Theoretical study of photodetachment processes of anionic boron clusters. II. Dynamics

Photodetachment bands of anionic boron clusters, Bn (n = 4,5) are theoretically examined here. The model Hamiltonians developed through extensive ab initio quantum chemistry calculations in Paper I are employed for the required nuclear dynamics study. While the precise location of vibronic lines and progression of vibrational modes within a given electronic band is derived from time-independent quantum mechanical studies, the broadband spectral envelopes and the nonradiative decay rate of electronic states are calculated by propagating wave packets in a time-dependent quantum mechanical framework. The theoretical results are in good accord with the experiment to a large extent. The discrepancies between the two can be partly attributed to the inadequate energy resolution of the experimental results and also to the neglect of dynamicspin-orbit interactions and computational difficulty related with detachment channels involving multi-electron transitions in the theoretical formalism

Theoretical study of photodetachment processes of anionic boron clusters. I. Structure

Photo-induced electron detachment spectroscopy of anionic boron clusters, B<SUP>−</SUP><SUB>4</SUB> and B<SUP>−</SUP><SUB>5</SUB>, is theoretically investigated by performing electronic structure calculations and nuclear dynamics simulations. While the electronic potential energy surfaces (X&#732;<SUP>1</SUP>A<SUB>g</SUB>, a&#732;<SUP>3</SUP>B<SUB>2u</SUB>, b&#732;<SUP>3</SUP>B<SUB>1u</SUB>, &#195;<SUP>1</SUP>B<SUB>2u</SUB>, c&#732;<SUP>3</SUP>B<SUB>2g</SUB>, and B&#732;<SUP>1</SUP>B<SUB>2g</SUB> of neutral B<SUB>4</SUB> and X&#732;<SUP>2</SUP>B<SUB>2</SUB>, &#195;<SUP>2</SUP>A<SUB>1</SUB>, B&#732;<SUP>2</SUP>B<SUB>2</SUB>, C&#732;<SUP>2</SUP>A<SUB>1</SUB>, D&#732;<SUP>2</SUP>B<SUB>1</SUB>, and E&#732;<SUP>2</SUP>A<SUB>1</SUB> of neutral B<SUB>5</SUB>) and their coupling surfaces are constructed in this paper, the details of the nuclear dynamics on these electronic states are presented in Paper II. Electronic structure calculations are carried out at the complete active space self-consistent field – multi-reference configuration interaction level of theory employing the correlation consistent polarized valance triple zeta basis set. Using the calculated electronic structure data suitable vibronic Hamiltonians are constructed utilizing a diabatic electronic basis and displacement coordinates of the normal vibrational modes. The theoretical results are discussed in relation to those recorded in recent experiments

Theoretical study of photodetachment processes of anionic boron cluster. III. B^-₇

Photodetachment spectroscopy of B-7 is theoretically studied in this paper. Calculated photodetachment bands are compared with the available experimental results and assigned to the vibronic structure of the electronic ground and excited states of the neutral B7 cluster. The complex structure of photodetachment bands is found to arise from many stable isomers of B-7 of different symmetry point group. In this study we focus on three most stable isomers of B-7 and examine their photodetachment bands. Extensive quantum chemistry calculations are carried out to establish the potential energy surfaces and the coupling surfaces of the electronic states of neutral B7 originating from each of the three isomers. A diabatic electronic ansatz is employed and the nuclear dynamics is studied both by time-independent and time-dependent quantum mechanical methods. Both agreements and discrepancies of the theoretical results with the experimental findings are discussed

Photophysics of fluorinated benzene. III. Hexafluorobenzene

A theoretical study of the photoabsorption spectroscopy of hexafluorobenzene (HFBz) is presented in this paper. The chemical effect due to fluorine atom substitution on the electronic structure of benzene (Bz) saturates in HFBz. State- of-the-art quantum chemistry calculations are carried out to establish potential energy surfaces and coupling surfaces of five energetically low-lying electronic (two of them are orbitally degenerate) states of HFBz. Coupling of these electronic states caused by the Jahn-Teller(JT) and pseudo-Jahn-Teller (PJT) type of interactions are examined. The impact of these couplings on the nuclear dynamics of the participating electronic states is thoroughly investigated by quantum mechanical methods and the results are compared with those observed in the experiments. The complex structure of the S1 ← S0absorption band is found to originate from a very strong nonadiabatic coupling of the S2 (of πσ* origin) and S1 (of ππ* origin) state. While S2 state is orbitally degenerate and JT active, the S1 state is nondegenerate. These states form energetically low-lying conical intersections (CIs) in HFBz. These CIs are found to be the mechanistic bottleneck of the observed low quantum yield of fluorescence emission, non overlapping absorption, and emission bands of HFBz and contribute to the spectral width. Justification is also provided for the observed two peaks in the second absorption (the unassigned “c band”) band of HFBz. The peaks observed in the third, fourth, and fifth absorption bands are also identified and assigned

