The role of proton transfer in the photoluminescence intermittency (PI) of single molecules of violamine R (VR) overgrown in potassium acid phthalate (KAP) crystals is evaluated in comparisons of protonated (KAP) and deuterated (DKAP) mixed crystals between 23 and 60 °C. The PI is analyzed by the construction of cumulative distribution functions that are statistically compared. We find that the on- and off-interval duration distributions change with isotopic substitution consistent with proton transfer contributing to the PI of VR. The on- and off-interval duration distributions have distinct temperature dependencies consistent with different mechanisms for dark state production and decay. Additional evidence for proton-transfer is provided by distributions of single molecule emission-energy maxima that reflect emission from protonated and deprotonated VR. A mechanism for the PI of KAP is presented, where the dark state is assigned to formation of the colorless, leuco form of VR, formed by proton transfer from VR to the KAP lattice, and decay of the dark state involves ring-opening promoted by proton transfer from KAP to VR. The distributed kinetics for dark-state production and decay are modeled using a log-normal distribution for the PI data in preference to a power-law previously assumed. A discussion of the log-normal distribution with regards to PI and proton transfer is presented.

The photochemistry of nitrosyl chloride (ClNO) dissolved in cyclohexane is investigated using ultrafast time-resolved infrared (TRIR) spectroscopy. Following 266 nm photolysis, the photochemistry is measured by following changes in optical density at frequencies spanning the N═O stretch fundamental transition. A photoinduced depletion in optical density is observed consistent with the depletion of ground-state ClNO. The depletion in optical density remains constant out to ∼50 ps demonstrating that ClNO photodissociation is not followed by recombination of the Cl and NO photofragments. In addition, no evidence for the formation of the ClON photoisomer is observed. These results stand in contrast to previous studies in acetonitrile where ClNO photolysis is followed by geminate recombination of Cl and NO, and by the production of ClON. These differences in ClNO photochemistry are proposed to arise from the population of different excited-states caused by solvent dependence of the ground-state potential energy surface minimum along the Cl–N stretch coordinate. Solvent-dependent vibrational relaxation and differences in strength of the solvent cage are also proposed to contribute to the solvent-dependent photochemistry. Finally, these results are placed in the context of recent models of ClNO photochemistry and role of this compound in tropospheric ozone production.

The environment and temperature-dependent photoluminescence (PL) intermittency or “blinking” demonstrated by single violamine R (VR) molecules is investigated in two environments: poly(vinyl alcohol) (PVOH) and single crystals of potassium acid phthalate (KAP). In addition, temperatures ranging from 23 °C to 85 °C are studied, spanning the glass-transition temperature of PVOH (Tg = 72 °C). The PL intermittency exhibited by VR is analyzed using probability histograms of emissive and non-emissive periods. In both PVOH and KAP, these histograms are best fit by a power law, consistent with the kinetics for dark state production and decay being dispersed as observed in previous studies. However, these systems have different temperature dependences, signifying two different blinking mechanisms for VR. In PVOH, the on- and off-event probability histograms do not vary with temperature, consistent with electron transfer via tunneling between VR and the polymer. In KAP the same histograms are temperature dependent, and show that blinking slows down at higher temperatures. This result is inconsistent with an electron-transfer process being responsible for blinking. Instead, a non-adiabatic proton-transfer between VR and KAP is presented as a model consistent with this temperature dependence. In summary, the results presented here demonstrate that for a given luminophore, the photochemical processes responsible for PL intermittency can change with environment.

Experimental and computational studies of the solvent dependence of the first molecular hyperpolarizability (β) for two donor-bridge-acceptor chromophores (CLD-1 and YLD156) are presented. Hyper-Rayleigh scattering (HRS) measurements are performed with 1907 nm excitation in a series of solvents with dielectric constants ranging from ∼2 (toluene) to ∼36 (acetonitrile). For both chromophores an approximately 2-fold increase in β is observed by HRS over this range of dielectric constants. Computational studies employing a polarized continuum model to represent the solvent are capable of reproducing this experimental result. The experimental and computational results are compared to the predictions of the widely employed two-state model (TSM) for β. Surprisingly, for the chromophores studied here the TSM predicts that β should decrease with increasing dielectric constant over the range investigated. The results presented here demonstrate that the TSM provides neither a quantitative nor qualitative description of the solvent dependence of β for CLD-1 and YLD156. The enhancement of β with increased dielectric constant suggests that modification of the dielectric surrounding the chromophore is one path by which the performance of nonlinear optical devices employing these chromophores may be significantly enhanced.

