Determination of the ground- and excited-state dipole moments of bromocresol purple in protic and aprotic solvents

• Excited- and ground-state dipole moments were determined for bromocresol purple.
• Modified Lippert–Mataga and Bilot–Kawski solvatochromic methods were used.
• Dipole moments were determined in both protic and aprotic solvents.
• Hydrogen bonding was shown to stabilize both ground and excited states.

Although it has been widely recognized that hydrogen bonds play a significant role in the photophysics of molecules, this phenomenon has rarely been applied to the solvatochromic method for determination of dipole moments. The difference in the dipole moment between the ground and excited state was determined in protic and aprotic solvents using both the Lippert–Mataga equation and the Bilot–Kawski equation for bromocresol purple, a molecule capable of hydrogen-bond donation and acceptance. The dipole change in protic environments was determined to be 15.2 ± 1.0 D for the Lippert–Mataga method and 9.2 ± 1.0 D for the Bilot–Kawski method, while the change in aprotic environments was 10.4 ± 1.0 D and 6.7 ± 1.0 D, respectively. Both methods highlighted the importance of hydrogen bonding in stabilizing increased charge-separation of the excited state, allowing for larger changes in dipole moments in protic environments. This study further validates a simple, rational modification to the commonly used methods that allows access to dipole-moment data on dyes with hydrogen-bonding capabilities through solvatochromic experiments.

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