Excited State Torsional Processes in Chalcogenopyrylium Monomethine Dyes.

Chalcogenopyrylium monomethine (CGPM) dyes represent a class of environmentally activated singlet oxygen generators with applications in photodynamic therapy (PDT) and photoassisted chemotherapy (PACT). Upon binding to genomic material, the dyes are presumed to rigidify, allowing for intersystem crossing to outcompete excited state deactivation by internal conversion. This results in large triplet yields and hence large singlet oxygen yields. To understand the nature of the internal conversion process that controls the activity of the dyes, femtosecond transient absorption experiments were performed on a series of S-, Se-, and Te-substituted CGPM dyes. For S- and Se-substituted species in methanol, rapid internal conversion from the singlet excited state, S1, occurs in ∼5 ps, deactivating the optically active excited state. The internal conversion produces a distorted ground-state species that returns to its equilibrium structure in ∼20 ps. For Te-substituted species, the internal conversion competes with rapid intersystem crossing to the lowest triplet state, T1, which occurs with a ∼ 100 ps time constant in methanol. In more viscous methanol/glycerol mixtures, the internal conversion to the ground state slows by 2 orders of magnitude, occurring in 500-600 ps. For Se- and Te-substituted species in viscous environments, the slower internal conversion rate allows a larger triplet yield. Using femtosecond stimulated Raman spectroscopy (FSRS) and time-dependent density functional theory (TD-DFT), the internal conversion is determined to occur by twisting of the pyrylium rings about the monomethine bridge. Evolution from the distorted ground state occurs by twisting back to the S0 equilibrium structure. The environmentally dependent photoactivity of CGPM dyes is discussed in the context of PDT and PACT applications.

Panchromatic Sensitization with ZnII Porphyrin-Based Photosensitizers for Light-Driven Hydrogen Production.

Three molecular photosensitizers (PSs) with carboxylic acid anchors for attachment to platinized titanium dioxide nanoparticles were studied for light-driven hydrogen production from a fully aqueous medium with ascorbic acid (AA) as the sacrificial electron donor. Two zinc(II) porphyrin (ZnP)-based PSs were used to examine the effect of panchromatic sensitization on the photocatalytic H2 generation. A dyad molecular design was used to construct a difluoro boron-dipyrromethene (bodipy)-conjugated ZnP PS (ZnP-dyad), whereas the other one featured an electron-donating diarylamino moiety (YD2-o-C8). To probe the use of the ZnP scaffold in this particular energy conversion process, an organic PS without the ZnP moiety (Bodipy-dye) was also synthesized for comparison. Ultrafast transient absorption spectroscopy was adopted to map out the energy transfer processes occurring in the dyad and to establish the bodipy-based antenna effect. In particular, the systems with YD2-o-C8 and ZnP-dyad achieved a remarkable initial activity for the production of H2 with an initial turnover frequency (TOFi ) higher than 300 h-1 under white light irradiation. The use of ZnP PSs in dye-sensitized photocatalysis for the H2 evolution reaction in this study indicated the importance of the panchromatic sensitization capability for the development of light absorbing PSs.

Rhodamine-Platinum Diimine Dithiolate Complex Dyads as Efficient and Robust Photosensitizers for Light-Driven Aqueous Proton Reduction to Hydrogen.

Three new dyads consisting of a rhodamine (RDM) dye linked covalently to a Pt diimine dithiolate (PtN2S2) charge transfer complex were synthesized and used as photosensitizers for the generation of H2 from aqueous protons. The three dyads differ only in the substituents on the rhodamine amino groups, and are denoted as Pt-RDM1, Pt-RDM2, and Pt-RDM3. In acetonitrile, the three dyads show a strong absorption in the visible region corresponding to the rhodamine π-π* absorption as well as a mixed metal-dithiolate-to-diimine charge transfer band characteristic of PtN2S2 complexes. The shift of the rhodamine π-π* absorption maxima in going from Pt-RDM1 to Pt-RDM3 correlates well with the HOMO-LUMO energy gap measured in electrochemical experiments. Under white light irradiation, the dyads display both high and robust activity for H2 generation when attached to platinized TiO2 nanoparticles (Pt-TiO2). After 40 h of irradiation, systems containing Pt-RDM1, Pt-RDM2, and Pt-RDM3 exhibit turnover numbers (TONs) of 33600, 42800, and 70700, respectively. Ultrafast transient absorption spectroscopy reveals that energy transfer from the rhodamine 1π-π* state to the singlet charge transfer (1CT) state of the PtN2S2 chromophore occurs within 1 ps for all three dyads. Another fast charge transfer process from the rhodamine 1π-π* state to a charge separated (CS) RDM(0•)-Pt(+•) state is also observed. Differences in the relative activity of systems using the RDM-PtN2S2 dyads for H2 generation correlate well with the relative energies of the CS state and the PtN2S23CT state used for H2 production. These findings show how one can finely tune the excited state energy levels to direct excited state population to the photochemically productive states, and highlight the importance of judicious design of a photosensitizer dyad for light absorption and photoinduced electron transfer for the photogeneration of H2 from aqueous protons.

A comparative study of the photophysics of phenyl, thienyl, and chalcogen substituted rhodamine dyes.

Although rhodamine dyes have been extensively studied for a variety of applications, many details of their photophysics are not yet fully understood, including the possible presence of a charge separated electronic state lying near the optically active excited singlet state and the role of twisting substituent groups in excited-state quenching. To address this, a large library of rhodamine dyes was studied in which the chalcogen is varied from O, to S and Se and the aryl group is either absent (in the pyronin series) or is a phenyl or thienyl substituent. Through an analysis of steady-state absorption spectroscopy, electrochemistry, X-ray crystallography, and quantum mechanical calculations, we show that the lowest unoccupied molecular orbital (LUMO) energy decreases in the O → S → Se series and when a phenyl or thienyl substituent is added. The reduction of the LUMO energy is larger for thienyl species in which the aromatic group has increased torsional flexibility. Excited state lifetimes and fluorescence quantum yields of these dyes in a high and low polarity solvent reveal dramatically different photophysics between chromophores with phenyl and thienyl substituents, due to a combination of torsional and inductive effects. In the pyronin and phenyl-substituted species, non-radiative decay can occur through an amine-to-xanthylium core charge separated state that is stabilized in a highly polar environment. In the thienyl derivatives, a lower energy excited state, which we term S'1, is accessed from S1via rotation of the aryl group and the excited state population rapidly equilibrates between S1 and S'1 in 6-30 ps. Preliminary photochemical hydrogen production data display the potential application of the thienyl derivatives for conversion of solar energy.

Deactivating unproductive pathways in multichromophoric sensitizers.

The effects of solvent and substituents on a multichromophoric complex containing a boron-dipyrromethene (Bodipy) chromophore and Pt(bpy)(bdt) (bpy = 2,2'-bipyridine, bdt =1,2-benzenedithiolate) were studied using steady-state absorption, emission, and ultrafast transient absorption spectroscopy. When the Bodipy molecule is connected to either the bpy or bdt in acetonitrile, excitation ultimately leads to the dyad undergoing triplet energy transfer (TEnT) from the redox-active Pt triplet mixed-metal-ligand-to-ligand' charge transfer ((3)MMLL'CT) state to the Bodipy (3)ππ* state in 8 and 160 ps, respectively. This is disadvantageous for solar energy applications. Here, we investigate two methods to lower the energy of the (3)MMLL'CT state, thereby making TEnT unfavorable. By switching to a low dielectric constant solvent, we are able to extend the lifetime of the (3)MMLL'CT state to over 1 ns, the time frame of our experiment. Additionally, electron-withdrawing groups, such as carboxylate and phosphonate esters, on the bpy lower the energy of the (3)MMLL'CT state such that the photoexcited dyad remains in that state and avoids TEnT to the Bodipy (3)ππ* state. It is also shown that a single methylene spacer between the bpy and phosphonate ester is sufficient to eliminate this effect, raising the energy of the (3)MMLL'CT state and inducing relaxation to the (3)ππ*.