles from a novel porphyrin dimer, which is known to exhibit both severe fluorescence quenching and two-photon cross section (TPACS) enhancement when assembled into macromolecules. We present here the first reported use of fiber aperture near-field optical microscopy (NSOM) for the purpose of characterizing directly the TPA of nonemissive nanoparticles, observing directly a 5-fold enhancement in TPA response. This assembly/characterization regime provides a fast and fully actualized method for the generation of low-scatter optical-limiting organic nanomaterials where domain size, morphology control, and TPA enhancement are all critical to application viability and unobservable via bulk measurements.Keywords: NSOM; organic nanoparticle; porphyrin dimer; self-assembly; two-photon;

New organometallic materials such as two-dimensional metallacycles and three-dimensional metallacages are important for the development of novel optical, electronic, and energy related applications. In this article, the ultrafast dynamics of two different platinum-containing metallacycles have been investigated by femtosecond fluorescence upconversion and transient absorption. These measurements were carried out in an effort to probe the charge transfer dynamics and the rate of intersystem crossing in metallacycles of different geometries and dimensions. The processes of ultrafast intersystem crossing and charge transfer vary between the two different classes of metallacyclic systems studied. For rectangular anthracene-containing metallacycles, the electronic coupling between adjacent ligands was relatively weak, whereas for the triangular phenanthrene-containing structures, there was a clear interaction between the conjugated ligand and the metal complex center. The transient lifetimes increased with increasing conjugation in that case. The results show that differences in the dimensionality and structure of metallacycles result in different optical properties, which may be utilized in the design of nonlinear optical materials and potential new, longer-lived excited state materials for further electronic applications.

Entangled photons generated by spontaneous parametric down-conversion (SPDC) have been used to investigate entangled two-photon absorption (ETPA) in multiannulene systems. The ETPA characteristics are shown to depend on the spatial orientation of the SPDC emission pattern. The expected dependence of the absorption rate on input flux is seen for emission patterns that exhibit spatial indistinguishability between the signal and idler photons, while no absorption is observed for a spatially distinguishable emission pattern. The amount of absorption of entangled photons is also seen to depend on the degree of overlap of the entangled photons for the indistinguishable conditions. Tunability of the entangled photon absorption can thus be achieved by utilizing the spatial characteristics of the entangled photon pairs.

Monolayer-protected metal nanoclusters (MPCs) were investigated to probe their fundamental excitation and emission properties. In particular, gold MPCs were probed by steady-state and time-resolved spectroscopic measurements; the results were used to examine the mechanism of emission in relation to the excited states in these systems. In steady-state measurements, the photoluminescence of gold clusters in the range of 25 to 140 atoms was considerably stronger relative to larger particle analogues. The increase in emission efficiency (for Au25, Au55, and Au140 on the order of 10−5) over bulk gold may arise from a different mechanism of photoluminescence, as suggested by measurements on larger gold spheres and rods. Results of fluorescence upconversion found considerably longer lifetimes for smaller gold particles than for larger particles. Measurements of the femtosecond transient absorption of the smaller clusters suggested dramatically different behavior than what was observed for larger particles. These results, combined with the result of a new bleach band in the transient absorption signal (which is presumably due to an unforeseen ground state absorption), suggest that quantum size effects and associated discrete molecular-like state structure play a key role in enhanced visible fluorescence of small clusters.

The conformational changes associated with the aggregation of proteins are critical to the understanding of fundamental molecular events involved in early processes of neurodegenerative diseases. A detailed investigation of these processes requires the development of new approaches that allow for sensitive measurements of protein interactions. In this paper, we applied two-photon spectroscopy coupled with time-resolved fluorescence measurements to analyze amyloid peptide interactions through aggregation-dependent concentration effects. Labeled amyloid-β peptide (TAMRA-Aβ1-42) was used in our investigation, and measurements of two-photon-excited fluorescence of the free and covalently conjugated peptide structure were carried out. The peptide secondary structure was correlated with a short fluorescence lifetime component, and this was associated with intramolecular interactions. Comparison of the fractional occupancy of the fluorescence lifetime measured at different excitation modes demonstrates the high sensitivity of the two-photon method in comparison to one-photon excitation (OPE). These results give strong justification for the development of fluorescence-lifetime-based multiphoton imaging and assays.

Femtosecond time-resolved absorption spectroscopy has been used to study the excited-state charge transfer dynamics in a set of self-assembled cyclic Fe(II)−bisterpyridine compounds with different π-conjugated ligands. By analyzing the dynamics, the internal conversion process involving a ligand-centered π−π* state to a lower lying metal-to-ligand charge transfer (MLCT) state was investigated. This is followed by intersystem crossing to the lowest MLCT state, which was found to occur at the ∼100 fs time scale. Vibrational cooling in the lowest MLCT state was found to occur on the 10s of femtoseconds time scale. The lowest MLCT state had an excited-state lifetime longer than 5 ns, indicating the possibility of light-induced excited-state spin trapping (LIESST).

The nature of one and two-photon absorption enhancement in a series of oligothiophene dendrimers, recently proposed for applications in entangled photon sensors and solar cells, has been analyzed using both theory (time dependent density functional theory calculations) and experiment (fluorescence upconversion measurements). The linear absorption spectra exhibit a red shift of the absorption maxima and broadening as a function of dendrimer generations. The two-photon absorption cross sections increase sharply with the number of thiophene units in the dendrimer. The cooperative enhancement in absorption two-photon cross sections is explained by (i) an increase in the excited-state density for larger molecules and (ii) delocalization of the low-lying excited states over extended thiophene chains. Fluorescence anisotropy measurements and examination of the calculated excited-state properties reveal that this delocalization is accompanied by a size-dependent decrease in excited-state symmetries. A substantial red shift of the emission maxima for larger dendrimers is explained through the vibronic planarization of the longest linear α-thiophene chain for the emitting excited state. For higher generations, the fluorescence quantum yield decreases due to increased nonradiative decay efficiency (e.g., intersystem crossing). The detailed information about the dendrimer 3D structure and excitations provides guidance for further optimizations of dendritic structures for nonlinear optical and opto-electronic applications.

