Excited States of Xanthene Analogues: Photofragmentation and Calculations by CC2 and Time‐Dependent Density Functional Theory

Abstract

Action spectroscopy has emerged as an analytical tool to probe excited states in the gas phase. Although comparison of gas-phase absorption properties with quantum-chemical calculations is, in principle, straightforward, popular methods often fail to describe many molecules of interest-such as xanthene analogues. We, therefore, face their nano- and picosecond laser-induced photofragmentation with excited-state computations by using the CC2 method and time-dependent density functional theory (TDDFT). Whereas the extracted absorption maxima agree with CC2 predictions, the TDDFT excitation energies are blueshifted. Lowering the amount of Hartree-Fock exchange in the DFT functional can reduce this shift but at the cost of changing the nature of the excited state. Additional bandwidth observed in the photofragmentation spectra is rationalized in terms of multiphoton processes. Observed fragmentation from higher-lying excited states conforms to intense excited-to-excited state transitions calculated with CC2. The CC2 method is thus suitable for the comparison with photofragmentation in xanthene analogues.

Why the lowest electronic excitations of rhodamines are overestimated by time‐dependent density functional theory

“…The computed S 1 states of RDs by TDDFT with GH GGA and meta‐GGA functionals are in the ππ* state and correspond to pure transitions from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). This result is in accordance with those of previous theoretical studies . Figure shows the computed HOMO and LUMO of Rhod101 by TD‐B3LYP as an example, and both the HOMO and LUMO are localized on xanthene moiety.…”

“…Designing and development of new RDs are still in progress despite the fact that many chemically modified RDs are commercially available . Previous theoretical studies found that the lowest excitation energies of RDs are overestimated systematically at approximately 0.4 eV by TDDFT approximations . Savarese et al also reported that the TD‐B3LYP‐D/6‐31 + G(d,p)/CPCM calculated the lowest excitation energy of rhodamine 110 (R110) in aqueous solution (~2.63 eV) and reproduces the experimental data (2.49 eV) .…”

“…In addition, the TD‐BP86 computed S 2 state of 5Rox is a mixed LE and CT state involving the CT from xanthene moiety to benzene moiety. However, the S 1 state of RDs should be a locally excited (LE) ππ* state, as demonstrated by previous studies . Therefore, as shown in Table and Supporting Information Table S1, the corresponding S 2 or S 3 states are listed to compare the experimental data.…”

“…In this work, we first evaluated the performance of TDDFT approximations with 21 density functionals in reproducing the lowest excitation energies of a series of RDs. The statistical analysis of our theoretical results shows that increasing the amount of HFX in exchange correlation functional can increase the predicted excitation energies; previous studies also came to a similar conclusion. Consequently, pure functionals without HFX provide the best performance with MAE at approximately 0.1 eV.…”

“…TDDFT with certain conventional approximate functional typically underestimates the excitation energies of charge transfer (CT) states, and range‐separated (RS) functionals can be used as alternatives . TDDFT also yields a systematic and large overestimation of the lowest electronic singlet excitation energies of linear cyanine, boron‐dipyrromethene (BODIPY) dyes that can be considered as a cis‐constrained cyanine, as well as rhodamine and rosamine dyes, regardless of either selected functionals, basis sets, or ground state geometries. A relatively large number of theoretical studies have been conducted to reveal the reason for the failure of TDDFT approximations in predicting the lowest excitation energies of linear cyanine dyes; this failure has been attributed to the difficulty of capturing the differential electron‐correlation effects between ground and excited states .…”