Photophysics of Diphenylacetylene: Light from the “Dark State”

Abstract

A weak band at the tail of the known tolane (diphenylacetylene, DPA) fluorescence spectrum in several solvents is assigned to the forbidden 1(1)A(u) → 1(1)A(g) transition on the basis of its lifetime (∼200 ps) and its fluorescence excitation spectra. The 1(1)A(u) state, generally called the dark state, is not truly dark. We report the temperature (T) dependence of DPA fluorescence quantum yields (ϕ(f)) in methylcyclohexane (MCH) solution and the fluorescence and phosphorescence quantum yields of DPA in glassy MCH at 77 K. Significant differences between fluorescence and phosphorescence excitation spectra reveal that, in addition to the 1(1)B(1u) ← 1(1)A(g) transition, the first DPA absorption band includes a transition to another excited state, most probably the 1(1)B(2u) state, from which intersystem crossing is more efficient. The T dependence of ϕ(f) values in MCH solution is shown to be consistent with the previously reported T dependence of the lifetimes of transient DPA singlet excited state absorptions in the picosecond time scale. Transient absorption decay rate constants in hexane, methylcyclohexane and decalin as a function of T are retreated. Application of the medium enhanced barrier model shows that the medium is fully engaged with the molecular motion that is involved in the activated nonradiative decay path of the 1(1)B(1u) state. In accord with theoretical calculations and experimental observations, that process is assigned to the diabatic internal conversion of the short-lived linear fluorescent π,π* (1(1)B(1u)) state, over a low intrinsic energy barrier, to the longer lived weakly fluorescent trans-bent π,σ* (1(1)A(u)), which is the precursor of the DPA triplet state. Absorption and fluorescence measurements in several solvents show that the 1(1)B(1u)-1(1)A(g) energy gap decreases linearly with increasing medium polarizability. Our results allow a more definitive state order assignment for DPA.

Carbazole‐Terpyridine Donor‐Acceptor Dyads with Rigid π‐Conjugated Bridges

“…This excited‐state scenario, in which both a linear “bright” π–π* state and a trans‐bent “dark” π–σ* state are located on the S 1 potential energy surface, resembles those already investigated for diphenylacetylene and for other organic molecules with C≡X (X=C or N) groups . However, in the present case, we did not succeed in finding a minimum for the trans‐bent π–σ* state of 2 since TD‐DFT optimizations failed.…”

“…Accordingly, the corresponding dyads can adopt a large number of virtually isoenergetic conformations, which are generated by the reciprocal orientations of the optimal dihedral angles between the different moieties of the dyads (Tables S1–S7). While the tolane unit in known to be planar, the biphenyl one is displays torsional angles of about 40°, depending on the environment . As a consequence, the dihedral angle between the plane of the carbazole and of the terpyridine moiety ( i. e ., α DA in Table ) can vary substantially within the different minima of a specific dyad, ranging from nearly coplanar to almost perpendicular conformations (as in the simplest case of 1 , see Table , Table S1 and Figure S1 in the Supporting Information).…”

Unusual Solvatofluorochromism of the Asymmetrically Substituted Aromatic Acetylene Derivative CNAacDMA**

“…They found that while the lowest‐energy singlet state of diphenylacetylene is the 1B 1u (ππ*) state in the linear D 2h symmetry, the πσ* state of the 1 1 Au symmetry in the bent C 2h symmetry is the lowest in energy. The transition from fluorescent ππ* state to weakly fluorescent πσ* state required crossing a small energy barrier [15] . The prediction arising from the calculation were confirmed experimentaly [26,31,32,33] .…”

Bridged Tolanes: A Twisted Tale

“…Emission from the "dark" state (1 1 A u → 1 1 A g ) is weak due to its bent, stilbene-like structure. 20 The triplet state (1 3 B 1u ) is linear with a lifetime of τ = 1 μs. 21−23 Tolanes twist either through sterically demanding groups 15,24−32 or by choosing a suitable tether, connecting the two phenyl rings.…”