Triplet BODIPY and AzaBODIPY Derived Donor‐acceptor Dyads: Competitive Electron Transfer versus Intersystem Crossing upon Photoexcitation

Abstract

The bis-iodo β-pyrrole-substituted BF 2 -chelated dipyrromethene, I 2 BODIPY, and its structural analogue BF 2 -chelated aza dipyrromethene, I 2 azaBODIPY, carrying a nitrogen at the mesoposition instead of carbon, were synthesized and characterized as new set of triplet sensitizers using different techniques. These sensitizers were further functionalized with fullerene, C 60 , at the central boron atom to build donor-acceptor conjugates. Using spectral, electrochemical, and computational methods, these conjugates were characterized, and the energy levels were established. Intersystem crossing to populate the triplet state was observed upon excitation of I 2 BODIPY and I 2 azaBODIPY, however, the measured rates of k ISC were found to be nearly two orders of magnitude higher for I 2 azaBODIPY (k ISC~1 0 11 s À 1 ) compared to I 2 BODIPY (k ISC~1 0 9 s À 1 ). The energetics, k ISC , and position of HOMO and LUMO levels was found to control the ability of the dyad to undergo electron transfer, although the donor-acceptor distances were virtually the same in both I 2 BODIPY-C 60 and I 2 azaBODIPY-C 60 conjugates. Freeenergy calculations revealed that the photoinduced electron transfer process was thermodynamically feasible from only the singlet excited states in both conjugates. Consequently, electron transfer from the 1 I 2 BODIPY* in competition with intersystem crossing was witnessed in the case of I 2 BODIPY-C 60 dyad while in the case of I 2 azaBODIPY-C 60 dyad, excitation of azaBODIPY led to a short-lived charge transfer state involving the catechol bridge followed by populating the low-lying 3 I 2 azaBODIPY* state without promoting the process of charge separation involving C 60 . The lifetime of the charge-separated states was in the ns range in the I 2 BODIPY-C 60 conjugate both in polar and nonpolar solvents.[a] S. subsequently reacted with NIS to make 1 d bearing iodo entities on the pyrrole rings. Finally, 1 d was reacted with C 60 in the presence of sarcosine in dry toluene to yield the dyad 1, according to Prato reaction. [16] The synthesis of 2 and the control compounds is given in Scheme 2. First, the BF 2 chelated (3,5-diphenyl-1H-pyrrol-2-yl) (3,5-diphenylpyrrol-2-ylidene)-amine, azaBODIPY was synthesized by literature methods. [17] This compound was further reacted with NIS in dry chloroform:acetic acid mixture to yield I 2 azaBODIPY, 2 b. In a separate experiment, azaBODIPY was reacted with 3,4-dihydroxybenzaldehyde in the presence of AlCl 3 to obtain the formyl phenyl dioxyboron derivative, 2 c. 2 c was further reacted with NIS to obtain 2 d. Finally, the electron acceptor fullerene was appended by reacting 2 d with fullerene and N-methyl glycine in dry toluene. It may be pointed here that reacting 1 b and 2 b directly with 3,4-dihydroxybenzaldehyde in the presence of AlCl 3 to obtain 1 d and 2 d resulted in complex reaction mixture due to the likely reaction of AlCl 3 with iodo entities.The newly synthesized compounds were fully characterized by MALDI-TOF-mass, 1 H and 13 C NMR techniques (see s...

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

“…Figure 5a shows the energy diagram for compound 1 having donor TPAa nd acceptor azaBODIPY entities.I n polar benzonitrile,C Sf rom 1 azaBODIPY* to yield TPAC + -azaBODIPYC À is thermodynamically possible with an energy of charge separated state of 1.31 eV (= ÀDG CR ). Formation of charge separation could also involve an intermediate CT state,asshown in Figure 5a.The energy of the CS state in this case is about 0.3 eV higher than that of 3 azaBODIPY* (E T % 1.0 eV), [17] and under such conditions,t he CS state could undergo charge recombination (CR) to populate the lowlying 3 azaBODIPY* state.T he 3 azaBODIPY* could relax back to the ground state slowly via the spin-forbidden triplet (phosphoresce) emission.…”

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

“…Such analysis resulted in two components, the first one had spectral features of the electron‐transfer product with a time constant of 7.5 ps. The second component with infinity time constant (>3 ns monitoring time window of our instrument) with a positive peak at 530 nm, characteristic of 3 BODIPY* which could be a product of direct intersystem of 1 BODIPY*. The short lived ion‐pair state could be attributed to the central acetylene bond favoring an electron‐hole recombination process.…”

“…In the case of SM1 , the only energetically favorable electron‐transfer process upon excitation of the S 0 →S 1 peak of BODIPY ( hν LE in Figure ) is the excited‐state reductive electron‐transfer resulting in BODIPY .− ‐PTZ .+ radical ion‐pair. Energy of this product is below that of 3 BODIPY* being 1.63 eV . In such a case, the electron‐transfer product could directly undergo back electron transfer to yield ground‐state product.…”

NIR‐Absorbing Donor–Acceptor Based 1,1,4,4‐Tetracyanobuta‐1,3‐Diene (TCBD)‐ and Cyclohexa‐2,5‐Diene‐1,4‐Ylidene‐Expanded TCBD‐Substituted Ferrocenyl Phenothiazines

“…The data was further analyzed by global analysist og enerate decay associated spectra.S uch analysisr esulted in twoc omponents, the first one had spectralf eatures of the electron-transfer product with atime constanto f7.5 ps. Thesecond component with infinity time constant (> 3nsm onitoring time window of our instrument) with ap ositive peak at 530 nm, characteristic of 3 BODIPY* [23] which could be ap roduct of direct intersystem of 1 BODIPY*. The short lived ion-pair state could be attributed to the central acetylene bond favoring an electron-hole recombination process.…”

“…In the case of SM1,t he only energetically favorable electrontransfer process upon excitation of the S 0 !S 1 peak of BODIPY (hn LE in Figure 5) is the excited-state reductive electron-transfer resultingi nB ODIPYC À -PTZC + radical ion-pair.E nergyo ft his product is below that of 3 BODIPY*b eing 1.63 eV. [23] In such a case, the electron-transfer product could directly undergo back electron transfer to yield ground-state product. The electrontransfer path from the 1 BODIPY*L Es tate to yield BODIPY-TCBDC À -PTZC + in the case of SM2 and BODIPY-DCNQC À -PTZC + in the case of SM3 is discussed earlier with the helpo fF igure 4.…”