Photoisomerization of simple merocyanines: a theoretical and experimental comparison with polyenes and symmetric cyanines

Abstract

The structure and electronic spectra of merocyanines are intermediate between those of acyclic polyenes and cationic symmetric cyanines of comparable sizes, shifting from one to the other as a result of a solvent polarity change. In this paper we address, both theoretically and experimentally, the question of how the photochemical behaviour of simple merocyanines compares with that of polyenes and symmetric cyanines. After a brief theoretical re-appraisal of the absorption maxima dependence on the chain length in the three classes of chromogens, we analyse the calculated potential-energy curves for two photoisomerization coordinates of a polyene, a symmetric cyanine and a merocyanine of comparable sizes. The results concerning both the energy curves and the nature of the relevant electronic states, particularly the TICT (twisted intramolecular charge transfer) character of the S 1 state at the perpendicular geometry, reveal the existence of a near relationship between merocyanines and symmetric ionic polymethines, traceable back to their being odd-alternant systems. The presence of a low-lying 1 (np*) state at the trans geometry and the associated solvent-modulated vibronic coupling with the 1 (p H p L *) state dominate the experimentally accessible photochemical behaviour of two simple merocyanines and make it peculiar with respect to those of both polyenes and symmetric ionic cyanines.y Dedicated to Professor Dr Z. R. Grabowski and Professor Dr J. Wirz on the occasions of their 75th and 60th birthdays. Fig. 1 Dominant resonance structures in strepto polymethinecyanines (SPC), strepto polymethinemerocyanines (SPMC) and acyclic polyenes (AP).

25th Anniversary Article: Design of Polymethine Dyes for All‐Optical Switching Applications: Guidance from Theoretical and Computational Studies

“…In the three‐term SOS expression (Equation , the N term is the dominant factor in the case of cyanines, as the key factors determining the magnitude of Re( γ ) are E ge and μ ge . For symmetric cyanines, E ge is directly proportional to 1/ N , and there is a steeper dependence on length than in polyenes or donor–acceptor systems . We note that the results of computational studies indicate that to reproduce the length dependence of E ge accurately, approaches incorporating double‐electron excitations (for example, CASPT2, coupled cluster methods, and in semi‐empirical CI techniques) are required with respect to methods that only incorporate single‐electron excitations (for example, TD‐DFT and INDO/SCI) .…”

Structure–Property Relationships for Exciton and Charge Reorganization Energies of Dipolar Organic Semiconductors: A Combined Valence Bond Self-Consistent Field and Time-Dependent Hartree-Fock and DFT Study of Merocyanine Dyes

“…Such tortional motion of photoexcited merocyanine dyes has also been verified by various experiments and identified as a major fluorescence quenching pathway, but also utilized for photoreactions by light-induced electrocyclization into spiro compounds. [60][61][62][63][64][65][66] The twisting motion predicted by our calculations is exemplified by a superposition of the planar groundstate structure and the twisted excited-state structure of MD353 in Figure 9. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 The exciton reorganization energies resulting from this induced twist motion are indeed by a factor of 10 higher than the effects discussed so far.…”

Photoisomerization among ring-open merocyanines. II. A computational study

“…14 Quantum chemical calculations of the potential-energy surfaces (PES) and the vibrational modes of 6-nitro BIPS in ground and excited state, are however needed to interpret the spectroscopic results. In this publication, we perform highlevel ab initio and density functional calculations in the S 0 and S 1 state along the reaction coordinate, which can be approximated by the rotation around bond 3 in Fig.…”