Origin of Intense Intramolecular Charge‐Transfer Interactions in Nonplanar Push–Pull Chromophores

Abstract

Planar push-pull chromophores featuring intense intramolecular charge-transfer (CT) interactions have been extensively studied in view of their potential applications in molecular electronics and optoelectronics. [1, 2] In contrast, only a limited number of nonplanar low-molecular-weight donoracceptor chromophores has been reported and the impact of nonplanarity on their p-conjugative and optoelectronic properties not been systematically investigated. [3, 4] Nonplanar CT chromophores tend to feature some desirable physical properties compared to their planar counterparts: they are usually more soluble, less aggregating, and more readily sublimable, forming amorphous, rather than crystalline films for potential use in optoelectronic devices. [2a, 5] We showed recently that donor-substituted alkynes undergo a formal [2 + 2] cycloaddition with electron-accepting olefins, such as tetracyanoethene (TCNE), [6][7][8] 7,7,8,8-tetracyanoquinodimethanes (TCNQs), [9][10][11] as well as dicyanovinyl (DCV) and tricyanovinyl derivatives, [12] followed by retroelectrocyclization, under formation of nonplanar push-pull chromophores featuring intense intramolecular CT and high third-order optical nonlinearities. Nonplanar, N,N-dimethylanilino (DMA) donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs), obtained by [2 + 2] cycloaddition of TCNE, produce high-optical quality amorphous films by vapor-phase deposition [13a] which in the meanwhile have found first application in silicon-organic-hybrid (SOH) waveguides.[13b] A key question raised was the origin of the intense intramolecular CT interactions in these push-pull chromophores, in view of their pronounced nonplanarity which is expected to lead to disruption of donor-acceptor p-conjugation and concomitant reduction in CT efficiency and optical nonlinearity.To investigate the effects of sterically enforced deconjugation on optoelectronic properties, we prepared two new series of push-pull chromophores (1 and 2, and 3-8), by adding TCNE or TCNQ to tetrathiafulvalene (TTF)-or ferrocenyl (Fc)-substituted alkynes, respectively. These strong electron donors have found numerous applications in intermolecular [14,15] and intramolecular CT systems, [16] but have not seen much use for activating alkynes electronically for the cycloaddition of TCNE or TCNQ. [17] TTF-appended TCBD 1 was obtained in 41 % yield as a deep blue solid by reaction of TCNE with TTF-substituted alkyne 9 in 1,2-dichloroethane at 80 8C (Scheme 1). The electronically more activated alkyne 10, with both TTF and DMA donor groups, reacted at room temperature to give 2 in 79 % yield. The ferrocene-substituted expanded TCNQs 3-8 were formed as black metallic-like solids in 41-81 % yield by regioselective cycloaddition between TCNQ and the acetylenic precursors 11-16. With the exception of 3 and 8, these push-pull chromophores are thermally stable up to 300 8C, as revealed by thermal gravimetric analysis (TGA); compound 6 can actually be sublimed without decomposition at about 250 8C/1 10 À6 Torr. Single crysta...

Postfunctionalization of aromatic polyamine by [2+2] cycloaddition of 7,7,8,8-tetracyanoquinodimethane with side chain alkynes

“…The first reduction potential (E red,1 ) of P2 (x ? y = 1) was clearly detected at -0.77 V. This potential is comparable to those of the small donor-acceptor molecules [42][43][44], indicating that the side chain chromophores of P2 are almost independent of each other. This is probably caused not only by the sp 3 -hybridized nitrogen atoms in the polymer main chain, but also by the significant twist between the repeating units.…”

Click synthesis of donor–acceptor-type aromatic polymers

“…The resulting poly(aryleneethynylene)s and poly(arylenebutadiynylene)s are an important class of conjugated polymers with fascinating optical and electrical properties [40,41]. In particular, ferrocene-containing poly(aryleneethynylene)s have been well studied [42,43], and the TCNE or TCNQ addition to ethynylferrocene derivatives has also been examined [22,23].…”

“…The additional important advantages of this reaction over the conventional azidealkyne click reaction are that no metal catalysts are necessary and the products feature strong chargetransfer (CT) bands in the visible absorption region, potent redox activities in both the cathodic and anodic directions, and related properties, e.g., excellent nonlinear optical effects [16,17]. The currently available donor groups for almost quantitative yields include aromatic amines [18][19][20][21], ferrocene [22,23], and azulene derivatives [24,25]. Diederich and others are now extensively expanding this chemistry by applying it to other acceptor molecules, such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) and the derivatives [26][27][28][29] as well as the dicyanovinyl and tricyanovinyl derivatives [30,31].…”

An investigation of the role increasing π-conjugation has on the efficiency of dye-sensitized solar cells fabricated from ferrocene-based dyes

“…The ferrocenyl groups in all dyes adopt an eclipsed D 5h conformation rather than the staggered conformation. 15,16 The electron densities of the HOMOs for all dyes are mainly localized on the ferrocenyl Table 1 Summary of optical and electrochemical properties of the dyes. moiety and are extended along to the benzothiadiazole moiety to the phenyl ring.…”