We report a systematic investigation of the optically excited vibrations in monolayer-protected gold clusters capped with hexane thiolate as a function of the particle size in the range of 1.1−4 nm. The vibrations were excited and monitored in transient absorption experiments involving 50 fs light pulses. For small quantum-sized clusters (≤2.2 nm), the frequency of these vibrations has been found to be independent of cluster size, while for larger clusters (3 and 4 nm), we did not observe detectable optically excited vibrations in this regime. Possible mechanisms of excitation and detection of the vibrations in nanoclusters in the course of the transient absorption are discussed. The results of the current investigation support a displacive excitation mechanism associated with the presence of finite optical energy gap in the quantum-sized nanoclusters. Observed vibrations provide a new valuable diagnostic tool for the investigations of quantum size effects and structural studies in metal nanoclusters.Keywords: acoustic vibrations; gold clusters; quantum size effect; thiolate

We present a systematic study of optical properties of a series of hexanethiolate-capped Au clusters of varying sizes using femtosecond transient absorption, time-resolved fluorescence, and two-photon absorption cross-sectional measurements. An abrupt change in optical properties and their trends has been found at the 2.2 nm size. Displacively excited vibrations with a period of 450 fs have been detected in the transient absorption signal for smaller clusters ≤2.2 nm. These results strongly suggest an emerging optical gap between the highest occupied and lowest unoccupied orbitals in the narrow size range at 2.2 nm.

The issue of macromolecular exciton delocalization length and fluorescence sensing of energetic materials is investigated and modeled from results of nonlinear optical and time-resolved spectroscopy. By using two- and three-photon absorption techniques the fluorescence quenching effects of an organic dendrimer for sensing TNT were carried out. The Stern−Volmer plots for the set of dendrimers were examined and a large quenching constant for the dendrimer G4 was obtained (1400 M−1). The quenching constant was found to increase with the dendrimer generation number. The mechanism for the enhanced sensitivity of the dendrimer system was examined by probing the exciton dynamics with femtosecond fluorescence up-conversion. Fluorescence lifetime measurements revealed a multicomponent relaxation that varied with dendrimer generation. Fluorescence anisotropy decay measurements were used to probe the exciton migration length in these dendrimer systems and for the large structure the excitation migration area covers ∼20 units. All of these results were used in a model that describes the exciton localization length with the fluorescence quenching strength. The use of time-resolved techniques allows for a closer and more detailed description of the mechanism of sensory amplification in organic macromolecules.

Femtosecond time-resolved fluorescence and transient absorption measurements have been carried out on a series of platinum acetylide complexes to unravel the dynamics of intersystem crossing and the formation of triplet states in real time as a function of chain length. Ultrafast inter system crossing with a time constant less than 100 fs has been observed for the case of short chain length platinum acetylide complex and this time constant increases with increasing the chain length. Apart from the singlet to triplet intersystem crossing, additional triplet state relaxation has also been observed which happens in picosecond time scale.

The nonlinear optical response in one-dimensional organic nanorods of N,N-dimethyl-4−4((4-(trifluoromethylsulfonyl)phenyl)ethynyl)aniline (DMFSPA) was investigated to probe the long-range interactions in the nanocrystals on the microscopic level. Differences in the linear and nonlinear optical properties are shown for two different morphologies of these organic crystals as well as for the chromophore in solution. The optimized nanocrystalline suspension had more than an order of magnitude increase in the two-photon excited fluorescence when compared to the solution phase of DMFSPA at similar chromophore densities. The one and two-photon properties of the nanocrystals and bulk crystals are compared by near-field scanning optical multiscope imaging. The images also provide insights into the formation of the nanorods during initial crystallization, changes in the optical response of the system with time, and the viability of these and similar nanomaterials for consideration in solid-state organic device applications. In addition to providing an imaging regime by which to assess this and other solid-state nanocrystalline organics, our investigation provides a simple and elegant method for enhancing the nonlinear optical response of organic materials by transition to nanoscale morphologies, without the need for additional chemical modification or synthesis.

An understanding of the molecular mechanisms of the newly characterized herpes simplex virus (HSV) B5 protein is important to further elucidate the HSV cell entry and infection. The synthetic peptide of B5 (wtB5) was functionalized with the nonlinear optical chromophore cascade yellow and its molecular dynamics was probed at physiological and endosomal pH (pH 7.4 and 5.5, respectively). Steady-state CD spectroscopy was utilized to characterize the peptides at different pH. These spectra showed structural changes in the peptide with time measured over several days. Nonlinear optical measurements were carried out to probe the interactions and local environment of the labeled peptide, and the increase in the two-photon cross section of this system suggests an increase in chromophore-peptide interactions. Time-resolved fluorescence upconversion measurements reflected changes in the hydrophilic and hydrophobic local environments of the labeled peptide-chromophore system. Ultrafast depolarization measurements gave rotational correlation times indicative of a reversible change in the size of the peptide. The time-resolved results provide compelling evidence of a reversible dissociation of the coiled coils of the wtB5 peptide. This process was found to be pH-insensitive. The data from this unique combination of techniques provide an initial step to understanding the molecular dynamics of B5 and a framework for the development of novel imaging methods based on two-photon emission, as well as new therapeutics for HSV.