Ultrafast Excited State Dynamics in 9,9′-Bifluorenylidene

Abstract

9,9'-Bifluorenylidene has been proposed as an alternative and flexible electron acceptor in organic photovoltaic cells. Here we characterize its excited state properties and photokinetics, combining ultrafast fluorescence and transient IR measurements with quantum chemical calculations. The fluorescence decay is ultrafast (sub-100 fs) and remarkably independent of viscosity. This suggests that large scale structure change is not the primary relaxation mode. The ultrafast decay populates a dark state characterized by distinct vibrational and electronic spectra. This state decays with a 6 ps time constant to a hot ground state that ultimately populates the initial state with a 20 ps time constant; these times are also insensitive to solvent viscosity. No metastable intermediate structures are resolved in the photocycle after population of the dark state. The implications of these results for the operation of 9,9'-bifluorenylidene as an electron acceptor and as a potential molecular switch are discussed.

Role of Dispersive Fluorous Interaction in the Solvation Dynamics of the Perfluoro Group Containing Molecules

“…A similar nature of time traces has been found for the C6H as well as C153 in MFE and TFE as shown in Figure S1. The observed decay behavior of time traces at the blue region and the rise at the red region of the emission spectrum are the characteristic features of the solvation process, and such features have been shown earlier for many different systems. ,− Time traces have been fitted with the sum of exponentials. Further, we have constructed the TRES for these two dyes in ETH, MFE, and TFE using their respective best fitting parameters obtained from their decay time traces.…”

Unconventional Synthesis of a CuI Rotaxane with a Superacceptor Stopper: Ultrafast Excited‐State Dynamics and Near‐Infrared Luminescence

“…Since the smallest molecule P shows the same transient spectrum as the free ligand 13 and the larger Cu I rotaxane 11 + , it is evident that also this second transient signal is associated with the DCNQ moiety and, for all the investigated compounds, it decays mono‐exponentially to the ground‐state minimum, with a lifetime of 1.3±0.2 ps. These transient features can be associated to two possible processes: 1) the population of T 1 by intersystem crossing within the DCNQ moiety or 2) the relaxation of the vibrationally hot S 0 state (hot‐S 0 ), populated by the ultrafast deactivation of S 1 through S 1 /S 0 conical intersections …”

“…These transientf eatures can be associatedt ot wo possible processes: 1) the population of T 1 by intersystem crossingw ithin the DCNQ moiety or 2) the relaxation of the vibrationally hot S 0 state (hot-S 0 ), populated by the ultrafast deactivation of S 1 through S 1 /S 0 conical intersections. [58,59] The first hypothesis requires ultrafast S 1 !T 1 intersystem crossing( i.e.,0 .4 ps), followed by another very rapid T 1 !S 0 spin-forbidden process (i.e.,1 .3 ps).I ndeed,i ntersystem crossing on the picosecond timescale and short lifetime of the lowestt riplet excited state have been detected in purely organic molecules, such as nitro polycyclic aromatic hydrocarbons [60,61] or xanthones. [62,63] In the present case, these two intersystem crossingp rocesses could be tentatively explained by 1) the virtually identical geometries adopted by the DCNQ moiety in both the minimum of S 1 and of T 1 ,a sd eterminedb y theoretical calculations in the case of P ( Figure S6 in the Supporting Information);2 )the extremelys mall adiabatic energy difference between T 1 and S 0 ,a sp roven by DFT calculations (i.e., DE = 0.56 eV in the case of P).…”

A Chemiluminescent Tetraaryl Diborane(4) Tetraanion

“…9,9′‐Bis(fluorenylidene) is currently attracting attention as non‐fullerene‐type electron acceptor in organic photovoltaic cells or building block for singlet fission materials. [ 58 , 59 , 60 , 61 , 62 , 77 , 78 , 79 ] Also in this light, the doubly boron‐doped congener [ 3 ] 2− , capable of accepting two additional electrons to form the highly charged 9,9′‐bis(borafluorenylidene) tetraanion [ 3 ] 4− in a reversible manner, is of fundamental interest. The dianion [ 3 ] 2− combines boron‐centered nucleophilicity with the rare ability to perform subsequent C−H‐activation reactions, whereas the reactivity of the tetraanion [ 3 ] 4− appears to be dominated by electron‐transfer events.…”