Bichromophores with scaffold linkers

Scaffold linkers with fixed conformation, such as decalin,^[69,70] steroid^[24,71–78]

or polynorbornyl^[67,79–84] systems allow the design of bichromophores with a rather well-defined chromophore distance and in some cases even fixed relative orientation.

Much attention was paid to bichromophores with linkers based on steroid structures.^[24,71–78] Vollmer and co-workers synthesized bichromophores with an anthrylthiophene group as energy donor and a bithienylporphyrin part as acceptor. The authors incorporated the steroid epi-androsterone or an α,α'-oligothiophene as linkers into bichromophores11and12, respectively (fig-ure 11).

S

S S

N NH N

HN BRIDGE

BRIDGE =

(R = H) R

R

(R = n-Bu) S

S S

11 12

Figure 11:Bichromophores with steroid based (11) and oligothiophene based (12) linkers.^[74]

The authors observe intramolecular energy transfer from the anthracene to the porphyrin moiety for both compounds (11and 12). In the case of the oligoth-iophene linker, the energy transfer is quantitative, whereas in the case of the rigid steroid linker the efficiency is lower, but still in the range of 99 %. These results indicate EET via the oligothiophene chain in compound12which is par-tially interrupted by the androstane linker in compound11. This interruption leads to some observable donor fluorescence. Since the EET is very efficient even in bichromophore11 with a non-conjugated linker, the authors assume that a

Chapter 2 BICHROMOPHORIC COMPOUNDS

Förster transfer dominates in both compounds. However, the authors cannot completely rule out through-bond interactions. Besides that, the presence of conformers may lead to different chromophore orientations and influence the FRET efficiency. Furthermore, the distance between the anthracene and por-phyrin group is shorter in compound12(19 Å) compared to11(21 Å).

Closs and co-workers designed a series of bichromophoric compounds using cy-clohexane or decalin as a linker (figure 12), and investigated the influence of the position and stereochemistry of the chromophore attachment to the linker.^[85]

H

H H

H

13 14

Figure 12:Bichromophores with cyclohexane and decalin linkers.^[69,85]

Initially, these authors studied intramolecular ET,^[85] but then also intramolec-ular TEET, in order to investigate similarities between both processes.^[69] They found for both processes that the transfer rate falls off exponentially with in-creasing the number of bonds separating the chromophores. Among stereoiso-mers the maximum rates were found for the equatorial-equatorial substitu-tion patterns. The logarithmic plots of the transfer rate versus the number of bonds separating the chromophores showed a slope ofβ=1.15/bond for ET and α=2.6/bond for TEET. The ratio ofα/β≈2 fits the prediction from the rather oversimplified model which treats TEET as two simultaneous occurring ET pro-cesses (see section 1.2.2).^[69,70]

N B N N

B N F F

F F

NMe₂

NMe₂ 15

Figure 13:An example of a bichromophore with a scaffold linker and free chromophore

Bichromophores with rigid and semi-rigid linkers 2.3 Ziessel and co-workers designed compound 15 based on 2,2’-disubstituted 9,9’-spirobifluorene as linker in combination with two boron dipyrromethane (BODIPY) chromophores as shown in figure 13.^[86] The authors tried to inhibit through-bond EET by incorporating an orthogonal connection (at the spirocen-ter) into the linker unit. While the distance was constant (20 Å), the spectral overlap integral was modified by addition of HCl creating the mono- and dipro-tonated forms of the dimethylamino groups:15(H⁺) and15(2H⁺). Experimental values of EET rates were compared with calculated values. For the neutral form of compound 15, the transition dipole moments were approximated as point-dipoles. The authors refrained from applying the point-dipole approximation, in the case of the protonated forms, because the wavefunctions were described as

"banana-like" in shape and therefore are unsatisfactorily approximated by point-dipoles. Instead, thetransition density cubeapproach^[87]was used to model the Coulombicinteractions and to estimate the EET rate. In this method, the ground and excited state wavefunctions of a chromophore (derived fromab initio calcu-lations) are combined to yield a three-dimensional transition density which is called the transition density cube (TDC). The TDCs of two chromophores are interacted to describe theCoulombiccoupling between them.^[88]While the cal-culated rates of the neutral compound15and the symmetric15(2H⁺) are in good agreement with the experimental values, the rates for15(H⁺) with an asymmet-ric push-pull energy acceptor differ considerably from the experimental values.