The connection between photoluminescence (PL) intermittency and excited-state kinetics is explored for 2′,7′-dichlorofluorecin (DCF) isolated in crystals of potassium acid phthalate (KAP) using time-tagged, time-resolved, time-correlated single-photon counting (T3R-TCSPC). In this technique, PL intermittency or “blinking” is measured in conjunction with the time of photon arrival relative to photoexcitation, allowing for the correlation of emissive intensities and excited-state decay kinetics of single molecules. The blinking trace is parsed into emissive and nonemissive segments using change-point-detection analysis, and the duration of these segments are used to quantify PL intermittency. The results presented here demonstrate that two populations of DCF exist in KAP, with one population demonstrating single-exponential excited state decay over the course of the blinking trace, and the other demonstrating stretched-exponential decay. Molecules demonstrating single-exponential decay also demonstrate modest intensity variation in the blinking trace. Correlation of the emission intensity and excited-state lifetimes demonstrates that for these molecules spectral diffusion is largely responsible for the evolution in emission intensity. In contrast, molecules demonstrating nonexponential excited-state decay vary in emission intensity. Correlation of the emissive intensities with the excited-state lifetimes demonstrates that these molecules undergo changes in both radiative and nonradiative decay rate constants as well as spectral diffusion. These observations suggest that DCF exists in two environments in KAP differentiated by the propensity for proton-transfer with the surrounding KAP matrix. The results presented here provide further insight into the origin of PL intermittency demonstrated by DCF in KAP and related systems.

The fluorescence intermittency or blinking of violamine R (VR) in poly(vinyl alcohol) (PVA) is measured for temperatures spanning the glass-transition temperature (Tg) of the polymer host. Probability distributions of on- and off-time durations are acquired and analyzed for temperatures ranging from 22.5 °C (Tg − 49 °C) to 85 °C (Tg + 13 °C). At all temperatures, the probability distributions are well described by a power-law. The on-time power-law exponent (mon) undergoes a modest increase with temperature, and the off-time power-law exponent (moff) varies little with temperature. Both observations are consistent with current electron transfer models for the production and decay of the non-emissive state. Surprisingly, both mon and moff demonstrated smooth variation as the temperature transitions through Tg. This observation suggests that the local VR environment is not significantly altered over the temperature range investigated here. The onset of deviation from power-law behavior is observed at Tg + 13 °C, consistent with thermally enhanced environmental fluctuations providing for the interchange of local environments that is rapid compared to blinking.

Large (1 cm3) potassium hydrogen phthalate (KAP) crystals grown in the presence of the fluorescent dye 2′,7′-dichlorofluorescein (DCF) show patterns of luminescence characteristic of selective inter- and intrasectoral zoning. Despite this selectivity, the polarization dependence of the luminescence of single DCF molecules inside the crystals indicated broad orientational distributions consistent with nonspecific mixed crystal growth mechanisms (Wustholz, K.; Kahr, B.; Reid, P. J. J. Phys. Chem. B, 2005, 109, 16357−16362). In an effort to reconcile this apparent discrepancy, KAP crystals were grown from 10−9 to 10−4 M DCF solutions and examined using confocal luminescence and polarized absorption microscopies, where possible. Single molecules and molecular ensembles were investigated. The fluorescence excitation dichroism was strongly dependent on the concentration of DCF, suggesting that different mixed crystal growth mechanisms were at work at different guest concentrations. Hottenhuis et al. ( J. Cryst. Growth, 1986, − 1989) had earlier established that certain trivalent cations bind preferentially to distinct kink sites of KAP. By blocking particular kink sites with Fe3+ or Ce3+ at high DCF-solution concentrations, the orientation of DCF in the crystals was modulated, thus establishing that the dye not only recognizes some propagating steps as opposed to others but also preferentially chooses between kinks propagating in opposing directions on the same step, evidence that kink selectivity plays a vital role in the dyeing of crystals.

