Modeling study of non-line-narrowed hole-burned spectra in weakly coupled dimers and multi-chromophoric molecular assemblies Mike Reppert a,b , Virginia Naibo b , Ryszard Jankowiak a, * a Department of Chemistry, Kansas State University, Manhattan, KS 66506, USA b Department of Mathematics, Kansas State University, Manhattan, KS 66506, USA article info Article history: Received 9 June 2009 In final form 8 October 2009 Available online 27 October 2009 Keywords: Hole burning Excitonic interactions Excitation energy transfer abstract Several different models have been proposed to explain the origin of the complex anti-hole features observed in hole-burned (HB) spectra of excitonically coupled systems such as photosynthetic com- plexes. This lack of consensus presents a serious constraint on the interpretation of HB spectra and the underlying electronic structures of these systems. To resolve this problem we present results of modeling studies of non-resonant HB spectra taking uncorrelated excitation energy transfer and excitonic interac- tions into account. Simplified analytical results are compared with Monte Carlo simulations in which excitonic interactions are explicitly taken into account in order to disentangle a number of distinct effects. It is shown that these effects can accurately account for both hole shapes and the broad anti-hole structure observed in excitonically coupled systems. We argue that these models will provide a necessary framework for probing the electronic structure of these systems via HB spectroscopy. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Hole burning (HB) spectroscopy is a low-temperature spectro- scopic technique which has been employed over the years in prob- ing the electronic structure and dynamics of a wide variety of systems [1–4]. Although much early HB work was performed on isolated chromophore systems (e.g. dye molecules dispersed in glassy matrices [1,2]), the technique has found numerous applica- tions in multi-chromophore systems as well, including both photo- synthetic complexes [2,3,5,6] and molecular aggregates [7–10]. The basic methodology of persistent HB spectroscopy involves the measurement of a low-temperature ‘‘pre-burn” absorption spectrum of a sample, followed by (typically laser-based) irradia- tion (the ‘‘burn”) and measurement of a final ‘‘post-burn” absorp- tion spectrum. The HB spectrum is obtained as the difference between the pre-burn and post-burn absorption spectra; its fea- tures are defined by light-induced changes in the electronic prop- erties of the sample. These changes are typically attributed to one of two mechanisms: in photochemical hole burning, the probed chromophores of the sample are chemically altered, for example, through an excited state tautomerization [1]; in non-photochemi- cal hole burning (NPHB), the changes in the absorption spectrum are most likely triggered by excited state tunneling between alter- nate physical configurations of the chromophore and its immediate environment, a situation commonly described in terms of a guest– host two-level system (TLS) [11,12]. Another NPHB mechanism involving TLS was proposed by Bogner and Schwarz [13]; in this mechanism hopping over the barrier occurs in the electronic ground state, using the thermal energy provided by radiation less decay of the electronic excited state. Although TLS have traditionally been used to describe a wide variety of features in NPHB spectra, it has been suggested that a more complete picture involving multiple accessible guest-chro- mophore configurations or multi-level systems (MLS) is required to explain many features of the NPHB process [4,11,12]. In both models, NPHB results from tunneling of the system from one min- imum of the potential energy surface to the other (or to one of sev- eral other minima in the case of MLS). After relaxation back to the ground state, the system is locked in the new post-burn configura- tion so long as the temperature is kept sufficiently low as to pre- vent thermal equilibration between the two minima (thermal hole filling). One of the defining features of NPHB is that absorption intensity is conserved [11]. That is, the integrated intensity of the anti-hole should be equal that of the hole (or, alternatively, the total integral of the hole spectrum should be zero). However, the shape and width of the anti-hole (photoproduct) distribution, especially in proteins, remains a subject of debate [14–16]. It is well known that the origin of such anti-holes is associated with the hierarchy of low 0301-0104/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2009.10.013 Abbreviations: EET, excitation energy transfer; FWHM, full-width at half maximum; MLS, multi-level system; HB, hole burning; NLN, non-line-narrowing; NPHB, non-photochemical hole burning; SDF, site distribution function; TLS, two- level system; ZPH, zero-phonon hole. * Corresponding author. Tel.: +1 785 532 6785; fax: +1 785 532 6666. E-mail address: ryszard@ksu.edu (R. Jankowiak). Chemical Physics 367 (2010) 27–35 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys