The photophysical properties of a newly synthesized unsymmetrically substituted zinc phthalocyanine derivative (1) bearing in its peripheral positions six n-hexylsulfanyl substituents and one amino-terminated n-hexylsulfanyl substituent were investigated. This mono-amino phthalocyanine exhibited a high tendency to form H-type aggregates in all of the investigated solvents: dichloromethane (DCM), tetrahydrofuran (THF) and dimethyl sulfoxide (DMSO). Several species of H-aggregates were present together in relatively broad concentration ranges in THF and DCM, whereas in DMSO they were observed separately depending on the concentration used. Despite the widely accepted non-emissive character of H-type dimers, the H-type aggregates of phthalocyanine 1 were highly emissive in all solvents: the fluorescence quantum yield in DMSO for the n-aggregate is equal to 0.05, whereas for the (n + 1)-aggregate it is 0.11. Upon (n + 1)-aggregation, the fluorescence lifetime of the n-aggregate increased from ca. 2.5 ns to 3.3 ns. Based on these results, the radiative lifetimes of both species were computed: 48 ns for the n-aggregate and 29 ns for the (n + 1)-aggregate. The determined oscillator strengths for the n-aggregate and the (n + 1)-aggregate in DMSO were 0.04 and 0.12, respectively. The observed emission of the H-type (n + 1)-aggregate was assigned to the radiative transition from the upper exciton state to the ground state, which could be rationalized by a constant thermal repopulation of the upper exciton state. The experimental findings were supported by theoretical calculations.

The accurate ground-state potential energy function of the beryllium dimer, Be₂ , has been determined from large-scale ab initio calculations using the single-reference coupled-cluster approach in conjunction with the correlation-consistent core–valence basis sets up to septuple-zeta quality. Results obtained with the conventional and explicitly-correlated coupled-cluster methods were compared. The scalar relativistic and adiabatic (the diagonal correction) effects were also discussed. The vibration–rotation energy levels of Be₂ were predicted and found to be as accurate as those determined from the empirical potential energy function [J. M. Merritt et al., Science, 2009, 324, 1548]. The potential energy function of Be₂ was determined in this study to have a minimum at 2.444 Å and the well depth of 935 cm⁻¹.

The equilibrium structure and potential energy surface of beryllium dihydride BeH2 in its ground electronic state have been determined from highly accurate ab initio calculations. The vibration-rotation energy levels of three isotopomers BeH2, BeD2, and BeHD were predicted using the variational method. The calculated spectroscopic constants are in remarkably good agreement with the existing experimental data (sub-cm−1 accuracy) and should be useful in a further analysis of high-resolution vibration-rotation spectra of all three isotopomers.

The hydrogen bond energies for 4H-1-benzopyrane-4-thione (BPT) in its ground and two lowest excited singlet electronic states have been determined using ab initio methods. It was shown that the BPT molecule can form, as an acceptor, four relatively strong hydrogen bonds with water molecules, leading to a stable complex in the ground electronic state S₀. The hydrogen bonds involving the sulfur atom from the thiocarbonyl group were found to be stronger than those involving the oxygen atom from the benzopyran moiety. The former hydrogen bonds were predicted to become significantly weaker upon excitation to the S₁ state and, in contrast, stronger upon excitation to the S₂ state. Calculated changes in the hydrogen bond energy due to the S₀ → S₁ and S₀ → S₂ excitation are in very good agreement with the experimental values obtained from the absorption solvatochromic study, according to a procedure proposed by us in [E. Krystkowiak, et al., J. Photochem. Photobiol. A: Chem., 2006, 184, 250]. The maxima of absorption spectra of the BPT–water hydrogen-bonded complex, calculated taking into consideration nonspecific solute–solvent interactions, are also in good agreement with the experimental results.

The accurate ground-state potential energy function of beryllium monohydride, BeH, has been determined from large-scale ab initio calculations using the multi-reference averaged coupled-pair functional (MR-ACPF) method in conjunction with the correlation-consistent core-valence basis sets up to septuple-zeta quality. The effects of electron correlation beyond the MR-ACPF level of approximation were taken into account. The scalar relativistic and adiabatic (the diagonal correction) effects, as well as some of the nonadiabatic effects, were also discussed. The vibration-rotation energy levels of three isotopologues, BeH, BeD, and BeT, were predicted to sub-cm⁻¹ accuracy.