Excited‐State Electron Transfer in 1,1,4,4‐Tetracyanobuta‐1,3‐diene (TCBD)‐ and Cyclohexa‐2,5‐diene‐1,4‐diylidene‐Expanded TCBD‐Substituted BODIPY‐Phenothiazine Donor–Acceptor Conjugates

Abstract

A new set of donor–acceptor (D–A) conjugates capable of undergoing ultrafast electron transfer were synthesized using 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY)‐substituted phenothiazine, SM1–SM3, by a Pd‐catalyzed Sonogashira cross‐coupling reaction and a [2+2] cycloaddition–electrocyclic ring‐opening reaction. The incorporation of 1,1,4,4‐tetracyanobuta‐1,3‐diene (TCBD) and cyclohexa‐2,5‐diene‐1,4‐diylidene‐expanded TCBD (abbreviated as DCNQ=dicyanodiquinodimethane) in BODIPY‐substituted phenothiazine resulted in significant perturbation of the optical and electronic properties. The absorption spectrum of both SM2 and SM3 showed red shifted absorption as compared to SM1. Additionally, both SM2 and SM3 exhibited a distinct intramolecular charge‐transfer (ICT) transition in the near‐IR region more so for SM3. The electrochemical study revealed multi‐redox processes due to the presence of redox‐active phenothiazine, BODIPY, TCBD or DCNQ entities. Using data from spectral, electrochemical and computational studies, an energy‐level diagram was established to witness excited‐state electron‐transfer events. Finally, evidence of electron transfer and their kinetic information was secured from studies involving a femtosecond transient absorption technique. The time constants for excited‐state electron‐transfer events in the case of SM2 and SM3 were less than 5 ps revealing ultrafast processes.

Charge stabilization via electron exchange: excited charge separation in symmetric, central triphenylamine derived, dimethylaminophenyl–tetracyanobutadiene donor–acceptor conjugates

“…[42][43][44][45][46][47][48] The photochemical behavior of a few donor-TCBD derived systems have been reported in the literature. [49][50][51][52][53][54][55][56] Although with high quantum yields, due to close proximity between the donor and acceptor entities, ultrafast charge separation and recombination was observed in these systems. That is, no charge stabilization could be accomplished.…”

One‐Photon Excitation Followed by a Three‐Step Sequential Energy–Energy–Electron Transfer Leading to a Charge‐Separated State in a Supramolecular Tetrad Featuring Benzothiazole–Boron‐Dipyrromethene–Zinc Porphyrin–C60

“…An energy‐level diagram was established to comprehend different photoinduced events in supramolecular tetrad, as shown in Figure 7. Energies of different states were calculated according to the Rehm–Weller approach, [25] as explained in our previous work [26] . From the earlier discussed steady‐state fluorescence studies, singlet–singlet energy transfer from directly excited 1 BTZ* ( hν 1 ) to BODIPY in BTZ‐BODIPY, BTZ‐BODIPY‐ZnP, and BTZ‐BODIPY‐ZnP:ImC 60 systems was clear (EnT1).…”

“…An energy-level diagram was established to comprehend different photoinduced eventsi ns upramolecular tetrad, as shown in Figure7.E nergies of different states were calculated according to the Rehm-Wellera pproach, [25] as explained in our previousw ork. [26] From the earlier discussed steady-state fluorescence studies, singlet-singlet energy transfer from directly excited 1 BTZ* (hn 1 )t oB ODIPY in BTZ-BODIPY,B TZ-BODIPY-ZnP, and BTZ-BODIPY-ZnP:ImC 60 systems was clear (EnT1). 1 BODIPY*, which wasp roduced as the product of energy transfer or by direct excitation (hn 2 ), could follow at least four photochemical eventsc ompetitively,n amely,f luorescence emission, ISC to populate 3 BODIPY*, charges eparation with the neighboring ZnP to yield the BTZ-BODIPYC À -ZnPC + charge-separated state (CS3), or singlet-singlet energy transfer to yield BTZ-BODIPY-1 ZnP * :ImC 60 (EnT2).…”

Interfacing High‐Energy Charge‐Transfer States to a Near‐IR Sensitizer for Efficient Electron Transfer upon Near‐IR Irradiation

“…[8] Based on this concept, few groups have been successful in building push-pull conjugates. [9][10][11] Research teams of Bottari, Torres,a nd Guldi have also reported phthalocyanine-TCBD and subphthalocyanine-TCBD donor-acceptor (D-A) systems. [10] Our groups have reported af ew TCBD functionalized chromophores,a nd established excited state charge separation useful for optoelectronic applications.…”

“…[10] Our groups have reported af ew TCBD functionalized chromophores,a nd established excited state charge separation useful for optoelectronic applications. [11] In general, in polar solvents,ICT transitions of push-pull systems occur at the low-energy side of the spectrum while that of LE transitions occur at the high-energy side of the spectrum. [6] This trend could be reversed if ahigh-energy ICT exhibiting species can be connected to anear-IR sensitizer.…”