Water-soluble [2.2]paracyclophane chromophores with large two-photon action cross sections

A series of α,ω-donor-substituted distyrylbenzene dimers held together by the [2.2]paracyclophane core were designed, synthesized, and characterized. Different substituents were chosen to modulate the strength of the donor nitrogen groups and to allow the molecules to be either neutral and soluble in nonpolar organic solvents or charged and water-soluble. The specific neutral structures are (in order of decreasing donor strength) 4,7,12,15-tetra[N,N-bis(6′′-chlorohexyl)-4′-aminostyryl]-[2.2]paracyclophane (1N), 4,7,12,15-tetra[(N-(6′′-chlorohexyl)carbazol-3′-yl)vinyl]-[2.2]paracyclophane (2N), and 4,7,12,15-tetra[N,N-bis(4′′-(6′″-chlorohexyl)phenyl)-4′-aminostyryl]-[2.2]paracyclophane (3N). The charged species are 4,7,12,15-tetra[N,N-bis(6′′-(N,N,N-trimethylammonium)hexyl)-4′-aminostyryl]-[2.2]paracyclophane octaiodide (1C), 4,7,12,15-tetra[(N-(6′′-(N,N,N-trimethylammonium)hexyl)carbazol-3′-yl)vinyl]-[2.2]paracyclophane octaiodide (2C), and 4,7,12,15-tetra[N,N-bis(4′′-(6′″-(N,N,N-trimethylammonium)hexyl)phenyl)-4′-aminostyryl]-[2.2]paracyclophane octaiodide (3C). Two-photon excitation spectra, measured using the two-photon induced fluorescence technique, show in toluene the following trend for the two-photon cross sections (δ): 3N > 2N > 1N. In water the δ values follow the same order, 3C ≈ 2C > 1C, but are smaller (approximately one-third). Significantly, the fluorescence quantum yield (η) in water decreases much more for 1, relative to 2 and 3. The two-photon action cross sections (δη) of 2C and 3C are 294 GM and 359 GM, respectively. These values are among the highest reported thus far. These results show that, to maximize the δη in this class of chromophores, one needs to fine-tune the magnitude of the charge transfer character of the excited state, to minimize fluorescence quenching in polar media.