The excited-state decay kinetics of single 2′,7′-dichlorofluorescein (DCF) molecules oriented and overgrown within crystals of potassium acid phthalate (KAP) are reported. Time-correlated single-photon counting measurements (TCSPC) of 56 DCF molecules in KAP reveal that single-exponential decay is exhibited by roughly half of the molecules. The remainder demonstrates complex excited-state decay kinetics that are well fit by a stretched exponential function consistent with dispersed kinetics. Histograms of single-molecule luminescence energies revealed environmental fluctuations and distinct chemical species. The TCSPC results are compared to Monte Carlo simulations employing a first-passage model for excited-state decay. Agreement between experiment and theory, on both bulk and single-molecule levels, suggests that a subset of the DCF molecules in KAP experience fluctuations in the surrounding environment that modify the energy barrier to proton transfer leading to dispersed kinetics.Keywords: dispersed kinetics; fluorescence intermittency; fluorescence lifetime; proton transfer; single molecule microscopy

The photochemistry of nitrosyl chloride (ClNO) in the solution phase is investigated using Fourier transform infrared (FTIR) and ultrafast time-resolved infrared (TRIR) spectroscopies. The NO-stretch fundamental transition for ClNO dissolved in cyclohexane, carbon tetrachloride, chloroform, dichloromethane, and acetonitrile is measured, with the frequency and line width of this transition demonstrating a strong dependence on solvent polarity. Following the photolysis of ClNO dissolved in acetonitrile at 266 nm, the subsequent optical-density evolution across the entire width of the NO-stretch fundamental is measured. Analysis of the optical-density evolution demonstrates that geminate recombination of the primary photofragments resulting in the reformation of ground state ClNO occurs with a quantum yield of 0.54 ± 0.06. In addition, an increase in optical density is observed at 1860 cm−1 that is assigned to the NO-stretch fundamental transition of the photoisomer, ClON, having a formation quantum yield of 0.07 ± 0.02. This work represents the first definitive observation of ClNO photoisomerization in solution. Finally, essentially no evidence is observed for significant vibrational excitation of the NO fragment following photodissociation, in marked contrast to the behavior observed in the gas phase. An environment-dependent dissociation scheme is proposed in which the interplay between solvent polarity and the location of the ground state potential-energy-surface minimum along the Cl−N coordinate provides for the optical preparation of different excited states thereby affecting the extent of NO vibrational excitation following photolysis.

The blinking dynamics of single violamine R (VR) molecules embedded in crystals of potassium acid phthalate (KAP) are analyzed using threshold and change-point detection (CPD) methods (Watkins, L. P.; Yang, H. J. Phys. Chem. B 2005, 109, 617). Analysis employing thresholding resulted in power-law distributions of on and off times corresponding to a power-law exponent of ∼2, consistent with a distributed kinetics model for population and depopulation of the nonemissive state. When the same emission time traces were analyzed using the CPD method, a power-law exponent of ∼1.5 is obtained. In addition, multiple emission states are observed using CPD, inconsistent with a simple two-state blinking model, and indicative of spectral diffusion. The role of spectral diffusion in the distributed blinking kinetics of KAP/VR is investigated by spectrally decomposing the emission using a dichroic mirror. Combining the CPD method with this experiment yielded the emission energy, intensity, and temporal duration of blinking events. A wide distribution of emission energies is observed, consistent with molecules experiencing a variety of dielectric environments within the crystal host. Time-dependent fluctuations in the spectral decomposition are observed, corresponding to spectral diffusion. Blinking events exhibited by single VR molecules in KAP are correlated, an effect referred to as memory. To our knowledge, this is the first reported observation of memory for a molecular system. Positive correlations are observed for consecutive on times and consecutive off times. In addition, adjacent on and off times demonstrate anticorrelation. These observations support the observation of spectral diffusion in this crystal environment, with this diffusion contributing to population and depopulation of the nonemissive state.

