Energy Transfer and Emission Quantum Yields of Organic−Inorganic Hybrids Lacking Metal Activator Centers

Abstract

This work discusses quantitatively the energy transfer mechanism that occurs in the white-light emission of sol−gel derived amine- and amide-functionalized hybrids. The white-light photoluminescence (PL) results from a convolution of the emission originated in the NH/CO groups of the organic/inorganic cross-links with electron-hole recombinations occurring in the siloxane nanoclusters, both emissions typical of donor−acceptor pairs. Two model compounds that reproduce separately the two hybrid's emissions were synthesized and characterized by X-ray diffraction, 29Si/H/13C magic-angle spinning NMR, diffuse reflectance, Fourier transform−IR, and photoluminescence spectroscopy to support their use as organic and inorganic structural models for the two counterparts of the hybrids. The comparison between the lifetimes of the two emissions of the inorganic and organic model compounds with those of the hybrids, the Arrhenius dependence with temperature of the siliceous-related lifetime in the hybrids, and the nonexponential behavior of the decay curve of the siliceous-related emission under lower excitation wavelengths are experimental evidence supporting the occurrence of energy transfer in the hybrids. This energy transfer rate is quantitatively estimated for d-U(600) (the diureasil host with smaller number of polymer repeat units) generalizing the ideas proposed recently for the intramolecular energy transfer between singlet and triplet ligand levels and ligand-to-metal charge transfer states in lanthanide coordination compounds. The dipole−dipole energy transfer rate between the two emitting centers is 1.3 × 109 s-1, larger than the value estimated for the transfer rate mediated by the exchange mechanism, 3.7 × 108 s-1. The predicted room-temperature emission quantum yield of that diureasil hybrid is comparable to the corresponding experimental value (7 ± 1 %), pointing out a strong dependence of the radiative component values of the two emissions with temperature, induced by the glass−rubber phase transition of the hybrid's polymer chains.

Tuning the Photoluminescence of Silsesquioxanes with Short Substituted Urea Bridges

“…[1][2][3][4][5][6][7][8][9][10][11][12] In particular, materials bearing urea groups in the organic bridge have received considerable attention in recent years due to their white light photoluminescence (PL). [13][14][15][16][17][18][19][20] Photoluminescent ureabridged silsesquioxanes are usually synthesized by the carboxylic acid solvolysis of the corresponding precursors containing terminal trialkoxysilyl groups. [16,18] In this process condensation reactions can be carried out in the absence of water, [21][22][23] leading to hybrid materials exhibiting high PL quantum yields.…”

Natural halloysite nanotubes as mesoporous carriers for the loading of ibuprofen

“…In the spectra of Hal/120-A, the signals at 43.7 (and 48.5), 21.3 (and 24.2 and 30.2), and 9.6 ppm are attributed to the carbon atoms at positions I, II, and III in the propyl groups (Fig. 5b), respectively; and the signal at 17.5 ppm is attributed to the carbon atom at position V in the ethoxyl groups [63][64][65][66]. Note that the 13 C MAS NMR spectra are more qualitative than quantitative, because the NMR peak areas for the three carbons in propyl group and for the two carbons in ethoxyl group are not accurately equivalent.…”

Self-Structuring of Lamellar Bridged Silsesquioxanes with Long Side Spacers

“…Energy transfer between these two distinct emitting centers was quantitatively estimated for the diureasil host with the smallest number of polymer repeat units (d-U(600)) yielding rate values of 1.3 Â 10 9 s À1 (dipoleÀdipole mechanism) and 3.7 Â 10 8 s À1 (exchange process). 64 Because these energy transfer rates strongly depend on the distance between the two emitting centers, the similar lifetimes of the siliceous-related component in the long-chain analogue d-U(2000) and C 10 C 11 C 10 suggest a similar average distance between the siliceous domains and the urea cross-links, indicating that the side spacers in C 10 C 11 C 10 most probably adopt gauche conformations. Furthermore, the 4-fold increase of the radiative transition decay rate (inverse of the lifetime at 10 K) in C 10 C 11 C 10 relative to d-U (2000) is probably associated with the increasing vibronicÀspinÀorbit interaction 64 due to a distinct distribution of hydrogen bonds induced by the increase in the length of the side spacers.…”

“…64 Because these energy transfer rates strongly depend on the distance between the two emitting centers, the similar lifetimes of the siliceous-related component in the long-chain analogue d-U(2000) and C 10 C 11 C 10 suggest a similar average distance between the siliceous domains and the urea cross-links, indicating that the side spacers in C 10 C 11 C 10 most probably adopt gauche conformations. Furthermore, the 4-fold increase of the radiative transition decay rate (inverse of the lifetime at 10 K) in C 10 C 11 C 10 relative to d-U (2000) is probably associated with the increasing vibronicÀspinÀorbit interaction 64 due to a distinct distribution of hydrogen bonds induced by the increase in the length of the side spacers. The latter result proves that the emission features of the urea-related component of diurea-cross-linked BSs are sensitive to the length of the side spacers (we recall that there are only three methylene groups in the side spacers of the diureasils versus 10 methylene groups in those of the C 10 C n C 10 hybrids).…”