Substituent controlled spectroscopy and excited state topography of retinal chromophore models: fluorinated and methoxy-substituted protonated Schiff bases

Abstract

Ab initio multireference second-order perturbation theory computations are used to explore the spectroscopic behaviour (i.e. absorption and emission) and the structure of two isolated (i.e. in vacuo) short chain retinal chromophore models (i.e. the 2-cis-penta-2,4-dieniminium and all-trans-epta-2,4,6-trieniminium cations), which have been modified using fluorine or methoxyl substituents as representative examples of electron withdrawing and electron releasing groups, respectively. A systematic analysis has been performed for the systems substituted in all positions along the chain. Significant effects on the excited state structure of the chromophore and its absorption and emission features are unveiled by comparison with previously reported results for the corresponding unsubstituted cations in vacuo. Indeed, it is demonstrated that (i) substituents may affect the chromophore absorption/emission energy depending on the substituted position along the chain and the specific electronic effect of the substituent (with fluorine and methoxyl inducing opposite effects), (ii) substituted odd- and even-numbered positions along the chain behave in a different way in agreement with the experiments, and (iii) the initially relaxed geometry in the excited state, addressing the excited state dynamics of the chromophore out of the Franck–Condon region, may change. A rationale for these effects is shown, which provides a crude basis for understanding experimental spectroscopic observations and allows one to speculate and draw conjectures about the intrinsic photochemical behaviour (namely the photoisomerization efficiency/selectivity) of substituted retinal protonated Schiff bases.

Acknowledgements

Funds have been provided by the University of Bologna (Funds for Selected Research Topics), MIUR (funds ex 60%), and MURST Cofin 2003 (projects: ‘Reazioni stereoselettive promosse da nuovi sistemi catalitici e loro modellistica'). M.G. is also grateful for a FIRB (RBAU01L2HT). We wish to thank CINECA and INSTM for granted calculation time.

Notes

†The anomalous SP point for 2 is not stable at the CASSCF level (see figure 6). Therefore this structure is directly drawn from the corresponding structure found in the C4-methoxylated system (see figure 7(b)), and a single point CASSCF calculation has then been performed to have the Mulliken charges distribution reported on scheme 3.