Shiva K. Ramini

Mechanisms of reversible photodegradation in disperse orange 11 dye doped in PMMA polymer.

We use amplified spontaneous emission (ASE) and linear absorption spectroscopy to study the mechanisms of reversible photodegradation of 1-amino-2-methylanthraquinone (disperse orange 11-DO11) in solid poly(methyl methacrylate). Measurements as a function of intensity, concentration, and time suggest that ASE originates in a state (be it a tautomer or a vibronic level) that can form a dimer or some other aggregate upon relaxation, which through fluorescence quenching leads to degradation of the ASE signal. Whatever the degradation route, a high concentration of DO11 is required and the polymer plays a key role in the process of opening a new reversible degradation pathway that is not available at lower concentrations or in liquid solutions. We construct an energy level diagram that describes all measured quantities in the decay and recovery processes and propose a hypothesis of the nature of the associated states.

A self healing model based on polymer-mediated chromophore correlations

Here we present a model of self healing in which correlations between chromophores, as mediated by the polymer, are key to the recovery process. Our model determines the size distribution of the correlation volume using a grand canonical ensemble through a free energy advantage parameter. Choosing a healing rate that is proportional to the number of undamaged molecules in a correlated region, and a decay rate proportional to the intensity normalized to the correlation volume, the ensemble average is shown to correctly predict decay and recovery of the population of disperse orange 11-DO11 (1-amino-2-methylanthraquinone) molecules doped in PMMA polymer as a function of time and concentration as measured with amplified spontaneous emission and linear absorption spectroscopy using only three parameters that apply to the full set of data. Our model also predicts the temperature dependence of the process. One set of parameters should be characteristic of a particular polymer and dopant chromophore combination. Thus, the use of the model in determining these parameters for various materials systems should provide the data needed to test fundamental models of the underlying mechanism responsible for self healing.

Experimental tests of a new correlated chromophore domain model of self-healing in a dye-doped polymer

Temperature-dependent photodegradation and recovery studies of Dipserse Orange 11 (DO11) dye dissolved in poly(methyl methacrylate) and polystyrene polymer hosts are used as a test of a new model that builds on the recently proposed correlated chromophore domain model. [Ramini and Kuzyk, J. Chem. Phys., 2012, 137, 054705.] Our model posits that dye molecules form domains or aggregates. The nature of aggregation or how it mediates self-healing is not yet well understood. In this paper we present evidence that supports the hypothesis that the dye molecules undergo a change to a state with higher dipole moment whether it be a tautomer or a TICT state and hydrogen bond with the amines and keto-oxygens of the polymer. Groupings of such molecules in a polymer chain form what we call a domain, and interactions between molecules in a domain make them more robust to photodegradation and mediate self-healing. All of the data is found to be consistent with our new model in the time–temperature–intensity–concentration domain.

Experimental verification of a self-consistent theory of the first-, second-, and third-order (non)linear optical response

We show that a combination of linear absorption spectroscopy, hyper-Rayleigh scattering, and a theoretical analysis using sum rules to reduce the size of the parameter space leads to a prediction of the imaginary part of the second hyperpolarizability of the dye AF-455 that agrees with the experimental data gathered through two-photon absorption spectroscopy. Our procedure, which demands self-consistency between several measurement techniques and does not use adjustable parameters, provides a means for determining transition moments between the dominant excited states based strictly on experimental characterization. This is made possible by our new approach that uses sum rules and molecular symmetry to rigorously reduce the number of required physical quantities.

The role of the polymer host on reversible photodegradation in Disperse Orange 11 dye

The photodegradation and recovery process in anthraquinone chromophores have been characterized using am plified spontaneous emission (ASE) in several Anthraquinone derivatives doped in poly(methyl methacrylate) (PMMA). To understand the mechanisms of self healing in disperse orange 11 (DOll) doped in PMMA and the role of the host polymer, we investigated reversible photodegradation in DOll doped in copolymer composed of different percentages of methyl methacrylate (MMA) and Styrene. The results suggest that the host polymer is an important factor in determining the distribution of domain sizes of dye molecules.

