  The light-harvesting excitonic properties of poly(phenylene ethynylene) (PPE)
extended dendrimers (tree-like {\pi}-conjugated macromolecules) involve a
directional cascade of local excitation-energy transfer (EET) processes
occurring from the "leaves" (shortest branches) to the "trunk" (longest
branch), which can be viewed from a vibronic perspective as a sequence of
internal conversions occurring among a connected graph of nonadiabatically
coupled locally-excited (LE) electronic states via conical intersections. The
smallest PPE building block able to exhibit EET, the asymmetrically
meta-substituted PPE oligomer with one acetylenic bond on one side and two
parallel ones on the other side (2-ring and 3-ring para-substituted
pseudo-fragments), is a prototype and the focus of the present work. From
time-dependent density-functional theory (TD-DFT) electronic-structure
calculations of the molecule as regards its first two nonadiabatically coupled,
optically active, singlet excited states, we built a (1+2)-state-8-dimensional
vibronic-coupling Hamiltonian (VCH) model for running multiconfiguration
time-dependent Hartree (MCTDH) wavepacket relaxations and propagations,
yielding both steady-state absorption and emission spectra, as well as
real-time dynamics. The EET process from the shortest branch to the longest one
occurs quite efficiently (about 80% quantum yield) within the first 25 fs after
light excitation and is mediated vibrationally through acetylenic and quinoidal
bond-stretching modes together with a particular role given to the central-ring
anti-quinoidal rock-bending mode. Electronic and vibrational energy
relaxations, together with redistributions of quantum populations and
coherences, are interpreted through the lens of a nonadiabatic perspective,
showing some interesting segregation among the foremost photoactive degrees of
freedom as regards spectroscopy and reactivity.
