Photoinduced Excited-State Energy-Transfer Dynamics of a Nitrogen-Cored Symmetric Dendrimer: From the Perspective of the Jahn–Teller Effect

Abstract

We report an interesting view to understand the ultrafast excited-state energy-transfer (EET) process in the D 3 -symmetric dendrimer tris(4-ethynylphenyl)amine (TEPA) from the perspective of the well-known E⊗e Jahn−Teller (JT) effect. Upon excitation to two lowest excited states (S 1 and S 2 ) with doubly degenerate E symmetry, two sets of e vibrational modes, dihedral angle twist and strong pyramidalization near the nitrogen core, lead to the JT distortion and symmetry lowering. Through the excited-state dynamics simulation with the onthe-fly surface-hopping approach at the TDDFT level, we find that the system may either travel three equivalent minima of S 1 state or undergo the nonadiabatic transitions between S 1 and S 2 states. These motions induce the ultrafast EET among different branches and the reorientation of the transition dipole moments, finally leading to the ultrafast fluorescence anisotropy decay. This energy-transfer mechanism can provide some interesting insights on the excited-state dynamics of large dendrimers with three equivalent branches and transition metal complexes with C 3 symmetry. ■ INTRODUCTIONOrganic dendrimers show attractive photovoltaic properties such as efficient photoharvesting, 1,2 unidirectional energy transfer, 3 enhanced two-photon absorption, 4−6 and high photoluminescence quantum yield. 7,8 These properties make them potentially useful in photovoltaic materials for solar energy conversion, 1,3,9 fluorescence sensors, 10,11 organic lightemitting diodes, 12,13 and organic lasers. 8,14 The fundamental photophysics and photochemistry of these novel dendritic macromolecular architectures involve the exciton formation and movement (localization or delocalization), 15−18 especially the intramolecular excited-state energy transfer (EET) between different subunits (branches). 3,19−22 A number of experimental studies have been carried out to study the intramolecular EET between different subunits for branched molecules. 23−25 In particular, the organic dendrimers with rotational symmetry (such as the existence of C 3 or C 4 rotational axes) have attracted considerable attention since the efficient EET is expected among those equivalent branches. With the development of laser technology, time-dependent fluorescence depolarization may provide useful information on the energy-transfer processes of such high-symmetric systems. 25−29 This special technology allows us to monitor the orientation change of the emission dipole with respect to the absorption dipole, which is caused by the variation of local excitation with time. Goodson III and collaborators studied the intramolecular EET mechanism of several dendrimers, such as T-NPTPA, 25,29 A-DSB, 30,31 G n , 32 and so on. With timeresolved spectroscopy, they reported that a group of tribranched molecules with a nitrogen core show a very fast energy transfer among different branches on the ∼30−100 fs time scale. Through the investigation of a series of branched molecules with similar building blocks (chromophores) and different core un...

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

“…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.…”

Surface hopping methods for nonadiabatic dynamics in extended systems

“…where g tot and g i are obtained through Equation (25) and Equation (27), respectively. In the denominator of Equation ( 27), both P a (t + dt) and Re U aa w a t ð Þw * a t + dt ð Þ Â Ã can be very close to zero when trivial crossings happen with the active state as in type 2 and type 4 surface crossings (see Figure 3), resulting in numerical instability.…”

“…Ehrenfest also fails to capture the correct long‐time dynamics when the quantum and classical subsystems are strongly coupled . Starting from Tully's seminal work of the fewest switches surface hopping (FSSH) in 1990, trajectory surface hopping has attracted growing attention in many different research fields, for example, molecular collisions, proton transfer, photoisomerization, singlet fission, energy transfer, excited state nonradiative decay, charge transport, and other nonequilibrium processes …”

“…[13][14][15] Ehrenfest also fails to capture the correct long-time dynamics when the quantum and classical subsystems are strongly coupled. 16 Starting from Tully's seminal work of the fewest switches surface hopping (FSSH) in 1990, 12 trajectory surface hopping has attracted growing attention in many different research fields, for example, molecular collisions, 17,18 proton transfer, 19,20 photoisomerization, [21][22][23] singlet fission, 24,25 energy transfer, 26,27 excited state nonradiative decay, 28 charge transport, 9,29 and other nonequilibrium processes. [30][31][32][33][34][35] In the framework of the standard FSSH algorithm, there are four major procedures at each time step of the simulation, including the classical time evolution of the nuclear coordinates and momenta, the electronic wavefunction propagation, the stochastic surface hopping guided by the quantum fluxes, and the nuclear velocity adjustment to ensure successful surface hops.…”

Photoinduced Ultrafast Intramolecular Excited-State Energy Transfer in the Silylene-Bridged Biphenyl and Stilbene (SBS) System: A Nonadiabatic Dynamics Point of View

“…[25][26][27][28][29][30] With this tool, interesting results were derived to understand the photo-induced reactions of organic photo-voltaic systems, particularly their excited-state charge transfer and energy transfer processes. 28,[31][32][33][34] In this work, we employed the on-the-fly trajectory surface-hopping (TSH) method at the TDDFT level to study the intramolecular EET process of a model system: the silylene-bridged biphenyl and stilbene system (SBS). This SBS system serves as a prototypical system to identify similar EET processes between the donor and acceptor units of silylene-bridged biphenyl and stilbene co-polymers.…”