Testing the diffusion hypothesis as a mechanism of self-healing in Disperse Orange 11 doped in poly(methyl methacrylate)

In this work, we show that reversible photodegradation of Disperse Orange 11 doped in poly(methyl methacrylate) is not due to dye diffusion—a common phenomenon observed in many dye-doped polymers. The change in linear absorbance due to photodegradation of the material shows an isobestic point, which is consistent with the formation of a quasi-stable damaged species. Spatially resolved amplified spontaneous emission and fluorescence, both related to the population density, are measured by scanning the pump beam over a burn mark. A numerical model of the time evolution of the population density due to diffusion is inconsistent with the experimental data, suggesting that diffusion is not responsible.

Recent progress in reversible photodegradation of Disperse Orange 11 when doped in PMMA

We report observations that dye-doped PMMA polymer with the organic dye Disperse Orange 11 exhibits self healing after photodegradation by continuous optical pumping whereas in liquid solution, degradation is permanent. This observation illustrates the important role of the polymer matrix in facilitating recovery of the dye molecules. In this work, we report on linear optical absorbance studies that confirm the existence of a quasi-stable state that is not formed in liquid solution. Studies as a function of dye concentration and temperature support our hypothesis of the role of molecular interactions in the decay and healing process that is mediated by the polymer host.

Correlation between molecular structure and self-healing in aseries of Anthraquinone derivatives doped in PMMA polymer

We observe that many different derivatives of anthraquinone chromophores doped in PMMA self heal after undergoing photodegradation. We are interested to know the mechanisms that are responsible for photodegradation and photorecovery, which are not yet fully understood. We used fluorescence and absorption spectroscopy as a probe of the photodegradation and recovery process while the temperature dependence is used to determine the energies of the species involved. We hypothesize that the host polymer mediates the formation of a quasi-stable state. In this scenario, once photo - damaged by intense pump laser, the molecules non radiatively decay into a tautomer state by intra molecule proton transfer, which subsequently leads to the formation of a damaged species - leading to decay of the fluorescence intensity. This hypothesis is consistent with our observation. The temperature dependent fluorescence decay and recovery studies give an insight about the different energy levels participating in optical excitation, decay and recovery. Comparing the experimental parameters such as decay and recovery rates of the fluorescence signal associated with the evolution of peaks in the fluorescence and absorbance spectrum helps us understand correlations between the efficiency of the recovery process and the structures of the dye molecules. Based on the temperature and the time-dependent observations of fluorescence and absorption, we validate qualitatively a new theoretical model which qualitatively takes into account the observed behavior and sheds light on the underlying mechanism. Preliminary measurements show good agreement with the theoretical model. More careful experiments and calculations are in process for further validation of the model.

Imaging studies of photodamage and recovery of anthraquinone derivatives doped into PMMA

One of the efficiency-limiting factors of optical devices used at high intensities is photo damage to the optical materials. In devices that use organic dyes, photo damage causes irreversible damage to the chromophores, deteriorating efficiency, and eventually causes failure. Our present work focuses on monitoring degradation and recovery of anthraquinone dye doped PMMA thin films with a digital imaging apparatus, with the goal of understanding the mechanisms. Our results suggest the possibility of making optical components more resistant to photodamage, and capable of self recovery, removing the necessity to constantly replace components damaged by high intensity light.

Correlated aggregate model of self-healing in dye-doped polymers

Self healing of chromophores in a dye-doped polymer after photodegradation is a counterintuitive process based on the nearly universal observation that molecular damage is a thermodynamically irreversible process. We propose a new simple model of this phenomenon that takes into account all observations, including the effects of concentration, temperature, and bystander states. Critical to this model are correlations between chromophores, perhaps mediated by the polymer, which actively favors the undamaged species in analogy to Bose-Einstein condensation. We use this model to predict the behavior of decay and recovery experiments as measured with amplified spontaneous emission and absorption spectroscopy.