Combination of Transient 2D-IR Experiments and Ab Initio Computations Sheds Light on the Formation of the Charge-Transfer State in Photoexcited Carbonyl Carotenoids

Year: 2014

Authors: Di Donato M., Segado Centellas M., Lapini A., Lima M., Avila F., Santoro F., Cappelli C., Righini R.

Autors Affiliation: LENS (European Laboratory for Nonlinear Spectroscopy) via N. Carrara 1, 50019 Sesto Fiorentino (FI), Italy; INO (Istituto Nazionale di Ottica), Largo Fermi 6, 50125 Firenze, Italy; Dipartimento di Chimica “Ugo Schiff”, Università di Firenze, via della Lastruccia 13, 50019 Sesto Fiorentino (FI), Italy, Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy; Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Risorgimento 35, I-56126 Pisa, Italy, CNR-Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organo Metallici (ICCOM-CNR), UOS di Pisa, Area della Ricerca, via G. Moruzzi 1, I-56124 Pisa, Italy, University of Málaga, Physical Chemistry, Faculty of Science, Málaga, 29071, Spain

Abstract: The excited state dynamics of carbonyl carotenoids is very complex because of the coupling of single- and doubly excited states and the possible involvement of intramolecular charge-transfer (ICT) states. In this contribution we employ ultrafast infrared spectroscopy and theoretical computations to investigate the relaxation dynamics of trans-8’-apo-beta-carotenal occurring on the picosecond time scale, after excitation in the S-2 state. In a (slightly) polar solvent like chloroform, one-dimensional (TID-IR) and two-dimensional (T2D-IR) transient infrared spectroscopy reveal spectral components with characteristic frequencies and lifetimes that are not observed in nonpolar solvents (cyclohexane). Combining experimental evidence with an analysis of CASPT2//CASSCF ground and excited state minima and energy profiles, complemented with TDDFT calculations in gas phase and in solvent, we propose a photochemical decay mechanism for this system where only the bright single-excited 1B(u)(+) and the dark double-excited 2A(g)(-) states are involved. Specifically, the initially populated 1B(u)(+) relaxes toward 2A(g)(-) in 200 fs. In a nonpolar solvent 2A(g)(-) decays to the ground state (GS) in 25 ps. In polar solvents, distortions along twisting modes of the chain promote a repopulation of the 1B(u)(+) state which then quickly relaxes to the GS (18 Ps in chloroform). The 1B(u)(+) state has a high electric dipole and is the main contributor to the charge-transfer state involved in the dynamics in polar solvents. The 2A(g)(-) -> 1B(u)(+) population transfer is evidenced by a cross peak on the T2D-IR map revealing that the motions along the same stretching of the conjugated chain on the 2A(g)(-) and 1B(u)(+) states are coupled.

Journal/Review: JOURNAL OF PHYSICAL CHEMISTRY B

Volume: 118 (32)      Pages from: 9613  to: 9630

More Information: The authors acknowledge the Italian Ministero dell’Istruzione dell’Universita e della Ricerca’ (FIRB Futuro in Ricerca 2010, RBFR10Y5VW to M.D.D. and C.C.; PRIN 2010-2011 2010ERFKXL Frontiers studies in molecular spectroscopy and dynamics: from simple molecular systems to supramolecular aggregates and advanced materials to F.S. and R.R.). A.L. acknowledges the financial support of the Regione Toscana through the found POR-FSE 2007-2013 obiettivo 2 asse IV, Project EPHODS. F.A. acknowledges support from EU People Program, Marie Curie Actions (G.A. No. 246550). The financial support of the Cassa di Risparmio di Firenze is also gratefully acknowledged. C.C. acknowledges support from COST (Action CODECS: COnvergent Distributed Environment for Computational Spectroscopy). The authors thank R. Improta (IBB-CNR, Napoli) and C. Angeli (University of Ferrara) for fruitful discussions.
KeyWords: Multiconfigurational Perturbation-theory; Internal-conversion Lifetime; Excited-states; Stretching Mode; Energy-transfer; Dynamics; Spectroscopy; Excitation; Peridinin; Polyenes
DOI: 10.1021/jp505473j

Citations: 15
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