Excitation-dependent electron exchange energy and electron transfer dynamics in a series of covalently tethered N,N-bis(4′-tert-butylbiphenyl-4-yl)aniline – [C₆₀] fullerene dyads via varying π-conjugated spacers

Abstract

Femtosecond time-resolved fluorescence and transient absorption studies are reported for three newly synthesized covalently linked N,N-bis(4'-tert-butylbiphenyl-4-yl)aniline (BBA) and pyrrolidinofullerenes (C60)-based donor-π conjugated bridge-acceptor dyads (D-B-A) as functions of the bridge length (7.1, 9.5 and 11.2 Å for Dyad-1, Dyad-2 and Dyad-3), dielectric constants of the medium and pump wavelengths. In polar solvent, ultrafast fluorescence quenching (kEET ≥ 2 × 1012 s-1) of the BBA moiety upon excitation of the BBA moiety (320 nm) is observed in the dyads and is assigned to a mechanism involving electron exchange energy transfer (EET) from 1BBA* to C60 followed by electron transfer from BBA to 1C60*. Cohesive rise and decay dynamics of conjugated BBA˙+-C60˙- anion pairs confirm the involvement of a distance independent adiabatic charge-separation (CS) process (kCS ≥ 2.2 × 1011 s-1) with near unity quantum efficiency (φCS ≥ 99.7%) and a distance-dependent non-adiabatic charge-recombination (CR) process [kCR ∼ (1010-108) s-1]. In contrast, excitation of the C60 moiety (λex = 430 to 700 nm) illustrates photoinduced electron transfer from BBA to 1C60*, involving non-adiabatic (diabatic) and distance-dependent CS (kCS in the range of 0.59-1.78 × 1011 s-1) with 98.86-99.6% (Dyad-3-Dyad-1) quantum efficiency and a CR process with kCR values [kCR ∼ (1010-108) s-1] up to three orders greater than kCS of the respective dyads. Both the processes, CS and CR, upon C60 excitation and the CR process upon BBA excitation show distance dependent rate constants with exponential factor β ≤ 0.5 Å-1, and electron transfer is concluded to occur through a covalently linked conjugated π bridge. Global and target analysis of fsTA data reveal the occurrence of two closely lying CS states, thermally hot (CShot) and thermally relaxed (CSeq) states, and two CR processes with two orders of different rate constants. Careful analysis of the kinetic and thermodynamic data allowed us to estimate the total reorganization energy and electronic coupling matrix (V), which decrease exponentially with distance. These novel features of the distance independent adiabatic CS process and the distance-dependent diabatic CR process upon donor excitation are due to extending the π-conjugation between BBA and C60. The demonstrated results may provide a benchmark in the design of light-harvesting molecular devices where ultrafast CS processes and long-lived CS states are essential requirements.

Excitation-Wavelength-Dependent Light-Induced Electron Transfer and Twisted Intramolecular Charge Transfer in N,N-Bis(4′-tert-butylbiphenyl-4-yl)aniline Functionalized Borondipyrromethenes

“…The synthesis of the BBA–BODIPY dyads, Dyad-1 , Dyad-2 , and Dyad-3 , discussed in the present work is shown in Scheme , and the experimental details are given in the Supporting Information. The precursors 2 , 4 , and 5 bearing an aldehyde functional group, which are further used for the preparation of the dyads and the control compounds, BBA and Tol-BODIPY , were synthesized according to the literature procedures. , , The synthesis of the dyads was initiated by preparing dipyrromethanes, obtained by the condensation of the corresponding aldehydes with pyrrole, followed by further treatment of the dipyrromethanes with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), triethylamine, and BF 3 etherate to afford the crude compounds as black reaction mixtures. Purification of the crude compounds by column chromatography yielded the dyads as orange crystals.…”

“…To construct an efficient donor–acceptor system, several pyrrolic macrocycles, such as porphyrins, phthalocyanines, sub-phthalocyanines, , corroles, etc., and non-porphyrin-like chromophores, such as phenothiazines, , benzothiazole, triphenylamine, , etc., are used as energy/electron donors, while fullerenes, quinolines, and perylenediimides are commonly used as electron acceptors. Among these systems, porphyrins are the most attractive species due to their structural similarities with natural pigments like chlorophylls and bacteriochlorophylls, which behave as light-storing chromophores and electron donors in the electron donor–acceptor molecular models. ,, On the other hand, borondipyrromethene (also known as BODIPY) and its structural analogue, aza-borondipyrromethene (aza-BODIPY), stand out as the promising molecules in light harvesting antenna and electron donors/acceptors in electron transfer cascades .…”

“…Triphenylamine derivatives are emerged as interesting precursors for small-molecule-based functional materials for applications in organic electronics. − Due to their low oxidation potentials, they are effectively used as hole-transporting material in organic light-emitting diodes − and organic solar cells . In N , N -dialkylamino derivatives, the twisted geometry around the nitrogen atom decreases the π-donor strength of the amino group through orbital coupling , and affects the fluorescence properties in the solid state and prevents nonradiative π–π aggregate formation. − Among the family of triphenylamines, N , N -bis­(4′- tert -butylbiphenyl-4-yl)­aniline, abbreviated as BBA, emerged as a strong electron donor that has an intriguing ability to stabilize the charge-separated state and exhibit the cation radical at 745 and 900 nm . A few studies involving the BBA-fullerene dyads and their photoinduced charge transfer and charge recombination dynamics upon excitation of the fullerene have been reported. − In these lines, our research groups have recently reported systematic ultrafast fluorescence and transient absorption studies in three covalently linked BBA-fullerene dyads.…”

“…A few studies involving the BBA-fullerene dyads and their photoinduced charge transfer and charge recombination dynamics upon excitation of the fullerene have been reported. − In these lines, our research groups have recently reported systematic ultrafast fluorescence and transient absorption studies in three covalently linked BBA-fullerene dyads. In these systems, excitation-dependent electron exchange energy transfer from 1 BBA*→C 60 followed by an electron transfer from BBA→ 1 C 60 * leading to the formation of the charge-separated state BBA +• – BODIPY –• , which was observed in polar solvents …”

“…4,13−18 Systematic investigations involving the importance of electron donor and acceptor entities, their spatial arrangement in donor−acceptor systems, and the shape and size of the linker connecting the donor and acceptors provided the crucial information for the strategic employment of these systems for capturing and storing light energy. 19−21 To construct an efficient donor−acceptor system, several pyrrolic macrocycles, such as porphyrins, 22 phthalocyanines, 23 sub-phthalocyanines, 24,25 corroles, 26 etc., and non-porphyrinlike chromophores, such as phenothiazines, 27,28 benzothiazole, 21 triphenylamine, 29,30 etc., are used as energy/electron donors, while fullerenes, quinolines, 31 are commonly used as electron acceptors. Among these systems, porphyrins are the most attractive species due to their structural similarities with natural pigments like chlorophylls and bacteriochlorophylls, which behave as light-storing chromophores and electron donors in the electron donor− acceptor molecular models.…”