Conformational Effects on the Dynamics of Internal Conversion in Boron Dipyrromethene Dyes in Solution

Abstract

Electronic energy transfer plays a crucial role in many natural processes, ranging from self-protection [1] to repair mechanisms [2] and energy transduction. [3] One of the primary reasons for employing such indirect excitation is that ideal reagents for inducing chemical change, such as electron transfer or conformational exchange, rarely possess optimal light-absorbing properties.[4] This simple strategy of equipping an elaborate photochemical device with an ancillary photon collector, programmed to absorb over a broad spectral range and to emit at a discrete wavelength, is also relevant to artificial photosynthetic systems. Indeed, such strategies may have relevance to the design of dye-sensitized solar cells [5] and certain organic light-emitting diodes.[6] Similar processes also appear relevant to conducting polymers [7] and solar concentrators.[8] A further, and possibly important, role for lightharvesting units is to promote the harmless dissipation of ultraviolet (UV) photons, arising from increased UVB radiation at the Earths surface because of stratospheric ozone depletion that would otherwise initiate photo-degradation. The latter realization has led to the recent investigation of cases where an upper-lying excited state (e.g., S 2 ) of a strongly fluorescent dye functions as the energy acceptor for high-energy photons while the lowest-energy excited state acts as sensitiser for the photonic device.[9] Complementary reports have shown that the S 2 states of certain dyes, notably carotenes [10] and metallo-porphyrins, [11] can be used as energy donors.In terms of protective mechanisms and device efficacy, it is essential that the S 2 state formed through energy transfer undergoes rapid electronic relaxation (i.e., internal conversion) to the corresponding S 1 state, without opening new routes for deleterious or antagonistic chemical steps. Although the S 2 states of most organic molecules tend to be very short-lived, there are notable exceptions [12] and few rules other than the generic Englman-Jortner energy-gap law [13] by which to predict their lifetimes. Even less is understood about how the internal conversion dynamics depend on the molecular structure and the local environment. We now report on internal conversion in two boron dipyrromethene dyes for which the meso-phenyl ring is either constrained (Bodipy) or free to rotate (Robod), the latter class of dye being popular rheology probes. [14] Our particular interest lies with understanding the effect of nuclear motion on the ultrafast dynamics of upper-lying excited states. Earlier work has concluded that deactivation of the higher-lying (i.e., S 3 and/or S 2 ) excited states and the simultaneous population of the S 1 states of such dyes occur on timescales of a few hundreds of femtoseconds while vibrational cooling of the S 1 state occurs over approximately 20 ps. [15] The two dyes, Robod and Bodipy (Scheme 1), differ by virtue of the substitution pattern at the pyrrole units. This structural change has an important effect on the photophysical p...

Fluorescence Quenching in BODIPYs Having Ir‐ and Rh‐Tethered Complexes

“…As noted above, hydrogen-substituted BODIPY derivatives 7a and 7c are nonemissive species because of the well-known effect of the unrestricted aryl rotation attached to the C8-carbon atom. [8] Figure 6. Emission spectra of 7.…”

“…The lack of coplanarity between both moieties of the dyad is mainly due to the steric hindrance of the indacene methyl groups, as reported for related non-organometallic BODIPY derivatives. [7,8] Density functional theory (DFT) calculations [9] on 8a, lacking the indacene methyl groups, indicate that both fragments are more planar in the gas-phase equilibrium geometry (dihedral angle of -52.5°). This subtle structural difference makes 8a and the rest of the hydrogen-substituted BOD-IPY derivatives nonemissive (see below) due to the unrestricted C8-aryl rotation.…”

“…This subtle structural difference makes 8a and the rest of the hydrogen-substituted BOD-IPY derivatives nonemissive (see below) due to the unrestricted C8-aryl rotation. [8] The influence of the structure of compounds 8 and 9 on their photophysical properties was then studied by using a combination of experimental and computational tools. Figure 3 shows the UV/Vis spectra of compounds 8a-d together with non-metallated BODIPY derivatives 7a and 7b recorded at 298 K in CH 2 Cl 2 with a concentration of 1 × 10 -6 mol L -1…”

Solid‐State Emission from Mono‐ and Bichromophoric Boron Dipyrromethene (BODIPY) Derivatives and Comparison with Fluid Solution

“…The absorption spectrum recorded for single crystals of 2 exhibits a broad transition centred at around 522 nm which is reminiscent of that seen for 2 in dilute solution (Figure S25). There is also a weak, broad transition centred at around 400 nm that corresponds to excitation into upper‐lying excited singlet states . Unlike the case in fluid solution, the solid‐state spectrum shows notable absorption at longer wavelength (Figure S25).…”

“…The absorption spectrum recorded for a single crystal of BC2 shows a maximum at 545 nm but with quite pronounced absorption at longer wavelengths (Figure ). Excitation to higher‐lying singlet states is seen at around 390 nm . The absorption profile in the 450–560 nm window can be analysed in terms of Gaussian components with the aid of the crystallographic data (Figure ).…”