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Time resolved luminescence measurements on polydiacetylenes

Christian Blumer

In this work time resolved luminescence measurements on the red phase of 3BCMU are presented. These polydiacetylenes form an ordered and isolated one dimensional model system. This work is divided in two parts. First we present an excitation spectrum in chapter 4. In the second part, radiative lifetimes are examined at temperatures down to 2 K (chapter 5). We present studies on excitation spectra on the red phase of 3BCMU up to an excitation energy of 850 meV above the zero phonon excitonic line. The vibronic modes of the double and triple bond of the carbon of the backbone and their combinations are identified. Compared to luminescence spectra a small anharmonicity is found. Also a relatively small Huang-Rhys factor is obtained, a parameter describing the decrease in intensity of the successive multiple of a vibronic mode. This is a result expected for such a complex system as a polymer. Furthermore, any signature of the band edge is absent in the spectrum. This result is interesting, as the theory predicts a suppression of the van Hove singularity at the band edge. Experimental results on other one-dimensional systems on this subject are rare. The radiative lifetime is a parameter which is dependent on the dimensionality of the structure. In the special case of one-dimensional systems it is predicted to be proportional to the square root of the temperature. Measurements of other one-dimensional systems show that the law is broken at relatively high temperatures (around 30 K) because of fluctuation of the confinement potential, which leads to localisation. For our sample, the radiative lifetime follows the mentioned law to a temperature down to 2 K. However, we show that in samples of less quality, mainly due to ageing, the law is not valid anymore. This ageing of the sample is caused by a higher polymerisation rate and defect in the crystalline structure.

EPFL2006

Semiclassical Approach to Photophysics Beyond Kasha’s Rule and Vibronic Spectroscopy Beyond the Condon Approximation. The Case of Azulene

Tomislav Begusic, Seonghoon Choi, George Cameron Fish, Jacques-Edouard Moser, Antonio Prlj, Julien Roulet, Jiri Vanicek, Marius Wehrle, Zhan Tong Zhang, Tomas Zimmermann

Azulene is a prototypical molecule with ananomalousfluorescence from the second excited electronic state,thus violating Kasha’s rule, and with an emission spectrum thatcannot be understood within the Condon approximation. To betterunderstand the photophysics and spectroscopy of azulene andother nonconventional molecules, we developed a systematic,general, and efficient computational approach combining thesemiclassical dynamics of nuclei withab initioelectronic structure.First, to analyze the nonadiabatic effects, we complement thestandard population dynamics by a rigorous measure ofadiabaticity, estimated with the multiple-surface dephasing representation. Second, we propose a new semiclassical method forsimulating non-Condon spectra, which combines the extended thawed Gaussian approximation with the efficient single-Hessianapproach. S1←S0and S2←S0absorption and S2→S0emission spectra of azulene, recorded in a new set of experiments, agree verywell with our calculations. Wefind that accuracy of the evaluated spectra requires the treatment of anharmonicity, Herzberg−Teller,and mode-mixing effects.

2020

Efficient Semiclassical Dynamics for Vibronic Spectroscopy beyond Harmonic, Condon, and Zero-Temperature Approximations

Tomislav Begusic, Jiri Vanicek

Understanding light-induced processes in biological and human-made molecular systems is one of the main goals of physical chemistry. It has been known for years that the photoinduced dynamics of atomic nuclei can be studied by looking at the vibrational substructure of electronic absorption and emission spectra. However, theoretical simulation is needed to understand how dynamics translates into the spectral features. Here, we review several recent developments in the computation of vibrationally resolved electronic spectra (sometimes simply called "vibronic" spectra). We present a theoretical approach for computing such spectra beyond the commonly used zero-temperature, Condon, and harmonic approximations. More specifically, we show how the on-the-fly ab initio thawed Gaussian approximation, which partially includes anharmonicity effects, can be combined with the thermo-field dynamics to treat non-zero temperature and with the Herzberg-Teller correction to include non-Condon effects. The combined method, which can treat all three effects, is applied to compute the S1 ← S0 and S2 ← S0 absorption spectra of azulene.

2021