Electronic Excitations of Simple Cyanine Dyes: Reconciling Density Functional and Wave Function Methods

Area of Science:

• Computational Chemistry
• Theoretical Spectroscopy
• Dye Chemistry

Background:

• Simple cyanine dyes present challenges for theoretical excited-state methods.
• Experimental spectra of these dyes are difficult to predict accurately.
• Standard theoretical arguments fail to explain observed spectral deviations.

Purpose of the Study:

• To compute and compare the lowest vertical excitation energies of cyanine dyes using various theoretical methods.
• To analyze the impact of computational parameters like basis set and active space.
• To investigate excited-state relaxation effects to reconcile theoretical predictions with experimental spectra.

Main Methods:

• Complete active space second-order perturbation theory (CASPT2)
• Quantum Monte Carlo methods (QMC)
• Coupled cluster linear response (CC3)
• Time-dependent density functional theory (TDDFT), including B2PLYP

Main Results:

• Wave function methods yield comparable excitation energies; CASPT2 gives the lowest.
• The B2PLYP scheme shows improved agreement with CC3 and DMC compared to other TDDFT flavors.
• Discrepancies with experimental spectra are attributed to nonvertical transitions, not solely theoretical method limitations.

Conclusions:

• Direct comparison of vertical excitations and experimental absorption maxima is not feasible due to nonvertical transitions.
• Previous agreements with CASPT2 were artifacts of specific computational choices.
• Accurate prediction of cyanine dye spectra requires accounting for excited-state relaxation and nonvertical effects.