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

• Published in: Journal of the Chinese Chemical Society (Taipei) 2019-09, Vol.66 (9), p.1105-1118
• Main Authors: Wang, Shao‐An, Chen, Wei‐Hao, Syue, Ming‐Lun, Tan, Kui‐Thong, Chen, I‐Chia
• Format: Article
• Language: English
• Subjects: charge–transfer state; Coupling (molecular); Dependence; Emissions control; Excitation; Ground state; Isomerization; Mathematical analysis; merocyanine; Organic chemistry; Quantum chemistry; time‐resolved spectroscopy
• ISSN: 0009-4536; 2192-6549
• DOI: 10.1002/jccs.201900121

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. Relaxation of L‐Mero4 is proposed to lie along the C=C trans–cis isomerization pathway of the π‐π* surface. In a polar solvent, the energy of the perpendicular conformer (p) is decreased to couple to the conical intersection to the ground state surface. Conversion from trans to p conformer achieves an equilibrium condition resulting in a biexponential recovery of the ground state.