Modification of Optical Properties and Excited-State Dynamics by Linearizing Cyclic Paraphenylene Chromophores

Abstract

Cyclic and bent conjugated molecular systems have tunable optical, structural, and dynamical features that differentiate them from their linear counterparts. Examples of such systems are [n]­cycloparaphenylenes (CPPs), which consist of nanorings composed of n para-linked benzene units. Circular geometry and tunability of π-orbital overlaps and bending strains enrich them with unique physicochemical and electronic properties compared to those of the corresponding linear oligoparaphenylenes. Herein, we explore the changes of these properties on alkyl-tethered-p-heptaphenylenes by modifying the methylene tether lengths from 1 to 19 carbons, leading to a gradual linearization of the conjugated backbone conformation. For this purpose, the photoinduced internal conversion processes of different alkyl-tethered-p-heptaphenylenes are simulated using nonadiabatic excited-state molecular dynamics. We found that the greater the strain introduced on the conjugated system, the slower the electronic and vibrational energy relaxation process. All bent p-heptaphenylenes exhibit similar patterns of intramolecular energy redistribution that finally spatially localize the exciton on phenylene units in the middle of the conjugated chain. This behavior is opposite to the random exciton localization previously reported for [n]­CPPs. Moreover, the nonadiabatic S2 → S1 electronic transition activates specific collective asymmetric vibrational excitations that promote periodic oscillatory evolution of the excitonic wave function before an excessive energy dissipates into the bath degrees of freedom.

Exciton Spatial Dynamics and Self-Trapping in Carbon Nanocages

“…The distributions reveal an increase in the average value of the dihedral angles with increasing nanocage size. This is in agreement with trends previously reported for CPPs and other related bent and curved conjugated molecular systems ,− and is a consequence of the relaxation of the bending strain of phenylene units due to a reduction of the backbone curvature along longer phenylene bridges. It is interesting to note that the dihedral angle distributions display double peaks.…”

“…Moreover, previous NEXMD simulations yield spectra and exciton migration properties in good correspondence with experimental results across a variety of conjugated molecules undergoing quite distinct exciton relaxation pathways, 35 which is very encouraging with regard to the application of the present methodology to the simulation of the photoinduced dynamics of carbon nanocages. It has been successfully applied to simulate the photoinduced dynamics of the family of [n]CPPs and other related nanohoops, 5,23,37,46 providing good agreement with more accurate quantumchemical calculations. Previous NEXMD simulations were able to associate the efficient fluorescence in large [n]CPP hoops with the exciton self-trapping due to strong vibronic coupling, breaking the Condon approximation and overriding the optical selection rules.…”

“…Organic conjugated materials are a broad class of electronically functional chemical compounds used across a variety of technological applications. − Many of these materials possess tunable optical and electronic properties, offering a way to customize the design of the material for a specific application. − To this end, establishing structure–property relationships for light absorption and emission features is extremely desirable. , These relationships can be very complex, as they involve the entire photoexcitation evolution and relaxation via nonadiabatic dynamics that leads to an emissive state. For example, localization of the final fluorescent state following photoexcitation in spiro-linked conjugated polyfluorene stabilizes its light emission .…”

NEXMD Software Package for Nonadiabatic Excited State Molecular Dynamics Simulations

“…The utility and accuracy of the NEXMD development version were exemplified by its application to a variety of systems including oligomers and polymers, − dendrimers, − light harvesting complexes (LHCs), ,, energetic materials, − and others . Additionally, our previous publications documented the development of theoretical methodology, algorithms, and analyses featured in NEXMD. − ,,− In this article, we outline the various functionalities of the NEXMD package derived from the development version and released for open public use and the associated getexcited.py python support package.…”

Exciton-vibrational dynamics induces efficient self-trapping in a substituted nanoring

“…[46][47][48][49][50][51][52][53][54][55][56][57][58][59][60] These methods have been previously applied to simulate the photoinduced electronic energy relaxation and redistribution in CPPs and related nanohoops. 61,62 Particularly, the NEXMD (Nonadiabatic EXcited-state Molecular Dynamics) software package 44 has been extensively applied to a variety of CPPs, 5,8 and related nanohoops, 26,63 nanobelts 64 and nanocages. 65 These previous investigations have revealed that structural modifications introduced in CPPs by various substitutions and insertions induce changes in the relative efficiency of the internal conversion process involving dynamical changes of the structures such the local planarization of the chain and concomitant changes in the specific exciton spatial localization (self-trapping) 53 occurring during the excited-state dynamics.…”