Rapid relaxation pathway of the excited state of linear merocyanines in solutions

Abstract

Excited‐state relaxation of linear merocyanine dyes in solution is investigated using time‐resolved spectroscopy techniques and quantum chemical calculations. The merocyanine L‐Mero4 and phenyl‐substituted P‐L‐Mero4 have a S‐trans and S‐cis structure, respectively, consisting of indole moiety as the donor, indandione as the acceptor, and the tetramethine as the bridge. The time‐correlated single‐photon counting (TCSPC) picosecond measurements after excitation at wavelength 515 nm to the ππ* state yield emission curves with a short component τ1 in the range of 27–160 ps and a second component τ2 of 200–780 ps for L‐Mero4. In P‐L‐Mero4, τ1 lies in the range of 18–150 ps and τ2 220–520 ps. The subfemtosecond transient absorption measurements yield a short component around 0.4–1.4 ps, and the second/third components are similar to those in the TCPSC measurements. The analysis of the experimental data demonstrates that the ground state recovery exhibits a biexponential rise and rapidly indicates that the conversion back to the electronic ground state provides a fast, nonradiative pathway. Quantum chemical calculations on the electronic structures and their dependence on the molecular confirmation are performed. We identify the excited states and the relaxation path along the twist of the center double bonds in tetramethine that might be the nonradiative pathway. The C=C double bond is weakened in the ππ* state. The phenyl substitution in the conjugated double bond weakens this C=C bond, lowers the isomerization barrier, increases the nonradiative rate, and reduces the emission quantum yield. In polar solvents, the energy of the perpendicular conformer along the trans–cis isomerization path is increased to achieve less coupling to the ground state surface. Because of the small barrier to the trans form, these two conformers establish an equilibrium condition. The trans form, which lies at a lower energy, gains more population and thus has a higher emission yield.

Photoisomerization of linear merocyanines in salt solutions

“…Under 515 nm wavelength excitation, the S 1 1 ππ* (ππ* on tetramethine) was accessed. [ 1,2 ] Most emission curves displayed multiple exponential decay components and some had rise components. All were fitted with multi‐exponential functions convoluted with an IRF to attain time constants.…”

“…This component suffered quenching in high salt concentrations, resulting in shorter τ 3 and a smaller quantum yield. According to the DFT calculation of Wang et al, [ 2 ] the position of p lies above the crest of the potential energy surface of the ground state along the twisting angle of CC, as shown in Figure 4. Going to the cis conformer has a slightly greater barrier than going back to the trans conformer, and the decay of p via the nonadiabatic pathway to the ground state is also slow because of the large energy gap.…”

“…[ 1 ] Their fluorescence intensities varied with the viscosity and polarity of solvent, can serve as environmentally sensitive fluorescence probes. [ 1,2 ]…”

The origin of the solvent dependence of fluorescence quantum yields in dipolar merocyanine dyes

“…[25][26][27] In a recent data survey on distyrylbenzene derivatives an inverted energy gap law for the non-radiative decay rate was attributed to access to a conical intersection. 28 Conical intersections in the vicinity of the Franck-Condon point may impact the uorescence rate by opening an additional temperature-dependent channel for relatively slow non-radiative decay [29][30][31] and photoisomerization. 12,[32][33][34][35] If a barrier separates a stable minimum close to the Franck-Condon point from a conical intersection, it is legitimate to assume that the reactions in the excited state are also governed by the rules of thermodynamics, so that one can apply transition-state theory for estimating the temperature-dependent rates of non-radiative decay.…”

“…54 In merocyanines as well as in cyanines solvent dependencies of the quantum yields were observed and possible relaxation channels were discussed. 12,31,[54][55][56] In this work we aim to elucidate the reason for this behavior of merocyanines. Our hypothesis is that the structural relaxation that follows the displacement of the electron cloud in these push-pull chromophores aer photoexcitation might be hindered by a barrier to a conical intersection that is very sensitive to the polarity of the solvent.…”