Chapter 2 BICHROMOPHORIC COMPOUNDS

O O O

NH N

O

O O O

N O O

n Calix[n]arene

OR¹ R²

CH₂

16

Figure 14:General structure of calix[n]arenes and bichromophore 16 with calix[4]arene linker.^[89]

Anh and co-workers designed compound 16 based on calix[4]arene.^[89]

Calix[n]arenes are oligomers composed of para-substituted phenols which are linked by methylene bridges throughortho-positions (figure 14). The authors observed two possible conformers depending on the solvent: in solvents like cy-clohexane, toluene or chloroform a conformer withπ-stacking of the two chro-mophores was observed, while in THF 25 % of the molecules were found to be in an extended conformation with a larger chromophore separation. In the case of the π-stacked conformation photoexcitation of the perylene bisimide unit leads to ET fom the pyrene chromophore towards the electron-accepting pery-lene bisimide. In contrast, when exciting the pyrene unit in the extended con-former the calixarene scaffold acts as an electron donor and the pyrene unit acts as electron acceptor.^[89]

Bichromophores with rigid and semi-rigid linkers 2.3

Figure 15:a) Bichromophore17with rigid bridge and fixed chromophore orientation.

b) Frontier molecular orbital description of the photoinduced electron trans-fer (ET). HOMOs and LUMOs of electron donor (D) and acceptor (A) are named d, d* and a, a*, respectively.^[83,90]

Williams and co-workers designed bichromopohore17 with a norbornylogous bridge comprising linearly fused norbornane and bicyclo[2.2.0]hexane units connecting a fullerene moiety and a N,N'-dimethylaminophenyl group (fig-ure 15a).^[83]After photoexcitation of the fullerene C₆₀unit, they observed rapid charge separation in a polar solvent that gives a long-lived charge-separated state (with a lifetime of ca. 0.25 µs). Figure 15b shows a simplified frontier molec-ular orbital description of the photoinduced electron transfer (ET) where the electron acceptor has the lowest excited state energy level. The ratio between the rates of charge separation and charge recombination for this system in is kcs/kcr≈1400. They proposed to use this design of donor-bridge-fullerene (ac-ceptor) systems as molecular building block in optoelectronic devices.^[83,90]

Scholes and co-workers designed bichromophore18in which naphthalene and anthracene are connected by a rigid bis(norbornyl)bicyclo[2.2.0]hexane linker (figure 16a).^[80] Although direct Coulombic interactions between the lowest-energy excited states should be forbidden due to orthogonal donor and acceptor transition dipole moments, the authors observed and studied SEET from

naph-Chapter 2 BICHROMOPHORIC COMPOUNDS thalene towards anthracene.

a)

b)

S0-S1

S₀-S₁ S₀-S₂

S0-S2 18

Figure 16:a) Bichromophore 18with a rigid norbornyl-fused linker, and fixed chro-mophore orientation;^[80]b) Absorption transition dipole moments of naph-thalene and anthracene.^[6]

Normally EET between from naphthalene to anthracene is assumed to occur between the lowest electronically excited states: According to Platt’s nomen-clature,¹L_b for naphthalene and¹L_a for anthracene.^[91] To put it more simply, the relevant transitions are S₀→^S1for naphthalene and S₁→^S0 for anthracene.

As shown in figure 16b, the corresponding transition dipole moments are ori-ented along the long and the short axis of naphthalene and anthracene, respec-tively. Consequently, due to orthogonality of the donor and acceptor transition dipole moments, the direct dipole-dipole coupling between these states should not occur.^[81]

Belser and co-workers designed a Ru/Os bimetallic complex19with a rigid rod-like bridging ligand based on adamantane (figure 17).^[92]

N N N

N

Os(bpy)₂ (bpy)2Ru

4+

19

Figure 17:An example of a bichromophore with scaffold linker and fixed chromophore orientation.^[92]

The ligand possesC² symmetry and leads to bimetallic complexes with a linear arrangement of the linker and the two metals. Upon excitation of the mixed-metal compound Ru^II/Os^IIat 465 nm, two emission bands were observed, which

Applications of multichromophoric compounds 2.4 the Ru^II-metal center is strongly reduced, while the luminescense of the Os^II center is enhanced (compared to the Ru^II/Ru^IIand Os^II/Os^IIcomplexes). The au-thors conclude an intramolecular energy-transfer process in the mixed Ru^II/Os^II complex and determined a transfer rate ofk_EET=⁵·^107s-1.