Energy transfer and charge separation dynamics in photoexcited pyrene–bodipy molecular dyads

Abstract

The photophysical properties of two pyrene-bodipy molecular dyads, composed of a phenyl-pyrene (Py-Ph) linked to the meso position of a bodipy (BD) molecule with either H-atoms (BD1) or ethyl groups (BD2) at the 2,6 positions, are investigated by stationary, nanosecond and femtosecond spectroscopy. The properties of these dyads (Py-Ph-BD1 and Py-Ph-BD2) are compared to those of their constituent chromophores in two solvents namely 1,2-dichloroethane (DCE) and acetonitrile (ACN). Stationary spectroscopy reveals a weak coupling among the subunits in both dyads. Excitation of the pyrene (Py) subunit leads to emission that is totally governed by the BD subunits in both dyads pointing to excitation energy transfer (EET) from the Py to BD chromophore. Femtosecond fluorescence and transient absorption spectroscopy reveal that EET takes place within 0.3-0.5 ps and is mostly independent of the solvent and the type of the BD subunit. The EET lifetime is in reasonable agreement with that predicted by Förster theory. After EET has taken place, Py-Ph-BD1 in DCE and Py-Ph-BD2 in both solvents decay mainly radiatively to the ground state with 3.5-5.0 ns lifetimes which are similar to those of the individual BD chromophores. However, the excited state of Py-Ph-BD1 in ACN is quenched having a lifetime of 1 ns. This points to the opening of an additional non-radiative channel of the excited state of Py-Ph-BD1 in this solvent, most probably charge separation (CS). Target analysis of the TA spectra has shown that the CS follows inverted kinetics and is substantially slower than the recombination of the charge-separated state. Occurrence of CS with Py-Ph-BD1 in ACN is also supported by energetic considerations. The above results indicate that only a small change in the structure of the BD units incorporated in the dyads significantly affects the excited state dynamics leading either to a dyad with long lifetime and high fluorescence quantum yield or to a dyad with ability to undergo CS.

Using sulfur bridge oxidation to control electronic coupling and photochemistry in covalent anthracene dimers

“…This type of solvent polarity enhanced internal conversion has been previously reported for carotenoid derivatives and attributed to an excited singlet with some CT character 69,70. It is also possible that the internal conversion proceeds through an intermediate state with CT character whose decay rate is much more rapid than its population rate from the initially excited singlet state (inverted kinetics) 71–73…”

A Novel Conjugated Pyrene-BODIPY Dyad: Synthesis, Characterization and Properties

“…For this purpose, studies were carried out to improve the electronic and optical properties of pyrene derivatives by changing their molecular structure by substituting electron donor or acceptor groups (19)(20)(21)(22). Although many photoactive supramolecular systems containing two or more chromophores have been reported up to date, only a few articles about boron dipyrromethene-pyrene systems have been published (23)(24)(25)(26)(27).…”

Excitation/detection energy controlled anisotropy dynamics in asymmetrically cyano substituted tri-podal molecules

“…The anisotropy, when detected in the few tens of ps timescale, is a valuable tool for determining (i) the angle between the absorption and emission dipole moments (via the value of the initial anisotropy r 0 ), as well as (ii) the possible energy transfer/delocalization phenomena. 17,19,[25][26][27]31,34,35,53,[58][59][60][61][62][63][64][65][66] The latter phenomena are highly dependent on the initial excited state, that is populated, or the amount of excess energy that is provided to the system, especially in compounds with multiple chromophores.…”