Photoinduced dynamics in cycloparaphenylenes: planarization, electron–phonon coupling, localization and intra-ring migration of the electronic excitation

Abstract

Cycloparaphenylenes represent the smallest possible fragments of armchair carbon nanotubes. Due to their cyclic and curved conjugation, these nanohoops own unique photophysical properties. Herein, the internal conversion processes of cycloparaphenylenes of sizes 9 through 16 are simulated using Non-Adiabatic Excited States Molecular Dynamics. In order to analyze effects of increased conformational disorder, simulations are done at both low temperature (10 K) and room temperature (300 K). We found the photoexcitation and subsequent electronic energy relaxation and redistribution lead to different structural and electronic signatures such as planarization of the chain, electron-phonon couplings, wavefunction localization, and intra-ring migration of excitons. During excited state dynamics on a picosecond time-scale, an electronic excitation becomes partially localized on a portion of the ring (about 3-5 phenyl rings), which is not a mere static contraction of the wavefunction. In a process of non-radiative relaxation involving non-adiabatic transitions, the latter exhibits significant dynamical mobility by sampling uniformly the entire molecular structure. Such randomized migration involving all phenyl rings, occurs in a wave-like fashion coupled to vibrational degrees of freedom. These results can be connected to unpolarized emission observed in single-molecule fluorescence experiments. Observed intra-ring energy transfer is subdued for lower temperatures and adiabatic dynamics involving low-energy photoexcitation to the first excited state. Overall our analysis provides a detailed description of photo excited dynamics in molecular systems with circular geometry, outlines size-dependent trends and connotes specific spectroscopic signatures appearing in time-resolved experimental probes.

An ab initio multiple cloning approach for the simulation of photoinduced dynamics in conjugated molecules

“…In order to take these swaps into account, we analyse the overlaps calculated directly at every time step and swap the states when necessary. This includes both the swap of Ehrenfest amplitudes and the appropriate change of overlaps in eqn (22), so that the total wave-function remains the same.…”

“…9 An adequate theoretical treatment of such processes can be achieved by using direct or on-the-fly non-adiabatic molecular dynamics methods. [10][11][12] A sub-family of these approaches, based on trajectory surface hopping (SH) algorithms, [13][14][15][16] have been extensively used to study the photophysics and photochemistry of a wide variety of organic molecules: dendrimers, [17][18][19][20] nanohoops, [21][22][23] fluorenes, 24 fullerenes, 25 Ru(II)-based complexes, 26 chlorophylls, [27][28][29] retinal, 30 nucleotides [31][32][33][34][35][36][37] and so on. Different SH computational implementations are represented by NEWTON-X, 38,39 SHARC (Surface Hopping including ARbitrary Couplings), 40 PYXAID (PYthon eXtension for Ab Initio Dynamics) 41,42 and NEXMD (Non-adiabatic EXcited-states Molecular Dynamics), 12,43 among others.…”

Ultrafast Exciton Self‐Trapping and Delocalization in Cycloparaphenylenes: The Role of Excited‐State Symmetry in Electron‐Vibrational Coupling

“…Thed ipolar nature of TDMs and MO-TDMs is correlated with the asymmetric vibrational motion for the angular momentum conservation during electronic transitions.Specifically,the collective C=Cstretching motion of two hoop fractions in the opposite phase is suggested as an important vibrational motion for modulating benzenoid/ quinoid characters of [n]CPP hoops,asproposed by previous reports. [27,29] Based on our experimental and theoretical results,w ee mphasize that the (quasi-)symmetry of excitedstate wavefunctions determines the (non-)dipolar nature of TDMs and MO-TDMs,w hich implies the involvement of Tabelle 3: Calculated vertical transitions of [12]CPP obtained at the Franck-Condon geometry. E 0n , f 0n ,a nd hM 0n i refer to excitation energy, oscillator strength,and two-photon transitionmatrix element between S 0 and S n states, respectively.…”

“…[27,29] Coherent asymmetric motions along with the out-of-phase C = Cs tretching lead to ultrafast torsional reorganizations and develop localized excitons. [27,29] On the other hand, the relaxation process from two-photon allowed states should be carefully scrutinized since lower lying excited states intervene before reaching the potential surface of the S 1 state. [ 12]CPP is introduced as Angewandte Chemie Forschungsartikel ar epresentative to describe internal conversion processes from TPE states as the [n]CPPs studied herein share similar TDM and MO-TDM topologies ( Figures S5-12).…”

Ultrafast Exciton Self‐Trapping and Delocalization in Cycloparaphenylenes: The Role of Excited‐State Symmetry in Electron‐Vibrational Coupling

“…Similarly,f or [n]CPPs,adependence of exciton self-trapping on the degree of torsional freedoms has been suggested by theoretical calculations,which motivated us to experimentally study exciton (de)localization processes within [n]CPP hoops. [26,27] Herein, two distinct optical transitions that obey different symmetry selection rules for electronic transitions were employed to control local disorders within [n]CPP hoops as torsional angles are rearranged on ultrafast time scales by non-adiabatic coupling vectors.Plastic p-conjugated organic materials experience rapid modulations of microscopic structures by the thermal activation of coherent vibrations in the direction of non-adiabatic coupling when internal conversion processes occur. [27][28][29] In this regard, femtosecond (fs) broadband fluorescence (FL) upconversion measurements on as eries of [n]CPPs (n = 9-12;F igure 1) were conducted upon one-photon (OPE) and two-photon (TPE) excitations to track exciton self-trapping and (de)localization based on the exciton theory on bent chromophores.…”