First principles theoretical spectroscopy of methylene blue: Between limitations of time-dependent density functional theory approximations and its realistic description in the solvent

Abstract

Methylene blue [3,7-Bis(di-methylamino) phenothiazin-5-ium chloride] is a phenothiazine dye with applications as a sensitizer for photodynamic therapy, photoantimicrobials, and dye-sensitized solar cells. Time-dependent density functional theory (TDDFT), based on (semi)local and global hybrid exchange-correlation functionals, fails to correctly describe its spectral features due to known limitations for describing optical excitations of π-conjugated systems. Here, we use TDDFT with a non-empirical optimally tuned range-separated hybrid functional to explore the optical excitations of gas phase and solvated methylene blue. We compute solvated configurations using molecular dynamics and an iterative procedure to account for explicit solute polarization. We rationalize and validate that by extrapolating the optimized range separation parameter to an infinite amount of solvating molecules, the optical gap of methylene blue is well described. Moreover, this method allows us to resolve contributions from solvent–solute intermolecular interactions and dielectric screening. We validate our results by comparing them to first-principles calculations based on the GW+Bethe–Salpeter equation approach and experiment. Vibronic calculations using TDDFT and the generating function method account for the spectra’s subbands and bring the computed transition energies to within 0.15 eV of the experimental data. This methodology is expected to perform equivalently well for describing solvated spectra of π-conjugated systems.

Influence of non-adiabatic effects on linear absorption spectra in the condensed phase: Methylene blue

“…performed AHFC calculations of MB in vacuum at the TDDFT/B3LYP/def2-SVP level, which produces a large shoulder in line with experimental results, stemming purely from vibronic contributions to the S 1 state. [54] Our calculations over a range of functionals and lineshape approaches suggest that this large shoulder is anomalous, occurring only for this specific combination of full (non-TDA) TDDFT and B3LYP in vacuum, and could be due to the excited state geometry optimization yielding a state that is of mixed S 1 -S 2 character. Many results in their work indicate state mixing.…”

“…al. show that significant intensity-borrowing between S 1 and S 2 can occur for selected snapshots of MB in explicit solvent [54], suggesting that both non-adiabatic and solvent effects might play an important role in the experimental lineshape.…”

“…Yet in the same study their computation of the S 0 → S 1 Franck-Condon spectra in vacuo overestimates the shoulder intensity. [54] Here we expand on previous studies of MB by including explicitly quantum mechanical treatment of the solvent polarization on excitation and nuclear dynamics for vibronic lineshapes. Linear absorption spectra are computed using second and third order truncation of a cumulant expansion of the linear response function constructed from energy gap fluctuations along a molecular dynamics trajectory.…”

Applicability of the thawed Gaussian wavepacket dynamics to the calculation of vibronic spectra of molecules with double-well potential energy surfaces

“…Electronic and photoelectron spectroscopies are regularly used for studying electronic states of molecules and the associated potential energy surfaces. 1,2 To interpret or simulate vibronic features of measured spectra, researchers often invoke the harmonic approximation, [3][4][5][6][7][8][9][10] which is sometimes valid and greatly simplifies the calculations. However, there are many systems for which this approximation fails, including floppy molecules that exhibit large-amplitude motion along bending or torsional degrees of freedom.…”

Comparison of Linear Response Theory, Projected Initial Maximum Overlap Method, and Molecular Dynamics Based Vibronic Spectra: The Case of Methylene Blue

“…A recent study by de Queiroz et al of the S 0 → S 1 Franck-Condon vibronic spectrum in vacuo, obtained at the linear response TDDFT B3LYP/def2-SV(P) level of theory, predicts a very large vibronic shoulder. 45 However, very recent work by some of the authors suggests that the LR-TD-B3LYP S 0 → S 1 vibronic shoulder may be due to S 1 /S 2 state mixing and a potential breakdown of the Born-Oppenheimer approximation. 46 We here investigate this state mixing possibility more thoroughly and also present the ability of PIMOM to approximate excited states and absorption spectra that are not subject to state mixing and thus find that PIMOM is more amenable than LR-TDDFT to the use of the excited state Hessian for predicting vibronic spectra.…”