Infrared absorption (IR), Raman, and resonance Raman spectra have been obtained from 500 to 1700 cm−1 for 4-hydroxybenzylidene-2,3-dimethyl-imidazolinone (HBDI), an analog of the green-fluorescent protein (GFP) chromophore. Numerous transitions are evident in both the IR and Raman spectra, with the resonance Raman spectrum of HBDI dominated by a subset of transitions in the 1430–1700 cm−1 region. Assignment of the transitions in this frequency region to the corresponding normal coordinates is accomplished through computational studies employing density functional and Hartree–Fock theory. The computational results indicate that the vibrational transitions in this frequency range are dominated by in-plane stretching modes that are localized to the imidazolinone or tyrosine portions of the chromophore, rather than being delocalized over the entire chromophore. No evidence is obtained for significant excited-state structural evolution along the O–H stretching coordinate. The implications of these findings with respect to the excited-state proton transfer dynamics of GFP are discussed.

The first resonance Raman spectra of Cl2O dissolved in CCl4 are reported. The dependence of the resonance Raman spectrum on incident energy is investigated and found to be consistent with the photochemical production of both ClO and ClClO. Both of these photoproducts appear linearly with incident energy at low power, but the scattering intensity of ClO is found to increase quadratically at higher energies. This behavior is proposed to be consistent with photodissociation of Cl2O into ClO and Cl followed by partial geminate recombination of these fragments, resulting in ClClO formation. At higher excitation energies, photodissociation of ClClO is proposed to occur resulting in ClO production. Finally, the photochemical behavior of Cl2O is compared to that of OClO, where geminate recombination does not result in formation of the analogous ClOO isomer. This comparison suggests that although geminate recombination is a characteristic feature of halo-oxide photochemistry in condensed environments, the products formed by recombination are species dependent.

The fluorescence intermittency or blinking of violamine R (VR) in poly(vinyl alcohol) (PVA) is measured for temperatures spanning the glass-transition temperature (Tg) of the polymer host. Probability distributions of on- and off-time durations are acquired and analyzed for temperatures ranging from 22.5 °C (Tg − 49 °C) to 85 °C (Tg + 13 °C). At all temperatures, the probability distributions are well described by a power-law. The on-time power-law exponent (mon) undergoes a modest increase with temperature, and the off-time power-law exponent (moff) varies little with temperature. Both observations are consistent with current electron transfer models for the production and decay of the non-emissive state. Surprisingly, both mon and moff demonstrated smooth variation as the temperature transitions through Tg. This observation suggests that the local VR environment is not significantly altered over the temperature range investigated here. The onset of deviation from power-law behavior is observed at Tg + 13 °C, consistent with thermally enhanced environmental fluctuations providing for the interchange of local environments that is rapid compared to blinking.

The environment and temperature-dependent photoluminescence (PL) intermittency or “blinking” demonstrated by single violamine R (VR) molecules is investigated in two environments: poly(vinyl alcohol) (PVOH) and single crystals of potassium acid phthalate (KAP). In addition, temperatures ranging from 23 °C to 85 °C are studied, spanning the glass-transition temperature of PVOH (Tg = 72 °C). The PL intermittency exhibited by VR is analyzed using probability histograms of emissive and non-emissive periods. In both PVOH and KAP, these histograms are best fit by a power law, consistent with the kinetics for dark state production and decay being dispersed as observed in previous studies. However, these systems have different temperature dependences, signifying two different blinking mechanisms for VR. In PVOH, the on- and off-event probability histograms do not vary with temperature, consistent with electron transfer via tunneling between VR and the polymer. In KAP the same histograms are temperature dependent, and show that blinking slows down at higher temperatures. This result is inconsistent with an electron-transfer process being responsible for blinking. Instead, a non-adiabatic proton-transfer between VR and KAP is presented as a model consistent with this temperature dependence. In summary, the results presented here demonstrate that for a given luminophore, the photochemical processes responsible for PL intermittency can change with environment.