Vibronic coupling in the X&#732;<SUP>2</SUP>Π<SUB>g</SUB>–&#195;<SUP>2</SUP>Π<SUB>u</SUB> band system of diacetylene radical cation

Vibronic interactions in the two energetically lowest electronic states (X&#732;<SUP>2</SUP>Π<SUB>g</SUB>–&#195;<SUP>2</SUP>Π<SUB>u</SUB>) of the diacetylene radical cation (C<SUB>4</SUB>H<SUB>2</SUB><SUP>•+</SUP>) are theoretically examined here. The spectroscopy of these two electronic states of C<SUB>4</SUB>H<SUB>2</SUB><SUP>•+</SUP> has been a subject of considerable interest and measured in the laboratory by various groups. Inspired by numerous experimental data, we attempt here a detailed investigation of vibronic interactions within and between the doubly degenerate Π electronic states and their impact on the vibronic structure of each state. A vibronic coupling model is constructed in a diabatic electronic basis and with the aid of ab initio quantum chemistry calculations. The vibronic structures of the electronic states are calculated by time-independent and time-dependent quantum mechanical methods. The progression of vibrational modes in the vibronic band is identified, assigned, and compared with the literature data. The nonradiative internal conversion dynamics is also examined and discussed

Parallel versus twisted pentacenes: Conformational impact on singlet fission

We placed two pentacene chromophores at the termini of a diacetylene linker to investigate the impact of excitation wavelength, conformational flexibility, and vibronic coupling on singlet fission. Photoexcitation of the low-energy absorption results in a superposed mixture of states, which transform on an ultrafast time-scale into a spin-correlated and vibronically coupled/hot delocalized triplet pair 1(T1T1)deloc. Regardless of temperature, the lifetime for 1(T1T1)deloc is less than 2 ps. In contrast, photoexcitation of the high-energy absorption results in the formation of 1(T1T1)deloc lasting 1.0 ps, which then decays at room temperature within 4 ps via triplet–triplet annihilation. Lowering the temperature enables 1(T1T1)deloc to delocalize and vibronically decouple, in turn affording 1(T1T1)loc. In addition, our results suggest that the quasi-free rotation at the diacetylene spacer may lead to twisted conformations with very low SF quantum yields, highlighting the need of controlling this structural aspect in the design of new singlet fission active molecules.This work was supported by funding from the German Research Foundation (DFG) GU 517/27-1, the “Solar Energy goes Hybrid” Initiative of the Bavarian Ministry for Science, Culture and Education (SolTech), from the Ministry of Science, Innovation and Universities of Spain through the Beatriz Galindo Programme (MCIU-19-BEAGAL 18/0224) and project PGC2018-095953-B-I00, from the DFG through a research grant and the Mercator Programme, from the state of Baden-Württemberg through bwHPC and the DFG through Grants No. INST 40/575-1 FUGG (JUSTUS 2 cluster), from University Grants Commission (UGC), India (ref. no. F.30-547/2021(BSR)) and from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Foundation for Innovation (CFI).Peer reviewe

Solution-based intramolecular singlet fission in cross-conjugated pentacene dimers

We show unambiguous and compelling evidence by means of pump–probe experiments, which are complemented by calculations using ab initio multireference perturbation theory, for intramolecular singlet fission (SF) within two synthetically tailored pentacene dimers with cross-conjugation, namely XC1 and XC2. The two pentacene dimers differ in terms of electronic interactions as evidenced by perturbation of the ground state absorption spectra stemming from stronger through-bond contributions in XC1 as confirmed by theory. Multiwavelength analysis, on one hand, and global analysis, on the other hand, confirm that the rapid singlet excited state decay and triplet excited state growth relate to SF. SF rate constants and quantum yields increase with solvent polarity. For example, XC2 reveals triplet quantum yields and rate constants as high as 162 ± 10% and (0.7 ± 0.1) × 1012 s−1, respectively, in room temperature solutions