Abstract
In contrast to the ground state, the calculation of the infrared (IR) spectroscopy of molecular singlet excited states represents a substantial challenge. Here, we use the structural IR fingerprint of the singlet excited states of a range of coumarin dyes to assess the accuracy of density functional theory based methods for the calculation of excited state IR spectroscopy. It is shown that excited state Kohn-Sham density functional theory provides a high level of accuracy and represents an alternative approach to time-dependent density functional theory for simulating the IR spectroscopy of singlet excited states.
Highlights
Computational chemistry has impact right across molecular science
All four highest occupied molecular orbitals (HOMOs) have a significant contribution from the benzene rings and the substituent nitrogen atoms, while the lowest unoccupied molecular orbitals (LUMOs) are more localized on the cyclic-ester rings and oxygen atoms
This is consistent with an intramolecular charge transfer character (ICT) model for the S1 excited state and high fluorescent quantum yields have been observed for all the coumarins in acetonitrile,[46,47,48] indicating an ICT excited state in this polar solvent.[46,47,48]
Summary
Computational chemistry has impact right across molecular science. While computational methods for determining the structure and properties of molecules in their electronic ground state are well established to the extent that such calculations are often considered as standard practice,[1,2,3,4,5] a different picture emerges if one considers calculations for molecules in electronically excited states, singlet excited states. 7-aminocoumarins are commonly used as laser dyes due to their high quantum yields of emission and low lying ππ* excited states of essentially HOMO to LUMO character.[40,41,42,43,44,45] The first excited singlet states of the 7-aminocoumarins exhibit relatively strong solvatochromic behavior, which has been widely attributed to partial intramolecular charge transfer character (ICT) between the nitrogen atom and carbonyl oxygen atom following extensive studies using TDDFT,[41,42,43,44,45] together with emission and absorption spectroscopy.[46,47,48] Despite this charge separation, unsubstituted 7-aminocoumarins are too small for the partial ICT character to be considered as a long-range charge transfer excited state in TDDFT, and B3LYP performs adequately for calculating excitation energies.[41,42,43,44,45] This is important for comparing with TRIR data, as the B3LYP functional is known to reproduce ground state experimental frequencies well using both scaled harmonic[49, 50] and anharmonic[2, 51] techniques, while other commonly used functionals, such as PBE052 or CAM-B3LYP25 and other range corrected functionals often predict frequencies of significantly lower quality.[50, 53]
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